Coastline Retreat Research Articles

Intertidal assemblages on groynes along sandy shores in Portugal: Exploring the effects of orientation and distance to rocky shore

Coastal erosion is becoming increasingly problematic as sea level rise and coastal areas become more urbanised. In response, more defence structures such as groynes are being built, which are crucial for counteracting sediment loss and coastline retreat. Despite worldwide use, comparatively little is known about the factors that determine the species composition on such structures. In this study, the composition and abundance of intertidal benthic species on groynes was investigated as a function of groyne orientation (North vs. South) and distance to natural rocky shores (5 km vs. 42 km). While orientation showed no effect on benthic assemblages, distance to rocky shores was identified as a key factor influencing the assemblage composition. Macroalgae were found in greater abundance further away from rocky shores, while snail and barnacle species were found in greater numbers closer to rocky shores.

Past and Future Impacts of the Relative Sea Level Rise on the Seafront of Ancient Delos (Cyclades, Greece) and Flooding Scenarios by 2150

Sea level rise due to global warming is a continuing and, disappointingly, accelerating process which has already affected and will further impact coastal lowlands and the social and economic activities in these areas. Delos Island, situated in the middle of the Cyclades in the Aegean Sea, was considered the most sacred of all islands in ancient Greek culture and was a trading hub for the entire eastern Mediterranean. Uninhabited since the 7th century AD, and consistently the focus of research and touristic attention, the island is designated as an archaeological site and inscribed on the UNESCO World Heritage List. Previous studies on the relative sea level (rsl) changes suggest a steadily rising rsl during the last 6300 years, starting from a sea level of −4.80 ± 0.20 m in the Late Neolithic. The seafront of the ancient city of Delos is subject to the effects of rsl rise, which have caused significant coastline retreat and exposure to the northerly winds and waves, whereas parts of the coastal lowland, where the remains of the ancient city lie, are inundated, forming extended wetlands. The future impacts of rsl rise on the seafront of ancient Delos are illustrated on very-high-resolution digital surface models, evaluating both the flooding risk under different climatic projections, as provided by the IPCC AR6 report, and the ongoing land subsidence, as recorded by GNSS data. An rsl rise ranging from 87 cm (SSP1-2.6 scenario) to 148 cm (SSP5-8.5 scenario) is anticipated by 2150, requiring both resilience strategies and adaptation solutions as well as mitigation policies to cope with the effects of climate change.

Coastal Retreat on the Spanish Mediterranean Coast in a Climate Change Context: Effects of the Regulation of the Segura River at Its Mouth and the Coastal Sand Dune in Guardamar del Segura (Alicante, Spain)

Coastal retreat processes are usually associated with many anthropogenic actions, such as the regulation of river basins, the construction of hydraulic storm defence works in coastal areas and the building of housing on the beach. To all of this, we should also add the increase in sea levels due to the effect of climate change. The chosen area of study corresponds to the coastal area of the municipality of Guardamar del Segura, belonging to the Segura River Basin. The methodology applied in this study comprised the gathering of historical information, the extraction of data using GIS, the compiling of data using official organisations and the analysis of all these data from a geographical perspective. The obtained results show the chronology of the regulation works in the Segura Basin and their relationship with the reduction and negative trend in average ordinary flows (1940–2023) and the extraordinary, swelled flows recorded in the period 1994–2023. Furthermore, the coastlines from 1923 to 2023 were mapped, enabling us to determine the evolution of the coastline retreat processes experienced in the dune ridge of Guardamar del Segura and the increase in the frequency of impacts due to storms on Babilonia Beach. Finally, data on wind, waves and marine currents recorded at a gauging station were incorporated, enabling us to understand their impact on this coastal sector. The results obtained are discussed, and they indicate the need to incorporate data on sediment into the study in order to complete it. The conclusions reveal the existence of a relationship between all these anthropogenic elements in the beach erosion processes experienced in the village of Guardamar del Segura.

Simulation of a Wave Hydrodynamic Numerical Model on a Single Vertical Porous Breakwater

One of the problems in the coastal area is a change in the coastline in the form of a coastline retreat which results in the loss or damage of infrastructure and settlement facilities caused by extreme sea wave activity. One way to anticipate this problem is to build a breakwater. This research is focused on the effect of porosity and freeboard on the transmission and reflection of waves in a single vertical porous breakwater. Numerical modeling was done using a 3D Numerical Wave Tank (NWT) based on Smoothed Particle Hydrodynamic (SPH). The numerical model results show that the average transmission and reflection coefficients are 0.58 and 0.42. In addition, it can be seen that the freeboard and porosity influence the value of the transmission and reflection coefficients. Generally, the smaller the freeboard, the greater the transmission coefficient and the smaller the reflection coefficient. Meanwhile, the greater the porosity, the greater the transmission coefficient, and vice versa, the smaller the reflection.

Formation and evolution of barrier coast under condition of sand scarcity; an example from the Polish coast of the southern Baltic sea

The formation and evolutionary history of intracontinental, tideless sea barrier coasts is still poorly known in comparison to open-ocean and tidal sea barrier systems. This paper describes the morpho-geological structure of the barriers and its substrate along a 90-km section of the middle part of the Polish southern Baltic coast. The main goal of the study is a better insight into the evolution of the barriers in case of sand scarcity and relations to sea level changes. The study area was examined by analysis of the published and historical data from 120 borehole profiles supplemented by new 18 core drillings. In addition to lithological and biostratigraphical data, a large set of radiocarbon ages was acquired and analysed. The barriers during its whole middle and late Holocene history were retrogradational despite of varied geological and morphological features of its substrate. The average rate of barrier's retreat has shown considerable variation in a close relation to the rates of sea-level rise. During the period 8500-7500 yr b2k when the sea level rose by 6–5 mm/yr, the coast retreated ca. 4.6–3.5 m/yr. Between 7500 and 6000 yr b2k, the sea-level rise slowed down to ca. 1.7 mm/yr and the rate of coastline retreat fell to ca. 1.0 m/yr. During the last 6000 years, coastal retreat slowed down to ca. 0.3 m/yr as the average sea level rise was ca. 0.5 mm/yr. The permanent retrogradational history of the barrier coast, even in the case of sand availability in the barrier's substrate, is explained by the existence of a divergence zone of longshore currents and a related source zone for the alongshore sandy streams. The divergence zone is present within the study area not only today, but it has existed most probably also at least since the beginning of the late Holocene. As a result, the erosional trend (sand scarcity) dominates here, which contributes to the presence of retrogradational barriers.

EROSION PROCESS IN THE CARIBBEAN SANDY BEACHES

Beach erosion is a widespread phenomenon on Caribbean beaches, with rates of coastline retreat between 2.2 and 0.5 m/year and higher reports in some areas. Different investigations indicate the combination of natural and anthropogenic causes and highlight the current influence of climate change related sea level rise. It is recorded as a generalizing trend the landward displacement of the beach profile, with sand transfer above the dunes and sand bars in a process that fits better the model proposed by Dean and Maurmeyer (1983) than the recognized model by Brunn (1962).

Shoreline Change Analysis of the Eastern Coast of Ghana between 1991 and 2020

The Eastern Coastline of Ghana is facing intense natural and anthropogenic disturbances, which pose a serious threat to the coastal community, ecosystem, and livelihoods. This study assessed the shoreline changes occurring along the Eastern Coast of Ghana stretching 149 km from Laloi Lagoon West of Prampram to Aflao, Ghana. The study utilizes satellite images from Landsat 4TM, Landsat 7 ETM+, and Landsat 8 OLI taken between 1991 and 2020. Data pre-processing techniques using ENVI 5.3 included calibration, layer stacking, mosaicking, and supervised classification. Post-classification shorelines were extracted using ArcGIS 10.7, and the DSAS tool was used to determine the rate of change over the 29-year period. The results showed that the coastline experienced an average erosion rate of 9 m/y and a maximum rate of 24 m/y, however, the accretion rate (3 m/y) was much lower, reflecting general coastline retreat. Thus, some 25 coastal communities are highly exposed to shoreline erosion. Sustaining the coastal area may require coastline re-engineering interventions. This study recommends continuous monitoring of the shorelines to ensure the protection of livelihoods. Implementation of both hard engineering and ecosystem-based adaptation strategies may be required to achieve holistic results toward sustainable coastal management.

Topo-bathymetric behaviour of a beach controlled by a groyne field and a dune-seawall backshore

The advances in morphodynamics numerical modelling and the free availability of such updated models have made XBeach a powerful tool to predict the hydrodynamic and morphologic processes on sandy coasts. This paper addresses the impact of the sea-structure-sediments interactions in the morphological evolution of a coastal stretch which undergoes erosion and overtopping despite having a combined groyne field-dune-seawall defence scheme. Preliminary tests, tailored to conclude on the model mode-version to be applied in the case study, evidence the importance of considering the NH mode. After calibrated and validated, the model was applied for a set of cases to compute and discuss the effect of the individual defence scheme structural elements and the hydrodynamic driving factors, namely the wave energy and obliquity and the tidal range, on the morphodynamics. The results revealed that the seawall aggravates the erosion of the beach, despite being essential in the protection against overtopping and coastline retreat, and that the present configuration of the groyne field does not provide enough protection at the downdrift part of the stretch. The knowledge acquired can support the interpretation of the topo-bathymetric behaviour in similar coastal defence schemes existent worldwide.

Influence of Local Factors on Coastal Erosion: The Case of Vagueira Beach in Portugal

Vagueira Beach, on the central Portuguese coast, is known as one of the places in Europe most affected by coastal erosion. The area has suffered more than 156 m of coastline retreat from the period 1958 to 2001. With the aim of evaluating the influence of local factors on coastal erosion, this paper assesses the anthropogenic and natural factors that are related to the retreat of the coastline by adopting statistical correlation and regression analyses. Through Pearson’s correlation coefficient (r), it was observed that local factors, such as annual dredging at the Aveiro Port entrance (r = 0.93), the total length of groins in the Espinho–Vagueira section (r = 0.89), and storm events (r = 0.52), are directly related to coastline retreat in the area. A multiple linear regression model was developed in which coastline retreat is explained by these same factors over the period 1980–2006. With a coefficient determination of R2 = 0.91, it was observed that the length of groins (significant at the 1% level), the dredging of the port entrance (significant at the 5% level), and precipitation (as a proxy for storm events; significant at the 10% level) are significantly correlated with coastline retreat. Hence, it is shown that anthropogenic factors are the main drivers of coastline retreat in Vagueira Beach. This study provides an innovative approach for the assessment of coastal erosion, resulting in important information that could be used for decision-making related to coastal zone management as it allows us to understand in greater detail the main drivers of coastal erosion.

A new perspective on the impacts of Spartina alterniflora invasion on Chinese wetlands in the context of climate change: A case study of the Jiuduansha Shoals, Yangtze Estuary

Spartina alterniflora, an invasive plant, was introduced to the Chinese coastal zone in the early 90s. As an eco-engineering species, S. alterniflora not only alters saltmarsh species distributions, previously described as habitat degradation, but it also plays a vital role in coastal protection, especially for the development of recently emerged intertidal shoals. To provide a reference for coastal management under global change, we quantified the impact of the invasion process on provided ecological and coastal protection functions, exemplified at the emerging Jiuduansha Shoals (JDS) in the Yangtze Estuary. Results obtained by high-precision satellite monitoring and numerical modelling showed that the establishment and growth of S. alterniflora can exert considerable changes on local environment. The invasion of S. alterniflora to JDS wetland can be divided into three distinct phases, (1) establishment 1998–2003, (2) expansion 2003–2009, and (3) dominant 2009–2018 stages according to the changes in saltmarsh composition. Spatially, S. alterniflora continuously replaced Scirpus mariqueter, forcing S. mariqueter and Phragmites australis slowly to the lower and higher intertidal habitats, respectively. Notably, S. alterniflora expansion was the main driver that contributed to over 70 % of recent JDS wetland expansion even under sediment deficit conditions. Established S. alterniflora marsh (directly) dampens more waves because of aboveground stems, but it also causes more accretion and indirectly leads to higher “morphological” wave dampening. Thus, it increases coastal defense provided by the saltmarsh in the context of sea-level rise and strengthening storms. In conclusion, the role of S. alterniflora invasion to the local environment under global changes is controversial. For sustainable coastal management, we need context-dependent S. alterniflora management to maximize the benefit of coastal protection and minimize the impact on local ecology, especially in sediment-starving estuaries with expected coastline retreat.

An Approach for the Validation of a Coastal Erosion Vulnerability Index: An Application in Sicily

In recent decades, coastal erosion phenomena have increased due to climate change. The increased frequency and intensity of extreme events and the poor sediment supply by anthropized river basins (dams, river weirs, culverts, etc.) have a crucial role in coastal erosion. Therefore, an integrated analysis of coastal erosion is crucial to produce detailed and accurate coastal erosion vulnerability information to support mitigation strategies. This research aimed to assess the erosion vulnerability of the Sicilian coast, also including a validation procedure of the obtained scenario. The coastal vulnerability was computed by means of the CeVI (Coastal Erosion Vulnerability Index) approach, which considers physical indicators such as geomorphology and geology, coastal slope, sea storms, wave maxima energy flux and sediment supply to river mouths. Each indicator was quantified using indexes which were assessed considering transects orthogonal to the coastline in 2020. These transects were clustered inside natural compartments called littoral cells. Each cell was assumed to contain a complete cycle of sedimentation and not to have sediment exchange with the near cells. Physical parameters were identified to define a new erosion vulnerability index for the Sicilian coast. By using physical indexes (geological/geomorphological, erosion/sediment supply, sea storms, etc.), the CeVI was calculated both for each littoral cell and for the transects that fall into retreating/advancing coastal areas. The vulnerability index was then validated by comparing CeVI values and the coastline change over time. The validation study showed a direct link between the coastline retreat and high values of CeVI. The proposed method allowed for a detailed mapping of the Sicilian coastal vulnerability, and it will be useful for coastal erosion risk management purposes.

Analysis The Impact of The Coastal Protection Construction Development Plan On Shoreline and Estuary Changes

This beach, which is located in Aceh Province, especially Bireuen Regency, Jangka District, Kuala Ceurape, is an tourist beach area, aquaculture,e and also a settlement. Several efforts to control sedimentation and erosion have been carried out by constructing coastal protection buildings on the coast. However, in the estuary, there are no coastal protection structures such as jetties. Estuaries have quite high economic value because they function as connecting channels between sea and land. The height of the wave currents around the estuary can cause the accumulation of sediment or silt of the beach by erosion which can result in decreased utilization of the coast around the estuary. Therefore, it is necessary to analyze the impact of coastal protection construction planning to determine the formation of sedimentation and erosion that occurs around the coast. This study used the Delft3D application to simulate changes in coastlines and estuaries over a period of 5 years. The data used in this study are wind data for 10 years from 2012 to 2021, depth data, tidal component data, factor morphology calculations, significant wave height calculations (Hs,), and significant wave periods (Ts). This simulation uses 4 (four) wind directions, namely North, Northeast, East, and Northwest directions. The results of modeling simulations over a period of 5 years show that the beach condition with the planned construction of the coastal protection has progressed along the coastline by ± 77,053 meters and progress of around ± 77.421 meters on the coast. On the contrary, it experienced a coastline retreat of ± 143.076 meters and experienced a shoreline advance of around ± 84.647 meters in the estuary. So it can be concluded that the construction of the Jetty building alone is not enough to reduce the occurrence of erosion and sedimentation problems around the coast, especially in the estuary. There is a need for the construction of other coastal protection structures.

Migration and Diffusion of Surface Sediments in Bohai Bay: Evidence from Grain Size and Elements

Grain size and element content of surface sediment from Bohai Bay were analyzed to study the sediment migration and diffusion based on grain size trend analysis (GSTA) and discriminant function (DF). The sediment in the southern, central and western part of Bohai Bay mainly originates from the Yellow River, while that in the northern part of Bohai Bay mainly originates from the Luanhe River. The influence boundary between the Yellow River and the Luanhe River is estimated to be at 38°50′ N. In both the southern and northern parts of Bohai Bay, sediment is transported into the bay under the influence of prevailing waves, strongest waves and tidal remnants, resulting in sediment accumulation in western Bohai Bay. The coastal sediment of Bohai Bay is generally in a state of offshore movement, which is consistent with the large-scale coastline retreat in past decades found by previous studies.

Late Holocene Morphodynamic Feedback in Can Gio Mangrove Tide-Dominated River Mouth Systems, Vietnam

Can Gio (CG), a mangrove forest with a dense network of tidal creeks, gradually grew and spread seaward on a coastal platform, which was built since about 8 ka before present (BP). Along with this development, a sand ridge began to form and moved back with the shoreline withdrawal landward in the Late Holocene. This mangrove environment is likely abandoned from the mainland but was, however, the place for settlement of ancient Vietnamese a few centuries BC. The CG mangrove forest was severely destroyed during the American War and was restored since 1980. However, the historical change of the landscape along the Saigon-Dong Nai River (SG-DNR) since the Late Holocene is not completely unraveled. By analyzing sediment cores with a multiproxy approach, we investigated the recent geological development with regard to the variation of the intensity of the East Asian palaeomonsoon and regarding the accommodation space, as both regulate the development of this coastal environment. A recently significant shift in the coastline, mainly due to a change of hydroclimatic factors, was observed. A continuous coastline retreat occurred over the last millennium, changing the depositional environment and reshaping the CG mangrove landscape. Along the present coast and tidal channels, partially strong erosion and bank failures occur, alternating with accretion at other coastal sections. This development tends to increase progressively.

Global raster dataset on historical coastline positions and shelf sea extents since the Last Glacial Maximum.

Historical changes in sea level caused shifting coastlines that affected the distribution and evolution of marine and terrestrial biota. At the onset of the Last Glacial Maximum (LGM) 26 ka, sea levels were >130 m lower than at present, resulting in seaward-shifted coastlines and shallow shelf seas, with emerging land bridges leading to the isolation of marine biota and the connection of land-bridge islands to the continents. At the end of the last ice age, sea levels started to rise at unprecedented rates, leading to coastal retreat, drowning of land bridges and contraction of island areas. Although a growing number of studies take historical coastline dynamics into consideration, they are mostly based on past global sea-level stands and present-day water depths and neglect the influence of global geophysical changes on historical coastline positions. Here, we present a novel geophysically corrected global historical coastline position raster for the period from 26 ka to the present. This coastline raster allows, for the first time, calculation of global and regional coastline retreat rates and land loss rates. Additionally, we produced, per time step, 53 shelf sea rasters to present shelf sea positions and to calculate the shelf sea expansion rates. These metrics are essential to assess the role of isolation and connectivity in shaping marine and insular biodiversity patterns and evolutionary signatures within species and species assemblages. The coastline age raster contains cells with ages in thousands of years before present (bp), representing the time since the coastline was positioned in the raster cells, for the period between 26 ka and the present. A total of 53 shelf sea rasters (sea levels <140 m) are presented, showing the extent of land (1), shelf sea (0) and deep sea (NULL) per time step of 0.5 kyr from 26 ka to the present. The coastline age raster and shelf sea rasters have a global representation. The spatial resolution is scaled to 120 arcsec (0.333° × 0.333°), implying cells of c.3,704 m around the equator, 3,207 m around the tropics (±30°) and 1,853 m in the temperate zone (±60°). The coastline age raster shows the age of coastline positions since the onset of the LGM 26 ka, with time steps of 0.5 kyr. The 53 shelf sea rasters show, for each time step of 0.5 kyr, the position of the shelf seas (seas shallower than 140 m) and the extent of land. Both the coastline age raster and the 53 shelf sea rasters are provided as TIFF files with spatial reference system WGS84 (SRID 4326). The values of the coastline age raster per grid cell correspond to the most recent coastline position (in steps of 0.5 kyr). Values range from 0 (0 ka, i.e., present day) to 260 (26 ka) in bins of 5 (0.5 kyr). A value of "no data" is ascribed to pixels that have remained below sea level since 26 ka. All data processing was done using the R programming language.

Evolution of blowouts in artificial foredunes on Pingtan Island, China

AbstractThe appearance of a blowout can be an obvious sign of accelerated coastline retreat or dune erosion and in response to serious coastal erosion, an increasing number of artificial foredunes are being built. However, over time, numerous blowouts have also developed on artificial foredunes. To our knowledge, no studies have been reported on the evolution of blowouts in artificial foredunes, and in response we explore the aerodynamics and morphodynamics of artificial blowouts at different stages, by conducting field experiments on the Changjiang Ao coast of Pingtan Island, China. Where a foredune was built on an artificial clay structure, with topography and wind fields around three blowouts in different developmental stages measured, preliminary results revealed that: (1) the blowout morphology and airflow patterns show distinct seasonal changes; (2) the development of blowouts in artificial foredunes involves five stages, in contrast to the four stages observed in natural foredunes; and (3) blowout development is controlled by a two‐way feedback between aeolian processes and blowout morphological development, and the artificial structure can interfere with this feedback mechanism by altering the path of blowout development. The findings enhance our understanding of geomorphic process–response systems in artificial foredunes and provide a reference for coastal management and dune stabilization policy and practice.

A coupled model for sediment transport dynamics and prediction of seabed morphology with application to 1DH/2DH coastal engineering problems

Coastline retreat poses a threat to nearshore environment and the assessment of erosion phenomena is required to plan the coastal engineering works. The hydro-morphodynamic response of a beach to natural and artificial forcing factors differ considerably, as the nearshore processes are especially complex and depended on a multitude of parameters, including prevailing wave and hydrodynamic conditions, beach topography, sediment characteristics and the presence of coastal protection works. The present study serves the purpose of numerically evaluating nearshore morphological processes and ultimately assessing the capacity of coastal defence structures to control beach erosion. For this reason, a new sediment transport model including unsteady effects and swash zone morphodynamics, was coupled to the highly nonlinear Boussinesq wave model FUNWAVE-TVD, providing integrated predictions of bed level evolution, across various timescales of interest. The compound model was validated thoroughly against laboratory data and other numerical investigations. Overall, a good agreement between experimental and numerical results was achieved for a number of test cases, investigating the effects of different types of shore protection structures. The proposed integrated model can be a valuable tool for engineers and scientists desiring to obtain accurate bed level predictions, over complex mildly and steeply sloping sea bottoms composed of non-cohesive sediment particles.

Flooding of Sandy Beaches in a Changing Climate. The Case of the Balearic Islands (NW Mediterranean)

The fate of the beaches around the world has paramount importance as they are one of the main assets for touristic activities and act as a natural barrier for coastal protection in front of marine storms. Climate change could put them at risk as sea levels rise and changes in the wave characteristics may dramatically modify their shape. In this work, a new methodology has been developed to determine the flooding of sandy beaches due to changes in sea level and waves. The methodology allows a cost-effective and yet accurate estimation of the wave runup for a wide range of beach equilibrium profiles and for different seagrass coverage. This, combined with regional projections of sea level and wave evolution, has allowed a quantification of the future total water level and coastline retreat for 869 beaches across the Balearic Islands for the next decades as a function of greenhouse gases emission scenario. The most pessimistic scenario (RCP8.5) at the end of the century yields an averaged percentage of flooded area of 66% under mean conditions which increases up to 86% under extreme conditions. Moreover, 72 of the 869 beaches of the region would permanently disappear while 314 would be completely flooded during storm episodes. Under a moderate scenario of emissions (RCP4.5), 37 beaches would permanently disappear while 254 would disappear only during storm episodes. In both cases, the average permanent loss of beach surface at the end of the century would be larger than 50%, rising over 80% during storm conditions. The results obtained for the Balearic Islands can be extrapolated to the rest of the Mediterranean as the beaches in all the regions have similar characteristics and will be affected by similar changes in sea level and wave climate. These projections indicate that adaptation plans for beach areas should be put in place as soon as possible.

Enhanced Weathering and Erosion of a Cohesive Shore Platform Following the Experimental Removal of Mussels

The organisms inhabiting intertidal platforms can affect their weathering and erosion rates. Research on biotic influences on platform integrity has traditionally emphasized the role of bioeroders (i.e., organisms that scrap or bore into platforms via mechanical and chemical means). Yet, recent studies illustrate that covers of sessile organisms on the surfaces of intertidal platforms can have bioprotective effects by reducing the efficacy of physical weathering and erosion agents. Eroding cliffs fronted by cohesive shore platforms are a pervasive feature along the continental Argentinean coastline (37–52°S). In this study, we investigated how mussel (Brachidontes rodriguezii) cover mediates weathering and erosion of a cohesive, consolidated silt platform at Playa Copacabana (5 km north of Miramar, Buenos Aires Province; 38° 14′ S, 57° 46′ W). By means of mussel removal experiments, we found that mussel cover attenuates variations in platform surface temperatures, enhances moisture retention during low tide, reduces rates of salt crystallization within the pores of the platform material, and attenuates hydrodynamic forcing on the platform surface. Mussel removal also led to a 10% decrease in surface hardness and a 2-mm reduction in platform height after 5 months. Collectively, our findings indicate that mussel beds limit substrate breakdown via heating-cooling, wetting-drying, and salt crystallization and provide some of the first experimental field evidence for the direct impacts of biotic cover on platform erosion. As intertidal platforms protect the cliffs behind from the hydraulic impact of waves, which may be enhanced with future sea-level rise, we posit that the protection of platforms by mussels indirectly moderates coastline retreat, especially on soft cohesive shores.

Coastal Flooding in the Balearic Islands During the Twenty-First Century Caused by Sea-Level Rise and Extreme Events

Sea-level rise induces a permanent loss of land with widespread ecological and economic impacts, most evident in urban and densely populated areas. Potential coastline retreat combined with waves and storm surges will result in more severe damages for coastal zones, especially over insular systems. In this paper, we quantify the effects of sea-level rise in terms of potential coastal flooding and potential beach erosion, along the coasts of the Balearic Islands (Western Mediterranean Sea), during the twenty-first century. We map projected flooded areas under two climate-change-driven mean sea-level rise scenarios (RCP4.5 and RCP8.5), together with the impact of an extreme event defined by the 100-year return level of joint storm surges and waves. We quantify shoreline retreat of sandy beaches forced by the sea-level rise (scenarios RCP4.5 and RCP8.5) and the continuous action of storm surges and waves (modeled by synthetic time series). We estimate touristic recreational services decrease of sandy beaches caused by the obtained shoreline retreat, in monetary terms. According to our calculations, permanent flooding by the end of our century will extend 7.8–27.7 km2 under the RCP4.5 scenario (mean sea-level rise between 32 and 80 cm by 2100), and up to 10.9–36.5 km2 under RCP8.5 (mean sea-level rise between 46 and 103 cm by 2100). Some beaches will lose more than 50% of their surface by the end of the century: 20–50% of them under RCP4.5 scenario and 25–60% under RCP8.5 one. Loss of touristic recreational services could represent a gross domestic product (GDP) loss up to 7.2% with respect to the 2019 GDP.