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Prevention and control measures for coastal erosion in northern high-latitude communities: a systematic review based on Alaskan case studies

Min Liew 1,4 , Ming Xiao 1 , Benjamin M Jones 2 , Louise M Farquharson 3 and Vladimir E Romanovsky 3

Published 19 August 2020 • © 2020 The Author(s). Published by IOP Publishing Ltd Environmental Research Letters , Volume 15 , Number 9 Citation Min Liew et al 2020 Environ. Res. Lett. 15 093002 DOI 10.1088/1748-9326/ab9387

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1 Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, United States of America

2 Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775, United States of America

3 Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, United States of America

4 Author to whom any correspondence should be addressed.

Min Liew https://orcid.org/0000-0002-5156-4610

Benjamin M Jones https://orcid.org/0000-0002-1517-4711

  • Received 13 November 2019
  • Accepted 15 May 2020
  • Published 19 August 2020

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Method : Single-anonymous Revisions: 1 Screened for originality? Yes

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Erosion along high-latitude coasts has been accelerating in recent decades, resulting in land loss and infrastructure damage, threatening the wellbeing of local communities, and forcing undesired community relocations. This review paper evaluates the state of practice of current coastal stabilization measures across several coastal communities in northern high latitudes. After considering global practices and those in northern high latitude and arctic settings, this paper then explores new and potential coastal stabilization measures to address erosion specific to northern high-latitude coastlines. The challenges in constructing the current erosion control measures and the cost of the measures over the last four decades in northern high-latitude regions are presented through case histories. The synthesis shows that among the current erosion controls being used at high latitudes, revetments built with rocks have the least reported failures and are the most common measures applied along northern high-latitude coastlines including permafrost coasts, while riprap is the most common material used. For seawalls, bulkheads, and groin systems, reported failures are common and mostly associated with displacement, deflection, settlement, vandalism, and material ruptures. Revetments have been successfully implemented at sites with a wide range of mean annual erosion rates (0.3–2.4 m/year) and episodic erosion (6.0–22.9 m) due to the low costs and easy construction, inspection, and decommissioning. No successful case history has been reported for the non-engineered expedient measures that are constructed in the event of an emergency, except for the expedient vegetation measure using root-wads and willows. Soft erosion prevention measures, which include both beach nourishment and dynamically stable beaches, have been considered in this review. The effectiveness of beach nourishment in Utqiaġvik, Alaska, which is affected by permafrost, is inconclusive. Dynamically stable beaches are effective in preventing erosion, and observations show that they experience only minor damages after single storm events. The analysis also shows that more measures have been constructed on a spit (relative to bluffs, islands, barrier islands, and river mouths), which is a landform where many Alaskan coastal communities reside. The emerging erosion control measures that can potentially be adapted to mitigate coastal erosion in high-latitude regions include geosynthetics, static bay beach concept, refrigerating techniques, and biogeochemical applications. However, this review shows that there is a lack of case studies that evaluated the performance of these new measures in high-latitude environments. This paper identifies research gaps so that these emerging measures can be upscaled for full-scale applications on permafrost coasts.

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1. Introduction

High-latitude coastlines are influenced by several factors that set them apart from lower latitude coastlines including the presence of sea-ice and permafrost and are among the most dynamic in the world (Jones et al 2009a , Mars and Houseknecht 2007 , Overeem et al 2011 , Lantuit et al 2012 , Farquharson et al 2018 , Irrgang et al 2018 , Novikova et al 2018 In some cases, high rates of coastal change can threaten coastal infrastructure and communities and create a need for erosion prevention. Solomon and Covill ( 1995 ) and Cunliffe et al ( 2019 ) noted that, rather than a slow steady process, erosion can occur in an episodic manner with short-term erosion rates that greatly exceed the long-term average rate. Accelerating rates of erosion at high-latitudes can be attributed to a longer open-water period or sea ice decline (Overeem et al 2011 , Cai et al 2018 , Farquharson et al 2018 , Kwok 2018 ), shifting of shorefast ice to frazil ice (Aré et al 2008 ), shifting of multiyear ice to seasonal sea ice (Kwok 2018 ), sea surface temperature (Costard et al 2007 , Overeem et al 2011 ), more frequent or severe storms (Manson and Solomon 2007 ), and the warming and thawing of permafrost (Nelson et al 2001 , Romanovsky et al 2010 , Rowland et al 2010 , Grosse et al 2011 , Sinitsyn et al 2020 ). More recently, Jones et al ( 2018 ) demonstrated a sustained increase in erosion over the last decade along the highly dynamic permafrost-affected Drew Point section of the Beaufort Sea coastline and the complexities associated with the factors most responsible for interannual variability in permafrost bluff erosion. Farquharson et al ( 2018 ) also demonstrated the complexities of coastal changes in permafrost regions by highlighting the high spatial variability of these changes across various types of coastal morphology and an increasingly dynamic permafrost coast of northwest Alaska. Lantuit and Pollard ( 2008 ) also reported such complexities after observing a decrease in mean annual rate of shoreline erosion on Herschel Island, which is also a permafrost-affected region, but an increase in erosion rate for segments of these shorelines with high ground ice content.

The economic impacts of climate change on Alaska's infrastructure and land losses associated with coastal erosion have been quantified under various climate forcing scenarios (Cole et al 1999 , Larsen et al 2008 , Melvin et al 2016 ). In general, studies indicate that projected climate change could impose additional costs due to infrastructure damage in Alaska and across the pan-Arctic (Hinkel et al 2003 , Hjort et al 2018 ). Such climate-induced expenditures increase substantially as more comprehensive infrastructure inventories are considered. The uncertainties in climate projections (Friedlingstein et al 2014 , Nordhaus 2018 ) may also increase the uncertainties in these predicted expenditures. A recent study by Melvin et al ( 2016 ) showed that cumulative costs of climate-related damages to Alaskan infrastructure from year 2015 to 2099 were estimated to be $5.5 billion for Representative Concentration Pathway (RCP) 8.5 (representing the highest greenhouse gas emissions scenario projected by the Intergovernmental Panel on Climate Change (IPCC)) and $4.2 billion for RCP4.5 (representing stabilizing greenhouse gas emissions scenario).

Due to the severe impacts of coastal erosion on the lives and livelihood of northern indigenous communities and the high costs of implementing coastal protection, some villages have started to consider community-based relocation. The urgency and constraints of community-based relocations due to coastal erosion have been discussed in many studies (Shearer 2012 , Marino 2012 , Bronen and Chapin 2013 , Gorokhovich et al 2013 , Maldonado et al 2013 , Bronen 2015 ). These studies generally agree that erosion, which has been considered a slow geomorphological process, should now be included under the statutory definition of disaster, and relocations should be community-led and government-supported so that the displacement efforts are in agreement with the culture and traditional values of local communities (Bronen et al 2019 ).

In some northern high-latitude populated regions with high coastal erosion rates, plans were established for community relocations and efforts are underway for the construction of erosion control structures (Radosavljevic et al 2016 , Novikova et al 2018 , Irrgang et al 2019 ). The efforts, however, are insufficient to address coastal erosion in high-latitude regions, especially those influenced by permafrost, and are currently of little help to local residents in adapting to and transitioning into a new coastal norm that is characterized by high erosion rates and a more dynamic coastal system. This is because the erosion prevention structures were designed based on their local historical trends, while the trends are dynamically changing and the structures can no longer be effective. Given the social context of the indigenous people and their long history of ties to the land and sea, community-based relocations are often complex and not feasible. Even when relocation is attempted, such as the case in Shishmaref, Alaska, which is affected by rapid barrier island migration and coastal erosion, the planning can last for more than 10 years and the relocation costs are projected to be at least about $180 million (Marino 2012 ). Erosion prevention structures are still in need even in those plan-to-move communities to help reduce damages to current civil infrastructures while planning and implementation take place. Therefore, there exists an urgency of identifying potential adaptations for northern high-latitude coastal erosion and change. One method of identifying potential adaptations is through a systematic review on the prevention of coastal erosion in key northern high-latitude areas.

Coastal stabilization projects have been one of the most challenging types of construction faced by contractors globally due to the complexities of various environmental forcing factors such as storm impacts, sediment transport, and deposition patterns. Site specific risks, which are sometimes difficult to be distinguished from other factors, can lead to catastrophic failures of the coastal protection structure in question. Errors in field investigations, design, construction, and maintenance can also reduce the effectiveness of the measures. In addition, extremes of climate and weather, which are changing from historical values (Ayyub 2018 ), introduce more uncertainties to coastal engineering practice at local levels. Such projects, when located at northern high latitudes, are further complicated by the presence of sea ice, changes in sea ice extent, the harsh frigid environment, permafrost thaw, and thermokarst. Across the highly scattered scientific publications and gray literature, opinions and evidence regarding the effectiveness of long- and short-term high-latitude coastal erosion control vary greatly. While some studies (e.g. Ogorodov 2003 , Carter and Smith 2011 , Bronen and Chapin 2013 ) have argued that shoreline protection exacerbates land erosion on adjacent land, others (e.g. Andrachuk and Smit 2012 ) show evidence that erosion prevention measures are effective in the long run. A systematic review of northern high-latitude coastal erosion controls is therefore necessary to understand which measures have worked and which did not.

This paper reviews the current erosion control practices being applied globally, with a focus on northern high-latitude communities (primarily in Alaska due to data availability), and explores potential measures that can effectively prevent or slow coastal erosion in northern high-latitude communities. Within this paper we (1) summarize the challenges in constructing erosion control measures in northern high-latitude regions and the solutions proposed in the literature, (2) synthesize and conduct a meta-analysis on case histories from scientific and gray literature publications both in permafrost and non-permafrost regions, (3) discuss the rationale, benefits, limitations, and costs of applying each prevention technique, (4) explore emerging techniques and technologies, and (5) identify the knowledge gaps in prevention of coastal erosion at northern high latitudes. The innovative techniques include those that have been tested in full-scale, small-scale research projects, laboratory settings, and numerical models. The costs of the current coastal erosion prevention measures are included to reflect the unit cost increase over the years in Alaska. The goal of this research is to assess past attempts at prevention and control of erosion along northern high-latitude coasts, so that this review may be used as a reference for decision-making for mitigating the impacts of erosion in northern high-latitude coastal villages in the future.

2. Data compilation of erosion control measures

The case histories and research studies of erosion control projects in this study are compiled from journal and conference articles and government documents. The databases that have been searched include the Northern Region Projects by the Alaska Department of Transportation and Public Facilities (ADOT&PF), documentation of Alaska Baseline Erosion Assessment by U.S. Army Corps of Engineers (USACE), the Denali Commission Project Database, and the documentation of Emergency Watershed Protection Program by Alaska Natural Resources Conservation Service (NRCS). According to USACE ( 2009d ), other agencies such as the National Oceanic and Atmospheric Administration (NOAA), U.S. Geological Survey (USGS), Federal Emergency Management Agency (FEMA), Bureau of Land Management, and U.S. Forest Service only occasionally assisted with erosion control projects in the Alaskan coastal communities. As a result, most of the protection measures in the Alaskan coastal communities that are discussed herein are extracted from the USACE database.

3. General challenges of construction at northern high latitudes

There are many challenges in the construction in the remote northern high-latitude coastal regions, including remoteness of construction sites, limited construction material, and extreme environmental conditions (see summary in supplementary table S1 (available online at stacks.iop.org/ERL/0000/0000/mmedia )). Challenges can be grouped into three main categories: geographic challenges, engineering challenges, and socio-economic challenges. Geographic challenges include site remoteness, extreme weather, highly variable site conditions, and short construction period. The engineering challenges include unavailability of equipment and instrumentation, unavailability of local construction materials, and limited database (e.g. documentation of environmental parameters and soil parameters, design and construction guidelines of erosion control measures that are specific to permafrost coastlines, and case studies of well-engineered northern high-latitude coastal structures). The socio-economic challenges consist of policy inadequacy, low labor retention, and vandalism. Solutions that are proposed in this paper and by those from the literature are synthesized accordingly to each challenge in supplementary table S1.

4. Coastal erosion processes at northern high latitudes

Erosion control structures should be selected and designed according to the types of coastal settings at a site in order to optimize the structure performance. It is therefore important to understand coastal erosion processes that are specific to northern high-latitude sites. Permafrost-affected sandy beaches at northern high-latitudes are dominated by erosion processes common in non-permafrost regions such as steepening of foredunes and transport of beach sediments on gently sloping shores (Farquharson et al 2018 ). In contrast, where ice-rich permafrost is present, thermal abrasion is the main coastal erosion process. Aré ( 1988 ) defined thermal abrasion as erosion of ice-rich permafrost coasts due to combined mechanical and thermal action of waves at the under-water bluff base. Thermal abrasion then leads to the development of a wave-cut niche, leaving a cornice overhanging and eventually collapsing under its self-weight (Aré 1988 , Hoque and Pollard 2009 , 2016 ). This feature, which is unique to ice-rich permafrost coasts, is known as the block failure. Processes that are also unique to coasts affected by permafrost are thermally based and include thermal denudation and thermal settling. Thermal denudation is the destruction of shore cliffs under the action of thermal energy of air and solar radiation; while thermal settling is the deepening of littoral zone of the sea due to the thermal action of sea water, whose temperature depends on the air temperature, solar radiation, and ocean currents. As erosion progresses and leads to the landward migration of the shoreline, deeper terrestrial permafrost that persists below the level of coastal erosion may then become sub-sea permafrost (Overduin et al 2014 ).

The duration of coastal processes also varies seasonally and annually across the northern high-latitude coasts due to variability in the duration of sea ice coverage. In northern high-latitude regions not affected by sea ice, the open sea duration is year-long, similar to that at middle and low latitudes. In regions affected by sea ice, erosion is limited to the ice-free months but the degree of erosion is generally affected by seawater temperature and salinity and other arctic features such as pressure ridges, coastal geomorphology, and sea ice (Overduin et al 2014 ). Pressure ridges, if grounded with their keels extending to the nearshore bottom, can shelter shores from wave action and protect adjacent shorelines from further sea ice pressure (Taylor 1978 ). Nearshore profiles can easily be destabilized if the grounded pressure ridges are eroded by increased wave action owing to the decline in sea ice extent (Taylor 1978 ). Sea ice in various forms (e.g. ice pileup, landfast ice, and drift ice) can extensively modify the coastal morphology. For examples, ice pileup can gouge up sediments onto the land and restore beaches (Kovacs 1984 ) but can also erode the lower shoreface (Radosavljevic et al 2016 ) and damage coastal protection structures. Whereas, sea ice and landfast ice help prevent thermal abrasion (Mahoney et al 2007 , 2014 , Günther et al 2015 ). Unfortunately, the current decline in landfast ice and sea ice has rendered the permafrost coasts more susceptible to erosion. Such complex interactions among these arctic coastal features need to be accounted in the design of coastal protection structures.

In general, northern high-latitude coasts can be categorized into lithified (i.e. rocky) and unlithified coasts, depending on the regional histories of glaciation. Unlithified coasts, which have high ground ice content and are usually composed of fine-grained soils, are more susceptible to erosion. It was reported in Overduin et al ( 2014 ) that the annual mean rate of erosion (0.4–1.1 m/year) is the highest along the Beaufort and East Siberian coasts, where unlithified ice-rich low-height bluffs are the common coastal type. As expected, Canadian Archipelago, Greenland, and Svalbard, where lithified coasts dominate, have the lowest annual mean rates of erosion (0.0 m/year) (Overduin et al 2014 ). While the mean rates at these locations are low, it is important to note that they are averaged over a great distance of coastline and may not represent the local erosion.

5. Past attempts to improve understanding of coastal dynamics and site reconnaissance for mitigating coastal erosion at northern high latitudes

Understanding of high-latitude coastal dynamics is critical to the design of effective erosion control measures. In the early 2000s, the Arctic Coastal Dynamics (ACD) program involving all circumarctic countries was established and has significantly contributed to the understanding of such dynamics through a series of workshop proceeding publications. Arctic coasts have been regionally classified and correlated with their rates of erosion; attempts have been made to identify arctic coast types most susceptible to erosion. Along the Alaskan Beaufort Sea coast, Jorgenson and Brown ( 2005 ) concludes that exposed bluffs have the highest mean annual erosion rates (2.4 m/year), whereas lagoons have the lowest rates (0.7 m/year). Mean annual rates of erosion is also higher for silty soils (3.2 m/year). The sediments are mostly sands and silts and the bluff heights are low (2–4 m) (Jorgenson and Brown 2005 ). Moving eastward to the Beaufort-Mackenzie region in Canada (Solomon 2005 ) and Barents and Kara Sea key sites in Russia (Vasiliev 2003 , Vasiliev et al 2005 ), locations exposed to winds and high water level were also observed to erode at the highest rates. Erosion rates are also higher for coasts with high ground-ice content (Vasiliev et al 2005 , Solomon 2003 ). However, erosion can be reduced if there are natural protections such as bars and flats (Solomon 2005 , Gibbs and Richmond 2017 ).

Many have attempted to understand coastal dynamics along sea-ice and permafrost-affected coasts so that erosion and its impacts can be mitigated. Attempted approaches include developing an index of coastal erosion hazards (Solomon and Gareau 2003 , Jordan 2003 ), utilizing satellite data for coastal erosion monitoring (Budkewitsch et al 2004 ), developing numerical models (Hoque and Pollardz 2004 ), seismic reconnaissance (Skvortsov and Drozdov 2003 ), and digital terrestrial photogrammetry (Wangensteen et al 2007 ) to predict bluff failure and erosion rates. Other than the environmental forcing factors, impacts of industrial exploitation on coastal erosion have also been investigated (Rivkin et al 2003 , Ogorodov 2003 , Sturtevant et al 2004 ). Significant efforts have been made to quantify the current and historical erosion rates across the pan-Arctic (e.g. Jones et al 2009a , 2009b , Arp et al 2010 , Solomon 2005 , Lantuit and Pollard 2008 , Vasiliev et al 2005 , Lantuit et al 2011 , Günther et al 2012 ) so that hotspots of erosion hazard can be identified. Challenges of site investigation (Carter and Smith 2011 , Smith and Carter 2011 ), data collection (Mason et al 2012 ), construction (Carter and Smith 2011 , Smith and Carter 2011 ), and management (Smith 2008 ) in coastal regions in the Arctic were also reported.

6. Structural and non-structural erosion control measures

Damages from coastal erosion can be minimized by two different approaches: structural and non-structural measures. This paper includes only structural measures, which are defined in the United Nations General Assembly ( 2016 ) as measures that 'reduce or avoid possible impacts of hazards and achieve hazard-resistance and resilience in structures or systems.' Non-structural erosion control measures that 'use knowledge, practice or agreement to reduce risks and impacts' (United Nations General Assembly 2016 ) are not discussed in this paper. Some examples of the non-structural measures are joining the National Flood Insurance Program (Smith 2008 ), implementing No Adverse Impact (Monday and Bell 2007 ), zoning, buyout acquisition, recovery or excavation of cultural sites, remediation of contaminated sites, relocation or elevation of at-risk structures, emergency warning system and signage, erosion control management, graduate education (Smith 2006 ), individual research efforts (Smith 2006 ), and community-based relocation (USACE 2018 , Shearer 2012 , Marino 2012 , Bronen and Chapin 2013 , Gorokhovich et al 2013 , Maldonado et al 2013 , Bronen 2015 ).

All structural erosion control measures presented in this article are classified into several categories as shown in figure 1 . Two major categories are (A) techniques that are currently employed on northern high latitude coasts and (B) techniques that are not yet tested on northern high latitude coasts. Sub-categories that follow Categories A and B are techniques that have been implemented (1) in full scale, (2) in small-scale research projects or physical models, (3) in laboratories or numerical models, and (4) in small scale non-engineered projects. In this study, a structure is considered failing when it has been destroyed or displaced by environmental impacts (e.g. wave impacts, surges, sea-ice impacts, and permafrost thaw) and the repair would cost more than replacement. Another failure type refers to the ineffective measures where erosion exceeds the expected or predicted rates of erosion. A successful case refers to a measure that is not damaged or only slightly damaged when the encountered storm events are less severe than or equally severe to the design events. A measure is also considered successful if it is able to slow down or prevent erosion within the period of its designed service life when compared to adjacent sites with no coastal protection.

Figure 1.

Figure 1.  Categories of erosion control status used in this review.

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7. Current erosion controls at northern high latitudes

Most case studies presented in this section are affected by coastal erosion while some are affected by fluvial erosion. Although these riverine communities are not affected by sea-ice and other processes specific to the coasts, the erosion control projects are affected by some similar factors unique to the northern high-latitude regions including remoteness, weather, and soil condition. So, the riverine experience may be helpful to the coastal communities and is therefore included in the discussion. The locations of northern high-latitude communities and facilities are presented in figure 2 with permafrost distributions as the base map. Figure 2 is adapted from the permafrost distribution maps by Brown et al ( 2002 ) and Jorgenson et al ( 2008 ).

Figure 2.

Figure 2.  Locations of northern high-latitude communities and facilities. The locations are indicated as red markers on the map of permafrost distributions. The Alaska permafrost map is adapted from Jorgenson et al ( 2008 ), and the permafrost map outside of Alaska is adapted from Brown et al ( 2002 ). The sites in Svalbard are presented in the bottom frame. The legend indicates the permafrost distributions (i.e. continuous permafrost, discontinuous permafrost, isolated permafrost, and sporadic permafrost), glacier, water body, and unspecified regions.

Most of the measures that have been employed in Alaskan coastal villages and other infrastructure locations are hard structures such as revetments, bulkheads, seawalls, groins, and offshore berms. Figure 3 shows examples of these measures at northern high latitudes. The permafrost distributions (i.e. continuous, discontinuous, sporadic, and isolated permafrost) were identified for each site as indicated as superscripts in the column titled 'Location' in supplementary table S2. Thus far, revetments built with various types of materials have been the most common option in preventing coastal erosion at northern high latitudes as presented in supplementary table S2. The materials include rocks (figure 3 (a)), sandbags (figure 3 (b)), articulated concrete mats (figure 3 (c)), Core-loc™ units, timbers, and gabions. Among all materials, revetments built with rocks have been implemented and tested in the harsh Arctic environment and were observed to have fewer failure cases. Revetments built with sandbags have also been proven effective as temporary and semi-permanent measures. USACE ( 2009c ) has conducted physical models to further optimize the stability of revetments built with sandbags. In general, revetments usually require maintenance throughout their service life as they can be easily displaced and destroyed during storm events or by sea-ice floes. Despite this limitation, revetments are preferred over seawalls, bulkheads, and other offshore structures due to the low costs and easy construction, inspection, and decommissioning (USACE 2018 ).

Figure 3.

Figure 3.  Various types of erosion control measures that have been employed at northern high latitudes. The photos were all taken at communities in the northern high-latitude regions. Image courtesies of U.S. Army Corps of Engineers, Alaska District (a), (b), (f), Dustin Whalen (c), Alaska Division of Homeland Security and Emergency Management (d), Worley Parsons (e), Millie Hawley (©2005) (i), and Janet Mitchell and Center for a Better Life (j). (g) to (j) are non-engineered expedient measures carried out during emergency.

Seawalls (figure 3 (d)) and bulkheads (figures 3 (e)–(f)) have also been constructed in northern high-latitude coastal communities. Similar to revetments, bulkhead and seawalls protect permafrost bluff faces from the development of niches, a critical stage in shoreline collapse (Hoque and Pollard 2009 , USACE 2018 ). For bulkheads, only one successful case using vertical sheet-piles is found in the literature review (USACE 2009a ). Bulkheads made of Longard tubes™ and pipe-piles were easily subjected to vandalism (Shah 1982 ) and excessive deflection (USACE 2007a , 2009d ), respectively. Whereas, one successful case using a seawall made of logs is found to be effective in mitigating fluvial erosion where the crib walls were constructed (USACE 2007h ). Seawalls made of gabions were not as effective due to potential sagging and settlement (USACE 2009b ); while seawalls made of sheet-piles were not cost-effective, for example, a sheet-pile wall in a project proposal was estimated to cost more than $31 million (in 2007 US dollars) for a 1 km long coastline (USACE 2007k ). The groin system is a less common option in northern high-latitude coastal communities with only one case study presented in supplementary table S2. Due to their complex offshore construction, groins are especially vulnerable to damage from sea-ice (USACE 2018 ). This paper is unable to identify a successful case study that used groin systems alone to mitigate erosion impacts. The erosion in Tuktoyaktuk in northwestern Canadian Arctic was reduced when the groin systems were installed together with bulkheads (Shah 1982 ). Similar to groin systems, offshore berms are not preferred at northern high latitudes due to costly and complex offshore construction. This option was once considered in the feasibility report in Kivalina but was eventually eliminated (USACE 2007k ). Overall, failures of seawalls, bulkheads, and groin systems are associated with displacement, deflection, settlement, vandalism, and material ruptures as presented in supplementary table S2.

In the event of an emergency, sacrificial structures have been constructed as expedient measures to minimize the erosion as shown in figures 3 (g)–(j). The materials used in sacrificial structures include sandbags, supersacks, ripraps, sand berms, and root-wads. Although the materials are the same as those used in the construction of hard structures, these sacrificial structures are ineffective when they are not properly designed and constructed. When in short supply, these materials are substituted using scrap metal and concrete and other waste materials. Such materials can potentially cause pollution. Based on the case histories presented in supplementary table S2, these measures are often ineffective in preventing or slowing erosion and require maintenance after each storm event (USACE 2007a , 2007d , 2007f , 2007g , 2007 j, 2008b , 2018 , Jaskólski et al 2018 ) and may sometimes exacerbate the adjacent coastline due to material mining (Carter and Smith 2011 ). Root-wads and willow planting are the only case histories that have been effective in preventing erosion, but the project is located in Cordova, which is a non-permafrost region of Alaska affected by riverbank erosion as shown in table S2 (USACE 2007a , Smith 2008 ).

Soft structures such as beach nourishment and dynamically stable beaches have also been implemented at northern high-latitudes. Beach nourishment requires continual sources of sand and is effective only when there are existing sources of sand adjacent to the sites. This solution is rarely used in northern high-latitude communities given the lack of local sand sources, transportation challenges and costs, the depletion of local construction materials, and the environmental issues brought by sand mining. As a result, only a few beach nourishment projects have been carried out along northern high-latitude coasts and these are presented in supplementary table S2. Beach nourishment is effective in Homer, Alaska (a non-permafrost region), but ineffective in Utqiaġvik, Alaska (a continuous permafrost region) where the storm events are more intense and sea ice can interfere with sand transport of the nourished beaches.

The other soft structure that has been implemented in northern high-latitude coastal communities is the dynamically stable beach. Specifications of the three dynamically stable beaches constructed in Alaska were presented by Smith and Carter ( 2011 ) and are summarized in supplementary table S2. Coarser sediments such as rocks, concrete armor units, and cobbles were used. The main difference between a dynamically stable beach and a nourished beach is that the former is designed according to the wave direction and is designed to be shaped by future storm events to reach equilibrium over time. Only minimal maintenance is needed for dynamically stable beaches, depending on the intensity of the storm events, whereas another beach-filling is needed for the nourished beach to continue its function after the first storm event. According to Smith and Carter ( 2011 ), all three dynamically stable beaches (i.e. one in Unalakleet, a village located on a spit near a river mouth in the discontinuous permafrost region of Alaska, and two in Unalaska and Homer in the non-permafrost affected region) have performed well and do not require high maintenance after storm events.

8. Meta-analysis of current measures with site characteristics

8.1. coastal landforms.

Alaskan coastal communities mostly reside at landforms such as bluffs with narrow beaches, spits, islands, barrier islands, and river mouths. Among the various landforms, spits are the most frequently resided (5 out of 13 locations collected in this study). In figure 4 , the frequencies of northern coastal communities at various coastal landforms are plotted in light gray; the frequencies of erosion control case studies (total number n = 32) collected in this study were plotted in dark gray. The frequency of the case studies indicate that more measures had been repeatedly constructed in barrier islands and spits. In general, the soil materials in villages range from silts, fine sands, sands, to cobbles. Little Diomede Island, which is an exception, is rich in rocks and boulders.

Figure 4.

Figure 4.  Frequencies of the northern high-latitude coastal communities and case studies for various coastal landforms. The y -axis represents the frequency counts of the numbers of villages or the numbers of case studies collected in this study. The raw data used to compute the bar chart are obtained from the literature, mostly from Alaska Baseline Erosion Assessment Reports (UMIAQ and BDS 2015 , USACE 2007b , 2007d , 2007k , 2007l , 2009a , 2007f , 2007g , Jaskólski et al 2018 ).

8.2. Rates of erosion

Figures 5 (a)–(b) shows boxplots of the mean annual erosion rates and erosion per single storm event reported at sites where the erosion controls were implemented. Numeric data used to generate the plots are included in supplementary table S3. As illustrated in figure 5 (a), revetments and bulkheads have been employed for sites with a wide range of mean annual erosion rates (0.3–2.4 m/year). As expected, groin systems, which are an offshore structure, have been employed at sites with high mean annual erosion rates (2.3 m/year); whereas, beach nourishment, dynamically stable beaches, and other non-engineered measures have been employed at sites with low mean annual erosion rates (0.3–0.9 m/year). However, seawalls were employed at sites with low erosion rates (0.3–0.6 m/year). It could be possible that seawalls were constructed in these sites to slow erosion after extreme episodic erosion (figure 5 (b)). Similarly, for revetments and bulkheads, the erosion reported per single storm event also ranged widely from 3.0 m to 22.9 m. The episodic events of erosion were at the lower ends for beach nourishment, dynamically stable beaches, and other non-engineered measures (6.0–10.7 m) with an outlier of 22.9 m for non-engineered measures. Seawalls, groins, and berms had been constructed at sites with high episodic erosion (17.0–22.9 m).

Figure 5.

Figure 5.  Statistical distributions of erosion rates at sites where erosion control measures were employed. (a) Mean annual erosion rates in m/year. (b) Amount of land eroded per a single storm event during the extreme cases. For the types of measures, Re = revetments, Bu = bulkheads, Se = seawalls, Gr = groins, Be = berms (no mean annual erosion rate was reported for berms), NG = non-engineered measures, BN = beach nourishment, and DS = dynamically stable beaches. Mean values are indicated as cross markers, while outliers are empty circles. Medians are indicated by the horizontal lines in the boxes. The raw data used to compute the boxplots are obtained from the literature, mostly from Alaska Baseline Erosion Assessment Reports (UMIAQ and BDS 2015 , USACE 2007b , 2007d , 2007k , 2007l , 2009a , 2007f , 2007g , Jaskólski et al 2018 ).

8.3. Proportions of various types of measures

Figure 6 (a) shows the proportion of the types of erosion control measures employed in the northern high-latitude coastal and riverine communities. Based on a total of 53 cases, revetments were most frequently employed (41%) and non-engineered measures were the second (23%). The measures that came after non-engineered measures are seawalls (11%), bulkheads (9%), dynamically stable beaches (8%), beach nourishment (4%), groins (2%), and berms (2%). Among the 53 cases at northern high latitudes, 38 of them were employed within permafrost regions (including continuous, discontinuous, sporadic, and isolated permafrost). For the rest of the cases ( n = 15), although they are influenced by northern high-latitude climate, they are located within non-permafrost regions. For the cases in permafrost regions, the percentage of revetments increases to 44% whereas the percentage of non-engineered measures reduces to 16%. Similarly, figure 6 (b) shows the proportions of the types of materials. Among all the cases collected in northern high-latitude regions, riprap is the most frequently used material, accounting for 40% of the materials. Sand or sandbags and sheet piles are both the second (11% for each). For cases in permafrost regions, the percentage of riprap reduces to 34%, but both percentages of sand or sandbags and sheet piles increase to 13% and 16%, respectively. The percentage differences between counts in northern high-latitude regions and those in permafrost-affected northern high-latitude regions are indicated on the pie charts in brackets. The percentages of the measures (in table 1 , the column 'Grouped percentages') computed in this study are compared to those (in table 1 ; the last column) reported in a survey 'Alaska Community Erosion Survey' conducted by USACE ( 2009f ). In this survey (USACE 2009f ), the Alaskan coastal communities were asked to indicate the types of materials that have been used in their communities to prevent coastal erosion. The results reported in this paper, which were computed based on case studies reported in the literature, were comparable to the survey with riprap (including rocks and cobbles) being the most frequently used measures and geo-tubes being the least popular option.

Figure 6.

Figure 6.  Proportion of types of (a) erosion control measures and (b) materials employed in northern high-latitude communities. Values not in brackets represent proportions for all case studies at northern high latitudes; the values in brackets are the comparisons with those in permafrost areas. Values in red indicate higher percentages for the cases in permafrost regions compared to all northern high-latitude cases, and vice versa for values in green.

Table 1.  Percentages of various types of measures discussed in this study and percentages of measures covered by the 2009 USACE survey data.

8.4. Effectiveness of current measures reported in community survey

In the same survey by USACE ( 2009f ), the communities were asked about the effectiveness of the erosion control measures currently employed in their communities; 84% of the community respondents ( n = 44) indicated that the measures had been effective. The communities were further asked about whether there had been a failure in the past in their communities; 100% of the respondents ( n = 23) indicated that there had been a failure. In general, the communities reported that the current measures cannot fully prevent erosion, are not adequate for large scale protection, and have been heavily damaged due to lack of repair and maintenance. It is likely that these past measures were designed without adequate site investigations and without the considerations of the changing climate. As a result, these measures were only able to prevent mild constant erosion but not extreme erosion in an episodic manner and may fail in future storm events.

9. Costs of erosion control measures at northern high latitudes

The cost of erosion controls in Alaska has been escalating over time even for the expedient measures that were built during the planning of community relocations such as those in Kivalina and Shishmaref (USACE 2009d ). Supplementary table S4 shows the type of protection measure for each case history and the corresponding length of the structure, cost, and year of construction. The map of permafrost distributions developed by Brown et al ( 2002 ) and Jorgenson et al ( 2008 ) were used to identify permafrost distributions (i.e. continuous, discontinuous, sporadic, and isolated permafrost) for each site as indicated as superscripts in the column titled 'Communities in Alaska' in supplementary table S4. The cost per meter of erosion control measures that have been implemented or recommended in the feasibility studies are plotted in figure 7 over 40 years from 1979 to 2018. Inflation is accounted by converting the costs to 2019 US dollars using the Consumer Price Index Inflation Calculator developed by the U.S. Bureau of Labor Statistics ( 2019 ). It is important to note that the impacts and concerns on coastal communities and facilities predate the 40-year time interval, dating back to the well-known 1963 storm event in Utqiaġvik, Alaska; this storm event is widely used as a historical reference in the North American literature. However, this 40-year time interval was selected mainly because of the availability of reports and documents specific to coastal protection structures. The early review paper by Aré ( 1988 ) might have inspired more efforts (and therefore more documentation) to be made in the field of coastal erosion protection since then.

Figure 7.

Figure 7.  Cost per meter of coastal erosion controls from year 1979 to 2018 along Alaskan coasts. Inflation is accounted by converting the costs to 2019 US dollars using the consumer price index inflation calculator developed by the U.S. Bureau of Labor Statistics ( 2019 ).

As shown in figure 7 , the unit cost has increased (approximately 10% increase per year) in the past four decades. The costs of breakwaters (figure 7 , blue diamond-shaped markers) and bulkheads (figure 7 , purple circle marker) are above the average cost. The cost of gabion seawall construction is below average and beach nourishment falls on the average cost. Although the estimated cost of the beach nourishment seems to fall within the median range as shown in figure 7 , its cost is considerably high when compared to the average cost in mid- to low-latitude regions. For an 8 km shoreline in Utqiaġvik, AK (USACE 2018 ), for example, the initial cost of the beach nourishment was estimated at $297 431 000 (in 2018 dollars). This results in a cost of approximately $37 000 per meter for initial construction. However, in mid- to low-latitude regions, the initial construction cost for beach nourishment ranges from only $6600–$16 000 per meter of coastline (SAGE 2015 ). This infers that the initial cost to practice beach nourishment in a northern high-latitude region is at least 2 times the cost needed for a mid- to low-latitude region. Similarly, in mid- to low-latitude regions, the initial construction cost for bulkheads ranges from $6600–$33 000 per meter (SAGE 2015 ). However, the bulkhead in Dillingham, Alaska, which is a northern high-latitude village affected by fluvial erosion, costs about $53 000 per meter in 2019 US dollars (USACE 2009d ). This is equivalent to a cost that is approximately 1.6 to 8 times higher than the cost needed for a similar application at middle to low latitudes.

9.1. Meta-analysis of cost of erosion control measures

A statistical analysis was further performed on the case studies with reported construction cost (figure 8 ). As expected, breakwater, which is an offshore structure, has higher median cost than the other measures (figure 8 (a)). Such trend is much clearer when comparing breakwater to revetment but not the other measures since only one data point is available for bulkhead, seawall, and beach nourishment. The low unit cost of revetments may be one of the reasons that lead to the popularity of revetments (as illustrated by the high frequency of revetment in figure 8 (b)) in controlling coastal erosion in Alaska.

Figure 8.

Figure 8.  Statistical distributions of (a) unit cost in 2019 US dollars and (b) how frequently various types of coastal erosion controls are implemented. For the boxplots, mean values are indicated as cross markers; medians are indicated by the horizontal lines in the boxes. The vertical axis of chart (b) refers to the number of case studies.

Although breakwater, which is mostly constructed in non-permafrost regions, has a higher median cost, the cost (both unit and total costs) of erosion control measures in permafrost regions is higher (in figures 9 (a) and (b)). This is likely because a revetment costs more if constructed in a permafrost region, driving up the cost of overall measures. In addition, it is noted in figure 9 that the difference in medians between permafrost and non-permafrost is higher for the unit cost but not as much for the total cost. This is because the length of coastline protected by the measures in non-permafrost regions is generally longer.

Figure 9.

Figure 9.  Comparison of costs (in 2019 US dollars) of coastal erosion control in permafrost ( n = 15) and non-permafrost ( n = 7) regions. (a) Unit cost. (b) Total cost. Mean values are indicated as cross markers. Note: the mean total cost for measures in permafrost regions is skewed by an outlier (i.e. a beach nourishment project that was proposed in Utqiaġvik in 2018 and was projected to cost about $439 M in 2019 US dollars), which is not presented in the chart, so that the boxplots can be better illustrated.

10. Potential erosion control measures or techniques

The potential prevention measures or techniques that have been used in mid- to low-latitude regions or assessed for their potential application in Arctic systems are synthesized and categorized into four different applications: geosynthetics, static bay beach concept, refrigerating techniques, and biogeochemical application. Geosynthetics have been implemented in full-scale applications in various forms to control coastal erosion in mid- to low-latitude regions as shown in supplementary table S5. They were used as offshore structures (e.g. breakwaters) and onshore structures (e.g. sand-bagged seawall, sand-bagged revetments, and wrap-around revetments). Martinelli et al ( 2011 ) observed that high-density polyethylene (HDPE) sandbags when constructed as submerged barriers resisted a strong storm surge in Emilia-Romagna, Italy in December 1996 and stabilized a natural sandy beach profile. When constructed as a seawall, the sandbags resisted a significant wave height of 5 m (Corbella and Stretch 2012 ). Another type of geosynthetic application, revetments constructed using Geotube® units, prevented erosion at a localized segment of coastal bluff (Nickels and Heerten 1996 , Yasuhara et al 2012 ). Geotube® units can be a potential protection measure during emergencies given its short construction duration of less than one hour per Geotube® (Shin and Oh 2007 ). Another emerging geosynthetic application is geotextile wrap-around revetments (GWR). The GWR structure in Sylt Island in Germany was effective in resisting wave action during intense storm events in 1993 and 1994 and prevented coastal erosion by more than 10 m when compared to adjacent coastlines (Nickels and Heerten 1996 , Yasuhara and Recio-Molina 2007 , Yasuhara et al 2012 ). Although the sand-covered structure was exposed after the storm events, it was not damaged. Small-scale geosynthetic applications incorporating geotextile bags with conventional methods (e.g. breakwater) have been implemented in some northern high-latitude sites. The geotextile sandbags were designed and installed in Longyearbyen and Barryneset in Svalbard in 2005 for an experiment to develop geosynthetics products for cold climates (Caline 2010 ). The USACE ( 2009c ) started to consider using geosynthetic applications along the Alaskan coasts given the successful case studies across low to high latitudes.

The static bay beach concept (SBBC), as synthesized in supplementary table S6, is similar to the dynamically stable beach presented in supplementary table S2. This application mimics the shapes of natural bays and beaches. It prevents erosion by allowing a shoreline to reach static equilibrium through incorporations of natural or artificial physiographic features (preferably a headland). Several case studies in northern high-latitude regions have demonstrated the effectiveness of the static bay beach concept. While the principle employed is the same at both the permafrost and non-permafrost sites, relatively coarse fill materials were selected and used for the permafrost sites in order to dissipate greater wave energy. The benefits of the static bay beach are its low costs and its satisfactory performances in the long term when compared to other hard measures (Hsu et al 2010 , Carter and Smith 2011 ).

Refrigerating systems such as thermosyphons have traditionally been utilized at permafrost sites to improve the stability of roadbeds and embankments (Regehr et al 2012 ). There are three different types of thermosyphons: active, passive, and hybrid. An active thermosyphon uses a heat pump to transfer heat (Wagner 2014 ). A passive thermosyphon uses working fluid to transfer heat from its evaporator (in the ground) to its condenser and radiator (above the ground) when air temperature is lower than the ground temperature (Gudmestad et al 2007 ); this heat transfer process does not utilize external power and ceases during the summer when the air temperature is higher than the ground temperature. A hybrid thermosyphon integrates the functions of both active and passive thermosyphons and operates with natural convection during winter months (when air temperatures are sufficiently low) and with a heat pump during summer months (when air temperatures are above 0 °C). According to Dupeyrat et al ( 2011 ), an increase in ground temperature can cause the frozen ground to thaw. Such phase change of ice within the permafrost results in excess water content, which in turn reduces the cohesion and shear strength of permafrost. So, the erosion resistance of thawed soil decreases; as a result, the rate of erosion increases. In brief, erosion can be potentially prevented or reduced if ice-bonded sediments can be kept below its freezing point. To date, this technique has only been applied in small scale to improve the stability of critical infrastructure such as the communication towers in Kwigillingok, Alaska, the power plants in Utqiaġvik, Alaska, the hangar facility in Deadhorse, Alaska, and the college in Inuvik, Canada (Wagner 2014 ). Recently, a thermosyphon system with 3 meter spacings was also proposed to mitigate river erosion in Kotlik in the sporadic permafrost zone of Alaska, but the proposal was eventually eliminated due to the high cost (Roberts et al 2019 ). Although conventional thermosyphons (typically installed with 3 m spacings) cost about $1 M per km, they require minimal maintenance throughout their service life. Zottola ( 2016 ) proposed to use two-phase passive thermosyphons to alleviate coastal erosion through freezing near-thawing soils. The numerical models with soil and climate data from Drew Point and Utqiaġvik, Alaska as input parameters showed that thermosyphons are capable of slowing permafrost coastal erosion (Zottola 2016 ) but further refinement of the design is needed to optimize its cost. Detailed information for the case studies of thermosyphons are summarized in supplementary table S7.

The microbial application is an emerging technique that has been applied to control internal and surface erosion and its overall effectiveness has been rated highly (Dejong et al 2013 ). Many research studies have focused on developing the technique to be ready for real-world applications, and some studies have begun to investigate the performance of microbial application at coastal bluffs and sand dunes to mitigate coastal erosion (Phillips et al 2013 , Imran et al 2019 , Shahin et al 2020 ). Detailed information of the two case studies that may be applicable to the northern high-latitude sites are presented in supplementary table S8. One of them is a bench-scale project, investigating the performance of sporosarcina pasteurii in mitigating erosion of sandy soils of foreshore slopes and sand dunes (Salifu et al 2016 , Shanahan and Montoya 2016 , Shanahan 2016 ). Sporosarcina pasteurii is a bacterium with the ability to precipitate calcite and cement sand particles given a calcium source and urea, through the process of microbiologically induced calcite precipitation (MICP) or biological cementation. The treated soils showed above-moderate unconfined compressive strength and improved the performance of sand dunes under wave-simulation. The wave-action-induced erosion was significantly reduced when the sand was treated with a microbial solution (Salifu et al 2016 ). The other study was conducted using native microbes to cement sand to develop artificial beach-rocks that are durable enough to replace concrete structures in erosion control projects (Khan et al 2015 , 2016 ); coral sand and ureolytic bacteria from beach-rocks in Nago, Japan were used in the study. An average unconfined compressive strength of 12 MPa was achieved for the artificial beach rocks after 14 d of curing. This technique can potentially resolve the shortage of local quarries in remote northern high-latitude villages and control the escalating cost of rocks and ripraps.

11. Research gaps and challenges

To date, the potential erosion control measures and techniques described in the previous section have not been fully tested in the northern high-latitude regions. Some of them have not been evaluated even under the laboratory-simulated northern high-latitude conditions. Here we assess their strengths and weaknesses within the northern high-latitude environment and identify research gaps that need to be filled before field implementation.

11.1. Geosynthetics

Among the potential erosion control measures and techniques that we researched, geosynthetics currently have the greatest potential to be applied along the northern high-latitude coasts given their successful field applications in the past. Erosion has been controlled using geosynthetics from sparsely-located individual geosynthetic sandbags to well-designed seawalls constructed using geosynthetic materials. However, the main challenge of using geosynthetics is the prolonged UV exposure by 24 h daylight a day in the summer. Developing UV resistant and non-degradable geosynthetics is needed for northern high-latitude applications. Geosynthetic materials, which can be easily decommissioned, may be an effective option for temporary erosion control so that the threats to environment can be reduced (Sinitsyn and Recker 2019 ). However, if not properly monitored, the degraded geosynthetic materials can pollute the environment, posing threats to arctic wildlife. The degradation of geosynthetic materials in harsh and frigid northern high-latitude environments (e.g. impacts of ice-floes and prolonged exposure to subzero temperatures) can further complicate the problem by allowing brittle ruptures of sandbags to occur, resulting in sand leaking and excessive sand movement within the sandbags. Settlement and collapse of a seawall made of geosynthetic bags can occur if sand move excessively within those bags (Corbella and Stretch 2012 ). As a result, materials that remain flexible and non-brittle at low temperatures need to be identified; Caline ( 2010 ) therefore suggested that highly angular aggregates and those larger than 10 cm should be removed to avoid brittle puncture. Damage to geotextile sandbags by low temperatures or coastal ice can also be reduced by using thicker geotextile fabrics, but the seams on thick fabrics can be easily unpicked (Caline 2010 ). Therefore, the sewing machine and sewing threads need to be selected accordingly for the geosynthetics materials. In addition, the effects of sand movement on the overall stability of the geosynthetic structures need to be evaluated. The optimal sand-filled volume also needs to be determined for different combinations of locally-available filling materials and geosynthetics materials.

11.2. Static bay beach concept

One of the challenges of static bay beach concepts is to develop a process-based shape equation that can accurately simulate a northern high-latitude coastline with various coastal features. If simulated coastline profiles are not consistent with the real profiles, northern high-latitude beaches at static equilibrium will be difficult to maintain and final maintenance costs will vary substantially from their respective initial estimations. The current shape equation (i.e. Gainza et al 2018 ) is capable of modeling complex bathymetry incorporating influences of near shore islands, rock outcrops, and rocky platforms. For a shape equation to be effectively applied to a northern high-latitude coastline, effects of headlands, bluffs, barrier islands, spits, and ice ridges on the nearshore need to be considered in the equation. Another challenge is to identify the control points when applying a shape equation (Lausman et al 2010 ) as the final prediction of a static equilibrium shoreline highly depends on the initial selection. As those coastal features degrade due to thawing of permafrost or melting of ice, so do the controls points. The SBBC application also becomes more challenging when environmental forcing factors (e.g. sea ice, wave fetch, storm patterns) are changing due to climate change processes. Effects of these changes need to be reconsidered to predict the long-term equilibrium coasts at northern high latitudes.

11.3. Refrigerating system

The challenges of implementing thermosyphons are mostly associated with their up-scale field application in terms of high transportation costs and complex installation as compared with those of the traditional hard structures. Besides, given the high initial implementation costs of thermosyphons, they must be appropriately selected so that their capacities are flexible enough to account for the future climate and permafrost warming conditions. During summer months when air temperature is higher than the ground temperature, the heat pump of an active or a hybrid thermosyphon system needs to be activated to continue the heat transfer process. Heat pump operation may consume more energy if the mean annual air and ground temperatures continue to rise in a warming climate. The possibility of utilizing green energy such as the solar energy can be explored to lower the fuel consumption.

11.4. Microbial application

One of the concerns of applying MICP to the soils in northern high-latitude regions is the efficacy of the precipitate formation under cold temperatures. As reported in Cheng et al ( 2016 ), calcite can still form on grain surfaces at a temperature as low as 4 °C using Bacillus pasteurii although the amount of calcite formed per unit weight of sand at the temperature of 4 °C is 25% lower than that at 25 °C and the unconfined compressive strength is 56% lower. Performances of the calcite precipitation at 4 °C have also been assessed by Jiang et al ( 2016 ) using Bacillus megaterium and purified urease enzyme. B. megaterium was selected over Sporosarcina pasteurii as the former is more versatile and can grow at low temperatures (Jiang et al 2016 ). Efficacy of the calcite precipitation at temperatures lower than 4 °C needs to be evaluated in future studies to minimize the thermal disturbances to both the active layer and permafrost and to maintain the desired urealytic rates. Gomez et al ( 2015 ) developed a stiff crust, which has a high resistance to erosion, in the field using Sporosarcina pasteurii . This technique can be tried and tested at a bluff face to form a crust, which can potentially prevent niche development at the toe and insulate the permafrost beneath. However, the challenges include calcite degradation under freeze-thaw effects and wave impacts.

12. Choosing the optimal measures or techniques

The potential erosion controls are summarized in table 2 with their corresponding rationales, coastal types, tidal environments, and possible integration with conventional measures. The majority of the potential erosion controls are applicable to sandy coasts and are effective in a low energy setting. In the past Arctic coastlines (such as the North Slope of Alaska) were generally regarded as a low wave-energy environment where waves were damped by perennial sea ice cover (Owens et al 1980 ) these measures could be applicable to such shores. However, given that sea ice is now declining and high-latitude coastlines may transition into a more dynamic coastal system, these potential erosion controls may soon be inapplicable and need further development. Other than conventional solutions, the geosynthetic application could also be a good option for a high energy setting due to its several successful case histories in withstanding strong wave impacts and surges. Thermosyphon systems can also be used to dissipate high wave energy if integrated with sheet-pile walls or any equivalent measures. All of the potential erosion control measures and techniques discussed herein can be applied on the beach, except for the refrigerating systems, which are intended to be located on ice-rich permafrost bluff tops to keep them from thawing. The selection of erosion control measures is site-specific and is constrained by many factors such as cost, construction material availability, and contractors' skills and experience. At coastal sites where rocks are readily available, it may be more cost effective to continue to employ rock revetments to control erosion.

Table 2.  Rationales of the potential erosion controls and the applicable soil types, coast types, and tidal environments.

Other than effectiveness, many factors (e.g. total costs, construction material availability, and sustainability) should also be considered in selecting the optimal coastal erosion control measures for a site. The life cycle cost analysis (LCCA), which is a quantitative approach that selects optimal measures based on their total costs over the life cycle, has been the primary framework used by USACE and construction companies. The total cost in LCCA includes the initial construction costs, annual maintenance and repair costs, operating costs, and inspection costs. However, analyses based on merely the total costs may not be adequate and the environmental impacts should also be considered. Recently, the life cycle assessment (LCA), which assesses environmental performance and impacts of a measure over its life cycle, including raw material extraction, manufacturing, use, disposal and recycling, are of rising interest in civil engineering (Singh et al 2011 , Dong and Frangopol 2016 , Raymond et al 2019 ). A combined assessment that incorporates both environmental and socio-economic impacts can be helpful for choosing the optimal coastal erosion control measures and should be transferred to coastal and geotechnical engineering in northern high-latitude regions.

13. Conclusions

This study investigates the conventional techniques that are currently used for northern high latitudes and emerging erosion control techniques applied globally that can potentially be adapted to prevent or limit coastal erosion in northern high-latitude regions. Challenges of implementing erosion prevention measures in northern high-latitude regions are summarized into three categories: geographical, engineering, and socioeconomical categories; the corresponding solutions, which are proposed in the literature, are also systematically documented in this review.

Meta-analysis is conducted on the case studies collected from scientific and gray literature publications. Our analysis shows that revetments built with rocks have the least reported failures among the current erosion controls and are the most common (41% of all measures) and cheapest measures applied in northern high-latitude settings, when compared to other less successful hard measures such as seawalls, bulkheads, and groin systems. Riprap is the most frequently used materials, accounting for 40% of the measures. No successful case history has been reported for the non-engineered expedient measures that are constructed in northern high-latitude regions in the event of an emergency. The effectiveness of beach nourishment as a soft structure in permafrost regions is inconclusive. However, dynamically stable beaches, which are also a soft structure, are effective in preventing erosion; observations show that they experience only minor damages after single storm events. Based on the collected case studies, we found that spits are landforms most frequently resided by the Alaskan coastal communities and more measures had been constructed on spits. Revetments have been implemented at sites with a wide range of mean annual erosion rates and episodic erosion due to the low costs and easy construction, inspection, and decommissioning.

By analyzing the cost of current erosion control measures in Alaska, in regions with and without permafrost, we show that the unit cost of erosion control structures has been escalating over the past 40 years (approximately 10% increase in cost per year). We also found that both the median unit cost and median total cost are higher for measures implemented in permafrost regions than those in non-permafrost regions; both permafrost and non-permafrost case studies in this cost analysis are located within the northern high-latitude regions.

The potential erosion control measures and promising techniques synthesized in this study include geosynthetics, the static bay beach concept, insulation systems, and microbial applications. The potential of these emerging measures and techniques for full-scale application on northern high-latitude coasts are discussed after reviewing the available research studies on their efficacy and performances under the high-latitude conditions, especially under the existence of sea ice and permafrost. Integrations of these potential measures and techniques with conventional measures are recommended and possible combinations are presented. We also propose to use a combined assessment (i.e. life cycle cost analysis and life cycle assessment) that incorporates both environmental and socio-economic impacts for optimal measure selections. This study shows that a wide knowledge gap still exists in the field application of the new measures and techniques in northern high-latitude regions even though some of them have already been widely implemented in mid- to low-latitude regions. Through the synthesis of the research gaps and challenges, future research can be directed towards upscaling of the emerging erosion control measures and techniques in northern high-latitude coastal regions.

Acknowledgments

This work was supported by the National Science Foundation under Grant Nos. OPP-1745369, OISE-1927137 and OISE-1927553. Any recommendations or conclusions are those of the authors and do not necessarily reflect the views of the US Government. The mention of trade names or commercial products does not in any way constitute an endorsement or recommendation for use. The authors would like to thank the six anonymous referees for their significant comments and constructive suggestions.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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  • Confronting Shoreline Erosion on O‘ahu

Waves chip away at land below a beachfront home

Surf and sand on Sunset Beach

Each winter, surfers from around the world flock to Sunset Beach on O‘ahu's North Shore. The huge waves that break along the beach during winter months have made it the home of the prestigious Vans World Cup of Surfing, and scores of visitors travel to the area for the competition or just to watch the massive waves. Over the years, most North Shore homeowners have become accustomed to the awesome display of waves breaking just offshore from their beachfront homes.  

Eroding cliff below a former beachfront house

Intense waves and high sea level events contribute to coastal erosion, which threatened this house on the North Shore of O‘ahu.

Over the winter of 2013–2014, however, the ocean swells threatened disaster. The pounding waves and high tides caused severe erosion and loss of land in front of about 20 homes along Sunset Beach. To the people residing along the beach, the need to protect their homes was obvious. At the same time, local regulatory agencies were challenged with balancing public safety and protecting the natural beach environment.

Efforts to save beaches

For many years, the typical response to the threat of coastal erosion has been to protect the land by building a seawall. Over time though, this “solution” often leads to another problem—the loss of beaches. In Hawaiʻi, dunes and sandy plains provide a primary source of sand to sustain beaches where erosion moves them inland. When a retreating beach runs up against a hard structure such as a seawall, it has no place to go. Beach sands in front of the wall can erode away entirely, leaving adjoining beach areas and unprotected property vulnerable to accelerated erosion. Miles of beach on the island of O‘ahu have already been lost, and some 70 percent of beaches in Hawai‘i are actively eroding.

Coastal homes with rock seawalls and very little beach sand

Coastal erosion and “hard armoring” using sea walls has resulted in stretches of shoreline that have no beach, such as this location at Lanikai, O‘ahu.

Activities at Sunset Beach contribute substantially to the North Shore’s economy. The beach also provides ecosystem services, such as wildlife habitat and storm surge protection. Losing the beach to erosion or a seawall could trigger a cascade of negative impacts.

Sunset Beach utilizes a “soft” approach to manage erosion 

Government agencies and homeowners from Sunset Beach sought guidance from Dr. Bradley Romine, a coastal geologist and coastal management specialist with the University of Hawai‘i Sea Grant College Program and the Hawaiʻi Department of Land and Natural Resources. Recently, Romine has grown accustomed to taking calls from homeowners when waves start eroding into their shorefront property. Where possible, he suggests ecological, or “soft,” approaches to shoreline protection as an alternative to hard structures. In answer to questions about the North Shore issue, Romine explained that soft approaches have the potential to protect homes while simultaneously preserving beaches.

Bulldozers moving beach sand in front of houses

Construction equipment is used to restore dunes on the North Shore of O‘ahu. This is one example of “soft” alternatives to hard armoring that can preserve the natural character of beaches while protecting homes from damage during high wave and storm events. 

Dune in front of beachfront home

Newly restored dunes on O‘ahu's North Shore protected homes from high waves during the 2015–2016 winter. 

Historically, dunes had backed the beaches of the North Shore, but most had been built on or removed to make way for roads and housing developments. Following the winter of 2013–2014, Romine and his department assisted homeowners in obtaining permits to restore the sand dunes in front of their homes. In implementing the dune restoration, residents chose to maintain the beach and the key functions it serves at the same time as they protected their homes.

The 2015–2016 winter season was a test for the newly restored sand dunes. With massive El Niño-fueled winter surf hitting the North Shore, more than 11 miles of highway were closed for days due to wave overwash. Yet the sand dunes held, and the Sunset Beach homeowners who had opted for restoring the dunes were relieved to find that no structures were lost or permanently damaged.

Sea level rise exacerbates shoreline erosion

Despite the success on Sunset Beach, the State’s coastal land managers know that repairing dunes is not a solution for every situation. On many eroded shorelines, there simply isn’t enough sand available to rebuild dunes, and outside sources of beach-quality sand are limited and expensive. Additionally, continued sea level rise will increase the intensity of impacts from storms and high waves.

“The frequency and severity of coastal erosion and flooding events are only going to increase in the coming decades with climate change and sea level rise, so improving coastal community resilience makes sense for the present and the future,” noted Dr. Romine.

The State seeks solutions

The problems of shoreline erosion on the North Shore and around Hawaiʻi highlight how vulnerable the state’s communities and economy are to sea level rise and coastal hazards. The growing concern over threats brought by climate change has captured the attention of the public as well as elected officials, spurring a legislative response. In 2014, the State passed the Hawaiʻi Climate Adaptation Initiative. This legislative act declared that climate change is the paramount challenge of this century (State Act 83, 2014), and mandated the convening of a new Interagency Climate Adaptation Committee. The first task of the Committee and the Department of Land and Natural Resources (DLNR) is to develop a Sea Level Rise Vulnerability and Adaptation Report—a step ultimately intended to lead to a Climate Adaptation Plan for the State of Hawai‘i. 

Romine and his colleagues, through Hawaiʻi Sea Grant’s Center for Coastal and Climate Science and Resilience, are providing a critical link between the latest science at the university and decision makers in government to inform the development of the State Sea Level Rise Vulnerability and Adaptation Report.  Through this successful university-government collaboration, Hawaiʻi Sea Grant, in partnership with the DLNR and State Office of Planning, secured additional funding through the NOAA Regional Coastal Resilience Grants Program to expand on the State’s work under the Climate Adaptation Initiative.  In addition to providing increased capacity for understanding coastal hazards and sea level rise vulnerabilities, the NOAA-funded project will provide a close examination of local government planning systems and disaster recovery processes for opportunities to improve resilience to coastal hazards and sea level rise. 

Adapted from NOAA Sea Grant article "Community Resilience: Is Hawai‘i ready for the impacts from climate change?" See link under Additional Resources.

Sunset Beach is slammed by waves when high tide combined with high surf in the winter of 2013–2014. Photo courtesy of the University of Hawai‘i Sea Grant College Program (Hawai‘i Sea Grant).

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case study of coastal erosion

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  • Volume 21, issue 12
  • NHESS, 21, 3827–3842, 2021
  • Peer review
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case study of coastal erosion

Assessment of potential beach erosion risk and impact of coastal zone development: a case study on Bongpo–Cheonjin Beach

Changbin lim, tae kon kim, sahong lee, yoon jeong yeon, jung lyul lee.

In many parts, coastal erosion is severe due to human-induced coastal zone development and storm impacts, in addition to climate change. In this study, the beach erosion risk was defined, followed by a quantitative assessment of potential beach erosion risk based on three components associated with the watershed, coastal zone development, and episodic storms. On an embayed beach, the background erosion due to development in the watershed affects sediment supply from rivers to the beach, while alongshore redistribution of sediment transport caused by construction of a harbor induces shoreline reshaping, for which the parabolic-type equilibrium bay shape model is adopted. To evaluate beach erosion during storms, the return period (frequency) of a storm occurrence was evaluated from long-term beach survey data conducted four times per year. Beach erosion risk was defined, and assessment was carried out for each component, from which the results were combined to construct a combined potential erosion risk curve to be used in the environmental impact assessment. Finally, the proposed method was applied to Bongpo–Cheonjin Beach in Gangwon-do, South Korea, with the support of a series of aerial photographs taken from 1972 to 2017 and beach survey data obtained from the period commencing in 2010. The satisfactory outcomes derived from this study are expected to benefit eroding beaches elsewhere.

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Lim, C., Kim, T. K., Lee, S., Yeon, Y. J., and Lee, J. L.: Assessment of potential beach erosion risk and impact of coastal zone development: a case study on Bongpo–Cheonjin Beach, Nat. Hazards Earth Syst. Sci., 21, 3827–3842, https://doi.org/10.5194/nhess-21-3827-2021, 2021.

In recent years, erosion of sandy beaches has worsened in many countries due to development in the watershed and coastal zones, construction of artificial structures, storm impact, and climate change. Among these factors, the scale of coastal zone development has threatened beach safety due to (1) reduction of upstream sediment supply, (2) changes in nearshore wave fields following the installation of harbor structures, (3) inappropriate large-scale reclamation without preventive measures, and (4) decrease in beach width due to forest plantation and construction of roads and infrastructure.

Coastal erosion is often accompanied by environmental and social problems. In many developed countries, including South Korea, coastal environments have deteriorated, and the beaches have narrowed due to urbanization. However, because it is difficult to accurately quantify the cause of erosion and logically infer the mechanism, it does not fundamentally alleviate the motive but rather protects the eroding coast, causing further problems or wasting public investments. Therefore, it is imperative to evaluate the existing regulations for beach erosion control and guidelines for coastal development, as well as to incorporate environmental impact assessments into a comprehensive licensing system. To achieve these goals, an appropriate method is required to assess the risk of beach erosion and determine the most effective strategy.

In general, beach erosion may be caused by a decrease in sediment supply to a beach, shoreline reshaping within a littoral cell due to the construction of large structures, and by bar formation during storms. Because sedimentation problems on a sandy coast are multi-scale spatiotemporal processes associated with different mechanisms and the shoreline planform is constantly evolving (Stive et al., 2002, 2009; Miller and Dean, 2004), it is not only difficult to find publications that include all these mechanisms, but it is also difficult to discover good cases in which the cause of erosion is identified at various timescales and space scales. However, Toimil et al. (2017) simplified the shoreline migration by disassociating long-shore processes (e.g., Zacharioudaki and Reeve, 2011; Casas-Prat and Sierra, 2012), which are mostly responsible for long-term changes, from those induced in the cross-shore direction (e.g., Callaghan et al., 2008; Wainwright et al., 2015), which tend to produce changes in the short-term and over seasonal timescales. In addition, Ballesteros et al. (2018) have classified the main factors inducing coastal erosion into three components: long-term (associated with a timescale of several decades), medium-term (associated with a timescale from years to few decades), and episodic terms (associated with a timescale from days to months) on the basis of different processes acting at different timescales.

A beach can retain stability when the sediment budget is balanced within a closed littoral cell, such as in an embayed beach. Therefore, it is essential to analyze sediment transport in both alongshore and cross-shore directions (e.g., Inman and Jenkins, 1984; Bray et al., 1995). When the amount of sediment enters or leaves littoral cell changes, a new equilibrium volume of sediment is established within the cell accordingly (Dolan et al., 1987; Kana and Stevens, 1992; Pethick, 1996; Cooper, 1997; Cooper and Pethick, 2005). On the other hand, the amount of sediment supplied from a river and then lost into the open sea due to continuous wave action should also be regarded as the main component in the sediment budget. For example, a decrease in sediment discharge due to the construction of dams (Foley et al., 2017; Warrick et al., 2019) or an increase in sediment loss due to sand mining (Edward et al., 2006) has caused gradual shoreline retreat. In addition, Lee and Lee (2020) recently proposed an equation to calculate the beach width according to the law of mass conservation by placing variables to represent the main factors in the sediment budget.

It is well known that wave diffraction and changes in longshore sediment transport direction occur downdrift of a harbor where shoreline reshaping begins, resulting in updrift accretion and downdrift erosion. Numerous observations and studies have been conducted to assess and predict the longshore sediment transport rate in a wave-sediment environment (Komar and Inman, 1970; CERC, 1984; Kamphuis, 2002; Bayram et al., 2007). Empirical models have been used to estimate the equilibrium shoreline in areas affected by harbor breakwaters. Among them, the parabolic bay shape equation (PBSE; Hsu and Evans, 1989) for headland–bay beaches in static equilibrium has been recognized for its practicality in many countries and has been used for coastal management (USACE, 2002; Herrington et al., 2007; Bowman et al., 2009; González et al., 2010; Silveira et al., 2010; Yu and Chen, 2011; Anh et al., 2015; Thomas et al., 2016; Ab Razak et al., 2018a, b). Recently, Lim et al. (2021) extended the parabolic model (Hsu and Evans, 1989) to concave beaches in polar coordinates and proved the versatility of this model for embayed beaches.

Lastly, cross-shore sediment transport causes morphological changes in the beach profile due to storm waves, resulting in shoreline retreat. Many studies have been conducted to interpret geomorphological phenomena (Swart, 1974; Wang et al., 1975; Wright et al., 1985; Miller and Dean, 2004; Yates et al., 2009; Montaño et al., 2020). Recently, Kim (2021) proposed a method to estimate the erosion width based on the frequency of high waves using statistical analysis of GPS shoreline observation data collected seasonally for more than 10 years. He also devised the concept of horizontal movement of suspended sediments and applied a wave scenario model to analyze the response relationship between the convergent mean shoreline position of Yates et al. (2009).

The aim of this study is to propose a combined potential erosion risk curve (CPERC) for a beach from accumulating the potential risk of three different erosion components (Sect. 3), using a minimum set of field data (e.g., aerial photographs and shoreline survey data). The methodology is then applied to Bongpo–Cheonjin Beach in South Korea as part of the environmental impact assessment for planning coastal protection measures.

This paper starts with a general introduction in Sect. 1, followed by the definition of potential erosion risk and the concept of the combined potential erosion risk curve (CPERC) in Sect. 2. Section 3 explains the methods for assessing three different erosion factors: (1) sediment input from the watershed, (2) construction of harbor breakwater, and (3) storm impact. The methodology is then applied to the Bongpo–Cheonjin Beach in South Korea, a shallow embayment with a high risk of erosion, supported by aerial photographs taken between 1972 and 2017, 37 sets of seasonal shoreline survey data collected during 2008–2017, and NOAA's wave data, shown in tables and graphs in Section 4. Discussions are then presented in Sect. 5 to improve the accuracy when applying the method proposed in this study to a different coastal environment. Finally, concluding remarks are presented in Sect. 6. It is expected that this quantitative method for the assessment of beach erosion risk will benefit eroding beaches elsewhere in both developing and developed countries.

Recently, research on coastal impacts caused by extreme events, such as hurricanes, has increased in several countries including the United States and Europe (e.g., Beven et al., 2008; Kunz et al., 2013; Van Verseveld et al., 2015; Spencer et al., 2015). Among these, Ballesteros et al. (2018) proposed a methodology, framed within the source–pathway–receptor–consequence model (SPRC), which enables the identification of the main factors inducing coastal erosion at different timescales and their associated impact on the beaches on the Mediterranean coast. Toimil et al. (2017) conducted a probabilistic estimate of shoreline retreat to quantify the risk consequences due to climate change on a regional scale. Sanuy et al. (2018) also established an erosion risk assessment method based on a Bayesian network and obtained a method to reduce erosion by applying it to beaches in the Mediterranean. In addition, many studies have been conducted to evaluate coastal risks by analyzing and predicting various physical phenomena and effects using numerical models (e.g., Roelvink et al., 2009; McCall et al., 2010; Harley et al., 2011; Roelvink and Reniers, 2012).

However, most risk assessment methods are not only focused on extreme events but also require numerous data and techniques. Therefore, it may be impractical for coastal managers to apply these methods to field conditions for coastal erosion management. In this study, we present a method to assess the potential erosion risk induced by the combined action of processes acting at different timescales and with minimal basic survey data.

2.1  Definition of beach erosion risk

Many different definitions of risk have been proposed (Knight, 1921; Rasmussen, 1975; Kaplan and Garrick, 1981; Hansson, 2007; Hubbard, 2009). In technical contexts, the word “risk” has several specialized uses and meanings. Among them, risk is defined as the expected loss of the event, implying the product of the probability of an event and the loss of the event itself. It is the standard technical meaning of the term “risk” in many disciplines, and it is also regarded by some risk analysts as the only correct usage of the term (Hansson, 2007). In the same context, risk is usually assessed by the time-averaged amount of damage, and its evaluation is possible through time domain, frequency domain, and probability domain analysis. In the frequency domain, potential risk  R is defined as the product of consequence (i.e., factor or mechanism)  C and frequency  F such that

In this study, R  is the beach area likely to be damaged by erosion due to development in the watershed, on land, and in coastal waters. The frequency,  F in Eq. (1) corresponds to the frequency of erosion risk from the equilibrium shoreline to the landward erosion limit. Where several erosion causes (factors) exist, the total erosion risk is taken as the sum of the risk from each contributing factor.

2.2  Potential beach erosion risk

The consequence(s),  C in Eq. (1), was obtained by analyzing all the factors affecting the eroded beach surface area. As mentioned in the introduction, coastal erosion is caused by an imbalance in the sediment budget, construction of harbor breakwaters, and storm impacts on the shore. As such, the physical process that causes erosion is characteristically subdivided, so the erosion consequence  C is calculated from the sum of the independently assessed beach erosion area defined as the potential erosion area (PEA) and the potential erosion width (PEW). The former consists of the beach surface area reduced by (1) background erosion due to reduction in sediment input from the river called potential background erosion area (PBEA,  A b ), (2) alongshore shoreline reshaping due to harbor construction called potential reshaping erosion area (PREA,  A r ), and (3) retreat by episodic storm impact called potential episodic erosion area (PEEA,  A e ). The latter contains three components: the potential background erosion width (PBEW,  W b ), potential reshaping erosion width (PREW,  W r ), and the potential episodic erosion width (PEEW,  W e ), which are obtained by dividing each PEA component by the effective beach length. In the above, the width of erosion risk is measured shoreward with respect to the equilibrium original shoreline (EOSL), which can be obtained by determining a long-term average value prior to erosion due to coastal zone development.

Because the sediment budget is expressed in volumetric units, information on the vertical dimension of active beaches, defined as the sum of closure depth and berm height, is required for the conversion to the area unit of the beach surface. When a change in the total surface area of a beach in the littoral cell occurs, it is necessary to assess the PBEA and PREA to ascertain whether it is due to development in a watershed or coastal zone. If there is no change in the total beach surface area within a littoral cell, but the equilibrium shoreline is reshaped and irreversible erosion occurs, assessment of PREA is required. Finally, an assessment of the PEEA corresponding to recoverable episodic erosion is required. For the first two erosion factors, the concept of frequency is not required because beach erosion is irrecoverable, but for the third factor, the return frequency (period) of storm occurrence should be considered because wave heights and periods vary with the strength of the storm.

Each component in the PEA is a term that has units of area and is defined as the potential beach erosion area. Similarly, this definition gives the erosion width for all the three component factors as follows:

where A b , A r , A e , W b , W r , and W e  correspond to PBEA, PREA, PEEA, PBEW, PREW, and PEEW, respectively, as defined above, and  L b , L r , and L e  are the effective beach lengths for PBEA, PREA, and PEEA, respectively. The PBEA can be assumed to have a uniform effect along the coast; for convenience, it is assumed that the same erosion occurs along a coast due to storm impact, so L b  and L e  are equal to the length of beach  L . However, erosion due to shoreline reshaping occurs only in the erosion/accretion zone, so it is less than the beach length  L .

2.3  Combined potential erosion risk curve (CPER)

Prior to delimiting the landward boundary of an ideal combined potential erosion risk for a sandy beach, which is the sum of all potential erosion widths from the contributing components, the existing beach status must be clarified. For example, a beach may include a wide buffer zone in which no damage occurs, such as the back beach and dunes that will only be damaged by a storm for a specific number of years, and the beach profile can recover after the storm wanes. Conversely, if the extent of erosion is too large, the existing property and infrastructure may be damaged. The extent of the current beach width and the area on which protection is required must be thoroughly investigated.

For practical applications, a combined potential erosion risk curve (CPERC) can be constructed by plotting the consequence  C (e.g., combined potential erosion risk area) versus the combined potential erosion width, with respect to the shoreward distance from the average shoreline (i.e., EOSL). By expressing the EOSL in polar coordinates, and if the circle that best fits the current average shoreline is obtained, the center of the circle  O can be determined. As shown in Fig. 1, the average shoreline is located at  R o from the reference pole, the beach landward limit (dashed red line in Fig. 1) is located at  R ec from the origin, and each angle  α has different values depending on its boundary configuration. Therefore, if R o  and R ec  are determined for each angle  α , a CPERC is obtained using an appropriate equation according to the shoreward distance  r from the EOSL:

If the shoreline is not well fitted into a circle, as in the example in Fig. 1, after finding the curve that best fits the shoreline, it is appropriate to set the fitting curve as EOSL and r  in the direction perpendicular to the shoreline.

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Figure 1 Conceptual diagram of combined potential erosion risk (CPER) curve © Google Earth.

Next, the total beach erosion width,  W t , is calculated from the sum of all PEWs obtained from the method described above such that

The right-hand side of Eq. (5) includes the effects of (1) background erosion resulting from a decrease in sediment budget due to watershed development, sand dredging, or extraction, (2) alongshore sediment redistribution and shoreline reshaping due to harbor construction, and (3) short-term erosion due to episodic storms. Because the beach recovers after storm waves, the recoverable episodic erosion ( W e ) will have different values depending on its recurrent interval. When W t  is calculated, as shown in Fig. 2, the overall erosion consequence,  C t can be obtained from a CPERC, which represents the accumulated area likely to be damaged from the EOSL.

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Figure 2 Combined potential erosion risk curve (CPERC) constructed from three components of potential erosion width and area.

The abscissa  r in Fig. 2 represents the shoreward distance from the average shoreline (EOSL). If the combined potential shoreline retreat,  W t , in Eq. (5) is substituted by  r , the CPERC can also represent an area corresponding to consequence  C in Eq. (1). To calculate the CPERC area, the frequency related to the background PBEA and PREA can be regarded as one per year ( F br = 1 / yr ), whereas that for the PEEA ( F e ) depends on the frequency of storm occurrence. Therefore, the combined risk  R in Eq. (1) can be expressed as follows:

where C br = C ( W b + W r ) and C e = C ( W t ) - C br , as illustrated graphically in Fig. 2.

3.1  Background erosion from watershed and river (PBEA)

The PBEA ( A b ) accounts for beach erosion caused by a decrease in sediment supply from the river. For a sandy beach within a littoral cell (Lee and Lee, 2020), the law of mass conservation gives

where Q in  is the ratio of sediment discharge mainly flowing into the littoral cell from a point source such as a river, and Q out  is the rate of sediment loss that is steadily lost to the open sea mostly due to the action of waves. However, Q out  includes the rate of sand loss due to artificial offshore sand extraction such as sand mining or dredging. In a natural state without artificial coastal zone development, the representative  Q in is the sediment discharge rate from the river and is balanced with the loss of sand to the open sea due to the continuous wave action. The latter can be expressed as the product of the sediment loss constant  K and the beach sediment volume  V (Lee and Lee, 2020).

If the difference between the point source and the sink sediment discharge in the sediment budget, excluding the sand loss to the open sea due to wave action, is defined as  Δ Q p , the following equation is obtained.

When the amount of sediment in a littoral cell is in equilibrium, the sediment loss constant  K can be estimated as Δ Q p / V . Here, volume  V in the active beach can be approximated as the product of the vertical height of the littoral zone  D s and beach surface area  A . Assuming  D s , the sum of berm height and closure depth, is constant along a beach, Eq. (8) becomes

Many studies have been performed to determine the berm height and closure depth,  D s (Rosati, 2005; Cappucci et al., 2011, 2020; Pranzini et al., 2020). Although closure depth varies with wave climate and sediment particle size (Hallermeier, 1981), judging from the observed beach profile data, its value has been shown to remain reasonably constant over several decades.

Because the purpose of this study was to obtain the PBEA, Eq. (9) gives the beach surface area  A for a steady state ( d A / d t = 0 ) as follows:

where K  and D s  are coefficients representing the characteristics of a beach. Therefore, if Δ Q p  changes within a coastal environment where K  and D s  are constant, the beach surface area will change accordingly. When Δ Q p  before coastal zone development is set as  Δ Q p o , and if Δ Q p  is reduced by  α Δ Q p o , then PBEA ( A b ) can be expressed as a function of  α as

Here, the superscript “o” corresponds to the beach area before development. Once α  is obtained, PBEA can be calculated as described above. However, because of the difficulty in directly determining the α  value, additional information is required, such as any changes in land use, forestation, water storage capacity stored by dams, and river maintenance projects in the watershed (Yang and Stall, 1974; Karim and Kennedy, 1990; Wu and Xu, 2006; Slagel and Griggs, 2008; Gunawan et al., 2019).

Assuming A b  is uniformly distributed over the entire embayment with a curved length  L b , then the PBEW ( W b ) = A b / L b as shown in Fig. 3.

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Figure 3 Conceptual diagram for the PBEA caused by sediment reduction from river.

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Figure 4 Sketch of parabolic bay shape equation and relevant geometric parameters.

3.2  Reshaping of shoreline due to harbor breakwater (PREA)

Harbor construction on sandy coasts often changes the wave field, generating new wave diffraction and nearshore current patterns. It also causes “shoreline reshaping”, with downdrift erosion accompanied by updrift accretion. Although the amount of sediment may be maintained within a cell, the erosion risk area (called PREA) induced by the redistribution of littoral drift can be assessed by an empirical parabolic shoreline model of parabolic type (i.e., PBSE; Hsu and Evans, 1989). This model can be readily applied to predict the static bay shape on a downdrift beach with the breakwater tip as a control point. This equation (in polar coordinates) can be used to define two adjoining regions with a common tangent at the downdrift control point  E (Fig. 4):

where R 0  is the length of the control line ( F E ) joining the parabolic focus ( F ; wave diffraction point) and the downdrift control point  E , R ( θ )  is the radius from the focus to a point  Q on the equilibrium shoreline, a  is the perpendicular distance from the wave crest baseline to point  E , β  is the angle between the wave crest baseline and the line joining the focus and the control point, θ  is the angle between the wave crest baseline and the line connecting  F and  Q , and  C 0 , C 1 , and C 2  are the coefficients provided by Hsu and Evans (1989). An approximate expression for the PBSE is given by

Recently, Lim et al. (2021) extended the applicability of the PBSE with polar coordinates to concave coasts. In the present case, the actual equilibrium shoreline can be estimated by shifting the downdrift segment of the predicted bay shape landward, parallel to the existing shoreline, and equating the accreted area  A r + with the eroded area  A r - , as shown in Fig. 5. The accreted area, which is the PREA, can also be derived from Eq. (13), rendering

In Eq. (14) and Fig. 5, β ′  is the angle between the focus point (i.e., the breakwater tip) and the secondary breakwater. For application, Eq. (14) can be approximated as follows:

Then, PREW  W r can be calculated by dividing the accretion area  A r by  L r , which is the length from the focus point to the farthest point on the downdrift beach or the shoreline length in the erosion section (Fig. 5).

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Figure 5 PREA caused by shoreline reshaping due to harbor construction.

3.3  Episodic storm caused beach erosion (PEEA)

The PEEA is defined as a beach surface that is temporarily eroded by storms. However, it is also characterized by a gradual return of the beach profile to the original shoreline after the storm wanes. Figure 6 shows the variation in the mean beach profile with a near-constant depth of closure at Bongpo–Cheonjin Beach. This reveals that the statistical distribution of shoreline survey data collected four times each year follows a normal distribution. Although these surveys are intended to present seasonal changes in shoreline variability, they are unlikely to reflect short-term changes during storms; if a series of survey data is sufficient for including storm effect, the daily extreme value can be calculated by multiplying the probabilistic analysis results by a weighting factor of 1.5. The validity of this method was verified by applying Tairua Beach in New Zealand (Montaño et al., 2020), which has sampling data for more than 8 years.

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Figure 6 Variation in beach profile and shoreline position and the probability distribution (inset) at a beach in South Korea.

When the observed shoreline data follow a normal distribution, it can be applied to assess the maximum probable erosion occurring once in n  years with a probability of  1 4 n in a cumulative normal distribution curve, from which the frequency  F for a shoreline variable  x F can be estimated by

From Eq. (16), the shoreline position due to episodic erosion  S e is then calculated for a shoreline variation width  x F by

where μ  is the mean position of the shoreline, and σ  is the standard deviation of the shoreline variation width obtained from the data distribution curve. The PEEW with a certain return period can then be estimated statistically from the shoreline observation data such that

where the frequency  F ( x F ) corresponds to the frequency  F e in the potential erosion risk given in Eq. (6). However, since the shoreline was observed four times a year, it was approximated by multiplying by 1.5 to convert it into a daily statistical value of the variation for a 30-year return period (e.g., x 1 / 30 yr = 1.5 × 2.4 = 3.6 ).

Finally, PEEA ( A e ) is obtained by multiplying PEEW ( W e ) by its effective shoreline length  L e . The proposed method cannot be applied because there are no shoreline survey data or the amount of data is insufficient for statistical analysis. The PEEW can be estimated using an equilibrium beach profile (Dean, 1977) from storm wave and sediment particle size data (Kim and Lee, 2018).

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Figure 7 Aerial photograph of Bongpo–Cheonjin Beach in February 2021, showing harbors, river, shore protection structures, and static bay shapes, produced by the software MeePaSoL © Google Earth.

4.1  Site description

The quantitative assessment proposed in the present study was applied to Bongpo–Cheonjin Beach (38 ∘ 15 ′  N, 128 ∘ 33 ′  E), in the northeast of Gangwon-do (province), South Korea, where the small Cheonjin Harbor is located to the north and the large Bongpo Harbor to its south (Fig. 7). The beach is of a crenulated shape, approximately 1.1 km long, and is a closed littoral cell due to the existence of the breakwater (completed in November 2010) for Cheonjin Harbor in the updrift region and a group of natural rocks nearshore in the downdrift region. Because beach erosion often occurs due to increased swell and larger waves in winter, three segmented submerged breakwaters totaling 490 m in length (installed between November 2017 and November 2019) and one groin of 40 m (completed in July 2018) extending out from the rocks eventually transformed the beach into a stable embayment (Fig. 7).

The application of the software MeePaSoL (Lee, 2015) developed for the PBSE (Hsu and Evans, 1989) revealed that Bongpo–Cheonjin Beach is currently close to static equilibrium (using focus points  B and  C for the updrift and downdrift half of the beach shown in the yellow curve, respectively; Fig. 7).

In geomorphic terms, Bongpo–Cheonjin Beach has received predominant waves from approximately the 47 ∘  E direction (drawn by software MeePaSoL), whereas the prevailing wave direction in spring and summer is from 50 ∘  E and in autumn and winter from 30 ∘  E in the open sea. Therefore, longshore sediment transport prevails from north to south in autumn and winter, especially during periods of high wave action in winter, which has caused severe beach erosion.

4.2  PBEA due to development in watershed

The Cheonjin River watershed, which contains three rivers and covers an area of 69.51 km 2 is linked to the littoral cell at Bongpo–Cheonjin Beach. Although a series of developments in the watershed (e.g., construction of several small weirs, change in forest environment, and river maintenance projects) have had the potential to reduce the sediment input to the beach, its impact on the background PBEA and PBEW was found to be minimal upon analyzing a series of 10 aerial photographs of Bongpo–Cheonjin Beach (Fig. 8) spanning over 45 years from 1972 to 2017 (i.e., July 1972, November 1979, October 1991, June 1997, May 2005, November 2010, May 2011, September 2013, November 2015, and July 2017). The values of shoreline position, beach width, and beach area were extracted from three key locations (A, B, and C marked on each sub-panel in Fig. 8) and tabulated in Table 1. In addition, 37 sets of seasonal shoreline survey data collected during 2008–2017 and NOAA's wave data were also utilized, and the results are presented graphically in Fig. 9.

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Figure 8 Aerial photographs of Bongpo–Cheonjin Beach by year: (a)  July 1972, (b)  November 1979, (c)  October 1991, (d)  June 1997, (e)  May 2005, (f)  November 2010, (g)  May 2011, (h)  September 2013, (i)  November 2015, and (j)  July 2017. Images courtesy of the National Geographic Information Institute (The Province of Gangwon, 2018).

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Figure 9 Variations in beach area and width for Bongpo–Cheonjin Beach using aerial photographs.

From each aerial photograph, the average beach width was obtained by dividing the beach area by the shoreline length at the time of photographing. Therefore, depending on the incident wave conditions at that time, it may not be able to reflect the effect of shoreline retreat caused by cross-shore sediment transport. Nonetheless, statistical analysis indicates that the erosion width occurring at a frequency of 1 year is approximately 16 m at the Bongpo–Cheonjin Beach.

As shown in Fig. 9, since November 1979, total beach area at Bongpo–Cheonjin has remained around 31 800 m 2 , about the average of 31 821 m 2 , or higher after May 2005, except between November 1991 and May 2005, whereas beach width has maintained about 28 m or more, except in June 1997 when it was reduced to 26.6 m. Although small submerged weirs were built along Cheonjin River, its effect on the background sediment budget  A b is minimal due to the small storage capacity of the weirs. Since the estuary of the Cheonjin River is located outside the Bongpo–Cheonjin Beach, it is not expected to significantly influence on PBEW depending on the potential bypass of sediment from the beach at the north. Therefore, considering the net effect of all agents, at the decadal scale, the Bongpo–Cheonjin Beach can be considered (more or less) to be in equilibrium. Hence, the PBEW  W b may be ignored in this study.

4.3  PREA due to the construction of harbor breakwater

As shown in Fig. 9, the averaged beach width of the Bongpo–Cheonjin Beach appears to have remained at approximately 30 m for a long time after mid 2008 (by linear interpolation between May 2005 and November 2010) in spite of the regional shoreline advancing to form a static bay shape after the construction of the Cheonjin Harbor breakwater. During this period, shoreline reshaping resulted in sediment deposition in the vicinity of the breakwater (at updrift  A ) and accompanying erosion (at downdrift  C ) of the beach, as shown in Table 1.

https://nhess.copernicus.org/articles/21/3827/2021/nhess-21-3827-2021-f10

Figure 10 Calculation of PREA at Bongpo–Cheonjin Beach. Image courtesy of the National Geographic Information Institute (MOF, 2020).

Table 1 Variations in beach area at three key locations of Bongpo–Cheonjin Beach marked in Fig. 8 (The Province of Gangwon, 2018).

case study of coastal erosion

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The PREA can be approximated by the bay-shaped shoreline feature across the entire Bongpo–Cheonjin Beach (Fig. 10). First, the equivalent wave obliquity ( β ) from the tip of the harbor breakwater can be approximated from the geometry of indentation ( a ) in relation to the beach length ( L r ):

PREA  A r is then obtained by substituting the calculated  β with  β ′ , as indicated in Fig. 5 and Eq. (15):

For a =150  m (Fig. 10), Eq.  (20) gives A r =14 560  m 2 . The relationship between  β and  β ′ in Eq. (15) can be plotted (Fig. 11) to obtain the dimensionless PREA ( A r a 2 ) with values from 0 to 10. Alternatively, the value for A r / a 2 can be obtained graphically, as shown in Fig. 11. By equating  A r + with  A r - (Fig. 10), the beach erosion width  W r was estimated to be 17 m by inputting the beach length from the breakwater ( L r =850  m) into Eq. (2).

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Figure 11 Diagram for determining dimensionless PREA ( A r a 2 ) ranging from 0 to 10 in Eq. (15).

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Figure 12 PEEA at Bongpo–Cheonjin Beach, showing standard deviation  σ and mean encroachment  σ x F with a 30-year return period (within inset). Image courtesy of the National Geographic Information Institute (MOF, 2020).

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Figure 13 Estimation of combined potential erosion risk using the CPERC for Bongpo–Cheonjin Beach.

4.4  PEEA due to episodic storm

Routine shoreline surveys have been conducted at least four times per annum for beaches in Gangwon-do, South Korea, since the 2000s. More specifically, a total of 37 sets of seasonal data were collected over 10 years from 2008 to 2017 for the Bongpo–Cheonjin Beach. These data were plotted and fitted by a normal distribution (Fig. 12) to show local shoreline changes with a standard deviation of σ =5.5  m. Figure 12 also compares the alongshore distribution of the mean shoreline and eroded shoreline of the 30-year return period from statistical analyses ( x F =3.6 ). The beach width due to the PEEW is evaluated as the value with the range from 5.57 to 23.16 m ( 1 yr ≤ F e ≤ 100 yr ).

4.5  CPER curve for Bongpo–Cheonjin Beach

The potential erosion risk to a beach can be obtained by accumulating all the erosion risk widths from each contributing factor, resulting in a CPERC (Sect. 2.3 and Fig. 2). In Fig. 13, the CPERC accounts for the erosion risk distance from the EOSL. At Bongpo–Cheonjin Beach, the PBEW  W b and PREW  W r are estimated to be 0 and 17 m, respectively, thus representing the sum of the first two individual components W b + W r = 17  m. Furthermore, by calculating the combined erosion risk width  W t (Eq. 5) at 5 m intervals, up to 50 m, the corresponding values for consequence  C t are tabulated as in Table 2.

Table 2 Relationship between combined shoreline retreat  W t and consequence  C t for Bongpo–Cheonjin Beach.

case study of coastal erosion

Because PEEW  W e is a function of the return period (frequency) of storm occurrence, the total shoreline retreat ( W t ), consequence ( C t ), and erosion risk ( R ; Eqs. 1 and 6) are calculated for several specific return periods (in years) of storms, as shown in Table 3. In addition, Fig. 13 illustrates the consequence  C t per return period  T r ( 1 / F e ) , which is obtained using the CPERC, while Fig. 14 shows the variation in consequence and the combined potential erosion risk with respect to the storm return period at Bongpo–Cheonjin Beach.

Table 3 Potential erosion risk per return period  T r for Bongpo–Cheonjin Beach using CPERC.

case study of coastal erosion

Figure 14 Consequence  C and potential risk  R with respect to  T r at Bongpo–Cheonjin Beach.

Overall, from the analysis of potential beach erosion area and width for the three key factors at Bongpo–Cheonjin Beach, the PBEW may be considered insignificant; hence, W b ≈0 , but PREW ( W r ) is estimated to be 17 m following a 40 m extension to the breakwater for Cheonjin Harbor. In addition, the PEEW ( W e ) value is estimated to be between 5.57 and 19.75 m for the storm return period ( F e ) of 1 and 30 years, respectively. Upon applying the combined shoreline retreat ( W b + W r + W e ) to the CPERC, it yields the total eroded beach area ranging from 20.9 to 4969.4 m 2 (see Fig. 13 and Table 3). For a storm with a 30-year return period, this implies that a beach area totaling 4969.4 m 2 (or beach width of approximately 36.75 m) might be eroded once every 30 years, thus requiring appropriate engineering solutions (such as coastal setbacks, beach nourishment, or others) to conserve the coastal environment at Bongpo–Cheonjin Beach.

The limitations of the assessment method proposed in this study are briefly described, together with additional considerations, to enhance the applicability of this methodology to different coastal environments.

Although the purpose of this study is to apply an assessment method to Bongpo–Cheonjin Beach, which is a shallow embayment or a semi-closed littoral cell, the proposed method is not limited to headland–bay beaches. It is also applicable to open beaches with suitable modifications to the mechanisms examined in this study.

The proposed combined potential erosion risk curve (CPERC) includes an individual risk component assessed for background sediment from a river at updrift, a fishing harbor with breakwater extension, and storm waves in winter. The construction of CPERC is based on a simple arithmetic sum to represent the worst case scenario rather than a multivariable regression analysis. It cannot predict temporal changes in erosion risk. To improve the reliability of this method, the temporal beach change and the scale of each contributing factor versus time must be examined, especially from that induced by the episodic storm that occurs only sporadically. Conversely, the other two are either almost constant or increasing gradually.

For Bongpo–Cheonjin Beach, the potential background erosion width (PBEW,  W b ) is negligible, indicating that the variation in sediment supply from the watershed is minimal. However, after a large dam is constructed within a watershed, the time-dependent change in beach width must be considered. The theoretical solution given by Lee and Lee (2020) suggests the effects of the sand loss rate  K b into the open sea, and the decrease rate  α of the sediment supply to the beach can be expressed as

where α  and K b  are constant, and the corresponding beach area is assumed to converge to (1− α ) A o , where A o  is the initial area. Equation (21) shows that the beach area decreases rapidly at the beginning but converges to 95 % or more of the equilibrium state when t  is greater than 3 / K b  years.

To increase the accuracy of potential erosion width (PREW,  W r ) due to shoreline reshaping caused by breakwater construction for harbors, empirical formulae (e.g., the CERC, 1984, equation in the Shore Protection Manual) can be applied. Starting from the angle difference between the initial and equilibrium shoreline angles at the boundary of erosion and deposition, the temporal width change was obtained by applying an exponentially converging angle change to the formula for longshore sediment transport:

where W r u  is the ultimate beach width due to longshore sediment transport, and K r  is the rate of change in angle according to the time at the junction, which is estimated by dividing the longshore sediment transport rate by the beach length  L r and the vertical littoral height  D s in the formula for longshore sediment transport. The equilibrium shoreline angle due to harbor or coastal structures can be obtained based on the PBSE of Hsu and Evans (1989).

For potential beach erosion due to episodic storms (PEEW,  W e ) that can be recovered after the storm wanes, Yates et al. (2009) have confirmed that a linear relationship exists between the location of the shoreline and swell wave energy in field observations. Applying this recoverable process, the shoreline change model proposed by Miller and Dean (2004) can be expressed by the ordinary differential equation (Kim, 2021),

where K b  is the beach recovery factor, E b  is the wave energy at the breaking point, and a  is the beach response factor between the wave energy  E b and the mean shoreline. When the value of  K e , which is unique for each beach, is known, the temporal change in the shoreline can be estimated from Eq. (23) for a given wave energy. Alternatively, the SBEACH model may be used (Larson and Kraus, 1989; Larson et al., 1990).

This study presents a quantitative method for assessing the potential erosion area (PEA) and potential erosion width (PEW) due to development in the watershed, harbor construction, and storm impact. Aerial photographs, beach surveys, and NOAA wave data were applied to support the analysis while omitting sea-level rise. The results are used to produce a combined potential erosion risk curve (CPERC) for planning coastal protection or restoration projects which include the effectiveness of potential risk induced by storms in different return periods of occurrence. For example, the potential erosion risk due to storms (PEEW,  W e ) over a 30-year return period is estimated to be about 19.75 m (Table 3), which gives a total potential erosion risk width ( W t ) of 36.75 m. This is greater than the beach width of 30 m from the current averaged shoreline (EOSL), thus calling for engineering solutions to protect Bongpo–Cheonjin Beach. Because of the potential severity of the predicted beach erosion risk, for beach nourishment three submerged detached breakwaters (each 160 m long with a gap of 70 m) were constructed from November 2017 to November 2019, with a short groin (40 m) (Fig. 7). These have satisfactorily transformed Bongpo–Cheonjin Beach into a stable embayment since the completion of the engineering work.

By applying the risk assessment method presented in this paper, it is possible to determine the optimal strategy by comparing the total cost of risk to the eroding section with the average annual cost of erosion protection. Moreover, the proposed methodology is helpful not only for quantitatively assessing beach erosion risk but also for devising engineering countermeasures to mitigate the causes of erosion. Further research is recommended to apply the methodology described in this paper to beaches suffering severe erosion so that this method can be improved and benefit other coastal communities through its application.

No data sets were used in this article.

CL and JLL conceived the idea. CL, TKK, SL, and YJY participated in the field data collection. CL, SL, and JLL participated in data interpretation. All authors contributed to the writing of the final draft.

The contact author has declared that neither they nor their co-authors have any competing interests.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

This research was part of the “Practical Technologies for Coastal Erosion Control and Countermeasure” project supported by the Ministry of Oceans and Fisheries, South Korea (grant no. 20180404).

This paper was edited by Daniele Giordan and reviewed by two anonymous referees.

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  • Introduction
  • Beach erosion risk
  • Assessment of potential erosion area (PEA)
  • Case study at Bongpo–Cheonjin Beach
  • Concluding remarks
  • Data availability
  • Author contributions
  • Competing interests
  • Financial support
  • Review statement
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Review article, innovations in coastline management with natural and nature-based features (nnbf): lessons learned from three case studies.

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  • 1 Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, MD, United States
  • 2 Stevens Institute of Technology, Hoboken, NJ, United States
  • 3 Department of Civil Engineeering, University of Texas at Arlington, Arlington, TX, United States
  • 4 Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, MD, United States
  • 5 Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, MD, United States
  • 6 San Francisco Estuary Institute, Richmond, CA, United States
  • 7 South Bay Salt Pond Restoration Project, San Francisco, CA, United States
  • 8 US Army Corps of Engineers, Galveston, TX, United States
  • 9 Texas A&M University atGalveston, Galveston, TX, United States
  • 10 Delft University of Technology, Delft, Netherlands
  • 11 National Oceanic and Atmospheric Administration (NOAA), San Francisco, CA, United States
  • 12 Arcadis, Long Island City, NY, United States
  • 13 US Army Engineer Research and Development Center, Vicksburg, MS, United States
  • 14 San Francisco Estuary Partnership, San Francisco, CA, United States
  • 15 Graduate School of Architecture, Planning, and Preservation, Columbia University, New York, NY, United States
  • 16 Deltares, Delft, Netherlands
  • 17 Institute of Environmental Sciences CML, Leiden University, Leiden, Netherlands
  • 18 US Army Corps of Engineers, New Orleans, LA, United States
  • 19 Texas General Land Office, Austin, TX, United States

Coastal communities around the world are facing increased coastal flooding and shoreline erosion from factors such as sea-level rise and unsustainable development practices. Coastal engineers and managers often rely on gray infrastructure such as seawalls, levees and breakwaters, but are increasingly seeking to incorporate more sustainable natural and nature-based features (NNBF). While coastal restoration projects have been happening for decades, NNBF projects go above and beyond coastal restoration. They seek to provide communities with coastal protection from storms, erosion, and/or flooding while also providing some of the other natural benefits that restored habitats provide. Yet there remain many unknowns about how to design and implement these projects. This study examines three innovative coastal resilience projects that use NNBF approaches to improve coastal community resilience to flooding while providing a host of other benefits: 1) Living Breakwaters in New York Harbor; 2) the Coastal Texas Protection and Restoration Study; and 3) the South Bay Salt Pond Restoration Project in San Francisco Bay. We synthesize findings from these case studies to report areas of progress and illustrate remaining challenges. All three case studies began with innovative project funding and framing that enabled expansion beyond a sole focus on flood risk reduction to include multiple functions and benefits. Each project involved stakeholder engagement and incorporated feedback into the design process. In the Texas case study this dramatically shifted one part of the project design from a more traditional, gray approach to a more natural hybrid solution. We also identified common challenges related to permitting and funding, which often arise as a consequence of uncertainties in performance and long-term sustainability for diverse NNBF approaches. The Living Breakwaters project is helping to address these uncertainties by using detailed computational and physical modeling and a variety of experimental morphologies to help facilitate learning while monitoring future performance. This paper informs and improves future sustainable coastal resilience projects by learning from these past innovations, highlighting the need for integrated and robust monitoring plans for projects after implementation, and emphasizing the critical role of stakeholder engagement.

1 Introduction

There is a growing need to protect shorelines from coastal flooding due to accelerating numbers of floods due to sea-level rise ( Sweet et al., 2018 ) and a rapid increase in billion-dollar coastal storm disasters ( NRC 2014 ; Smith 2020 ). Sea-level rise in particular is predicted to have much larger impacts to coastal communities during the remainder of this century and into the future ( IPCC 2021 ). Traditional approaches to coastal protection largely have relied on “gray” infrastructure, such as seawalls, levees, and breakwaters, which may reduce the risk of flooding but may have adverse ecological impacts ( Bilkovic and Mitchell 2013 ) and alter physical dynamics resulting in downstream erosion ( de Schipper et al., 2020 ). In response, management strategies in the United States (US) and elsewhere have evolved and often incorporate natural, or “green,” approaches such as living shorelines ( Gittman et al., 2014 ; Sutton-Grier et al., 2015 ). Interest in infrastructure projects with natural and nature-based features (NNBF) for tackling these coastal resilience challenges is rapidly expanding. New initiatives are helping address this demand, including Engineering with Nature (EWN) from the US Army Corps of Engineers (USACE), “Building with Nature” in Europe ( Van Slobbe et al., 2013 ), and the World Association for Waterborne Transport Infrastructure (PIANC; The World Association for Waterborne Transport Infrastructure, 2018 ). The EWN Atlas volumes 1 and 2 ( Bridges et al., 2018 ; Bridges et al., 2021 ) present over 100 projects from around the world that integrate natural processes with engineering approaches. Project descriptions emphasize operational efficiencies, the use of natural processes to maximize benefits, and collaborations with partners and stakeholders.

Coastal ecosystem restoration, often with a goal of restoring fisheries, water quality benefits, and/or key habitat features has been occurring for decades, some of it at quite large scales ( DeAngelis et al., 2020 ). More recent NNBF efforts (which are also sometimes called “hybrid” infrastructure approaches) on the other hand differ in that they tend to have a focus on providing specific coastal resilience benefits, typically involving both habitat restoration components to the design as well as other engineering components ( Sutton-Grier et al., 2015 ). These NNBF projects are innovative in that they are not attempting to restore a fully functioning ecosystem, but instead are designed to restore very specific ecosystem functions for coastal resilience (such as erosion reduction and/or flood protection) while potentially providing some additional benefits. Additional NNBF hallmarks include a limited geographic setting, often near large human populations dependent on anticipated coastal resilience benefits, and constraints on budget. The critical human dimension involved in NNBF projects translates to community engagement in the outcomes and designs of the projects. Together, these NNBF characteristics are much more likely to push a project towards meaningful risk mitigation while also enhancing ecological and/or social resilience.

However, there are still many unknowns about the broader enterprise of NNBF-based coastal resilience, spanning design, funding, policy, co-production, implementation, and long-term monitoring and learning. Traditional gray infrastructure has been used to prevent flooding and erosion for decades and, as a result, there are standard design criteria for projects and a permitting system that is designed to easily and quickly provide project approval ( Sutton-Grier et al., 2015 ). In contrast to traditional coastal gray infrastructure, NNBF projects are more complex and challenging to design and implement, since they tend to be multifunctional with several goals and are often more dynamic due to the natural components of the projects (e.g., shifting sediments, vegetation changes) rather than being a static structure, and key questions remain as to how to design, fund, and permit projects. This is particularly important as communities and agencies increasingly look to incorporate NNBF into their shoreline management plans. In fact, the USACE has recently been given guidance for “equal consideration” of economic, social and environmental categories in their project planning and evaluation ( US Army Corps of Engineers, 2021 ).

One particular shortcoming of past NNBF projects has been limited stakeholder engagement. A great deal of the literature focused on community engagement in the context of coastal resilience has centered around disaster preparedness, rather than the specific conditions relevant to NNBF efforts. More than 2 decades ago, Mileti (1999) documented the challenges of externally designed hazard-mitigation strategies. Mileti (1999) noted the significant shift in understanding that these projects are not just a combination of the physical environment with engineering and infrastructure mitigation, but that communities are also central to identifying and implementing successful solutions. This shift towards less hierarchical planning was evident in varying degrees following the devastating impacts of Hurricanes Katrina and Harvey on the Gulf Coast and extended to participatory modeling ( Hemmerling et al., 2020 ), appreciation for multi-stakeholder participatory planning efforts ( Dunning 2020 ), and the critical value of local and traditional ecological knowledge in prioritizing data collection and modeling and decision-making frameworks ( Nichols et al., 2019 ). Stakeholder engagement that reaches “hard-to-reach” and underrepresented communities is particularly important to avoid unintended consequences of NNBF. For example, there are now several high-profile examples of “green gentrification,” such as East Boston Greenway ( Anguelovski and Connolly 2021 ), Chicago’s 606 rails-to-trails ( Rigolon and Németh 2018 ), and New York High Line ( Wolch et al., 2014 ). These projects led to rapid commercial and property development, escalating property values and eventual displacement of vulnerable community members. NNBF solutions can be more equitable when engagements reach vulnerable communities, infrastructure is designed with these communities’ input, and funds are distributed among these communities along with other tangential communities ( Heckert and Rosan, 2016 ). The examples presented here highlight the growing role of stakeholder engagement in disaster preparedness and planning, even though examples of community involvement are more limited in the case of NNBF focused projects.

In this study, we analyze three case studies of innovative NNBF coastal resilience projects on different US coastlines and coming from widely contrasting initiatives and sources of funding. These examples showcase the diverse ways that NNBF projects are imagined and implemented, with different features, designs, engineering strategies, funding sources, and stakeholder engagement. Our goal is to synthesize insights and lessons learned from these projects to inform future efforts and add to a growing knowledge base for NNBF implementation (e.g., Narayan et al., 2016 ; Morris et al., 2018 ; Vuik et al., 2018 ) similar to guidance for gray infrastructure.

The case studies were presented at a series of web panels from which we developed the case study descriptions and assessed keys to success and remaining challenges. The three case studies are 1) Living Breakwaters in New York (NY) Harbor, 2) the Coastal Texas Protection and Restoration Feasibility Study (Coastal Texas (TX) Study), and 3) South Bay Salt Pond Restoration Project in San Francisco Bay, California (CA) ( Figure 1 ). We begin by summarizing these innovative projects, all of which have created or will create in-water habitat within coastal natural/human resilience projects. We focus on the innovations to coastal design, the important role of stakeholder engagement in all three projects, and funding implementation for each project and then assess common themes that emerged. Within each theme, we discuss tips for success and/or areas of progress, as well as remaining challenges and knowledge gaps for future work.

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FIGURE 1 . Overview map of project locations.

A series of web panels was held in October and November 2020 entitled “Innovations in Nature-Based Systems for Coastal Protection” as part of Coastlines & People (CoPe) Research Coordination Network (RCN) funded by the US National Science Foundation. The web panels were recorded and can be accessed at https://www.umces.edu/cope/events . The final list of web panel titles and panelists, and Steering Committee members, is provided in Table 1 . The focus of these panels was initially developed by the Steering Committee, with the intention of featuring one project along each of the continental US’s ocean coastlines (East, West, Gulf) and one international project. Steering Committee members identified potential projects in each geographic region and contacted relevant partners to help select panelists. The goal was to select large-scale projects with diverse approaches that were not yet completely constructed and had willing panel participants. While there are many other projects we could have selected, we feel that the insights gained from three case studies selected highlight emerging themes that are broadly applicable to other NNBF projects.

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TABLE 1 . Steering Committee Members: Cindy Palinkas [University of Maryland Center for Environmental Science (UMCES)]; Philip Orton (Stevens Institute of Technology); Michelle Hummel (University of Texas at Arlington); William Nardin, Matthew Gray, Ming Li, Lora Harris (UMCES); Ariana Sutton-Greir (University of Maryland College Park).

The Steering Committee and panelists co-developed the focus and initial content of each panel, which followed the same structure of ∼30 min of introductory presentations by panelists followed by ∼45 min of discussion moderated by a Steering Committee member and including attendees as active participants. We focused these discussions on implementation and design, funding, and stakeholder engagement; from our perspective, these aspects make NNBF projects unique relative to gray infrastructure and are often the most challenging. We used a structured analysis approach and asked panelists to comment on these aspects, highlighting successes and challenges, as well as lessons learned from their experiences. More than 700 people registered for the series, with ∼200–300 attending the live sessions, from a variety of fields (e.g., academia; local, state, and federal agencies; non-profits; private industry) and geographies.

Using content shared prior to and during the panels, the Steering Committee developed descriptions for each case study and synthesized lessons learned across all panels. These were refined during a meeting that included all panelists and the Steering Committee and informed the rest of the paper. The first panel focused on the Sand Motor project in Netherlands, which is well described in the first volume of the EWN Atlas ( Bridges et al., 2018 ) and other publications (e.g., Stive et al., 2013 ; Brière et al., 2018 ; Luijendijk and van Oudenhoven 2019 ; de Schipper et al., 2020 ). Rather than repeating those details, we have chosen to omit it as a specific case study and instead focus on the other three projects. These three projects (described below) are in different phases of development. The Living Breakwaters project in New York Harbor has obtained funding and worked with stakeholders to refine the design plan; its construction began in August 2021. The Coastal Texas Protection and Restoration Project in the Gulf of Mexico is still in the study phase, awaiting submission to Congress for authorization of federal funding. If authorized, it will then proceed to the design and implementation phases. South Bay Salt Ponds in San Francisco Bay is the most mature project, having completed the initial phase of implementation in 2014. This project has a robust adaptive management plan, so that results from the first phase informs project designs and the science program for subsequent phases, including an established program for stakeholder engagement and long-term plans for monitoring and performance assessments.

3 Case Study Descriptions

3.1 living breakwaters—new york harbor.

The Living Breakwaters project is being built in the waters of Raritan Bay (Lower New York Harbor) along the southernmost part of Staten Island’s eastern shoreline ( Figure 2 ). The project area is a shallow estuary that has historically supported commercial fisheries and shell fisheries. The area was heavily impacted by Hurricane Sandy in October 2012, which damaged or destroyed an unprecedented number of homes and businesses and caused loss of life and significant harm to the local economy. In response, a design competition, Rebuild by Design, was launched by the Hurricane Sandy Rebuilding Task Force to “couple innovation and global expertise with community insight to develop implementable solutions to the region’s most complex needs” ( Grannis et al., 2016 ) ( http://www.rebuildbydesign.org/our-work/sandy-projects ). The Living Breakwaters project resulted from a winning entry to this competition, with the competition and initial design phase occurring in 2013–2014.

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FIGURE 2 . (A) The Living Breakwaters project will construct a series of breakwaters in Raritan Bay, offshore of Staten Island. (B) The breakwaters are designed to provide habitat for marine life, including oysters. (C) Sample breakwaters (shown in gray) include a main breakwater plus “reef streets” angled outward. These were tested using computational fluid dynamics modeling to evaluate and optimize designs for avoiding scour and sedimentation.

3.1.1 Innovative Coastal Design

The Living Breakwaters project innovates by integrating risk reduction, ecological enhancement, and social resilience ( Tschirky et al., 2018 ). The project consists of approximately 2,500 linear feet (∼760 m) of nearshore “breakwaters,” or partially submerged rubble-mound structures located between 790 and 1,800 ft (∼240 and 550 m, respectively) from shore ( Figures 2A,B ). With regards to risk reduction, the project addresses both event-based and long-term shoreline erosion to preserve or increase beach width and provides wave attenuation to improve safety and prevent damage to buildings and infrastructure. The breakwaters are designed to reduce the height of wind-driven waves reaching buildings and roads to less than 3 ft (∼1 m) during a 100-year storm event with up to 18 inch (∼45 cm) of sea-level rise (SLR). They are not designed to reduce storm surge but instead cause wind waves to break further offshore, reducing wave run-up onto land and potentially also reducing the effect of waves on the surge (termed “wave setup”). Even as the breakwaters are more frequently submerged by storm surges with higher SLR, hydrodynamic modeling indicates that they will continue to provide wave attenuation ( Marrone et al., 2019 ). The project also includes one-time sand replenishment to enhance beach width along the narrowest stretch of shoreline. Extensive computational fluid dynamics modeling and scaled physical laboratory modeling was utilized to optimize design, ranging from evaluation of the effectiveness of the entire set of breakwaters to reduce erosion and accrete beach over time, down to design of individual breakwaters to avoid scour and sediment accretion ( Figure 2C ; Marrone et al., 2019 ).

In addition to risk reduction, the project is also meant to increase the diversity of aquatic habitats, especially hard-structured habitats that can function much like the historical oyster reefs that once existed in Raritan Bay. In particular, the breakwaters were designed as rubble-mound structures with outer layers consisting of armor stones of varying sizes and ecologically enhanced concrete armor units that provide textured surfaces to promote biological activity and species recruitment. The structures also include “reef streets,” narrowly-spaced rocky protrusions on the ocean side of the breakwaters, to increase habitat diversity ( Marrone et al., 2019 ).

The benefits of detached breakwaters for coastal protection have been known for decades (e.g., Chasten et al., 1993 ), and oyster reefs have been gaining appreciation as a new NNBF option (e.g., Piazza et al., 2005 ; Reguero et al., 2018 ). However, their combination, the urban setting, and the social components of LB are innovations on these concepts. The project uses education, outreach, and workforce training to spread awareness about harbor restoration activities and to encourage stewardship of the harbor. It also aims to increase physical and visual access to the shoreline and nearshore waters for enhanced recreational use.

3.1.2 Stakeholder Engagement

The community-based design process engaged a range of stakeholders such as regional experts, government entities, elected officials, issue-based organizations, local groups and individuals. Stakeholder engagement during the Rebuild by Design Competition led to improved understanding of current vulnerability and future threats, while at the same time raising public expectations about grantees meeting grand challenges with constrained budgets ( Grannis et al., 2016 ). After LB was selected as winner of the Rebuild by Design Competition, the Citizens Advisory Committee (CAC) formed in 2015. The CAC intended to serve in a community-based advisory role to the project while leaving additional input from the public during public engagements and workshops. The NY Governor’s Office of Storm Recovery “encouraged applications from all variety of individuals and organizations in order to represent the diverse community of Staten Island and the region who the project will serve” ( https://stormrecovery.ny.gov/LBWCAC ). There were nine CAC meetings between July 2015 to July 2018.

Stakeholder input led to many adjustments to the project, including the project location, breakwater height, and an initial land-based “water hub” concept evolved and eventually changed form altogether to become a floating hub. Moreover, stakeholder input also informed project priorities and helped ensure the retention of critical features of the project, including ecological elements, through the design process when budgetary concerns often lead to loss of non-protective features of NNBF projects. Additionally, the iterative process of reviewing and updating designs with public input garnered greater public support for the projects over time.

3.1.3 Funding and Implementation

The project was implemented using $60M of Community Development Block Grant Disaster Recovery (CDBG-DR) funding as well as $14M of funding from the State of New York. An environmental impact statement (EIS) was completed in 2018, and necessary state and federal permits were secured soon thereafter. The project construction began in August 2021 with a projected completion date of Fall 2024.

3.1.4 Ecosystem Services and Connectivity

Given that a fundamental goal of the project is ecological enhancement, several ecosystem service benefits are part of the design. Ecosystem service values for the project were estimated using a biome-based spatial approach, using the net change in habitat area with areal habitat dollar values obtained from published literature sources ( NYS-GOSR 2021 ). Biomes with positive net change in value included oyster habitat/reef sustainability, increased productivity of commercial finfish and crustaceans, shoreline stabilization, water quality improvements (nitrogen removal and SAV enhancement), and refugia. The only negative (gross) ecosystem services were related to loss of relatively lower-value sandy subtidal habitat under the footprint of the breakwater structures ( NYS-GOSR 2021 ). Also, there were hopefully limited negatives with regard to ecological connectivity, since the breakwaters could cause increased long-term sedimentation and reduced circulation behind them.

3.2 Coastal Texas Protection and Restoration Study—Gulf of Mexico

The Coastal Texas Study was undertaken to address habitat loss and the range of hazards faced by coastal areas in the state, including erosion, sea-level rise, and storm surge ( Figure 3 ). It seeks to determine the feasibility of Coastal Storm Risk Management (CSRM) and Ecosystem Restoration (ER) measures to protect the state’s communities, critical economic functions, and environmental assets ( US Army Corps of Engineers and Texas General Land Office, 2020 ; https://coastalstudy.texas.gov ). The scope of the project covers the entire Texas coast, from the Sabine River to the Rio Grande River, including all coastal areas and interconnected ecosystems in the state’s 18 Gulf Coast counties.

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FIGURE 3 . (A) Overview of the Coastal Texas Study area. (B) The study proposes a multiple lines of defense approach for Galveston Bay that includes gulf defenses along the outer barrier island coast as well as bay defenses to provide residual risk reduction within the bay. The gulf defenses include (C) restored beach and dune systems along Bolivar Peninsula and (D) a gate system at Bolivar Roads, the primary connection between Galveston Bay and the Gulf of Mexico.

3.2.1 Innovative Coastal Design

The project aims to minimize economic damage from coastal storm surge, inland and Gulf shoreline erosion, and restore threatened and endangered critical habitats hydrology to key lagoons. This is accomplished through a multiple-lines-of-defense strategy that combines structural, nature-based, and non-structural features to provide coastal resilience through implementation of robust and redundant protective features similar to the “double-insurance” framework of Andersson et al. (2017) . A tentatively selected plan was identified in May 2018, followed by draft reports integrating feasibility and environmental impacts for public, policy, and peer review, with the goal of advancing the project to Congress for authorization of construction funding in 2022. The comprehensive plan consists of 1) an ER component that covers 6,600 acres (∼27 km 2 ) of the coast to restore fish and wildlife habitat, improve hydrologic connectivity, and create and restore oyster reefs, marshes, dunes, and islands that provide protection for communities and infrastructure; 2) a CSRM component for 2.9 miles (∼4.7 km) of beach nourishment on South Padre Island along the lower Texas coast; and 3) a final CSRM component for the Houston-Galveston region spanning 63 miles (∼101 km) of the upper Texas coast to reduce storm surge entering Galveston Bay. This largest component, referred to as the Galveston Bay Storm Surge Barrier System, deploys a multiple-lines-of-defense approach intended to offer redundancy with the goal of mitigating storm surge impacts and improving the resilience for residents, industry, and ecosystems in the Houston-Galveston region. It includes a 2.8-mile (∼4.5-km) long gated surge barrier system across the Galveston Bay entrance, improvements to the existing Galveston Seawall, and 43 miles (∼69 km) of beach and dune systems on Galveston Island and Bolivar Peninsula, as well as strategies to mitigate residual risk from bay water surges, including additional gate closures and pumping stations at Clear Lake and Dickinson Bay on the mainland, a ring barrier for the backside of the City of Galveston, and additional nonstructural improvements on the mainland including floodproofing and raising of at-risk structures. The ER components target eight locations along the coast and include the construction of 114 miles (∼183.5 km) of breakwaters, 15.2 miles (∼24.5 km) of bird rookery islands, 2,052 acres (∼8.3 km 2 ) of marsh, 12.3 miles (∼19.8 km) of oyster reef, and 19.5 miles (∼31.4 km) of beach and dune restoration ( US Army Corps of Engineers and Texas General Land Office, 2020 ).

3.2.2 Stakeholder Engagement

The scoping process included federal, state, and local agencies and tribal nations, which met monthly to discuss study details and progress. Additional interagency and international workshops were held to discuss alternatives, performance metrics, and adaptive management approaches, among other aspects. Prior to the COVID-19 pandemic, a series of face-to-face public hearings and outreach meetings were held to solicit public comments on the plan and to inform the public regarding project updates (recordings are available at https://coastalstudy.texas.gov/get-involved/public-meetings/index.html ). Community feedback led to changes to the plan, which originally included a floodwall over 17 ft (∼5.2 m) high to protect the barrier islands along Galveston Bay’s Gulf of Mexico shoreline. Local communities objected to this floodwall solution for a variety of reasons. After the USACE received more than 13,000 negative comments to this effect, they revised their plans and moved toward a more nature-based solution of beach and dune systems on the fronts of the barrier islands. It should be noted that this modification came with an increase of potential residual risks, but the tradeoffs offered an opportunity to better balance engineering performance, costs, benefits (i.e., returns on investment), and fewer environmental impacts resulting in a more socially acceptable solution. During the COVID-19 pandemic, the study team could not host face-to-face public outreach activities, and as a fallback developed an interactive GIS-based driven StoryMap system to offer the public an opportunity to engage with the study team virtually and explore the recommended plan through an interactive experience medium ( https://coastal-texas-hub-usace-swg.hub.arcgis.com/ ).

3.2.3 Funding and Implementation

The US Congress appropriated $20.6 million to USACE over the course of the study in cooperation with the Texas General Land Office (TGLO), the non-federal cost-share sponsor, to complete the study effort. The estimated construction first-cost (in 2021 dollars) for the recommended plan is $28.9 billion, with 69% of the cost for Gulf Coast defense in Houston-Galveston and South Padre Island, 22% for bayshore defense in Houston-Galveston, and 9% for ecosystem restoration. The estimated average annual operation, maintenance, repair, replacement, and rehabilitation costs are $131 million, which must be shouldered solely by the construction sponsor. Recent scholarly research demonstrated economic benefits of a coastal barrier for the communities along the upper Texas coast to outweigh its engineering costs ( Davlasheridze et al., 2019 ) and also looked at its significance in terms of buffering negative ripple effects on the economies of other states and the nation as a whole ( Davlasheridze et al., 2021 ). To proceed to construction, funding must be authorized and appropriated by Congress, and cost-share sponsors must be identified. The recent Senate Bill 1160, passed on 16 June 2021, authorized a creation of the “Gulf Coast Protection District,” a five-county taxing authority ( https://legiscan.com/TX/drafts/SB1160/2021 ) and corresponds to the latest developments towards realization of the coastal-defense system for upper Texas coast communities. The bill creates a formal mechanism for the district to partner with USACE and contribute towards funding, construction, and maintenance of a coastal barrier by taxing, issuing bonds, and other financial instruments. In addition, the Texas General Land Office will serve as an additional cost-share sponsor for the ER and South Padre Island components.

Design is expected to take 2–5 years to complete (per component), and construction is expected to take an additional 10–15 years after that. The project will be maintained for a minimum of 50 years by local sponsors. The average annual costs for operation, maintenance, repair, rehabilitation, and replacement during this period are estimated at $131 million. This includes funding for periodic nourishment of restored beaches and dunes on Bolivar Peninsula and West Galveston Island every 6–7 years. As the project moves into the next phase, the project team will continue to engage with stakeholders and the public at large through the interactive StoryMap tool.

3.2.4 Ecosystem Services and Connectivity

A main goal of the Coastal Texas Study is to improve hydrologic connectivity while restoring or creating fish and wildlife habitat and natural features to provide coastal protection for communities and infrastructure. Specifically, this includes a designed system to reduce storm surge entering Galveston Bay. The full project incorporates several types of restoration actions including marsh restoration, island creation/restoration, dune and beach restoration, oyster reef creation/restoration, and hydrologic restoration. Each proposed ER action was evaluated by simulating the change in number of habitat units available for target species, compared to the no-project condition. Tools such as the Habitat Evaluation and Assessment Tool (HEAT), Habitat Suitability Index (HSI), and the Wetland Value Assessment (WVA) were used based on the ecosystem type and species. Average annual habitat units calculated across the project planning period were then used to develop the final suite of ER actions described in Section 3.2.1.

Together, the ER components of the project are designed to provide a range of ecosystem services for Texas coastal communities. They contribute to the primary risk-reduction goals of the project by preventing shoreline erosion and reducing inundation of populated areas. In addition, these projects can enhance local water quality and provide habitat for a variety of species of commercial and recreational value, including brown shrimp, brown pelican, Kemp’s Ridley sea turtle, oyster, and spotted seatrout.

3.3 South Bay Salt Ponds—San Francisco Bay, California

This panel had a wider focus than the others, since restoration in San Francisco Bay often occurs within a regional context. The panel included experts from the NOAA National Estuarine Research Reserve System (NERRS), San Francisco Estuary Partnership, California State Coastal Conservancy, and San Francisco Estuary Institute, providing perspectives from federal, state, local, and non-profit stakeholders. Many San Francisco Bay projects take regional strategies into consideration as part of their planning and implementation. These strategies include the San Francisco Bay Subtidal Habitat Goals Project ( Subtidal Goals 2010 ), Baylands Habitat Goals Update ( Goals Project 2015 ), Comprehensive Conservation and Management Plan for the San Francisco Estuary (2016) , the San Francisco Bay Shoreline Adaptation Atlas ( Beagle et al., 2019 ), and the recent effort to establish a regional monitoring program through the Wetlands Regional Monitoring Program (WRMP; https://www.sfestuary.org/wrmp ). The WRMP Program Plan was released in April 2020, with the intention of full program implementation by 2022. The development of the WRMP included a process that engaged hundreds of experts around the Bay Area in designing the overall plan and the science framework. We focus on one specific project for the case study—South Bay Salt Pond Restoration Project (SBSP; https://www.southbayrestoration.org )—and provide insights from regional collaborations in the discussion of common themes below ( Figure 4 ). While the SBSP Restoration Project has been featured in other publications ( Chapple and Dronova 2017 ; Gies 2018 ; DeAngelis et al., 2020 ; also see https://www.southbayrestoration.org/news-items ), the project continues to evolve and has entered its second phase of construction.

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FIGURE 4 . Map of the South Bay Salt Pond Restoration Project in San Francisco Bay.

3.3.1 Innovative Coastal Design

The SBSP Restoration Project is the largest tidal wetland restoration project on the US West Coast (15,100 acres, ∼60.7 km 2 ), seeking to restore multiple former salt-production ponds back to natural conditions like tidal marshes and other aquatic habitats ( Valoppi 2018 ). In addition to restoration, the project will provide regional flood risk reduction by absorbing tidal energy instead of reflecting it, reducing tidal amplitudes far beyond the project, to varying degrees across all of San Francisco Bay ( Holleman and Stacey 2014 ).

The project integrates three main goals: 1) habitat restoration that focuses on a range of special-status species, primarily tidal-marsh species but also species (mainly birds) that used ponded areas during the salt production era; 2) protection from tidal flows brought closer to developed areas as leveed salt ponds are opened up; and 3) addition of wildlife-compatible public access features to connect people with the Bay while providing wildlife access and habitat. The SBSP Restoration Project is implemented on lands within a state ecological reserve (Eden Landing Ecological Reserve) and a national wildlife refuge (Don Edwards San Francisco Bay National Wildlife Refuge), and it is located in three counties, underscoring the importance of a regional approach to coastal management. Phase 1 occurred from 2007–2014, restored 3,000 (∼12.1 km 2 ) acres of tidal marsh, made improvements to >700 acres (∼2.8 km 2 ) of managed ponds to target pond-dependent wildlife, built islands and installed water management structures, and added 7 miles (∼11.3 km) of trails, mostly on levees, viewing platforms, a kayak launch, and historical exhibits. The project is in the earliest phases of Phase 2, which seeks to return tidal flows to additional areas, enhance pond habitat in other places, add trails, and integrate flood-protection projects with several external partner agencies. The SBSP Restoration Project includes an adaptive management program that uses a “restoration staircase” concept to address questions of adaptation and resilience, inserting intentional pauses to evaluate how habitats are evolving and how wildlife are responding. For example, before moving to Phase 2, the program worked with scientists to evaluate past performance and suggest possible adaptation measures to adjust project designs and refine the science program for Phase 2. These measures include anticipated effects of climate change and emerging technologies, as well as potential funding and communication mechanisms. Insights from this process and other regional planning efforts such as the Adaptation Atlas (developed by the San Francisco Estuary Institute; Beagle et al., 2019 ) help to identify types of adaptation strategies for consideration at specific sites.

Both the regional WRMP and the SBSP Restoration Project face potential challenges to success. For the WRMP, the biggest challenge is serving such a broad community of interest while remaining technically rigorous. There is a constant driving need to produce great science, but the process can overpower a Program like this one if all interested parties are at the table. For example, balancing trade-offs of serving such a broad community of stakeholders and of inclusion and focus when it comes to program development can be difficult. For the SBSP Restoration Project, challenges include climate change and other environmental changes that affect flooding and sediment supply to sustain the establishing tidal marshes. Project actions have the potential to increase bioavailable mercury and negatively impact the food web, as well as invasive species expansion. South Bay Salt Pond Restoration Project Adaptive Management Plan, (2007) is specifically designed to address these, and many other, uncertainties as the project continues, directly informing design and implementation of each phase of the project.

The marsh restoration areas are meant to have an indefinite/permanent useful life, as they are primarily habitat features in an obviously dynamic and ever-changing environment. They are not necessarily intended to provide any specific degree of coastal resilience or flood protection on their own. The public access features such as levee-top trails, boardwalks, viewing platforms, etc. have useful lives of 30–50 years, with the 30 being the official “intended” useful life of those features. The water control structures and pond levees/berms used in the managed pond enhancements usually need constant maintenance and/or repair/replacement on the order of a decade or so.

3.3.2 Stakeholder Engagement

The Wetland Regional Monitoring Plan (WRMP) uses a collaborative, consensus-based approach for regulators, land managers, and scientists making decisions together, starting with management questions that drive monitoring down to the level of metrics, protocols, and indicators, then bringing in new questions to update metrics and protocols. It is a prime example of combining the technical foundation for the work with public engagement, listening to the underserved communities that are adjacent to restoration projects and any other interested community members. Indeed, extensive community engagement is becoming a basic foundational practice for designing restoration projects and is one of the core best management practices.

For the SBSP Restoration Project, outreach and stakeholder engagement efforts are led by the California State Coastal Conservancy and the Consensus and Collaboration Program at California State University Sacramento. Outreach is a critical part of the entire project, since it is one of the largest restoration projects in the US and takes place in one of the most densely populated regions of California with many different user groups, interests, neighboring landowners, and stakeholders. The major venue for the public to provide advice and recommendations to the Project Management Team is through the Stakeholder Forum, a group of 25 individuals representing local businesses, advocacy groups, elected officials, recreational groups, and others. Input from extensive interviews with a wide range of stakeholders prior to implementation resulted in the current stakeholder process that provides opportunities for input at each phase of planning ( California State University Sacramento, 2003 ). This feedback, and recognition of the additional challenges of climate change on this system, led to recent recommendations to increase regional coordination and engagement to enhance adaptive management moving forward. This is recognized as an important theme for all regional planning. Indeed, the Baylands Goals Science Update for the San Francisco Bay area ( Goals Project 2015 ; https://www.sfei.org/projects/baylandsgoals ) made several recommendations for climate planning in the region that included centralizing data for better coordination and facilitating dialogue to promote information diffusion among stakeholder groups.

3.3.3 Funding and Implementation

In 2003, 15,100 acres (∼61.1 km 2 ) of commercial salt ponds were acquired from Cargill, Inc. for $100 million, funded by federal and state resource agencies and several private foundations. Funds for implementation of the South Bay restoration, flood management, and public access plan to date have come from a mix of sources, including local, state, and federal funds, as well as private funds from foundations or other non-governmental organizations. The largest sources of ongoing funding for restoration planning, design, permitting, and construction are competitive federal, state, and regional grant programs, matched by in-kind contributions from the project partner agencies.

The South Bay project is being implemented in multiple phases over 50 years, using a robust adaptive management plan (AMP) to determine how far the system can move toward full tidal action and associated tidal habitats, while still meeting the other Project Objectives ( Trulio et al., 2007 ). The AMP identifies restoration targets as well as triggers that may necessitate management actions. The organizational strategy includes an Executive Project Manager and Executive Leadership Group and a Project Management Team that regularly interacts with the Science Team, Regulatory and Trustee Agency Group, and Stakeholder Forum to ensure oversight and coordinate planning and implementation throughout the project ( https://www.southbayrestoration.org/page/who-we-are-collaborative-team ).

3.3.4 Ecosystem Services and Connectivity

The SBSP Restoration Project highlights the need to make sure habitats are not created for a single species but rather consider competing species’ needs. Indeed, one of the main goals of the SBSP Restoration Project is habitat restoration for a range of special-status species. While the focus is mostly on tidal marsh species, there is also a need to protect the many types of wildlife (mainly birds) that used the ponded areas during salt production. So, the project design needed to restore tidal marsh species while also providing for pond-dependent wildlife species and while connecting habitat via wildlife corridors. Another goal is to add wildlife-compatible public access features like trails and viewing areas to connect people with SF Bay and help them understand why restoration is needed. These goals and associated ecosystem services are not always compatible, making some trade-offs potentially necessary. For example, opening up the salt ponds and restoring tidal flows brought water closer to developed areas, resulting in a need to maintain or improve current levels of flood protection for those areas. This has entailed working with local flood protection agencies to incorporate their projects into the landscape with the restored sites.

Beyond the challenge of managing the competing goals for the project and the challenges of predicted effects of sea-level rise, the project provides an array of ecosystem services and greatly improved connectivity along the shoreline. The restoration supports baseline services and functions such as photosynthesis, nutrient cycling and provides habitat and nursery areas, increases biodiversity, and the transition zones designed into the project provide high tide refugia. Much of the habitat along the shoreline of San Francisco Bay has been reduced in size and suffered from fragmentation due to urban development. Increasing the area of tidal marshes is an important part of the design and will help to create larger, more connected patches of marsh habitat in the South Bay to allow movement of not only wildlife species, but of water, sediment and nutrients between the Bay and ponds that were previously restricted by berms and levees. Social and economic services are services that are especially important to people for cultural and social development and the Bay area will benefit from increased access to trails for hiking and biking, and birdwatching. This is a key issue for the region, and so it follows that evaluating how wetland restoration provides benefits to humans is one of the five guiding questions of the regional WRMP. This task will work to ensure that diverse voices are at the table in the WRMP process, and their interests are reflected in the suite of indicators monitored by the WRMP. Enhancing community engagement and ecosystem services evaluation will improve the ability of the WRMP to advance environmental justice and improve environmental conditions for communities disproportionately impacted by climate change and the loss of wetlands.

4 Discussion of Emerging Themes

Several common themes emerged from the three case studies that highlight factors contributing to and/or hindering success of innovative coastline management projects, depending on the context. We have organized the themes into two sections—areas of progress and remaining challenges. It is important to note that there have been advancements and challenges in every theme; the groupings are intended to guide readers rather than represent a hard boundary. Our goal is to glean lessons learned within each theme to inform future NNBF coastal resilience projects.

4.1 Areas of Progress

4.1.1 moving beyond single-benefit projects.

Historical approaches to coastal protection have focused on reducing potential damages from hazards such as flooding and erosion via gray infrastructure (e.g., levees, seawalls, and bulkheads) ( Griggs 2005 ; Spalding et al., 2014 ). Despite the immediate and often substantial risk-reduction benefits provided by these structures, they offer minimal co-benefits and can even cause loss of coastal habitat and associated ecosystem services ( Sutton-Grier et al., 2015 ). NNBF and hybrid approaches to coastal protection represent a promising alternative to gray infrastructure because of the many co-benefits that can be achieved, including wildlife habitat, recreation, water quality, and carbon/nutrient sequestration ( Bridges et al., 2015 ). Additionally, the USACE has determined that NNBF projects that involve very collaborative, multi-disciplinary partnerships including landscape architects, engineers, and applied scientists, not only result in improved NNBF projects, but also improved communication and support for these types of projects ( King et al., 2022 ). Hence, projects with multiple goals and multiple collaborators have many benefits.

Each case study started with an innovative framing, enabled through the funding sources themselves. The Rebuild by Design competition that resulted in the Living Breakwaters project encouraged innovation and broad interdisciplinary teams, with the goal of “promoting innovation by developing regionally scalable but locally contextual solutions that increase resilience in the region” ( https://stageipk.es.its.nyu.edu/initiatives/rebuild-by-design/ ). The competition awarded projects that included strong engagement of local communities and government stakeholders, driving projects to target a wider range of benefits than simple flood-damage reduction. In the case of the Coastal Texas Study, the authorization for the study was explicitly for “flood damage reduction” and “ecosystem restoration,” in contrast to other more typical feasibility studies (e.g., US Army Corps of Engineers, 2015 ; US Army Corps of Engineers, 2019 ) that were only authorized for damage reduction. The SBSP Restoration Project initiative sought co-benefits from the initial stages of planning for restoration, flood reduction and wildlife-friendly public access.

By leveraging nature-based and hybrid infrastructure, all three case studies move beyond a sole focus on safety and flood reduction to include multiple functions and benefits ( Van Veelen et al., 2015 ; O’Shaughnessy et al., 2020 ). Complete elimination of risk is not the goal, nor is it realistic given anticipated increases in the rate of sea-level rise and storm intensity; instead, each project provides meaningful risk mitigation while also enhancing ecological and/or social resilience. For example, in addition to providing wave attenuation and erosion reduction, the Living Breakwaters project also aims to increase biodiversity, enhance shoreline recreational opportunities, and raise awareness of coastal resiliency and ecological health. The Coastal Texas Study includes numerous components aimed at creating or restoring natural features that provide habitat in addition to acting as barriers to storm surge or waves. The SBSP Restoration Project and other regional projects in San Francisco Bay are focused on ecological restoration rather than an explicit risk reduction component, although projects do include measures to ensure that flood risk for adjacent communities and infrastructure does not increase as a result of restoration actions. Also, enhancing recreation opportunities can be an important aspect for community buy-in and obtaining funding from multiple sources. For example, the SBSP Restoration Project has public access as one of its main goals and includes trails and viewpoints in almost all of the project sites.

Multi-functional projects such as these can address the needs of a variety of stakeholders and enable multiple pathways for a project to be successful, even if some aspects of the project do not end up working as well as others. For example, there has been increasing awareness by the public that the restoration of ponds at the edge of SF Bay may provide some protection against sea level rise for critical infrastructure, global technology companies, and other Silicon Valley businesses. As these case studies demonstrate, communities and stakeholders value natural habitats and the services they provide and may be more willing to support coastal resilience projects that include co-benefits such as maintaining ecosystem integrity and recreational access. An example of increased public awareness and support of wetland restoration includes the passage of Measure AA (San Francisco Bay Clean Water, Pollution Prevention and Habitat Restoration Measure)—the nine counties of San Francisco Bay voted for a 20-year, $12/year parcel tax that will raise $500 million for restoration projects in the Bay. The personal connection to the Bay by voters was one of the major factors for its success ( https://www.sfbayrestore.org/overview ). Given the multifaceted and interdisciplinary nature of NNBF projects, successful design and implementation requires expertise and cooperation across a variety of fields and sectors. Local system knowledge is critical to apply successful strategies from other projects, adapting them to address site-specific conditions. The projects described here include teams spanning a broad range of participants from architecture, engineering, ecology, economics, and/or social science representing state/federal agencies, consulting firms, and academia. These multidisciplinary teams reflect the importance of integrated thinking that considers the physical hazards alongside ecological and social responses.

4.1.2 Creating Opportunities for Natural and Nature-Based Features Through Co-Production of Project Designs

Input from the public is critically important since coastal resilience projects not only affect local communities and the environment but also people’s lives. As a result of the potential negative side effects of protecting coasts with gray infrastructure, including degraded habitat, loss of shoreline access, and impacts on neighboring properties, gray projects have faced opposition from stakeholders and the public in the past ( Griggs 2005 ). For example, a project in Ventura, California to prevent shoreline erosion by constructing a seawall was opposed by local stakeholders, including the Surfrider Foundation and the California State Coastal Conservancy, in the 1990s. Instead, the interested parties agreed upon a managed retreat approach (Surfer’s Point Managed Shoreline Retreat; https://ventura.surfrider.org/surfers-point/ ) that allowed for habitat restoration and did not interfere with local hydrodynamics ( Judge et al., 2017 ).

In contrast, coastal resilience projects that have stakeholder engagement as one of their explicit goals can incorporate feedback early and often as the project progresses. The Coastal Texas Study is an excellent example of feedback shifting the project design from a more traditional, gray approach to a more natural/hybrid solution. That project initially proposed a floodwall that was opposed by local residents, who sent thousands of negative comments to USACE. As a result, the project was redesigned to use a beach and dune system to reduce flooding from the Gulf instead. This solution provides fewer risk-reduction benefits but is more acceptable to the local community and provides more NNBF co-benefits.

Frequent and effective communication between the project team, stakeholders, and the public can contribute to a more transparent process that includes opportunities for input and adjustments, helping to build trust and buy-in ( Paul et al., 2018 ). The case studies here, especially Living Breakwaters and the Coastal Texas Study, highlight the importance of clear communication. For example, in the Living Breakwaters project, being transparent in the process about what the project could and could not do was critical for developing trust with everyone involved. This project made it clear from the beginning that the goal was not to keep flood waters out of the area but rather to restore ecological systems, reduce the risk of erosion and wave damage, and enhance social outreach and education. The project team specifically engaged with stakeholders before truly beginning the design to establish project goals and trade-offs, ultimately producing hundreds of pages of information for the Environmental Impact Statement (EIS). For the Coastal Texas Study, following the release of the first Draft EIS, there was widespread misunderstanding among locals about the proposed plan, and misinformation was spread on social media. The project team learned from this experience and conducted a much more extensive outreach and public education campaign prior to the release of the revised Draft EIS, which resulted in more productive exchanges between the project team and the public.

4.1.3 Managing Uncertainty in Project Performance

Unlike engineered structures that have a set of well-defined design criteria, there are uncertainties in quantifying the capacity of nature-based systems to withstand extreme events and determining the breakpoints at which such a system is expected to either fail to provide its required engineering service or itself be destroyed due to the environmental conditions. This requires a flexible design approach that can not only satisfy short-term needs but also allow for future adjustments to meet long-term goals. It also requires a post-construction monitoring program to document the performance of these systems and may require a greater commitment to ongoing maintenance to achieve a desired level of protection than traditional approaches. Since these systems are innovative, guidance on expected outcomes (e.g., amount of sediment accretion that will occur over time) is lacking, especially compared to decades of experience with gray infrastructure, and engineers may have more comfort with materials that have documented factors of safety. It can also be a challenge to predict the long-term evolution of NNBF and to scale up from small-scale to larger-scale applications. For example, in the Sand Motor project in Netherlands, the uncertainty in the predicted evolution is a mixture of both uncertainty in model formulations in current state of the art models and the uncertainty in future (wave) forcing ( Kroon et al., 2020 ). This is especially important since there is a large natural variability in ecology and still many unknowns on how habitat attributes result in changes in biodiversity and species richness.

Thus, it is critical that learning from NNBF projects also be one of the multi-functional goals, so that we can learn as much as possible from every project. For example, because there is no “one-size-fits-all” natural infrastructure design for all contexts ( Sutton-Grier et al., 2015 ) and the coastal resilience ecosystem services provided by natural infrastructure vary by geomorphic setting and event conditions ( Saleh and Weinstein 2016 ), one main research focus that is still greatly needed is better understanding of what approaches and strategies work well in which conditions ( Smith et al., 2020 ). Additionally, we need to know how to effectively implement NNBF projects for coastal resilience, and how projects can address uncertainties and evaluate or weigh different components (e.g., using Ecosystem Services framework). Projects should define goals for long-term adaptability in the planning of the project and establish specific performance metrics and clearly defined goals ( Arkema et al., 2017 ; Bayulken et al., 2021 ). In the Living Breakwaters project, modeling of waves, storm surge, and sediment movement in water and then onshore was a critical component to developing the design, and monitoring the project will be key to understanding how well the project is functioning for both ecological and risk reduction goals. In the SBSP Restoration Project, understanding how salt pond restoration would impact flooding and flood risk to human development has been key to planning to maintain or improve flood protection for those communities as part of the restoration design.

There is a general focus on “no-regrets” strategies by assessing adaptability to climate change via stress tests under higher sea levels. The Coastal Texas Study evaluated project alternatives under low, intermediate, and high SLR scenarios through 2,135 and includes a plan for monitoring and adaptively managing the ecosystem restoration components of the project to ensure that project objectives are met across the lifetime of the project. It also utilizes a multiple-lines-of-defense approach to provide redundancy in coastal protection and address possible failure modes. For example, possible breaching of the dune barrier system during large storms is addressed by including bay-side defenses and by elevating structures. The SBSP Restoration Project explicitly includes adaptive management, so that lessons learned from Phase 1 can be incorporated into Phase 2 plans and future phases along with new insights from emerging science and technology. Adaptive management via engaging scientists and stakeholders is a key part of the WRMP in San Francisco Bay, in which lessons learned from designing, implementing, and monitoring for one project will inform other projects at local and regional scales.

4.1.4 Expanding Beyond the Project Scale to a Regional Perspective

Possibly because of the stakeholder engagement and the focus on multiple benefits of each project, as well as the need to design multiple features to achieve many aspects of resilience, NNBF projects are often part of a suite of projects to build resilience across a broad region. This is a strength of the NNBF approach, because there can be a larger focus on how individual projects fit together into a coastal system designed for resilience. Connectivity of multiple projects along a coastline can be part of the design of NNBF projects particularly in urban settings in order to counter past loss of coastal ecosystems and biodiversity ( Aguilera et al., 2020 ). For example, in the Coastal Texas Study, there are many different resilience approaches across an entire region that are combined into one larger project with multiple goals—restoring fish and wildlife habitat; improving hydrologic connectivity; creating and restoring oyster reefs, marshes, dunes, and islands that provide protection for communities and infrastructure; renourishing beaches; installing a new tidal gate; and improving a seawall. In the SF Bay, once developed, the WRMP would integrate monitoring, reporting, and data-sharing across a wide range of projects to improve uniformity, consistency, currency, and other aspects of data. The project itself is also taking into account that as a result of the restoration of tidal flows, flood risk on different parts of the landscape is changing, and so maintaining and improving flood protection is part of the design of the project to help address the changes taking place across the landscape as restoration reconnects parts of the landscape that were previously separated by the salt ponds. Taking a regional approach to resilience and incorporating NNBF projects is a critical, forward-looking step in improving coastal resilience.

4.2 Remaining Challenges

4.2.1 navigating permitting and policy barriers.

A significant barrier to the widespread implementation of innovative NNBF projects is the permitting process for new designs, which is often complex and may need quite a lot of lead time. Permits may be required at the local, state, and/or federal level. There may be many steps, and regulators may be seeing this type of innovative project for the first time. Again, the importance of being transparent and doing outreach and education as part of these projects applies to getting regulatory and community buy-in. For example, a key to facilitating the permitting process for the Living Breakwaters project was to have a clear purpose and need statement that included all of the project benefits (risk reduction, ecological enhancement, and social resilience) and to have a robust and transparent dialog with the regulatory community early and often to help craft a permitting path that was appropriate for the project’s innovative approaches.

Many panelists described the frustrations and challenges with regulatory barriers that made projects more difficult. Oftentimes the policies are well-meaning environmental regulations that aim to protect people and ecosystems, and yet make it challenging to be innovative in the coastal resilience setting. For example, regulations around the use of “fill” (e.g., sand or silt dredged from a location often for maintenance needs such as navigable shipping channels) in wetlands were initially designed to protect wetlands and generally did not include anticipated effects of climate change or potential opportunities to use fill to enhance or restore coastal environments when they were implemented. Because they are legal requirements, regulatory agencies may have to undergo a lengthy legal process to grant permits to allow use of fill in wetlands. Even given the challenges of addressing this barrier, it is important to change the conversation on fill from seeing it only as a pollutant to considering it as a possible asset to NNBF projects. Restrictions on fill use was a challenge to the Living Breakwaters project design. In San Francisco Bay, the Bay Conservation and Development Commission, the regional body charged with managing the Bay coastline, is doing just that and changing its policy to allow the use of fill for restoration and environmental enhancement projects while still maintaining restrictions on the use of fill for development projects. At the federal level, USACE is increasingly exploring opportunities to incorporate NNBF into coastal risk-reduction projects, as evidenced by the Coastal Texas Study and EWN efforts, including the use of fill for beneficial reuse. However, there are still limitations in the use of dredged material across multiple projects that will require innovations in policy and blending of funds. Thus, more research on the appropriate use of fill is needed to inform potential changes in regulations that could facilitate more NNBF projects. This type of innovation in the policy space is going to continue to be needed to support more NNBF projects.

Successful permitting and implementation of multi-objective projects may also require a rethinking of how project planning is conducted within and between mission-focused agencies, as projects that move outside of the stated mission may be difficult to justify and fund. This siloing of projects within existing agency boundaries to avoid “mission creep” may miss out on opportunities to achieve multiple benefits across a range of objectives. Thus, while there has already been some progress to address these barriers, they will likely continue to be a challenge for innovative coastal resilience projects until these innovations become more of the “norm.”

4.2.2 Funding

Several panelists identified funding as a critical need for both project development and long-term monitoring after implementation. This is a continuing challenge, especially in the US, where funding for resilience projects is primarily often tied to the disaster-response cycle ( NRC, 2014 ). The Living Breakwaters project is an excellent example of a project that arose after a major disaster; if it had been implemented before Hurricane Sandy, it may have protected the coastline from some damages. It is important as a society that we start making investments in resilience projects that are not tied to the disaster-response cycle. We need to be anticipatory and fund the development and implementation of these projects during “blue-sky” periods before the next big storm (e.g., Reguero et al., 2020 ). This would allow projects to not be rushed and to have more certainty about funding opportunities.

When NNBF projects are funded in a post-disaster context, there is often a firm and short timeline on funds, prohibiting post-installation monitoring. Yet such monitoring can help define “success” of a project and inform other projects or other phases of the existing projects. In response to a vacuum of long-term monitoring of NNBF projects after Hurricane Sandy, a New York State-funded team and stakeholders recently co-created a state-level monitoring program ( Wijsman et al., 2021 ).

An ongoing monitoring program can be planned during project design that is rigorous but not so much that it becomes onerous or burdensome to fund and/or conduct over the years. For example, the Living Breakwaters project was designed to enable long-term monitoring to learn more about which specific breakwater designs and parameters are more likely to be effective to help inform adaptive management efforts and future projects. However, future funding will still be required for the actual research and measurements in this project. The SBSP Restoration Project offers a success story in which the investment of funding for long-term monitoring in the first phase of the project has informed the planning for the next phase, as well as providing valuable insights for other projects in the region.

Cost-sharing requirements can also place a limitation on the long-term maintenance and monitoring of NNBF projects. For example, in the Coastal TX Study, the ecosystem restoration alternatives were initially meant to include funding for nourishment if needed in response to changing conditions and sea-level rise. However, the cost of maintaining ecosystem restoration measures, which are required to be self-sustaining, could not be cost-shared, so this was later removed from the recommended plan. If nourishment is deemed necessary during the post-project monitoring period, the required work will have to be authorized separately at that time. Without proper funding to maintain ecosystem restoration components, the long-term survival and performance of these systems may be constrained. In contrast, features that also included a risk-reduction component, such as the beach and dune restoration, do include funding for periodic nourishment to maintain protective benefits.

4.2.3 Defining and Assessing Project Costs and Benefits

There is also a need to broaden the thinking on cost-benefit analyses. These studies are commonly used to assess options and trade-offs for project alternatives, and yet have traditionally only included the costs of construction and the benefits of reducing flood risks provided by projects (e.g., Aerts 2018 ; Reguero et al., 2018 ; Waryszak et al., 2021 ). With the burgeoning of innovative NNBF projects with multi-functional goals, it is key that cost-benefit analyses include all the benefits these projects provide, including ecological, recreational, and other benefits, although these co-benefits can be hard to quantify in monetary terms ( Sutton-Grier et al., 2015 ; Seddon et al., 2020 ). Each of our case studies addressed more than benefit-cost ratios for risk reduction, often in response to community input. In the Coastal Texas Study for example, the cost-benefit analysis included benefits to wildlife in addition to risk reduction benefits. For Living Breakwaters, the project team is specifically planning to monitor fish productivity on the underwater portions of the breakwaters to quantify the ecological benefits. In some cases, data may be lacking or very limited to inform cost-benefit analyses of these multiple benefits; however, this is one of the reasons why monitoring many aspects of these NNBF projects is key. We will learn from these projects and that new understanding of co-benefits can help inform cost-benefit analyses of future projects and address some of the hesitancy to give permits.

One of the unique elements of the Coastal Texas Study and Living Breakwaters project is a focus on co-benefits in the provision of dune habitat and oyster reefs, respectively. Without the context of serving as engineering structures, these elements alone might be considered ecosystem restoration. Here, the NNBF context reimagines a practical approach to incorporating these elements into levees and breakwaters and asks us to consider the phenomenological roots of “restoration” (see Hilderbrand et al., 2005 ; Hobbs et al., 2009 ; Hertog and Turnhout 2018 ). Certainly, these efforts are not restoring the system back to some historical state, but they replace lost restoration functions, and in this way, they lie in a grey area of restoration (e.g., “novel ecosystems”; Hobbs et al., 2013 ).

One recommendation for future projects and efforts in coastal resilience is to think specifically about what we can learn from each of these innovative projects because additional data on the performance, community outreach/stakeholder engagement, and socioeconomic components of NNBF projects is key to improving future projects ( Smith et al., 2020 ; Bayulken et al., 2021 ). Collecting similar types of data from these and future projects would enable us to learn as much as possible from each project. We have included recommendations for the types of benefits or ecosystem services that NNBF projects should consider including in their monitoring plans ( Table 2 ).

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TABLE 2 . Potential services provided by NNBF ( Bridges et al., 2015 ).

This is not a comprehensive list by any means, but is a good starting point for thinking about the types of data we should collect across projects to help with project comparisons and to inform future planning and analyses. Considering a consistent and more complete set of services across NNBF projects could facilitate the evaluation, selection, and permitting of similar future projects. The goal of this case-study analysis, as well as the suggestions for data collection for future projects, is to facilitate additional innovative coastal resilience projects across the US and around the world.

Author Contributions

This paper was co-developed by the Steering Committee members and Panelists listed in Table 1 . The Steering Committee wrote the first draft of the paper, led by CP, after which it was reviewed for feedback and input by the Panelists. All authors participated in subsequent rounds of revisions and editing. All authors have read and agreed to the published version of the manuscript.

We acknowledge funding support from National Science Foundation through the CoPe (Coastlines and People) RCN (Research Coordination Network): Advancing Interdisciplinary Research to Build Resilient Communities and Infrastructure in the Nation’s Estuaries and Bays CoPe RCN grant: ICER 1940273. CP was partially supported by the Grayce B. Kerr Fund. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Conflict of Interest

JM is employed by Arcadis.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors would like to thank many colleagues, students, and staff for fruitful discussions on this topic. We especially thank the attendees of the web panel series, particularly those who actively participated via questions and comments in the chat and brought out additional facets of the projects beyond our initial panel planning. Constructive comments from 2 reviewers helped improve the original manuscript. This is contribution #6113 of the University of Maryland Center for Environmental Science.

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Keywords: coastal resiliency, restoration, stakeholder engagement, NNBF design, NNBF monitoring

Citation: Palinkas CM, Orton P, Hummel MA, Nardin W, Sutton-Grier AE, Harris L, Gray M, Li M, Ball D, Burks-Copes K, Davlasheridze M, De Schipper M, George DA, Halsing D, Maglio C, Marrone J, McKay SK, Nutters H, Orff K, Taal M, Van Oudenhoven APE, Veatch W and Williams T (2022) Innovations in Coastline Management With Natural and Nature-Based Features (NNBF): Lessons Learned From Three Case Studies. Front. Built Environ. 8:814180. doi: 10.3389/fbuil.2022.814180

Received: 12 November 2021; Accepted: 25 March 2022; Published: 27 April 2022.

Reviewed by:

Copyright © 2022 Palinkas, Orton, Hummel, Nardin, Sutton-Grier, Harris, Gray, Li, Ball, Burks-Copes, Davlasheridze, De Schipper, George, Halsing, Maglio, Marrone, McKay, Nutters, Orff, Taal, Van Oudenhoven, Veatch and Williams. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Cindy M. Palinkas, [email protected]

This article is part of the Research Topic

Natural and Nature-Based Features for Flood Risk Management

Aug 04, 2022

New High-Resolution Study on California Coastal Cliff Erosion Released

  • Climate Change
  • Earth Sciences

The first study to analyze California’s coastal cliff retreat statewide using high-resolution data has found that cliffs receded faster in the north than elsewhere in the state during the study period.

But the study, which covered 866 kilometers (538 miles) of cliffs, detected erosional hotspots in central and Southern California as well.

“It’s probably the largest high-resolution cliff erosion study ever conducted, and provides the first detailed erosion rates for several parts of the state,” said coastal geomorphologist Adam Young of Scripps Institution of Oceanography at UC San Diego. Young and Zuzanna M. Swirad of the Polish Academy of Sciences in Warsaw recently published their study in the journal Geomorphology .

Data from that study — funded by the California Ocean Protection Council and California Department of Parks and Recreation plus previous research funded by California Sea Grant — are now available on the website California Coastal Cliff Erosion Viewer . Users can browse any cliff in the state to see its past rate of erosion and related retreat statistics. The website is designed for coastal planning and development decision-makers, but the information may also be of interest to members of the research community, and the general public.

“Communities and critical infrastructure are located on the cliff top. It is really important to understand the hazard of cliff collapse,” said Swirad, a former postdoctoral scholar at Scripps Oceanography.

Cliffs account for most of California’s 1,646 kilometers (1,023 miles) of coastline. Landslides and collapses have caused injuries and multiple deaths in recent decades. Coastal infrastructure potentially at risk includes highways, public coastal access points, homes, military bases, wastewater treatment plants, power plants, and railways. The State of California recently allocated $300 million toward moving 1.7 miles of tracks away from an eroding bluff in Del Mar.

In their new study, Swirad and Young created one-meter digital elevation models and evaluated the cliff erosion and retreat between 2009-2011 and 2016 in five-meter (16.4 foot) segments along 866 kilometers (538 miles) of California’s coast. Erosion was detected along more than half of the cliffs.

Rates were typically higher in the north of the state and for cliffs with beachfront. The statewide retreat rates of both cliff faces and cliff tops averaged about six centimeters (two inches) annually. However the average values mask the effects of episodic collapses. Cliff top retreat rates exceeded five meters (16 feet) per year in some areas of Rancho Palos Verdes south of Los Angeles, an area south of Point Arguello near Vandenberg Air Force Base, Big Sur in central California, and Martin Beach south of San Francisco. Northern California locations with high retreat rates include Usal Beach, King Range, Centerville Beach, the McNeil Creek area north of Trinidad Head, and an area about two miles north of the Klamath River.

Image of the California Coastal Cliff Erosion Viewer web page

Included in the analysis were data collected with airborne LiDAR (Light Detection and Ranging), an advanced laser-imaging technology, during 2009-2011 and 2016. New machine-learning techniques that Swirad developed helped reduce the project’s manual processing and analysis time, expediting the large-scale study. 

Large cliff failures can occur episodically making cliff-retreat projections based on averaged historical data challenging. In addition, retreat measurements made only at the cliff top do not capture landslides in the middle or bottom of cliffs. However, using LiDAR, Swirad and Young were able to measure both the cliff top change and changes within the cliff face.

The new results were compared to one of Young’s previous studies of southern and central California and yielded similar statistical results of cliff face retreat. These statistics help account for episodic erosional events, and help improve model predictions.

“Because we found statistical agreement with the previous time period, 1998 and 2009-2010, we may be more confident that the statistical approach is the way to do it,” Swirad said.

New statewide data collection began last year. This will provide a third time span to see if the statistical results remain consistent with those from the earlier periods. Still, the researchers find it difficult to isolate events that happen seasonally or annually such as El Niño events because the statewide data sets span five to 10 years. Such a large time span makes it hard for them to interpret the processes driving erosion such as pounding waves, groundwater, and rainfall.

“We need to survey the coast more frequently so we can better track how the coast is evolving and improve model predictions,” Young said. “We have the ability and technology to do this.”

Young noted that coastlines are the only place on Earth where ocean, land, and atmosphere intersect.

“That makes it a fascinating place to study,” he said.

Young is also leading a team at Scripps on a new program that aims to accelerate the science behind coastal bluff failures. The research, authorized through State Assembly bill AB-66 , will support enhanced coastal monitoring to better understand the timing of bluff failures.

About Scripps Oceanography

Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.

About UC San Diego

At the University of California San Diego, we embrace a culture of exploration and experimentation. Established in 1960, UC San Diego has been shaped by exceptional scholars who aren’t afraid to look deeper, challenge expectations and redefine conventional wisdom. As one of the top 15 research universities in the world, we are driving innovation and change to advance society, propel economic growth and make our world a better place. Learn more at ucsd.edu.

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UH Hilo Stories

‘A‘ohe pau ka ‘ike i ka hālau ho‘okahi │ One learns from many sources │ A publication from the Office of the Chancellor, University of Hawai‘i at Hilo

Case study on UH Hilo research of coastal erosion published in “U.S. Climate Resilience Toolkit”

The innovative research combines historic aerial photos, current drone imagery, and topographic surveys to discover coastal changes around Hawai‘i Island. The results of the study are informing policymakers who are crafting a new setback policy for coastal development.

Bethany walks on the rocky shore line.

By Susan Enright . 

The results of a collaborative research project led by a graduate student at the University of Hawai‘i at Hilo have been published as a case study in the U.S. Climate Resilience Toolkit , a website where the public can find information and tools to understand and address climate risks. The website offers information from all across the United States federal government in one easy-to-use location. The case study also has been published regionally on the Pacific Islands Regional Climate Assessment (PIRCA) website .

The UH Hilo study examines shoreline migration—the rate that changes occur, gradually or suddenly—on Hawai‘i Island and is being used by county planners and policymakers to develop a more comprehensive and effective coastal development plan.

Scott Laursen

The published case study, “Reality Check: Collaborative Research Contributes to Real-Life Policy Decisions,” is co-authored by Scott Laursen , program specialist at the UH Pacific Islands Climate Adaptation Science Center . The center was established by the U.S. Department of the Interior in 2011 as a consortium hosted by UH Mānoa, UH Hilo, and the University of Guam.

The case study is co-authored by Bethany Morrison , a planner on land use with the County of Hawai‘i who collaborated on the research.

Bethany Morrison

“We do not have adequate knowledge of Hawai‘i Island’s shoreline to be able to assess and adapt to the vulnerabilities from sea level rise and related hazards,” Morrison explains. “The goals of this project will help us to address these challenges. More specifically, Hawai‘i County will have a first phase of shoreline change rates and sea-level rise projections for three different types of shorelines.”

Laursen says that local managers such as Morrison, who work with regional land and seascapes, are also working within the social norms and values of the communities that utilize those ecosystems.

“Directly involving local professional networks within every stage of the scientific method roots research products within the place-based experiences of these natural and cultural resource managers,” Laursen explains in an email. “[This] increases the probability that these products will be utilized. Resource managers like Bethany can be thought of as ‘custodians of context’ throughout the research process, ensuring the immediate utility of research output.”

The innovative research in this particular project combines historic aerial photos, current drone imagery, and topographic surveys to discover coastal changes around Hawai’i Island. The results of the study are informing policymakers who are crafting a new setback policy for coastal development.

UH Hilo responds to an urgent request from the county

In 2016, responding to an urgent need to reevaluate set back policy in county code, planning experts in the County of Hawai‘i Office of Planning initiated a collaborative two-year research project funded by UH Hilo’s Manager Climate Corps, a program of the Pacific Islands Climate Adaptation Science Center. A research team was assembled, made up of Morrison and researchers from UH Hilo, to examine and quantify historic and contemporary rates of change along different types of shorelines on Hawai‘i Island, and then to model observed changes into the future through sea level rise impacts.

On the research team was UH Hilo graduate student Rose Hart —who had completed an undergraduate internship involving research on shoreline setback policies—who was the driving force behind the project. Also on the team was Ryan Perroy , associate professor of geography and director of UH Hilo’s Spatial Data Analysis and Visualization Labs . Perroy is an expert and award-winning researcher in innovative equipment and methods, notably in the use of drones to survey areas of the island to collect data on Rapid ‘Ōhi‘a Death and the recent lava flows in lower Puna .

Rose Hart and Ryan Perroy stand by Welsome sign to AAG conference.

Hart, Perroy, and Morrison worked closely together to develop the detailed methodologies and field schedules. Over two years, the research team combined existing datasets (historic aerial photos) with new data (drone imagery collected by Hart and Perroy) along with topographic surveys to quantify past and present rates of coastal change. These data were then merged with sea level rise projections and other geospatial data to estimate future impacts along the coastline using a Geographic Information System platform.

Also collaborating on the project was Steve Colbert , chair and associate professor of marine science at UH Hilo, and Charles H. “Chip” Fletcher III , associate dean and professor at the School of Ocean and Earth Science and Technology at Mānoa.

Sharing the data

In May 2018, the research team presented their findings to the county planning department, which administers planning regulations for the entire island and provides technical advice to the mayor, planning commission and county council. Based on analyses at Hāpuna, Honoli‘i, and Kapoho, the team offered suggestions for how the county could use scientific data to create place-based setbacks. Predictive models that combined a designated shoreline setback distance, place-based shoreline erosion rates, and structural life expectancy were suggested for calculating setbacks for locations similar to Hāpuna and Honoli‘i. Conversely, elevation-based setbacks were suggested for low-lying coastal communities such as Kapoho.

Landslide below house on cliff.

Throughout the project, the research team worked directly with community members in each of the three study site locations. These interactions were particularly effective along the Hāmākua Coast. The project became so familiar to these communities that it was directly integrated into the Hāmākua Community Development Plan, which called for guidelines for a practical definition of “top of cliff,” a term in shoreline policy that has yet to be clearly defined. Developing common understandings is essential to monitoring and predicting erosion along unstable clifflines that can pose dramatic threats to community infrastructure.

Looking to the future and the uncertainty associated with climate change, the combined results of the completed study will be used to develop policies that are increasingly adaptive to present and future coastal change.

Read the full case study .

Story by Susan Enright, a public information specialist for the Office of the Chancellor and editor of UH Hilo Stories. She received her bachelor of arts in English and certificate in women’s studies from UH Hilo.

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Coastal erosion at Happisburgh, Norfolk

Landslide case study

Happisburgh, on Norfolk’s North Sea coast, is a village with a population of 1400 people in about 600 houses. The village contains a notable stone church dating from the 14th century, an impressive manor house, listed buildings and a famous red and white striped lighthouse.

Although now a coastal village, Happisburgh was once some distance from the sea, parted from the coast by the parish of Whimpwell, long since eroded away. Historic records indicate that over 250 m of land were lost between 1600 and 1850.

More recently the village was affected by the tragic floods of 1953 that claimed the lives of 76 Norfolk residents.

Happisburgh location map

Happisburgh, Norfolk location map. BGS © UKRI.

Coastal defences built at Happisburgh have slowed down the rate of retreat. However, large sections are now in disrepair. Sea-level rise and climate change, including increased storminess, may also increase the rate of erosion. Agriculture and tourism contribute significantly to the economy of the village and surrounding hinterland and this is threatened by the receding cliff line that, prior to the construction of a rock embankment at the northern end of the survey site, had claimed at least one property per year plus significant quantities of agricultural land.

Figure 1 The eroding coast at Happisburgh in Norfolk.(Photo: © Mike Page)

The eroding coast at Happisburgh in Norfolk. © Mike Page.

Figure 2 Cliff top position in 2004 — this had retreated a further 20 metres in 2007. 1992 aerial photograph © Environment Agency, reproduced with kind permission of the Shoreline Management Group, (Anglian Region).

Cliff-top position in 2004. This had retreated a further 20 m in 2007. 1992 aerial photograph © Environment Agency, reproduced with kind permission of the Shoreline Management Group (Anglian Region).

Detailed geology

The cliffs at Happisburgh range in height from 6 to 10 m and are composed of a layer-cake sequence of several glacial tills, separated by beds of stratified silt, clay and sand (Hart, 1987; Lunkka, 1988; Hart, 1999; Lee, 2003). The basal unit within the stratigraphic succession at Happisburgh is the How Hill Member of the Wroxham Crag Formation . These deposits are typically buried beneath modern beach material but are periodically exposed following storms (Figure 3). They consist of stratified brown sands and clays with occasional quartzose-rich gravel seams that are interpreted as intertidal/shallow marine in origin.

Unconformably overlying these marine deposits are a series of glacial lithologies deposited during several advances of glacier ice into the region during the middle Pleistocene (about 780 to 430 ka BP) (Lee et al., 2002; Lee et al., 2004). The survey site has a tripartite geological succession.

The Happisburgh Till Member crops out at the base of the cliffs and its base is frequently obscured by modern beach material: it has a maximum thickness of 3 m. The Happisburgh Till Member is a dark grey, highly consolidated till with a matrix composed of a largely massive clayey sand with rare (less than one per cent) pebbles of local and far-travelled material.

The upper surface of the till undulates and comprises a series of ridges and troughs upon which the overlying Ostend Clay member crops out. This unit is between 2.3 and 3.4 m thick and consists of thinly laminated light grey silts and dark grey clays.

In turn, these beds are overlain by 2 to 4 m, of weak, stratified sand ( Happisburgh Sand Member ) with occasional silty clay horizons.

Figure 3 The geology of the cliffs at Happisburgh.

The geology of the cliffs at Happisburgh. BGS © UKRI.

Coastal erosion

It is likely that the Norfolk cliffs have been eroding at the present rate for about the last 5000 years, when sea level rose to within a metre or two of its present position (Clayton, 1989). Therefore, the future predictions of sea-level rise and storm frequency due to climate change are likely to have a profound impact on coastal erosion and serious consequences for the effectiveness of coastal protection and sea defence schemes in East Anglia in the near future (Thomalla and Vincent, 2003).

Rapid erosion of the cliffs at Happisburgh means that we can observe processes that, for other sites, may normally take thousands of years. This means that we can look for patterns in the erosion at Happisburgh, which may help our understanding of sites elsewhere that are eroding more slowly.

Survey results

As part of a programme of work monitoring coastal erosion and landsliding at several sites around the coast of Great Britain, we surveyed the cliffs adjacent to the village of Happisburgh in Norfolk with LiDAR .

The resulting computer model enables volume calculations and observations to be made as to the way in which the coast is eroding. The results from the survey provide data for models of coastal recession.

Figure 4 3-D laser scan solid models for 2001 to 2006 at Happisburgh. 2001 is yellow ranging annually to dark blue (2006).

3D laser scan solid models for 2001 to 2006 at Happisburgh. 2001 is yellow ranging annually to dark blue (2006). BGS © UKRI.

From this survey, the following conceptual model has been proposed.

In winter, erosion caused by groundwater as seen in the gullying of the cliff face, coupled with increased seasonal storminess, causes small-scale, frequent, shallow landsliding in the Happisburgh Sand Member. The Happisburgh Sand Member is easily eroded and undercutting of the cliff toe reduces slope stability and cliff failure occurs. The beach surface is low and scouring of the upper surface of the till extends the till platform.

In summer, the beach surface is higher and covers the ‘winter platform’. Wave attack is the dominant form of erosion accompanied by landsliding in the Happisburgh Sands.

Figure 5 Cross-section at Happisburgh, showing cliff and platform stratigraphy.

Cross-section at Happisburgh, showing cliff and platform stratigraphy. BGS © UKRI.

The cliff surface profiles show that the erosion process is non-uniform, involving the cyclic formation of a series of embayments that continually enlarge. This could imply landsliding processes involving block falls, mudflows and running sand.

Figure 6 Process model for embayment formation at Happisburgh.

Process model for embayment formation at Happisburgh. BGS © UKRI.

Publications

Poulton, C V L. 2004. Disappearing coasts . Planet Earth , Summer 2004, 26–27.

Poulton, C V L, Lee, J R, Jones, L D, Hobbs, P R N, and Hall, M. 2006. Preliminary investigation into monitoring coastal erosion using terrestrial laser scanning: case study at Happisburgh, Norfolk, UK . Bulletin of the Geological Society of Norfolk , Vol. 56, 45 – 65.

Hobbs, P R N, Pennington, C V L, Pearson, S G, Jones, L D, Foster, C, Lee, J R, and Gibson, A. 2008. Slope Dynamics Project Report: Norfolk Coast (2000 – 2006) . British Geological Survey Open Report OR/08/018. (Nottingham, UK: British Geological Survey.)

Happisburgh November 2009.

Happisburgh, November 2009. BGS © UKRI.

Happisburgh November 2009.

Happisburgh, 2007. BGS © UKRI.

Happisburgh 2007.

Happisburgh, 2005. BGS © UKRI.

Happisburgh 2005.

Happisburgh, 2004. BGS © UKRI.

Happisburgh 2004.

Happisburgh, 2003. BGS © UKRI.

Happisburgh 2003.

Happisburgh, 2002. BGS © UKRI.

Happisburgh 2002.

Happisburgh rock bund, 2006. © Mike Page.

Happisburgh rock bund 2006.

Happisburgh rock bund, 2004. © Mike Page.

Happisburgh rock bund 2003.

Happisburgh rock bund, 2003. © Mike Page.

Happisburgh rock bund 2002.

Happisburgh rock bund, 2002. © Mike Page.

Landsliding due to toe removal at Happisburgh.

Landsliding due to toe removal at Happisburgh. BGS © UKRI.

Contact the Landslide Response Team

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fall at Pennington Point. (Photo: © Eve Mathews)

Landslide case studies

The landslides team at the BGS has studied numerous landslides. This work informs our geological maps, memoirs and sheet explanations and provides data for our National Landslide Database, which underpins much of our research.

Debris flow on A83

Understanding landslides

What is a landslide? Why do landslides happen? How to classify a landslide. Landslides in the UK and around the world.

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How to classify a landslide

Landslides are classified by their type of movement. The four main types of movement are falls, topples, slides and flows.

Storegga landslide

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Landslides in the UK, around the world and under the sea.

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Climate, Environment and Disaster in Developing Countries pp 375–390 Cite as

An Assessment on Effects of Coastal Erosion on Coastal Environment: A Case Study in Coastal Belt Between Kalu River Mouth and Bolgoda River Mouth, Sri Lanka

  • Kirishanthan Punniyarajah 4  
  • First Online: 01 February 2022

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Part of the Advances in Geographical and Environmental Sciences book series (AGES)

The Southwest coast of Sri Lanka has become more prone to extreme coastal erosion, posing numerous environmental and social-economic threats. Therefore, the main focus of this research was on assessing the effects of coastal erosion on the coastal environment in the coastal belt between the mouths of Kalu River  and Bolgoda River in Sri Lanka. For the purpose of data collection, Thermal Infrared Sensor images (2003–2018) were collected through the United States Geological Survey. In addition, questionnaire survey and key informant interviews were also conducted. The data collected via questionnaire survey and interviews were analysed statistically and thematically respectively. The results of the study revealed that destroying coastal landforms, saltwater intrusion, loss of green belt and wildlife and coastal flooding were recognized as the key environmental issues due to coastal erosion. Among them, coastal landforms are more prone to erosion in the study area, especially at Kalido beach, Kalutara. The current research is clearly highlighted that accelerating coastal erosion severely affects the balance and values of the coastal environment. Thus, coastal conservation and protection bodies, plans, and programmes should immediately take necessary action to mitigate the environmental issues in the coastal belt.

  • Coastal erosion
  • Coastal environment
  • Loss of landforms
  • Kalido beach

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Asian Development Bank and The World Conservation Union Sri lanka (2002) Regional technical assistance for coastal and marine resources management and poverty reduction in compendium report of high priority areas–Sri Lanka Component. Asian Development Bank and IUCN-The World Conservation Union

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Luijendijk A, Hagenaars G, Ranasinghe R, Baart F, Donchyts G, Aarninkhof S (2018) The State of the World’s Beaches. Scientific Reports, 8 (1).

Wickremaratne H (1985) Environmental problems of the coastal zone in Sri Lanka. Econ Rev 8–16

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Wickramagamage P (2011) Evolution of the Kalu Ganga-Bolgoda Ganga flood plain system, Sri Lanka. J Geol Soc Sri Lanka 14:41–53

Wijayawardane ISK, Ansaf KMM, Ratnasooriya AHR, Samarawickrama SP (2013) Coastal erosion: investigations in the Southwest coast of investigations on sediment transport. In: international conference on sustainable built environment, pp 13–14

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Woodroffe CD (2003) Coasts: form, process and evolution. Cambridge University Press, Cambridge

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Punniyarajah, K. (2022). An Assessment on Effects of Coastal Erosion on Coastal Environment: A Case Study in Coastal Belt Between Kalu River Mouth and Bolgoda River Mouth, Sri Lanka. In: Jana, N.C., Singh, R.B. (eds) Climate, Environment and Disaster in Developing Countries. Advances in Geographical and Environmental Sciences. Springer, Singapore. https://doi.org/10.1007/978-981-16-6966-8_20

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Improvement of a coastal vulnerability index and its application along the Calabria Coastline, Italy

  • Daniela Pantusa 1 ,
  • Felice D’Alessandro 2 ,
  • Ferdinando Frega 3 ,
  • Antonio Francone 4 &
  • Giuseppe Roberto Tomasicchio 4  

Scientific Reports volume  12 , Article number:  21959 ( 2022 ) Cite this article

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The present paper further develops a coastal vulnerability index formulation (CVI) previously proposed by the authors by integrating a new variable and redefining three variables to improve the suitability of the index for low-lying coasts. Eleven variables are divided into three typological groups: geological, hydro-physical process and vegetation. The geological variables are: geomorphology, shoreline erosion/accretion rates, coastal slope, emerged beach width, and dune. The hydro-physical process group includes: river discharge, sea-level change, mean significant wave height and mean tide range. The vegetation variables are: vegetation behind the back-beach and coverage of Posidonia oceanica . The index was applied to a stretch of the Ionian coast in the province of Crotone in the Calabria region (Southern Italy), and a vulnerability map was produced. A geography information system (GIS) platform was used to better process the data. For the case study area, the most influential variables are shoreline erosion/accretion rates, coastal slope, emerged beach width, dune, vegetation behind the back-beach, and coverage of Posidonia oceanica . The most vulnerable transects are those near urban areas characterized by the absence of dunes and vegetation. Statistical and sensitivity analyses were performed, and the proposed CVI was compared with the previous formulation proposed by the authors and with two other CVI methods present in the literature.

Introduction

Coastal areas are dynamic environmental and socioeconomic systems that provide services including wildlife habitat, erosion and flooding protection, and economic and recreational activities. These areas are vulnerable to a number of factors, including erosion, subsidence and changes in hydrology, and are often characterized by increasing population densities 1 .

Defining simple predictive approaches is essential for assessing and categorizing responses to progressive changes in coastal dynamics around the world.

Gornitz, White and Cushnam 2 and Gornitz 3 originally proposed a coastal vulnerability index (CVI), as an index-based method to assess coastal vulnerability to climate change, particularly to sea level rise (SLR). Thieler and Hammar-Klose 4 proposed a new CVI formulation that modified the original index developed by 2 , 3 , resulting in a widely used tool for applications at different territorial scales. Furthermore, several CVI formulations have since been proposed with modifications and integrations of the original physical parameters to adapt the index to the particular characteristics of the coastal area.

Specifically, applications have been carried out for different coastal areas worldwide, such as the U.S. coast 5 , 6 , 7 , Indian coast 8 , 9 , 10 , 11 , Australian coast 12 , 13 , African coast 14 , 15 , 16 , 17 , and European coast 18 , 19 , 20 . Formulations combining physical and socioeconomic variables have also been developed 21 , 22 , 23 , 24 , 25 , 26 .

In this context, the present paper resumes and continues a previous CVI formulation developed by Pantusa et al. 27 and proposes improvements particularly suitable for low-lying coasts. The interest in low-lying coasts is motivated by the desire to (i) identify their high and growing exposure to different hazards, such as, storm surges, flooding, and sea level rise 28 , 29 , and, consequently, (ii) design coastal interventions 30 , 31 , 32 , 33 , 34 and (iii) orient planning strategies. The improved CVI index has been applied to a stretch of the Mediterranean low-lying coast , in the Calabria region, Italy, an area impacted by intense human activity and naturally exposed to climate and environmental changes that determine coastal erosion, extreme events, sedimentation decrease and degradation of some habitats (e.g. coastal dunes, coastal cliffs or coastal terraces) 35 , 36 , 37 , 38 .

With specific reference to low-lying coasts, several CVI formulations have been proposed in the literature. Some studies refer to the variables and formulation originally proposed by 2 , 3 , 4 , while others include additional parameters.

Several studies are summarized in Table 1 , with specific reference to the adopted method, geographical location, variables and remarks.

As described in Table 1 , some applications to low-lying coasts 8 , 41 , 42 consider only the variables originally proposed by 4 , while the index proposed by 24 adds a further geological variable. These applications focus on physical vulnerability and do not integrate further specific variables for low-lying coasts.

Other indices 16 , 24 , 40 , 43 extend the original CVI formulation, also taking into account socioeconomic factors. Therefore, these indices, which combine physical and socioeconomic variables, assess coastal vulnerability by a different approach compared to that proposed in this work.

In the approach proposed by Bagdanavičiūtė et al. 39 , the index relies almost exclusively on geological variables and uses also some variables suitable for low-lying coasts. Similarly, the index proposed by Sekosky et al. 20 includes some variables specific for low-lying coasts.

Compared to these indices, the novelty of the proposed index is the combination of all the variables originally proposed by 4 with further specific variables that take into account the recognized factors of interest for low-lying coasts from climate-related changes: dunes, vegetation, and marine ecosystems 44 . The addition and combination of these variables with the original CVI geological and physical variables allows us to undertake a more specific analysis of the coastal response to sea natural hazards and coastal inundation that constitute threats, especially for low-lying coasts. A more in-depth analysis, especially at the local scale, can be useful to better identify the most vulnerable transects and can better support policy and management tools, especially when investment budgets are limited.

As mentioned above, the present work further develops a CVI index previously proposed by the authors that integrates a new variables and redefines three variables. The variables are divided into three typological groups: geologic, hydro-physical process and vegetation.

Statistical analysis was carried out to evaluate the correlation and multicollinearity between the considered variables, together with a sensitivity analysis of the variable ranking score, aggregation formulation and overall CVI classification method. Considering the three typological groups of variables, statistical analysis was also performed to identify the groups that predominantly influenced the overall coastal vulnerability.

Finally, an application of the CVI formulation previously proposed by the authors 27 and of two other CVI methods proposed in the literature by Ružić et al. 45 and Palmer et al. 46 has been carried out for the case study area, and the comparison between the results obtained using the different formulations has been described.

This section provides details on the new proposed CVI formulation describing the variables used, the ranking score and the mathematical formulation applied to construct the synthetic index. This section also describes the statistical and sensitivity analysis carried out for the preparation of this work, and the application and comparison of other CVI formulations present in the literature for the case study area.

CVI formulation

A stretch of coast is divided into a number of transects (or cross-sectional profiles of the beach) to assess its vulnerability. Each transect is characterized by a control area 0.5 km wide. A relative vulnerability score is assigned to each variable based on the potential magnitude of its contribution to physical changes on the coast 2 , 3 , 4 . Variables are ranked on a linear scale from 1 to 5 in order of increasing vulnerability. A mathematical formulation is used to construct the synthetic index and the CVI values are classified into four different groups using percentiles of the distribution as limits.

The newly proposed CVI formulation uses the variables described in the following table (Table 2 ).

For the CVI method previously proposed by the authors 27 , a table reporting the variables used and the ranking scores is included as Supplementary materials (Table S.1 ).

The integration of the variables “Dune” and “Vegetation behind the back-beach” is aimed at allowing a more in-depth evaluation of the ability of the coastal system to dissipate wave energy and better respond to extreme events. The “Dune” variable considers both aspects of dune width and dune type. Dune width, already considered in the previous formulation 27 , is important for the conservation of the coastal zone, as it increase its resilience 47 , 48 , 49 , 50 ; the temporal dominance of the wave collision regime, wherein volume loss from the dune occurs through dune retreat without overtopping, suggests that dune width must be considered an important factor of coastal defence 51 . Although it constitutes one of the most important factors of resistance to wave attacks and erosion, dune systems offer a different degrees of resistance/resilience in relation to their consolidation and typology 52 , 53 . Therefore, this variable combines these two aspects to characterize the overall role of dunes in coastal protection.

Similarly, the variable “Vegetation behind the back-beach” considers the two aspects of width of the vegetation behind the back-beach and type of vegetation. The width of the vegetation, already considered in the previous formulation 27 , is related to the coastal defence against storm events and inundation; the type of vegetation is also important, as it plays a crucial role in the defence of coastal areas and infrastructures along the coast by decreasing wave run-up and decreasing sand loss by wave backwash 54 . Furthermore, the size, shape and stability of dunes are strongly controlled by vegetation cover. Therefore this variable combines these two aspects to take into account the overall effects of vegetation on wave energy dissipation and erosion reduction.

In the same way, the variable “Coverage of Posidonia oceanica ” allows us to take into account the implications of these meadows for coastal defence. The Posidonia oceanica meadows constitute an effective barrier for coastal defence from erosion due to both seabed stabilization and wave motion damping 55 , 56 . Consequently, the presence of these meadows and their degree of coverage are both relevant, as they affect on the protection capacity of the beaches.

Finally, new variable, “River discharge”, it has been added considering that water and sediments discharged from rivers into the sea impact sedimentation processes and dynamic balance, particularly in low-lying and sedimentary coasts 57 . However, it should also be highlighted that anthropogenic factors, such as, river basin regulation works (especially dams), determine changes in sediment transport processes and water discharge. In such cases, the potential role of this variable may be limited and therefore require a more careful evaluation of local features and peculiarities in the specific study area.

Below is a description of the variables and relative ranking scores.

The geological variables are:

Geomorphology This variable expresses the relative erodibility of different landform types (e.g., rocky cliffs, and sandy beaches) along the coast and requires information on the spatial distribution of landform types and their stability 4 , 27 . Ranges of vulnerability scores are defined according to the relative susceptibility of a given landform to physical change and are those proposed by 4 ; the ranking score assigns the lower vulnerability score to rocky and cliff coasts and the higher vulnerability score to landform types such as barrier beaches, sand beaches, and deltas.

Coastal slope This variable is an indicator of the relative vulnerability to inundation and of the potential rapidity of shoreline retreat 27 . Its ranking scores have been defined considering previous studies carried out for the Mediterranean coast 18 , 41 , 42 . In particular, the ranges used here refer to those proposed by López Royo et al. 18 . Considering inundations, storm surges and associated land losses, a coast characterized by a gentle slope is considered more vulnerable than a coast with a steeper slope. Consequently, for the case study area, the highest score (very high vulnerability) is assigned at coastal slope < 2% and the lower score value (very low vulnerability) for coastal slope > 12%. Low vulnerability is considered on slopes 4–2%, moderate vulnerability for slopes 8–4%, and high vulnerability for slopes 12–8%.

Shoreline erosion/accretion rates This variable assesses the state of erosion or accretion 27 . Ranking scores have been defined considering those proposed by Karymbalis et al. 42 . In particular, shoreline change characterized by erosion rates <  − 1.5 m/year is considered very high vulnerability, while accretion rates > 1.5 m/year are considered very low vulnerability. Low vulnerability is considered for accretion rates of 1.5–0.5 m/year, moderate vulnerability for values between 0.5 and − 0.5 m/year, and high vulnerability for values between − 0.5 and − 1.5 m/year.

Emerged beach width Ranges of vulnerability scores are defined in consideration of characteristics endemic to the Italian and Mediterranean areas 58 and of the fact that a wider beach has a greater ability to dissipate wave energy and to better defend against extreme events. Therefore, for the case study area, the vulnerability score of emerged beach width > 100 m is considered very low. Beach widths from 50–100 m are considered low vulnerability, 25–50 m moderate vulnerability, 25–10 m high vulnerability, and < 10 m very high vulnerability.

Dune A relative score is assigned separately to both the dune width and the type of dune.

For the dune width, the relative scores are defined considering that wide dunes constitute a greater defence factor than narrow dunes during storms and extreme events. Therefore, the relative score varies from 1 (dune width > 100 m) to 5 (dune width <25 m) according to class intervals of 25 m.

Regarding the type of dune, the classification described in NSW (Department of Land and Water Conservation) 52 is used as a reference and considers three typical features of a dynamic beach system: incipient dune, foredune and hind dune. In particular, in the absence of a dune system the highest relative score is assigned (score 5), while score values equal to 4, 3 and 2 are assigned in the case of incipient dunes, foredunes and hind dunes, respectively.

A mathematical formulation is used to combine the relative scores assigned separately to dune width and type of dune, as described below:

where D DW and D TD are the relative scores assigned to dune width and type of dune, respectively, and D is the total value associated with the variable. According to this mathematical formulation the value of D varies from 1 to 5, and corresponds to the same ranking score vulnerability for the “Dune” variable.

The hydro-physical process variables are:

River discharge This variable is added to consider the aspect of discharge and sediment transport that affects the morphodynamics of coastal systems 57 , 59 . The ranking scores, are defined considering the peculiar characteristics of the Calabrian territory; they are applied to the following categories: stream (river discharge less than 1 m 3 /s), small river (river discharge between 1 m 3 /s and 5 m 3 /s), medium river (river discharge between 5 m 3 /s and 20 m 3 /s) and large river (river discharge greater than 20 m 3 /s). Considering that CVI provides a numerical basis for comparing transects to identify those where vulnerability may be relatively greater, the presence of a river in the transect is a factor that can contribute into reducing vulnerability in the area. Therefore, the highest vulnerability score is assigned in the absence of rivers/streams, while high, medium, low and very low vulnerability scores are assigned in the case of streams, small rivers, medium rivers and large rivers, respectively. River discharge is used because it is closely associated with sediment transport 59 .

Relative sea level change This variable is derived from the time series of sea level records at each tide gauge station along the coast; it includes both eustatic sea-level rise and regional sea-level rise due to isostatic and tectonic adjustments of the land surface. Relative sea-level change data are a historical record, and thus show change for only a recent time scale 4 . In this work ranking scores are defined in agreement with those proposed by Karymbalis et al. 42 . The rate of relative sea-level rise is ranked considering the values < 1.8 mm/yr as very low vulnerability, the values 1.8–2.5 mm/year as low vulnerability, the values 2.5–3.0 mm/year as moderate vulnerability, the values 3.0–3.4 mm/year as high vulnerability and the values > 3.4 mm/year as very high vulnerability.

Mean significant wave height This variable is used as a proxy for wave energy, which drives the coastal sediment budget 4 . The wave energy increases as the square of the wave height, and therefore, the vulnerability score assumes an increasing value as the height of the waves increases. For the case study area, the ranking scores are those proposed by Karymbalis et al. 42 and vary from very low (< 0.3 m) to very high (> 1.2 m) according to class intervals of 0.3 m.

Mean tide range This variable is linked to both permanent and episodic inundation hazards. Some researchers 3 , 43 consider that coastline with a large tidal range must be assigned a high vulnerability classification, while microtidal coasts receive a low vulnerability rating. The main reasoning is that a large tidal range delineates a broad zone of intertidal area that will be most susceptible to inundation following long-term sea-level rise. Moreover, a large tidal range is associated with strong tidal currents that influence coastal behaviour. Instead, other studies adopt the opposite assumption. In this work, the vulnerability ranges are defined in accordance with those proposed by Thieler and Hammar-Klose 4 . This assumption is based on the concept that, in general, microtidal (tide range < 2.0 m) and macrotidal (tide range > 4.0 m) are characterized by high and low risk, respectively. The reasoning is based primarily on the potential influence of storms on coastal evolution, and their impact relative to the tide range. For example, on a tidal coastline, there is only a 50 percent chance of a storm occurring at high tide. Thus, for a region with a 4.0 m tide range, a storm with a 3 m surge height is still up to 1 m below the elevation of high tide for half a tidal cycle. A microtidal coastline, on the other hand, is essentially always “near” high tide and therefore always at the greatest risk of inundation from storms 4 . It is worth highlighting that this assumption has also been used for other studies present in the literature 5 , 6 , 10 , 17 , 41 . The Mediterranean area is a microtidal environment and the coast of Calabria has a tidale range < 1 m. Rankings scores vary from very low (> 6 m) to very high (< 1 m)

The vegetation variables are:

Vegetation behind the back-beach Regarding the width of vegetation behind the back-beach, it should be noted that vegetation is determined by clear and obvious signs of flora, indicated by the green area behind an emerged beach (back-beach); it may be interrupted in places where it intersects with infrastructures such as roads, and houses. 27 ; Its relative score is defined to indicate that the degree of coastal vulnerability decreases with the increase in width of vegetation behind the back-beach due to the vegetation influence on sediment transport and wave impact. It should be noted that in the case of sparse vegetation a careful evaluation of the specific characteristics of the area must be considered; therefore, in this case it is necessary to carefully analyse the special coverage and prevailing vegetation and evaluate whether to assign a more precautionary higher vulnerability score. The relative scores assigned considering the width of the vegetation are the following: width > 400 m, relative score 1; width from 400 to 200 m, relative score 2; width from 200 to 100 m, relative score 3; width from 100 to 50 m, relative score 4; width < 50 m, relative score 5. Regarding the type of vegetation, three types of vegetation are considered in relation to their different actions of defence and protection of the coast from inundation: primary species, secondary species and tertiary species. Primary species consist of herbs, grasses and creepers; secondary species consist of shrubs, ground plants and short-lived trees; and tertiary species consist of long-lived trees 52 . The relative scores are defined considering the three types of vegetation. The highest relative score is assigned in the absence of vegetation and decreasing values for primary species (relative score 4), secondary species (relative score 3) and tertiary species (relative score 2) in relation to their different action of coastal defence.

A mathematical formulation is used to combine the relative scores assigned separately to the width of vegetation behind the back-beach and the type of vegetation, as described below:

where V DV and V TV are the relative scores assigned to the width of vegetation behind the back-beach and the type of vegetation, respectively, and V is the total value associated with the variable. According to this mathematical formulation, the value of V varies from 1 to 5, which correspond to the same ranking score vulnerability for the variable.

Coverage of Posidonia oceanica This variable takes into account the presence of these meadows and their degree of coverage. Posidonia oceanica is a marine phanerogam endemic to the Mediterranean basin that forms extended meadows along its coasts in a bathymetric surface to 0–40 m depth in clear waters 60 . It plays a crucial role in the physical equilibrium of a large portion of the Mediterranean coasts. Although its benefits occur mainly below seagrass meadows, where it stabilizes the sea bottom by reducing sediment mobilization, the presence of Posidonia oceanica has effects on coastal protection, stabilization of the shoreline and beach morphology 55 , 56 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 ; furthermore, studies have documented the effect of reducing wave energy. Large-scale flume experiments on artificial Posidonia oceanica meadows and studies using field data have been carried out to measure wave attenuation and energy dissipation in intermediate and shallow waters while also considering seagrass density. These studies confirm the attenuation, transmission and energy dissipation of waves induced by Posidonia oceanica meadows 68 , 69 , 70 , 71 .

Therefore, this variable takes into account these positive effects on coastal protection related to the presence of coverage of Posidonia oceanica . In the previous CVI formulation proposed by the authors 27 , Posidonia oceanica was assessed only in relation to its presence/absence; in this work Posidonia oceanica is evaluated considering its bottom coverage, which affects its ability to mitigate damage caused by storm surges and inundations. Bottom coverage is expressed as a percentage of the seabed surface covered by living plants. The ranges have been defined by the percentage of coverage of these meadows with respect to the seabed area considering cells of 0.5 km x 1 km. In the absence of Posidonia oceanica meadows the highest vulnerability score is assigned, while high, medium, low and very low vulnerability values are assigned using coverage percentages <25%, 50–25%, 75–50%, and >75%, respectively.

Table 3 shows the range of vulnerability for the 11 variables.

The CVI is obtained by the square root of the product of the vulnerability scores assigned to each variable divided by the total number of variables:

where a = geomorphology, b = coastal slope, c = shoreline erosion/accretion rates, d = emerged beach width, e = dune, f = river discharge, g = relative sea-level change, h = mean significant wave height, i = mean tide range, j = vegetation behind the back-beach, k = coverage of Posidonia oceanica.

The CVI values obtained considering the vulnerability associated with each variable according to the formulation described in Eq. ( 3 ) have been divided into four categories of vulnerability:

Low vulnerability (green).

Moderate vulnerability (yellow).

High vulnerability (orange).

Very-high vulnerability (red).

These categories are obtained by considering the 25th, 50th and 75th percentiles of the distribution of the CVI values across the transects 4 .

Statistical and sensitivity analysis

The variance inflation factor (VIF) is used to measure the severity of multicollinearity in regression analysis 72 . A VIF value > 10 is considered an indicator that the collinearity among the variables is too high and t the regression output is unreliable; therefore, several studies recommended a value of 10 as the maximum acceptable VIF limit 73 , 74 , 75 .

Considering overall vulnerability, Pearson’s product-moment correlation coefficient values are computed to show the most influencial group of variables.

To evaluate the response of the index to changes in the score ranking, aggregation formulation and vulnerability classification method, a local sensitivity analysis is performed.

In particular, the categorical scale that uses equal classes defined between min–max values of the distribution and the percentile method is considered to compute different score rankings for some variables. Correlation coefficients are computed to compare the output of scores for each considered variable. A high correlation denotes that the variable is lightly sensitive to changes in the ranking scoring, while a low correlation denotes that the variable is highly sensitive to change. First, sensitivity analysis is carried out considering each variable separately, and subsequently, sensitivity analysis is also carried out considering the variations in overall CVI results using the two score ranking simultaneously for all considered variables. Regarding the ranking scores analysed, it is worth noting that the use of equal intervals and percentiles of the distribution values, representing the relative vulnerability of the coast, are considered as they are the most commonly used formulations in categorical scale methods and benchmarking approaches.

Regarding the aggregation formulation, sensitivity analysis is performed by testing two different formulations: the average sum of squares and the modified product mean.

Finally, sensitivity analysis is performed on the overall CVI vulnerability classification method using equal classes between min–max values of the distribution as it is a method also used in several applications.

Statistical and sensitivity analyses are performed using the Octave 5.2.0 software ( https://ocatve.org ).

Application and comparison of three methods for the case study area

It should be noted that there is no a unique approach to assessing coastal vulnerability, and it might be useful to select and customize the variables considering the site-specific conditions to make the overall CVI results truly useful for vulnerability analysis and intervention/investment planning. Similarly, according to Koroglu et al. 19 , it might be useful to consider site or region-specific ranking categories to develop reliable tools for local coastal management.

Therefore, in this work, a comparison of the results of the application for the case study area considering the previous CVI formulation proposed by the authors 27 , the CVI method proposed by Ružić et al. 45 , and the CVI formulation proposed by Palmer et al. 46 , is carried out.

Regarding the CVI method proposed by Ružić et al. 45 , the variables considered are: geologic fabric, coastal slope, beach width, significant wave height, land use. The CVI is computed as the square root of the vulnerability scores assigned to each variable product divided by the total number of variables, emphasizing the geological fabric’s importance:

where a = geologic fabric, b = coastal slope, c = beach width, d = significant wave height, e = land use. The index gives priority to the geological fabric considered the most important factor of coastal negative changes, and it was developed to analyze an area characterized by complex geomorphology 46 .

Regarding the method proposed by Palmer et al. 46 , the physical variables used are: beach width, dune width, distance to 20 m isobath, distance of vegetation behind the back beach, percentage of outcrop. The relative physical coastal vulnerability is computed as follows:

where a = beach width, b = dune width, c = distance to 20 m isobath, d = distance of vegetation behind the beach, e = percentage of outcrop, f = additional weighting of highly vulnerable sites, g = additional weighting if cell intersects an estuarine area. The index refers to the relative coastal vulnerability of coast to erosion and extreme weather events, and it was validated by the comparison to historical data from past coastal erosion event impact sites 47 .

For these two CVI methods, a table reporting the variables used and the ranking values is included as Supplementary materials (Table S.2 ).

Description of the case study area

The study area is located in the province of Crotone, Calabria region, southern Italy, and extends for a length of 20 km (Fig.  1 ).

figure 1

Case study area. Torretta di Crucoli – Cirò Marina, Calabria region, Italy. The map was created using QGIS 3.29 software ( https://qgis.org ).

This area is representative of a typical Mediterranean coastline and has two large urban settlements that have been significantly developed since the start of the second half of the twentieth century due to the growth of the agricultural and fishing sectors, and the related processing industries. Craft and recreational activities have also arisen, linked to seaside tourism. In this coastal stretch, there is an important railway affected by threats of service disruption during storm events. The typology of coastal beaches is low-lying sandy beach characterized by fine and medium-sized sand granulometry.

The area has been divided into 39 transects with a width of approximately 500 m, mainly consisting of a sandy beach with dune systems, among which there are areas of important naturalistic interest, such as the “Dunes of Marinella”, which has constituted a Site of Community Importance (SCI) since 1999, as defined in the European Commission Habitats Directive (92/43/EEC). This area represents one of the largest (up to 800 m) coastal dune/paleodune systems in Calabria 76 . This system is characterized by well-sorted, medium to fine brown sands, sometimes reddish in color. Hydrography of the area consists mainly of ditches and streams. The coastal slope is low, and most of the transects are in a state of erosion. The relative sea-level change is 4 mm/year, the mean significant wave height is 1.12 m, and the tide regime is microtidal. The first 4 transects consist of sandy beaches and are located in the town of Torretta di Crucoli; the following 3 transects have incipient dunes and foredunes and areas where the dune system has been damaged by buildings. The next 13 transects are in the “Dunes of Marinella” area and consist mainly of hind dunes, reaching to the “Madonna di mare” locality. The following 6 transects are characterized by incipient dunes. The next 11 transects are characterized by wider beaches and hind dunes, though the dunes of three transects have been damaged over the years. The last 2 transects are sandy beaches near the town of Cirò Marina. Figure  2 shows the case study area and related transects.

figure 2

Case study area and related transects. The map was created using QGIS 3.29 software ( https://qgis.org ).

The data sources used to define all the variables used for the CVI formulation are described in the following table (Table 4 ).

Results statistical and sensitivity analysis results

“Geomorphology”, “Relative sea-level change”, “Mean significant wave height”, and “Mean tide range” assume constant values for the case study area. Preliminarily, these variables were excluded; then, the VIF values are computed, and were within 10; The obtained VIF values are the following: coastal slope (1,25); shoreline erosion/accretion rates (1,15); emerged beach width (1,18); dune (3,02); river discharge (1,09); vegetation behind the back-beach (2,92); coverage of Posidonia oceanica (1,57).

Regarding statistical analysis, in addition to the VIF calculation described above, Pearson’s product-moment correlation coefficient values were computed to identify the most influential group of variables on the overall CVI value (Fig.  3 ). The correlation coefficient values between the overall CVI values and the variables of the three groups (Geological, Hydro-physical process and Vegetation) show that the Geological and Vegetation variables, which are characterized by the greatest spatial variability, are characterized by a high relationship (r = 0.851 and r = 0.826, respectively); therefore, these two groups have the most influence on the overall CVI values, while the Hydro-physical process variables have less influence on the physical vulnerability of the coast (r = 0.180).

figure 3

The scatter plot shows the results of Pearson’s product-moment correlation considering the three groups of variables and the overall CVI value with the correlation coefficient value (r) highlighted.

Regarding sensitivity analysis on ranking score, variables characterized by a constant value for all transects and those that included qualitative data were excluded. As previously mentioned, two different ranking scores have been tested: categorical scores using equal classes defined considering the min–max values of the distribution, and the percentile rank (20th, 40th, 60th, 80th).

The following table (Table 5 ) shows the correlation coefficients calculated to compare variations in the score ranking outputs for each considered variable. Furthermore, it shows the correlation coefficient of the overall CVI output, simultaneously considering all the variables tested and their ranking scores. The table also shows the correlation coefficients computed to compare variations in the aggregation method and in the vulnerability categories rank.

Results of the CVI application

As mentioned above, for the “Geomorphology” variable, the value is constant for all transects at very high vulnerability, while for “Coastal slope” the values vary between 0.94% (min) and 4% (max) with transects at high and very high vulnerability scores. Regarding “Shoreline erosion/accretion rates”, most transects are at very high and high vulnerability. Only two transects are at very low vulnerability. Almost all the transects are in a state of erosion, with a minimum retreat value of approximately 0.4 m and a maximum value of approximately 7 m. Regarding the other transects, the minimum coastal accretion value is approximately 0.5 m while the maximum value is approximately 15 m.

“Emerged beach width” includes very low vulnerability and high vulnerability, and it varies between the minimum value of approximately 22 m and the maximum value of approximately 204 m with a prevalence of transects at moderate vulnerability value, characterized by a beach width of about 50 m.

Regarding the variable “Dune”, it should be highlighted that dune width varies from a minimum of 91.5 m to a maximum of 470 m, while the dune system is present in 32 transects, 17 of which are of hind dune type, 8 of which are foredune types and the remaining are incipient dunes. The overall “Dune” is classified as very-low to very high vulnerability.

Regarding the variable “River discharge”, most transects are at very high vulnerability, few have values of high vulnerability and one transect is within the moderate vulnerability score range; the surface water circulation of the study case area mainly consists of drainage ditches, except for 7 transects characterized by torrential rivers, 6 of which are classified as “stream” and one as “small river”.

“Relative sea-level change” is constant for all transects (4 mm/year) at very high vulnerability, “Mean significant wave height” is constant at high vulnerability (1.12 m), while “Main tide range” is constant at very high vulnerability (microtidal).

Regarding the variable “Vegetation behind the back-beach”, the maximum width of vegetation value is approximately 1100 m with an average value of approximately 350 m, and most of the transects are characterized by tertiary vegetation, except in the transects near the towns and for the transect without the dune system. Overall, this variable is classified as very low to very high vulnerability.

Finally, the “Coverage of Posidonia oceanica ” includes transects at high vulnerability, few transects at very high vulnerability and one transect that is within the moderate vulnerability score category; in particular, Posidonia oceanica is absent in 9 transects, and in the other transects the maximum coverage is 27% with average coverage values of approximately 5%. The vulnerability values are mainly high, followed by very-high vulnerability values.

The following figure (Fig.  4 ) shows the score value maps of the transects based on each variable.

figure 4

Score value maps for each variable: ( a ) geomorphology, ( b ) coastal slope, ( c ) shoreline erosion/accretion rates, ( d ) emerged beach width, ( e ) dune, ( f ) river discharge, ( g ) relative sea-level change, ( h ) mean significant wave high, ( i ) mean tide range, ( j ) vegetation behind the back-beach, ( k ) coverage of Posidonia oceanica . The map was created using QGIS 3.29 software ( https://qgis.org ).

The estimated CVI values vary between 75.38 (min) and 1167.75 (max). The CVI mean is 485.01 and the median is 412.86. Table 6 shows the vulnerability category.

Figure  5 shows the obtained CVI category for each transect.

figure 5

CVI category for each transect. The map was created using QGIS 3.29 software ( https://qgis.org ).

A table describing the vulnerability score values associated with each variable and the estimated CVI value for each transect is included in the Supplementary materials (Table S.3 ).

Results of the comparison of three methods for the case study area

Regarding the comparison with the previous formulation proposed by the authors 27 , the results obtained have been compared to highlight the effects of the added variables to better characterize the case study area.

The aim of a comparison with the two other methods is to test the sensitivity of these indices regarding the characteristics of the study case area and to assess whether the newly proposed formulation is better suited to the specific characteristics of the case study coastline, a typical low-lying coast of the Mediterranean. The CVI method proposed by Ružić et al. 45 was developed to analyse a stretch of the Mediterranean coast, the Croatian eastern Adriatic coast; this coast is characterized by coastal retreat, slope instability and erosion conditions, similar to the case study area described in this paper. The CVI method proposed by Palmer et al. 46 , was chosen because it uses some variables similar to those proposed in this paper and has been applied for a coastline mainly consisting of low-lying sandy beaches.

Regarding the application and comparison of the proposed index with the previous formulation 27 , the results are shown in Fig.  6 , while Fig.  7 shows the results of the comparison with the two other methods.

figure 6

Proposed CVI formulation and previously CVI formulation 27 . The map was created using QGIS 3.29 software ( https://qgis.org ).

figure 7

Proposed CVI formulation and CVI formulations proposed by Ružić et al. 46 and Palmer et al. 47 . The map was created using QGIS 3.29 software ( https://qgis.org ).

The results of the comparison of the proposed index with the previous authors’ CVI 27 and the other two formulations 45 , 46 , are summarized in two tables included as Supplementary materials (Tables S.4 , S.5 ).

Regarding the statistical analysis, the VIF values are within 10; therefore, this confirm that all the variables can be included in the index calculation. The results of the sensitivity analysis show that for all the considered variables, the correlation coefficients indicate a reduced sensitivity to variation in ranking score. The coverage of Posidonia oceanica has a lower correlation coefficient than the other variables, which indicates a greater sensitivity to changes. Considering the overall CVI output, the coefficient values are moderate with a greater sensitivity for the categorical scale, which uses equal classes. Regarding the sensitivity analysis of the aggregation method, the results of the two different mathematical formulations tested, show high correlation coefficients values. Similarly, the sensitivity analysis on the overall vulnerability categories rank denotes a low sensibility to change (r = 0,898).

Regarding the results of the CVI application, transects at low vulnerability are generally characterized by a wide or moderate emerged beach width and dune width. Dunes are generally of the hind dune type with a large or moderate width of vegetation behind the beach, mainly of the tertiary type. In particular, this category comprises 9 transects; transects 13, 14, and 18 are characterized by low vulnerability values mainly due to the presence of the dune system of “Dune of Marinella” consisting of width hind dunes and width secondary and tertiary vegetation, respectively. Transects 28, and 33–37 are at low vulnerability mainly due to the width of beaches and to the values of “Dune” and “Vegetation behind the back-beach” variables.

Transects at moderate vulnerability, similar to those at low vulnerability, are generally characterized by a wide or moderate emerged beach width. Dune width is generally moderate with hind dune and foredune types. The width of vegetation behind the beach is generally wide for all the transects, and the vegetation is of primary and secondary types. Compared to the previous category, these transects are characterized by lower coastal slopes and greater shoreline erosion conditions. Transects 8, 10–12, 16, 17 and 20 fall within the area of the “Dunes of Marinella” and therefore consist of areas with wide beaches and dunes; however these transects have a lower coastal slope and therefore a greater vulnerability value associated with this variable, a greater erosion condition and Posidonia oceanica is absent in some transects (16, 17). A similar condition concerns transects 27, 29 and 32 which are the last two transects of this category.

The transects at high vulnerability, are in areas with shoreline erosion conditions, the presence of small width dunes, primary or secondary vegetation and a low percentage of coverage of Posidonia oceanica , as in the case of transects 21, 23, and 25. This category also includes: transect 4, which is mainly characterized by a reduced beach width, without dunes or vegetation; transect 15, which has high vulnerability essentially due to the moderate beach width value and dune system damage; and transects 9 and 19, which are characterized by a small beach width, absence of streams and high erosion conditions. The last two transects of this category, 30 and 31, are mainly characterized by significant erosion conditions and the destruction of the dune system that in recent years, however, has begun to cover.

Finally, transects at very high vulnerability are characterized by high vulnerability values for almost all variables. In particular, for transects 1–3, and 5–7, the CVI values are very high due to the presence of an urbanized area with a reduced beach width, small distance of vegetation behind the beach, and absence of dune (or incipient and foredune type for transects 5 and 6); in the case of transects 22, 24 and 26, the CVI values are due mainly to the high erosion condition, by the presence of small width incipient dunes, by primary vegetation and by a low percentage of coverage of Posidonia oceanica. The last two transects of the category, 38 and 39, near the town of Cirò Marina, are at very high vulnerability, as these stretches are characterized by a moderate emerged beach width and a high vulnerability values for the remaining variables.

Regarding the comparison with the previous CVI formulation proposed by the authors, it should be noted that in transects 6, 22, 23 and 26, the CVI values increase in the present formulation, as all the added variables are characterized by high and very high vulnerability values.

Regarding transects 8, 15, 21 and 31, the vulnerability values increase compared to the previous formulation, varying from low to moderate vulnerability for transects 8 and 31 and from moderate to high vulnerability for transects 15 and 21. This increase in vulnerability is certainly due to Posidonia oceanica which has a high vulnerability value; even if the coverage of Posidonia oceanica results low, in the previous CVI formulation, the value associated with this variable would have been 1 (presence: very low). Instead, in the new formulation, the reduction in the defensive effects of the Posidonia oceanica is taken into account, since its coverage percentage is low. The river discharge variable also has high and very high values, which affects the increase in CVI value, while the other variables have moderate and low values.

Transects 9, 16 and 17 decrease the vulnerability value in the present formulation, varying from very high to high vulnerability (transect 9) and from very high to moderate (transects 16 and 17). For these transects, Posidonia oceanica had no influence, as it was not present. This variation in vulnerability is essentially due to the presence of hind dunes and primary and secondary vegetation which constitute an important barrier with respect to flooding for low-lying coasts.

Finally, for transects 18, 32, 34, 36 and 37 the decrease in vulnerability is due mainly to the values of the “Dune” and “Vegetation behind the back-beach” variables.

Overall, the comparison carried out showed that there are differences in vulnerability values due to the added variables. In particular, the dune, vegetation and coverage of Posidonia oceanica have a significant influences.

Regarding the method proposed by 45 , the geologic fabric variable assumes a constant value for all transects, similar to the geomorphology variable used in the index proposed in this paper, while the results relating to the coastal slope are different. In particular, considering the method proposed 45 , this variable assumes values between very low and low vulnerability (70% and 30% of the transects, respectively) while in the index proposed here, the values vary between high and very high vulnerability (51% and 49% respectively). This difference is due to the different ranking values and to the inverse score scale between the two indices. The beach width variable assumes constant values at low vulnerability considering the index proposed by 46 , while in the model proposed in this paper this variable assumes all the values from very low to very high vulnerability; this difference is due to the difference in the ranking values. The significant wave height variable assumes a constant value equal to high vulnerability while for land use the values vary from low to very high vulnerability (69%, 13%, 5%, 13% of transects, respectively). Regarding the overall CVI values obtained for each transect, it should be noted that this index shows a small variation in vulnerability classes. Approximately 49% of the transects are at low vulnerability, approximately 33.3% are at moderate vulnerability, 7.7% are at high vulnerability and 10% are at very high vulnerability. This small variation in vulnerability classes could make the index a weak tool for decision and management making processes for coastal areas, such as that of the case study, subject to different natural hazards and with limited resources to invest.

Regarding the index proposed by 46 , the beach width variable varies among all the vulnerability score classes, with a prevalence for the highest value of vulnerability (approximately 69% of transects). Similarly, the dune width variable assumes values between all vulnerability score classes but with a prevalence of transects at very low and low vulnerability (35,9% and 30,8% respectively). Regarding the distance to the 20 isobath variable, no transect has values at very low vulnerability while their prevalence assumes values of low vulnerability and high vulnerability in similar percentages. Regarding the width of the vegetation variable, the values vary among all vulnerability score classes but with a prevalence of transects at low vulnerability (approximately 56.4% of transects), while all transects have a percentage of outcrops corresponding to the highest value of the vulnerability score. Regarding the overall CVI values, approximately 25.6% of the transects are in the low vulnerability category, 35.9% are in the moderate vulnerability category, 23.1% are in the high vulnerability category and 15.4% are in the very high vulnerability category. Comparing the obtained results for the two methods, it should be noted that in the method proposed here, approximately 31% of the transects are at the same vulnerability category as that proposed by 46 , while approximately 46% of the transects show an increase in the vulnerability category when compared with that proposed by 46 . In particular, transects 1, 2 and 3 are in the very high vulnerability category in the present CVI formulation and in the high vulnerability category in the formulation proposed by 46 . This difference is due to the very high value of vulnerability score for all the other variables of the present CVI formulation. In transects 5, 6 and 7, all the variables except for two in the formulation proposed by 46 have high vulnerability score values, while in the case of the present formulation, most of the variables have very high vulnerability score values leading to a greater vulnerability category in the present formulation. Similar considerations are for transects 16 and 17.

Regarding transects 9, 19, 22, 24, 25 and 30, while in both methods the common variables assume similar vulnerability score values, the increase in vulnerability category for the CVI proposed here is due to the high and very high values of all the other variables.

Regarding the transects characterized by a lower vulnerability in the present CVI index, it should be noted that in both methods the common variables assume similar vulnerability score values and the increase in vulnerability category considering Palmer’s method is due to the additional weighting of the intersects in an estuarine area, according to the mathematical formulation of the method, which is the case for transects 13, 18, 21, 28. Instead, for transects 14 and 37 the difference in vulnerability category is due to the low vulnerability score for some of the CVI variables in the present method.

Overall, the two methods are comparable even if the results of the application with the method proposed by 46 show a lower number of transects in the very-high vulnerability category and a greater number in the moderate category; in the case, for example, of transects 6 and 22, characterized by high erosion conditions, low coastal slopes, no river/stream, low width of vegetation, and a very low percentage of Posidonia oceanica coverage, they variables result in a moderate vulnerability category, while with the index proposed by the authors, the transects are at very high vulnerability; this result appears more realistic for these transects. Overall, the use of variables such as dune and vegetation, coverage of Posidonia oceanica , coastal slope, and shoreline erosion accretion rates allow to obtain a more sensitive index with respect to inundation and extreme events; the index is therefore robust and useful for areas such as that of the case study, which are affected by different natural hazards and where planning and management actions must address the needs for economic development and the limitation of available financial resources to reduce threats and promote area development.

Finally, the obtained results have also been analysed considering the Regional Hydrogeological Plan of the Calabria Region ( https://geoportale.regione.calabria.it ); this plan uses a mathematical approach based on a set of data and parameters, to investigate different vulnerability threats that affect the regional territory such as flooding, coastal erosion, and hydraulic risk. It also includes programs, and structural and nonstructural interventions. In the Regional Hydrogeological Plan of the Calabria region areas between transects 1–7, 21–26, 38–39 are considered highly vulnerable to erosion, inundation and extreme events and are defined as areas that need more attention in planning and management activities. Considering the proposed CVI, transects 1–7, and 38–39, are at very high vulnerability (excepts transect 4), in agreement with the Regional Hydrogeological Plan, while the area between transects 21–26 has high and very high vulnerability. Therefore, the results of the proposed CVI formulation provide evidence similar to the results of the Regional Hydrogeological Plan and identify the areas that should be prioritized for interventions. It should be noted that a lack of other similar studies for the case study area did not allow for further comparisons and considerations.

It is worth noting that for transects 1–7, in the previous formulation proposed by the authors 27 , some transects are at high vulnerability; considering the index proposed by 45 , one transect is at high vulnerability, while two are at moderate vulnerability, while in the formulation proposed by 46 , almost the transects are at high vulnerability and one at moderate vulnerability. Therefore, the three formulations compared tend to consider the area least vulnerable. Regarding transects 21–26, the obtained results using the previous formulation 27 are predominantly at moderate-high vulnerability; the method proposed by 45 considers this area as a whole to be at low-moderate vulnerability, while Palmer's index considers it at higher vulnerability with transects at moderate, high and very-high vulnerability. Finally, considering the transect 38–39, in all formulations considered, the obtained results are similar (transects at very high vulnerability) even if Ružić’s formulation considers these transects to be at high vulnerability.

Therefore, the overall obtained results with the new CVI formulation find evidence with the real vulnerability of the study area and with the most vulnerable transects, and they better characterize the area than the other formulations.

Conclusions

This paper proposes a coastal vulnerability index, in continuation with a previous work by the authors, suitable for low-lying Mediterranean coasts, and a case study has been presented. The main conclusions of the study are as follows.

The study area is characterized by low slopes and predominantly erosive conditions. The beaches of the study area are quite wide, and the majority of the area is characterized by a dune system comprising hind dunes, foredunes and incipient dunes; there are some stretches where the dune system has been damaged. The type of vegetation present is mainly tertiary vegetation: the width of the vegetation has mainly large values. The area is characterized by the presence of a few ditches and streams, and the coverage of Posidonia oceanica is low or even absent in various transects. The obtained results have shown that the most vulnerable areas are those near urban centers characterized by the absence of a dune system and vegetation behind the beach, while the low vulnerability transects are mainly characterized by wide/moderate emerged beach width, and the low vulnerability is also due to the characteristics of the dune system and vegetation. Sensitivity analysis showed that the index is lightly sensitive to changes in ranking scores, aggregation formulation and vulnerability classification method.

In coastal areas, such as that of the case study, characterized by low-lying coastlines, erosive conditions, and low slopes, affected by anthropogenic pressures, infrastructure, and growing tourist and economic activities, the proposed index could analyse the relative vulnerability of the different transects of the investigated coastal area, hence providing a reliable tool for local coastal management; this aspect is even more important to limited investment budgets. There is a need to define in detail the transects with the greatest vulnerability for which investments should be primarily focused. The lack of information and of systematic data collection on the Calabrian territory, prevented application to a longer stretch of coastline. Aware of this limitation, the aim of this work was to test the index, to assess its feasibility and initially apply it to identify the vulnerable coastal areas that should be prioritized. Future research objectives include the application of the proposed index to a larger spatial scale with greater variable heterogeneity.

Regarding the comparison with the previous formulation proposed by the authors the variables “Dune” and “Vegetation” has provided more information on the ability of the method to counteract the action of floods/storm surges and the dissipation of wave energy; in addition, for an area characterized by shoreline erosion and changes in land use, the integration of the variable “River discharge” allowed us to account for the effects of rivers to limit erosive conditions. The redefinition of Posidonia oceanica also had a significant influence on the final results of the application, as it allowed for a more detailed analysis compared to only estimating its presence/absence, asits defence action varies in relation to its coverage degree. A comparison of the results showed that the contribution of these variables, completes the previous formulation, and the application showed a difference in the vulnerability categories with a greater number of transects at very high vulnerability and moderate vulnerability and a reduction in those at low vulnerability. These results find evidences on the real vulnerability of the territory.

The comparison with the other two methods also indicated that the results of the proposed index appear more realistic for the case study area and that the set of variables used provides a robust analysis of the different threats and defence/protection factors that influence the overall vulnerability value for the typology of coastal area studied. To find evidence on the real vulnerability of the territory, a regional study has also been considered; the lack of other studies on this issue in the case study area did not permit further comparisons. Overall, the results of this first application appear consistent with the study area conditions, and the aim of future research is to validate the proposed index by comparing it with more complex numerical models to make the index a useful tool for coastal planning and management.

Data availability

The dataset used and/or analysed during the current study is available from the corresponding author on reasonable request.

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Conceptualization and Methodology: G.R.T., D.P. and A.F.; Formal analysis: D.P.; Investigation D.P., F.D., F.F., A.F.; Data curation and Visualization D.P., F.D., F.F., A.F.; Writing—original draft D.P.; writing—review and editing, G.R.T., D.P., F.D., F.F., A.F.; supervision, G.R.T.

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Pantusa, D., D’Alessandro, F., Frega, F. et al. Improvement of a coastal vulnerability index and its application along the Calabria Coastline, Italy. Sci Rep 12 , 21959 (2022). https://doi.org/10.1038/s41598-022-26374-w

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Hornsea Case Study

Location of hornsea.

Hornsea is a small coastal town on the Holderness Coast , located between Bridlington and Withernsea. A 2.9km stretch of shoreline fronts the town of Hornsea. Hornsea consists of a high-density urban development containing residential and various tourist-related properties. Hornsea’s local economy is dependent on tourism and recreation, as well as incorporating a small fishing industry.

Geology of Hornsea

Hornsea lies upon boulder clay . This material is unconsolidated till was deposited by glaciers during the last ice age 18,000 years ago. The boulder clay consists of about 72% mud, 27% sand and 1% boulders and large pebbles.

Coastal Features at Hornsea

The groynes on Hornsea beach ensure wide and relatively steep beaches. The beach material is made up of sand and shingle. The exposed boulder clay cliffs to the north give way to a seawall along the front of the town. The boulder clay cliffs are again exposed to the south of the town.

What are the reasons for coastal management at Hornsea?

There are several reasons why planners have chosen to hold the line at Hornsea. These include:

  • Hornsea Mere, Yorkshire’s largest natural lake, is an important recreational site for visitors and residents and one of the key features for the marketing of Hornsea. It is also of special wildlife interest and is designated a European Special Protection Area (SPA);
  • Its high population density, with a population of 8,327 (Office for National Statistics (ONS), 2020 mid-year estimate), and
  • The wide range of infrastructure already in place in the town.

What is the coastal management strategy at Hornsea?

The position of the coastline at Hornsea has been artificially fixed since existing coastal defences were erected in the early 1900s. The current coastal management plan is to hold the line at Hornsea. This means coastal defences will be maintained and replaced to protect the town.

A range of management techniques has limited coastal erosion at Hornsea. The map below shows a range of management strategies and the impact these are having further along the coast.

Hard defences in the form of a concrete seawall and timber groynes afford protection , and an ongoing refurbishment programme ensures this has continued. The image below shows groynes to the south of Hornsea.

Coastal management at Hornsea

Coastal management at Hornsea

The groynes trap sediment transported by longshore drift , providing a wide sandy beach. This provides effective protection from destructive waves.

The sea wall provides effective protection from destructive waves. Rock armour has recently been placed along a considerable stretch of the sea wall to absorb wave energy and increase the lifespan of the sea wall.

The videos below show the importance of the seawall at Hornsea, particularly during a spring tide combined with strong winds!

A stone and steel gabion and a concrete revetment have been erected to the southern end of Hornsea. This helps protect Longbeach Leisure Park. The gabion absorbs wave energy, protecting the boulder clay cliffs behind.

More recently, rock armour has extended the coastal defences to the south of the town. This absorbs wave energy, protecting the boulder clay cliffs below the Longbeach Leisure Park park.

The image below shows coastal defences to the south of Hornsea. These include groynes, extending at right angles out into the sea, and rock armour can also be seen to the south.

View north from the south of the defences

View north from the south of the defences

What are the effects of coastal management at Hornsea?

Positive impacts of coastal management at Hornsea

The hard engineering solutions provide effective protection from coastal erosion. The defences withstand stormy winters.

The groynes have ensured a wide, sandy beach which attracts thousands of visitors to the town, supporting many local businesses, including Longbeach Leisure Park, a holiday park to the south of Hornsea.

The wide sandy beach allows boat owners to launch, supporting the local fishing industry and leisure users.

Negative impacts of coastal management at Hornsea

Where the defences end to the south of Hornsea, erosion rates have rapidly increased. The downdrift beach is starved of material, trapped behind groynes. Therefore, the unprotected, soft boulder clay is rapidly eroding. There is little beach material to the south of the defences, so even neap tides (low-high tides) reach the base of the cliffs.

The image below shows the impact of the terminal groyne.

Terminal groyne, rock armour and cliff slumping

Terminal groyne, rock armour and cliff slumping

The photograph below was taken to the south of Hornsea. The camera is pointing northeast. You can see the impact of erosion to the south of the coastal defences. This has led to conflict between the owners of the Longbeach Leisure Park and the East Riding of Yorkshire County Council, who are responsible for managing coastal defences at Hornsea.

Take a look at the Hornsea gallery and video collection.

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Related Topics

Use the images below to explore locations along the Holderness Coast.

Flamborough

Geos and mass movement at Flamborough

Spurn Point

Spurn Point

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Tethys Logo: Environmental Effects of Wind and Marine Renewable Energy

The Potential of Wave Energy Conversion to Mitigate Coastal Erosion from Hurricanes

Wave energy conversion technologies have recently attracted more attention as part of global efforts to replace fossil fuels with renewable energy resources. While ocean waves can provide renewable energy, they can also be destructive to coastal areas that are often densely populated and vulnerable to coastal erosion. There have been a variety of efforts to mitigate the impacts of wave- and storm-induced erosion; however, they are either temporary solutions or approaches that are not able to adapt to a changing climate. This study explores a green and sustainable approach to mitigating coastal erosion from hurricanes through wave energy conversion. A barrier island, Dauphin Island, off the coast of Alabama, is used as a test case. The potential use of wave energy converter farms to mitigate erosion due to hurricane storm surges while simultaneously generating renewable energy is explored through simulations that are forced with storm data using the XBeach model. It is shown that wave farms can impact coastal morphodynamics and have the potential to reduce dune and beach erosion, predominantly in the western portion of the island. The capacity of wave farms to influence coastal morphodynamics varies with the storm intensity.

IMAGES

  1. Holderness coastal erosion case study

    case study of coastal erosion

  2. Holderness coastal erosion case study

    case study of coastal erosion

  3. Holderness coastal erosion case study

    case study of coastal erosion

  4. Holderness coastal erosion case study

    case study of coastal erosion

  5. Holderness coastal erosion case study

    case study of coastal erosion

  6. Holderness coastal erosion case study

    case study of coastal erosion

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