Multi-decadal Shoreline Change Research Articles

Multi-decadal shoreline changes in Eastern Ghana—natural dynamics versus human interventions

Human infrastructures, such as dams, seawalls, and ports, can affect both the sedimentary budget and nearshore hydrodynamics, enhancing and accelerating the loss or gain of coastal sediments. Understanding the processes and factors controlling beach morphodynamics is essential for implementing adequate adaptation strategies in coastal areas, particularly in those regions where coastal protection measures are scarce. This study analyzes shoreline changes in the Keta Municipal District, located in southeastern Ghana (West Africa). This area is characterized by the sedimentary input of the Volta River, forming a river delta situated to the west, i.e., updrift, of our study site. Following the construction of two dams (Akosombo and Kpong) on the Volta River in 1965 and 1982, groins and revetments have been built along the coast between 2005 and 2015 to reduce the high rates of coastal erosion in this area. Here, we explore the influence of these dams and the hard protection constructions on beach morphodynamics using historical maps and satellite images complemented by a shoreline survey undertaken with a differential GNSS in 2015. The multi-decadal evolution between 1913 and 2015 reconstructed for 90 km of shoreline suggests that local erosion rates in the region predate the construction of the two dams on the Volta River, indicating that these structures might not be the primary driver of coastal erosion in this area, as previously suggested. We emphasize that delta dynamics under conditions of high-energy longshore drift, modified by anthropogenic drivers such as sand mining, play a key role in the long-term evolution of this coast. Our results also show that the infrastructures built to halt coastal erosion result in localized erosion and accretion down-current along the coastline towards the border with Togo, highlighting the need for a transnational perspective in addressing the problems caused by coastal erosion.

Seasonal to Multi-Decadal Shoreline Change on a Reef-Fringed Beach

This study investigates the shoreline dynamics of a Caribbean reef-lined beach by utilizing a long-term satellite dataset spanning 75 years and a short-term, high-frequency dataset captured by a fixed camera over 3 years. An array of statistical methods, including ARIMA models, are employed to examine the impact of storms and potential cyclical influences on the shoreline dynamics. The findings indicate that significant storm events trigger a substantial retreat of the vegetation limit, followed by a slow recovery. Given the current frequency of such major events, complete recovery may take several decades, resulting in a minor influence of cyclones on the long-term erosion trend, which remains moderate. The short-term shoreline evolution is primarily driven by the annual cyclicity of the still water level, which generates an annual oscillation—an insight not previously reported. In the context of climate change, alterations to sea-level rise and cyclone frequency could disrupt the observed dynamic equilibrium at different timescales. Such changes could result in an alteration of existing cyclicities, disturbance of recovery periods, increased long-term shoreline retreat rates, and potentially affect overall coastal resilience over time.

A quantitative analysis of multi-decadal shoreline changes along the East Coast of South Korea

South Korea's east coast is facing several issues related to coastal erosion because of sea-level rise, typhoon-induced storm surges, and various coastal development projects. In recent decades, high storm waves have frequently appeared on the east coast, causing casualty, beach erosion, and coastal infrastructure damage, drawing significant public attention. Thus, we analyzed the multi-decadal shoreline changes to understand the coastal dynamics and the forces responsible for the spatio-temporal changes along the 173 km coastline. The shorelines covering 38 years between 1984 and 2022 were derived from Landsat images and the change statistics, i.e., linear regression rate (LRR), endpoint rate (EPR), weighted linear regression (WLR), and net shoreline movement (NSM), were calculated at a 100 m alongshore intervals using Digital Shoreline Analysis System (DSAS), revealed several distinct behaviors of shoreline position. The long-period (1984–2022) assessment showed an average shoreline change rate (LRR) of 0.17 m/year with an estimated mean erosion and deposition rate of −0.57 and 2.07 m/year, respectively. The long-term surface gain and loss of the backshore region exhibited that the net surface gain of the east coast is 421.13 ha, and the net loss is 181.82 ha. The assessment of decadal shoreline changes showed a cyclic pattern of erosion (from 1984–1990 and 1999–2010) and accretion (from 1990–1999 and 2010–2022). Furthermore, a secondary level of investigation was conducted to address a wider variety of coastal behaviors by segmenting shoreline change rates based on coast types and average slopes along coastlines. It was observed that the frequent coastal deformation is associated with a flatter beach compared to a steep one. This study found that the artificial structures constructed along the east coast have not completely solved or stopped the erosion issues but shifted it from one location to another. The analysis of local and regional shoreline changes had shown that typhoon-induced storm surges, high storm waves, and anthropogenic activities like encroachment and the development of artificial coastal structures were the primary drivers of coastline changes along the east coast. Finally, we proposed a decision-making classification scheme that can be used to determine the mechanism of decision for protective and preventive measures against further coastal deterioration.

Improving multi-decadal coastal shoreline change predictions by including model parameter non-stationarity

Our ability to predict sandy shoreline evolution resulting from future changes in regional wave climates is critical for the sustainable management of coastlines worldwide. To this end, the present generation of simple and efficient semi-empirical shoreline change models have shown good skill at predicting shoreline changes from seasons up to several years at a number of diverse sites around the world. However, a key limitation of these existing approaches is that they rely on time-invariant model parameters, and assume that beaches will evolve within constrained envelopes of variability based on past observations. This raises an interesting challenge because the expected future variability in key meteocean and hydrodynamic drivers of shoreline change are likely to violate this ‘stationary’ approach to longer-term shoreline change prediction. Using a newly available, multi-decadal (28-year) dataset of satellite-derived shorelines at the Gold Coast, Australia, this contribution presents the first attempt to improve multi-decadal shoreline change predictions by allowing the magnitude of the shoreline model parameters to vary in time. A data assimilation technique (Ensemble Kalman Filter, EnKF) embedded within the well-established ShoreFor shoreline change model is first applied to a 14-year training period of approximately fortnightly shoreline observations, to explore temporal variability in model parameters. Then, the magnitudes of these observed non-stationary parameters are modelled as a function of selected wave climate covariates, representing the underlying seasonal to interannual variability in wave forcing. These modelled time-varying parameters are then incorporated into the shoreline change model and tested over the complete 28-year dataset. This new inclusion of non-stationary model parameters that are directly modelled as a function of the underlying wave forcing and corresponding time scales of beach response, is shown to outperform the multi-decadal predictions obtained by applying the conventional stationary approach (RMSEnon-stationary = 11.1 m; RMSEstationary = 254.3 m). Based on these results, it is proposed that a non-stationary approach to shoreline change modelling can reduce the uncertainty associated with the misspecification of physical processes driving shoreline change and should be considered for future shoreline change predictions.

Coastal transgression and regression from 1980 to 2020 and shoreline forecasting for 2030 and 2040, using DSAS along the southern coastal tip of Peninsular India

This study explains the multi-decadal shoreline changes along the coast of Kanyakumari from 1980 to 2020. The shorelines are extracted from the Landsat images to estimate the shoreline dynamics and future predictions using Digital Shoreline Analysis System (DSAS). By the estimation of End Point Rate (EPR) and Linear Regression Rate (LRR), it is quantified that the maximum erosion is 5.01 m/yr (EPR) and 6.13 m/yr (LRR) consistently with the maximum accretion of 3.77 m/yr (EPR) and 3.11 m/yr (LRR) along the entire coastal stretch of 77 km. The future shoreline predicted using the Kalman filter forecasted that Inayam, Periyakattuthurai and Kodimunai are highly prone to erosion with a shift of 170 m, 157 m and 145 m by 2030 and 194 m, 182 m and 165 m by 2040 towards the land. Also, the western coast is highly prone to erosion and it is predicted that certain villages are prone to loss of economy and livelihood. The outcome of this study may guide the coastal researchers to understand the evolution and decision-makers to evolve with alternative sustainable management plans in the future.

Natural vs. Anthropic Influence on the Multidecadal Shoreline Changes of Mediterranean Urban Beaches: Lessons from the Gulf of Cagliari (Sardinia)

Urban Mediterranean beaches are often characterized by a fragile and unstable equilibrium that can be easily altered by ongoing climate change and by the increase in human pressure. This may pose serious threats to the survival of beach systems that cannot accommodate these modifications. In this paper, the spatio-temporal shift of the shoreline was investigated along two urban beaches in the Gulf of Cagliari (Poetto and Giorgino; southern Sardinia, western Mediterranean Sea) across a time frame of 62 years (1954–2016). The Digital Shoreline Analysis System (DSAS) ArcGIS™ extension was used to extract different statistical parameters which allowed us to quantify the erosion and accretion rates. These data were further examined in relation to a number of anthropic and natural forcings in order to disentangle the factors controlling shoreline evolution. Eight sectors with interchanging net erosive and accretion trends were identified along the Poetto and Giorgino beaches. In six decades, some sectors of the two study sites appeared to have undergone great shoreline modification as a result of the intense anthropogenic activities impacting these coastal areas. The westernmost portions of both beaches were found to be the most vulnerable to erosion processes; such conditions were likely controlled by the interplaying of local hydrodynamics and by the intense coastal development which affected these sectors. The highest retreat rates (mean end point rate (EPR) = −0.51/year) were recorded in the western limit of Giorgino beach. Along the western limit of Poetto beach, EPR erosion rates (mean EPR = −2.92/year) considerably increased in the years after the artificial beach nourishment carried out in 2002, suggesting that the majority of the nourished material was lost offshore or partly redistributed along the beach. Coastal structures, urban development, river catchment modification, industrial and port activities, beach cleaning and touristic and recreational activities have been identified as the ongoing causes of coastal alteration. If these factors remain constant, under projected climate change scenarios, these beaches are at risk of further increased flooding and erosion. In this context, the application of DSAS appeared as an essential tool, supporting a monitoring system able to provide understanding and, potentially, predictions of the short- to long-term evolution of these beach systems.

Multi-decadal shoreline change in coastal natural world heritage sites – a global assessment

Natural World Heritage Sites (NWHS), which are of Outstanding Universal Value, are increasingly threatened by natural and anthropogenic pressures. This is especially true for coastal NWHS, which are additionally subject to erosion and flooding. This paper assesses shoreline change from 1984 to 2016 within the boundaries of 67 designated sites, providing a first global consistent assessment of its drivers. It develops a transferable methodology utilising new satellite-derived global shoreline datasets, which are classified based on linearity of change against time and compared with global datasets of geomorphology (topography, land cover, coastal type, and lithology), climate variability and sea-level change. Significant shoreline change is observed on 14% of 52 coastal NWHS shorelines that show the largest recessional and accretive trends (means of −3.4 m yr−1 and 3.5 m yr−1, respectively). These rapid shoreline changes are found in low-lying shorelines (<1 m elevation) composed of unconsolidated sediments in vegetated tidal coastal systems (means of −7.7 m yr−1 and 12.5 m yr−1), and vegetated tidal deltas at the mouth of large river systems (means of −6.9 m yr−1 and 11 m yr−1). Extreme shoreline changes occur as a result of redistribution of sediment driven by a combination of geomorphological conditions with (1) specific natural coastal morphodynamics such as opening of inlets (e.g. Río Plátano Biosphere Reserve) or gradients of alongshore sediment transport (e.g. Namib Sea) and (2) direct or indirect human interferences with natural coastal processes such as sand nourishment (e.g. Wadden Sea) and damming of river sediments upstream of a delta (e.g. Danube Delta). The most stable soft coasts are associated with the protection of coral reef ecosystems (e.g. Great Barrier Reef) which may be degraded/destroyed by climate change or human stress in the future. A positive correlation between shoreline retreat and local relative sea-level change was apparent in the Wadden Sea. However, globally, the effects of contemporary sea-level rise are not apparent for coastal NWHS, but it is a major concern for the future reinforcing the shoreline dynamics already being observed due to other drivers. Hence, future assessments of shoreline change need to account for other drivers of coastal change in addition to sea-level rise projections. In conclusion, extreme multi-decadal linear shoreline trends occur in coastal NWHS and are driven primarily by sediment redistribution. Future exacerbation of these trends may affect heritage values and coastal communities. Thus shoreline change should be considered in future management plans where necessary. This approach provides a consistent method to assess NWHS which can be repeated and help steer future management of these important sites.

Multi-decadal shoreline change analysis in coast of Thiruchendur Taluk, Thoothukudi district, Tamil Nadu, India, using remote sensing and DSAS techniques

Shoreline position has been forwarding or retrograding due to many factors, among them climate change and human interferences play a major role. The shoreline changes can be analyzed by various methods. In the present study, Linear Regression Rate (LRR) method is applied for the demarcation of rate of shoreline changes along the coast of Thiruchendur Taluk, Thoothukudi district, by using Digital Shoreline Analysis System (DSAS). It is an extension tool in ArcGIS. The length of the coast is 45 km. The coastal zone is further divided into three coastal zones based on their administrative division (panchayat union). The zones are Alwarthirunagiri coast, Thiruchendur coast, and Udankudi coast. These zone divisions are done for detailed mapping. Multi-date of remotely sensed data was used for mapping the details. These data are georeferenced with the UTM projection of WGS1984 and digitized the shorelines with the FCC color image. Transect lines were generated using DSAS tool. The transect lines starting from 752 to 1210 were used for present study to assess the shoreline changes. The tool had generated the LRR Statistic reports on each transect lines for all shorelines. The statistic report has been classified into five classes, viz., high erosion, low erosion, stable, low accretion, and high accretion. Finally, the shoreline has classified with LRR reports. The overall result shows that, low accretion has notied at 36 km (79%), high accretion perceived at 8.7 km (8.7%), and stable has observed in 8.3 km (8.3%), along the coast. Whereas, low erosion was noticed around 1.4 km (3.1%), and no high erosion was noticed along the study area. Thiruchendur and Udankudi coastal zones are mostly perceived low rate of shoreline changes with the rate from 1.02 to 3.96 m/year, happen in 18.9 km and 16.5 km, respectively. But, Alwarthirunagiri coast zone, low rate of erosion is noticed along the 1.4-km length of the shore. The study has concluded that, most of the Thiruchendur Taluk coastal areas are under the low-rate accretion, and the same area is observed as erosion along the coast due to both natural and human activities. Thiruchendur Taluk has major coastal cities, and a large number of people live around the coast. There are numerous natural resources and tourist places around the coast. Therefore, this coastal area shoreline changes studies should be examined.

Multi-decadal Shoreline Change Interpreted from Aerial Photographs in Ilgwang Beach, South Korea

Kim, C.W.; Shim, W.J., and Yim, J., 2019. Multi-decadal shoreline change interpreted from aerial photographs in Ilgwang Beach, South Korea. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 316-320. Coconut Creek (Florida), ISSN 0749-0208.The aim of this study is to use multi-temporal aerial photographs spanning a period of 70 years from 1948 to observe shoreline changes at Ilgwang beach in the northeastern part of Busan, which is an area that has excellent economic, social, and natural attributes. Photographs were enhanced using geometric correction, and the shoreline and beach width were extracted using a wet/dry line. Decadal changes were statistically analyzed using the Digital Shoreline Analysis System, 4.3 (DSAS). The sediment budget of the watershed was calculated using the Revised Universal Soil Loss Equation (RUSLE) and a correlation analysis was then conducted against beach width. The impact of typhoon Thelma which occurred in 1987 was strongly observed, and its occurrence prevented any clear trends in shoreline changes from being observed between 1948 and 2017, although a dike constructed on northern part of the beach caused strong deposition in transect 1 from 2009. In addition, the beach width was found to be relatively narrow at the both end of the beach and in the center where a small stream flows. The annual rate of change in beach width was 0.31 m in the northern part, 0.23 m in the central part, and 0.25 m in the southern part meaning sedimentation. The sediment budget of the watershed was positively correlated with beach width; and this result highlights the importance of proficient watershed management with respect to beach management. It was evident that the artificial structure constructed in the middle part of the beach had an influence on sediment transport, and changes related to the typhoon were evident; these results indicate that shoreline changes are affected by various factors, including the human impact on the watershed and natural conditions, and it is thus necessary to use a spatially integrated management approach that involves all these factors.

Understanding Interactions between Shoreline Changes and Reef Outer Slope Morphometry on Takapoto Atoll (French Polynesia)

Collin, A.; Duvat, V.; Pillet, V.; Salvat, B., and James, D., 2018. Understanding Interactions between Shoreline Changes and Reef Outer Slope Morphometry on Takapoto Atoll (French Polynesia). In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 496–500. Coconut Creek (Florida), ISSN 0749-0208.Low-lying atoll reef islands host populations who are highly exposed to increasing ocean-related pressures resulting from accelerated sea-level rise, ocean warming and acidification. Since 2010, changes in the position of atoll reef island shoreline were estimated on more than 400 Pacific islands using multi-date image analysis. These studies have emphasized island persistence and highlighted the major role of local controls, i.e., human activities and tropical cyclones, in shoreline change. However, they provide limited knowledge on the ecological-morphological linkages driving island change. In order to contribute filling this gap, will be studied the statistical relationships between multi-decadal shoreline change and high-resolution outer slope morphometry of the eastern windward coast of Takapoto Atoll (Tuamotu Archipelago, French Polynesia). Shoreline change was estimated using 1969 airborne and 2013 spaceborne imagery, while outer slope morphometry was mapped at 2 m spatial resolution using 2016 spaceborne imagery. Nine classes of shoreline changes (N=1 402), ranging from >50 m accretion to >50 m erosion, were compared with averages of depth and slope metrics, derived from satellite-based bathymetry mapping. A strong positive correlation between shoreline erosion versus (i) depth and (ii) slope averages was obtained (R2=0.93 and R2=0.96, respectively): the deeper/steeper outer transects are, the more eroded shoreline transects are. Those findings suitably match the theory of wave propagation in shallow/gentle waters describing an increase in cross-shore sediment transfer with shallowness, thus explaining spatial variations in shoreline response. The implications of those original results for reef islands' future under climate change will be discussed.

Modelling the Response of Atoll Reef Islands to Multi-Millennial Sea Level Rise from the Last Glacial Maximum to the Coming 10kyr: the Case of Mururoa Atoll (Tuamotu, French Polynesia)

ABSTRACT Le Cozannet, G.; Duvat, V.; Salvat, B., Etienne, S.; Terorotua, H.; Garcin, M.; Lecacheux, S., and Montaggioni, L., 2018. Modelling the Response of Atoll Reef Islands to Multi-Millennial Sea Level Rise from the Last Glacial Maximum to the Coming 10kyr: the Case of Mururoa Atoll (Tuamotu, French Polynesia). In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 511–515. Coconut Creek (Florida), ISSN 0749-0208. Composed of biodetritic sediments and lying just a few meters above present sea-level, atoll reef islands are liable to be highly exposed to coastal flooding and shoreline erosion. Nevertheless, the analysis of multi-decadal shoreline change has shown that most reef islands either remain stable in area or are expanding within the context of current sea level rise. This article addresses the key issue of future atoll-island persistence using a simple mor...

Multi-decadal shoreline change and beach connectivity in a high-energy sand system

ABSTRACTUnderstanding how and why the world’s coastlines are changing is a pressing international concern in a context of rising sea levels, increased climatic variability and intensifying coastal development. Medium to long-term records of coastal change are rare worldwide and often limited to individual beach compartments. This study presents a 70-year (1940–2010) aerial imagery record to compare decadal changes in shoreline position across four high-energy west coast beaches near Auckland, New Zealand/Aotearoa: Whatipu, Karekare, Piha and Te Henga (Bethells). The common exposure of these adjacent mesotidal beaches to changing wind and wave conditions might, if they present the dominant controls on shoreline position, be expected to produce synchronous change. Whatipu (935 m), Piha (32 m) and Te Henga (52 m) showed net overall progradation for the study period, while Karekare retreated slightly (−4.1 m). All except Whatipu underwent periods of beachwide erosion. Shoreline change was not coherent between beaches, despite similar exposure to variations in wind, wave and sea level. Variable sediment supply from northward littoral drift is implicated as the primary control on decadal-scale shoreline change for these beaches, highlighting the importance of local context in influencing shoreline response to changing environmental conditions.

Multi-decadal shoreline changes on Takú Atoll, Papua New Guinea: Observational evidence of early reef island recovery after the impact of storm waves

Hurricanes, tropical cyclones and other high-magnitude events are important steering mechanisms in the geomorphic development of coral reef islands. Sandy reef islands located outside the storm belts are strongly sensitive to the impact of occasional high-magnitude events and show abrupt, commonly erosive geomorphic change in response to such events. Based on the interpretation of remote sensing data, it is well known that the process of landform recovery might take several decades or even longer. However, despite the increasing amount of scientific attention towards short- and long-term island dynamics, the lack of data and models often prevent a robust analysis of the timing and nature of recovery initiation. Here we show how natural island recovery starts immediately after the impact of a high-magnitude event. We analyze multi-temporal shoreline changes on Takú Atoll, Papua New Guinea and combine our findings with a unique set of published field observations (Smithers and Hoeke, 2014). Trends of shoreline change since 1943 and changes in planform island area indicate a long-term accretionary mode for most islands. Apparent shoreline instability is detected for the last decade of analysis, however this can be explained by the impact of storm waves in December 2008 that (temporarily?) masked the long-term trend. The transition from negative to positive rates of change in the aftermath of this storm event is indicative of inherent negative feedback processes that counteract short-term changes in energy input and represent the initiation of island recovery. Collectively, our results support the concept of dynamic rather than static reef islands and clearly demonstrate how short-term processes can influence interpretations of medium-term change.

Multi-decadal shoreline changes in response to sea level rise in the Marshall Islands

Abstract Low-lying reef islands are considered highly vulnerable to the impacts of climate change. Accelerating rates of sea level rise as a result of anthropogenic climate change are expected to destabilise islands and threaten to render entire nations uninhabitable. Using historic aerial photographs and recent high-resolution satellite imagery, shoreline changes on six atolls and two mid-ocean reef islands in the Republic of the Marshall Islands were analysed. Results reveal that since the middle of the 20th century more shoreline has accreted than eroded, with 17.23% showing erosion, compared to 39.74% accretion and 43.03% showing no change. The net result of these changes was the growth of the islands examined from 9.09 km2 to 9.46 km2 between World War Two (WWII) and 2010. Analyses of shoreline changes since the 1970s show that shorelines are accreting albeit at a slower rate, with rates of change between the 1970s and 2010 of 0.29 m/dec compared with 0.77 m/dec between WWII and 1970s. The observed shoreline changes occur in the context of locally rising sea level. As sea level continues to rise there is a critical need for regular monitoring of reef islands in order to better understand the spatio-temporal variability of reef island change and guide future adaptation efforts within atoll nations.

Wave energy fluxes and multidecadal shoreline changes in two coastal embayments in Denmark

Spatial patterns of multidecadal shoreline changes in two microtidal, low-energetic embayments of southern Zealand, Denmark, were investigated by using the directional distribution of wave energy fluxes. The sites include a barrier island system attached to moraine bluffs, and a recurved spit adjacent to a cliff coast. The barrier island system is characterized by cross-shore translation and by an alignment of the barrier alongshore alternating directions of barrier-spit progradation in a bidirectional wave field. The recurved spit adjacent to the cliff coast experienced shoreline rotation through proximal erosion and distal lateral accretion in a unidirectional wave climate. The multidecadal shoreline changes were coupled to a slope-based morphological coastal classification. All erosive shores occurred within a narrow range of onshore and offshore coastal slopes. The alongshore variability of directional distributions of wave energy fluxes furthermore outlined potential sediment sources and sinks for the evolution of the barrier island system and for the evolution of the recurved spit.

Multidecadal Shoreline Changes in Denmark

Kabuth, A.K.; Kroon, A., and Pedersen, J.B.T., 2014. Multidecadal shoreline changes in Denmark. Multidecadal shoreline changes along ca. 7000 km coastline around Denmark were computed for the time interval between 1862 AD and 2005 AD and were connected with a geomorphological coastal classification. The shoreline data set was based on shoreline positions from historical and modern topographic maps. Coastal landforms were identified on a digital terrain model in combination with aerial photographs. Two shoreline-change computation methods were evaluated at a test site, aiming for optimized time efficiency and accuracy of the countrywide application: a Nearest Neighbor search and a cross-shore transect method based on the ArcGIS-based Digital Shoreline Analysis System (DSAS). The cross-shore transect method was more robust and performed better in the detection of local extremes in shoreline changes, which was crucial for the scope of the mapping. Countrywide shoreline-change distances and rates were, therefore, computed with the DSAS method. Patterns in coastline dynamics were identified through the connection of shoreline-change rates with the occurrence of coastal landforms. Short-term changes and alterations of shoreline evolution through coastal structures were not resolved in this study. Because of the long time span covered, the relative errors originating from data and method are acceptable. The scope of the mapping was to provide a coastal management tool that allows screening for critical sites with respect to coastal erosion. As the first countrywide quantification of historical shoreline changes around Denmark, the mapping can contribute to enhanced adaptation and mitigation strategies in response to increased risks of erosion and flooding under a changing climate.

Nearshore shore-oblique bars, gravel outcrops, and their correlation to shoreline change

This study demonstrates the physical concurrence of shore-oblique bars and gravel outcrops in the surf zone along the northern Outer Banks of North Carolina. These subaqueous features are spatially correlated with shoreline change at a range of temporal and spatial scales. Previous studies have noted the existence of beach-surf zone interactions, but in general, relationships between nearshore geological features and coastal change are poorly understood. These new findings should be considered when exploring coastal zone dynamics and developing predictive engineering models. The surf zone and nearshore region of the Outer Banks is predominantly planar and sandy, but there are several discrete regions with shore-oblique bars and interspersed gravel outcrops. These bar fields have relief up to 3 m, are several kilometers wide, and were relatively stationary over a 1.5 year survey period; however, the shoreward component of the bar field does exhibit change during this time frame. All gravel outcrops observed in the study region, a 40 km longshore length, were located adjacent to a shore-oblique bar, in a trough that had width and length similar to that of the associated bar. Seismic surveys show that the outcrops are part of a gravel stratum underlying the active surface sand layer. Cross-correlation analyses demonstrate high correlation of monthly and multi-decadal shoreline change rates with the adjacent surf-zone bathymetry and sediment distribution. Regionally, areas with shore-oblique bars and gravel outcrops are correlated with on-shore areas of high short-term shoreline variability and high long-term shoreline change rates. The major peaks in long-term shoreline erosion are onshore of shore-oblique bars, but not all areas with high rates of long-term shoreline change are associated with shore-oblique bars and troughs.