Coastal Retreat Research Articles

Unforeseen cascading effects of an inlet opening

The opening of the Cacela Inlet (southern Portugal) in 2010 led to unforeseen effects observed after 2017, including an extreme acceleration of the retreat of the inland lagoon margin from about 0.2 to 2 m/year. This was a consequence of the development of a large flood delta in an area of limited accommodation space, forcing the main tidal channel to move inland. The coastal retreat currently affects a flat sandy area that separates the old and inactive Cacela cliff from the lagoon. Between 2035 and 2040, the currently inactive Cacela cliff is likely to become active again, posing a potential risk of damage to a medieval fortress and the existing settlement of Cacela Velha, an unforeseen cascading effect of the opening of the inlet. In order to prevent instability and damage to this legally protected area of national and public interest, several coastal management measures will be required.

Onshore sediment transport enhancement and evolution of bedforms: Laboratory experiments of beach ploughing

Progressive coastal retreat has been an issue exacerbated in recent years due to climate change. Sand is eroded from beaches during the winter and partially recovered during summer by slow accretion processes. The development of new working with nature techniques that produce enhanced beach accretion could help recover most of the sand lost during winter and thus reduce the impact of climate change on beaches. The presence of bedforms contribute to increasing onshore sediment transport, but few studies have been performed to quantify their effect. In this study, the evolution and effect of artificially created bedforms on onshore sediment transport were analysed in prototype-scale laboratory experiments. The tested bedforms mimicked a beach ploughing of the intertidal area, with a wavelength of 1.6 m and height of 0.25 m, corresponding to the ploughing dimensions that a tractor can perform. Two tests were performed with the same initial morphology, medium sand (D50 = 0.318 mm), sea state conditions (Hs = 0.3 m, Tp = 7 s) that produced accretion, and different water levels that represent two tidal states. The experimental flume was longitudinally split into two equal channels of 1 m wide, allowing the simultaneous simulation of a natural control geometry and a ploughed geometry, facilitating the comparison and assuring the very same sea conditions. The presence of ploughed bedforms produced two effects: (1) an acceleration of natural accretion rates reaching 40%, and (2) onshore sediment transport due to the migration of the bedforms. The acceleration of natural accretion was explained by the extra bottom roughness induced by the bedforms, which produced more wave dissipation through bottom friction and thus more accretive conditions. The ploughed height decreased exponentially as waves broke over the crest of the ridges, which almost disappeared after 2–3 h of wave action. As a result, the extra bottom roughness also decreased as time passed. Consequently, the nature-assisted beach enhancement technique of ploughing should be applied at each low tide to produce a cumulative effect. Plough bedforms migrated onshore at a rate of approximately 0.2 m/h during the first hour, mobilizing onshore up to 61 kg m−1 h−1 of sediment. Ripples appeared on the tops of the ridge crests and migrated faster onshore, contributing to the migration of the ploughed bedforms. These results demonstrated the importance of considering bedforms while studying accretion processes and the potential of ploughing as an innovative strategy of working with nature to enhance beach recovery.

Detection of Shoreline Changes With Automated Water Extraction Index (AWEI) In Galur District, Kulon Progo Regency

Abstract Monitoring shoreline change is one of the efforts in the management and protection of the coastal environment. Through digital image processing as the utilization of shoreline extraction index can be used as a reference in measuring the rate of shoreline change. Galur District is one of the sub-districts in Kulon Progo Regency that experiences the highest abrasion process, the worst abrasion that has occurred in recent years is located at Trisik Beach where abrasion threatens the existence of stalls, turtle conservation sites and also coastal vegetation that grows along the beach. The purpose of this study is to determine changes in the coastline starting in 2013, 2018, and 2023 and also to analyze numerically through DSAS the rate of change in the coastline both the influence of abrasion and accretion processes in Galur District. The method used in this research is Quantitative Descriptive assisted by Landsat 8 image transformation using Automated Water Extraction Index (AWEI) and measuring the rate of shoreline change using DSAS based on EPR and NSM values. The results of this study show that Banaran Village has the highest shoreline retreat changes where in 2013 - 2018 the shoreline retreat reached 70 meters and in 2018 - 2023 the coastal retreat reached -75.7 meters, while for the rate of change in Karang Sewu Village in 2013 - 2018 the shoreline retreat reached -33.2 meters and in 2018 - 2023 showed the highest abrasion of -33.5 meters.

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.

Coastal retreat and sea-cliff dynamic on the North Atlantic coast (Gerra Beach, Cantabrian Coast, Spain)

Gerra is small beach located at the foot of a cliff on the Western coast of Cantabria (North of Iberian Peninsula). It has a sandy distal part and a proximal part at the foot of the cliff made up of boulders, supported by rockfalls and landslides from the cliff. Claystones, limestones and marls of the Triassic and Cretaceous ages form the cliff. To study geomorphological processes, a geomorphological map was drawn up; analysis of the beach deposits as texture, granulometry and Atterberg limits; analysis of photogrammetric flights between 1957 and 2017; drone flights between 2017 and 2020; and geomatic monitoring of the cliff and the beach by means of a terrestrial laser scanner (TLS) between 2012 and 2019. The processes involved in the coastal dynamic and retreat of the cliff and beach have been established, as well as the rates of erosion and retreat, which coincide with other beaches on the Cantabrian Coast. The current dynamics are characterized by the moderate regression of the coastline and greater regression and dynamism at the cliff-top, which implies the gradual degradation of the cliff due to loss of verticality. Among the factors involved in the cliff degradation, the main changes in the present and future evolution of the sea cliff can only be ascribed to sea level rise.

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.

(Un)managed retreat? Perspectives on biophysical and social dynamics of coastal retreat on O‘ahu, Hawai‘i

As an island community with world famous beaches, Hawai‘i’s coastal managers face significant challenges in the face of sea level rise (SLR). Shoreline laws have recently been updated to limit further shoreline hardening, making Hawaiʻi a salient case study to understand the challenges and opportunities around managed retreat. We interviewed 42 private sector, civil society, and government actors involved with coastal governance and anchored our conversations to three distinct communities on O‘ahu. Factors that affected participant perspectives on the viability of retreat per site hinged on: 1) physical/natural barriers and opportunities; 2) perceptions of the public value of a beach; 3) the importance of existing infrastructure; and 4) the political influence of coastal homeowners. Participants’ views on who should inform the coastal planning process moving forward varied by site. In addition to residents, there was largely agreement that communities should include those with affinity and long-standing relationships with place.

Coastal response to Holocene Sea-level change: A case study from Singapore

The deceleration of early to mid-Holocene (10–7 cal. ka BP) relative sea-level rise (RSLR) played a key role in transforming coastal systems from estuaries to deltas. Palaeoenvironmental reconstruction of coastal evolution provide case studies that can help project the response of modern coastal systems to future RSLR. The response of deltas to future RSLR is particularly important to South, Southeast and East Asia which collectively contain 71% of the global coastal population living below 10 m in elevation and 75% of the global coastal floodplain population. However, few Holocene studies of equatorial delta systems exist.Here, we investigate the early to mid-Holocene coastal response to decelerating rates of RSLR through paleoenvironmental reconstruction of the Kallang River Basin, Singapore. We produce a multi-proxy (sedimentology, stable carbon isotope, XRF elemental ratios) record from sediment core MSBH01B to compare with the Holocene relative sea-level record for Singapore. We identify different phases of coastal response through the interplay between accommodation space (A) driven predominantly by RSLR and sedimentation rate (S).In the early Holocene rapid RSLR coupled with low sedimentation rates (A/S = 5.1 ± 0.3) led to mangrove disappearance in the Kallang River Basin coastal area within ∼300 years (9.5–9.2 cal. ka BP). Estuarine sediments were deposited from 9.2 to 8.8 cal. ka BP during continued high rates of RSLR coupled with highest sedimentation rates (A/S = 3.1 ± 0.8) as the coastline retreated. Prodelta sediments were deposited from 8.8 to 8.2 cal. ka BP during decreasing rate of RSLR and high sedimentation rates (A/S = 4.6 ± 5.2). Delta front sediments were deposited during this delta initiation phase from 8.2 to 7.6 cal. ka BP as during a period of low and consistent RSLR and sedimentation rates (A/S = 1.7 ± 0.2). Finally, a prograding delta started forming from 7.6 to 7.2 cal. ka BP during lowest rates of RSLR and sedimentation rates (A/S = 1.7 ± 0.2). Our record provides a case study of possible responses of modern delta systems under a spectrum of predicted sea-level rise scenarios and accompanying sedimentation rates. This study provides an estimated threshold A/S value of 1.7 for coastal retreat to inform policy and mitigation/adaptation measures for Singapore and a simple methodology to obtain local threshold values of other equatorial cities built on floodplain and/or delta systems.

Morphodynamic Types of the Laptev Sea Coast: A Review

The Laptev Sea coast has a unique high-latitude and dynamic landscape. The presence of low-temperature permafrost (below −7 °C) and its high ice content (up to 90%) determine a wide array of permafrost landforms and features. Under the actions of thermal abrasion and thermal denudation, high rates of coastal retreat are evident within this region. Local differences in the geological structure and sea hydrodynamic conditions determine the diversity of this sea coast’s types. In this review, we present the results of a morphodynamic classification and segmentation of the Laptev Sea coast. The integrated approach used in the classification took into account the leading relief-forming processes that act upon this coastal zone. The research scale of 1:100,000 made it possible to identify and characterize the morphologies of the coast and their spatial distributions within the study area. The presented original classification can be considered to be universal for the eastern Arctic seas of Eurasia; it may be used as a basis for further scientific and applied research.

Late Quaternary mangrove biogeography and paleoenvironments in the reef area of the South China Sea based on analysis of palynomorph assemblages

In this study, we analyse the development of mangroves around the South China Sea (SCS) over the last 40 kyrs BP. Findings are based on 208 palynological samples obtained from a sediment core (NS07–25) in the reef area of the SCS. These palaeo-records are interpreted in the context of 161 surface sediment palynological samples obtained from across the SCS that serve as a modern reference set. Employing the Near Analysis function in ArcGIS, mangrove pollen in modern surface sediment comprises two major assemblages: dominant Rhizophoraceae (subtropical, wind- and animal-pollinated taxa) in the northern SCS and diverse Sonneratia (tropical, animal-pollinated taxa) in the southern SCS. The distribution field of these modern mangrove pollen assemblages is bounded by the Nansha Islands and reefs, which may act to obstruct pollen transportation between the northern and southern parts of the SCS. In our late Quaternary core, concentrations of Rhizophoraceae and Sonneratia pollen assemblages gradually increase in the late stage of Marine Isotope Stage 3 and during the Last Glacial Maximum, as a result of falling sea level and mangrove colonization of the exposed marine shelf, whereas they rapidly decrease in the Deglacial period, coincident with an increase in sea level and coastal retreat. The occurrence of Rhizophoraceae pollen assemblages in the southern SCS during the Last Glacial Maximum indicates that these communities disperse southward in response to climate change. Our findings highlight the complex effects of growth conditions, varied pollinators, and transportation patterns on the preservation of Rhizophoraceae and Sonneratia pollen assemblages with implications for understanding change in climate and sea level since 40 kyr BP.

Assessment and management of coastal erosion in the marine protected area of the Rosario Island archipelago (Colombian Caribbean)

By utilizing historical satellite image databases, it was possible to assess the rates of coastal retreat in the Grande, Rosario, and Tesoro Islands from 2010 to 2021, due to the influence of climatic variables. It was discovered that the three islands have lost 0.4%, 1.6%, and 9.3% of their landmass, respectively. The notable increase in sea level rise, which has risen 6.2 mm/year since 2010, has emerged as the primary driving factor in the magnitude of erosion by altering the wave transformation patterns in shallow waters. This enables the wave trains to maintain more energy and intensify their height in surf zones, particularly during the dry season, where the wave height has increased by 12%. At the same time, the intensity of longitudinal currents has increased due to the new wave dynamics, leading to changes in the coastlines. Areas characterized by a coastal geomorphology of terraces and beaches, such as Tesoro Island, were found to be the most affected, unlike the cliffs of Grande and Rosario Islands. The study also revealed a lack of policies and concrete actions aimed at mitigating erosion in the coral islands of the Colombian Caribbean. Thus, a set of guidelines is proposed as a starting point to address this issue.

Examining the Hydro-Climatic Drivers of Lagoon Breaching and Healing in a Deltaic Barrier

As sea-level rise (SLR) and human-made interventions affect coastal currents and sediment transport, coastal barriers have become more vulnerable to the effect of storms, hurricanes, and climate variability. The response of each barrier is unique and depends on wave regime, coastline orientation, weather conditions, bathymetry, and type of human-made interventions, among other factors. In the Magdalena River deltaic barrier, located on the Colombian Caribbean coast, coastal erosion has caused the loss of hundreds of square kilometers of critical ecosystems, such as wetlands and lagoons, since the 1960s. This work aims to analyze the short-term drivers behind the observed loss of lagoons, particularly the drivers of lagoon breaching events and subsequent healing along the deltaic barrier. Lagoon breaching events and healings were detected using satellite imagery, and the timing of these events was related to prior local atmospheric, oceanographic, and fluvial conditions. The findings reveal that the dynamics of the lagoons are driven by extreme river discharges and energetic wave conditions associated with storms or hurricanes. Healing is driven by the sediment supplied by littoral currents and average waves punctuated by energetic events. The cumulative effect of breaching and healing has resulted in a deltaic barrier that has rolled over the lagoons, reducing their size over time. These findings provide a better understanding of the forces of coastal retreat and will help inform future management decisions of the coastal zone.

The impact of coastal erosion on the archaeology of the Cyrenaican coast of Eastern Libya.

Coastal erosion in Cyrenaica (Eastern Libya) represents a major problem for archaeology and heritage management. The area is rich in archaeological sites, often understudied or not fully documented, but also has extensive stretches of vulnerable eroding coastline. This study demonstrates the extent and impact of erosion via shoreline change assessment at two spatial scales. Firstly, wide area assessment using shorelines extracted from a time-series of medium-resolution Landsat imagery. Secondly, site-specific assessment using recent and historic Very High Resolution (VHR) satellite imagery. In both cases, extracted shorelines at different timesteps were compared using the Digital Shoreline Analysis System (DSAS) tool to quantify rates and magnitudes of shoreline movement. The results show extensive zones of erosion at and around the key ancient harbour sites of Apollonia, Ptolemais and Tocra. They also suggest increased rates of coastal retreat in recent years, which is likely linked to anthropogenic actions such as sand mining and urbanization. Forecasts based on present-day shoreline change rates, coupled with ground-level documentation of the vulnerable shorelines is used to identify archaeological features and structures which will likely be progressively damaged or destroyed over the next 20 years. The ability to actively protect archaeological sites is unclear, but there is a clear need for mitigation in the form of enhanced awareness of environmental problems (e.g. caused by sand mining) and more intensive survey/documentation of sites and areas which will be lost in the coming years.

Coastal erosion in Argentina: The retreating rates of southern South America

Tall cliffs are characteristic of the Patagonian and Tierra del Fuego coastline. They are receding at rates below 1 m/yr. Gravel-dominated beaches and spits are protecting the foot of these cliffs. At the Buenos Aires coastline, erosion also dominates. Sandy barriers constructed during the Holocene sea-level fluctuation are today under severe erosion, either natural or induced by human interventions. The Eastern Barrier is more threatened to sea level rise while the Southern barrier is located on top of former cliffs. Groyne fields were implemented but led to drift obstructions. Argentine coastal retreat is more extended and intense compared to other retreating rates of Brazil, Uruguay and Chile.

Coastal Realignment

While the concept of managed coastal retreat is now familiar to many, the future for rural coastal lowlands has received less attention.Planned processes of coastal realignment can create opportunities, including carbon sequestration, nature-based transformation of coastal interfaces, and evolution of increasingly unproductive farmland towards other beneficial activities. Our present planning system provides high-level policy support for these changes but is mired in detail and short on recognition that the coastal edge will advance inland. While the challenges are being addressed positively in some areas, including by, or in partnership with, iwi/hapū, there is a national lack of leadership in integrated management across the changing land–sea interface, land ownership remains problematic, and funding requirements remain unresolved. New legislation promises improved approaches and is urgently needed.

Automatic extraction of highly risky coastal retreat zones using Google earth engine (GEE)

Automatic extraction of highly risky coastal retreat zones using Google earth engine (GEE)

Instability mechanisms and evolution of a rocky cliff on the Atlantic coast of Spain

Predicting the response of rocky coasts to different erosional agents remains a great challenge at present. The episodic and discontinuous nature of the instability processes typical of hard bedrocks makes it difficult to make predictions based on observations over short research periods. This work aims to contribute to the understanding of the geomorphological evolution of rocky cliffs by means of a case study of a geologically complex cliff (developed on quartzite and slate) located on the Atlantic coast of Spain. The analysis of high-precision topographic models and orthophotographs, the use of geomatics techniques and geomorphological characterization have made it possible to define a model of the cliff behaviour. The results indicate that the structure of the bedrock determines the type of instability processes affecting the cliff and the morphology of the associated deposits. Lithology is the other main conditioning factor: while slate is easily eroded, quartzite offers greater strength and its detached blocks act as an effective natural defence element protecting the cliff and slowing down the coastal retreat. The evolution model established for this cliff explains the absence of retreat in the study period (2003–2022) and confirms the important role of local factors in cliff evolution.

Interrogating private property rights and path dependencies for coastal retreat

For too long, implicit acceptance of and assumptions about colonial property rights have directed the flow of adaptation policy pathways. Though there is increasing recognition of the challenges that flow from private property rights for effective, long term adaptation planning, there is yet to be sustained and systematic attention to how property rights act as a mechanism that entrenches maladaptive tendencies by further embedding path dependency. This is complicated by law, which on the one hand is thought of as a vehicle to help institutionalise adaptative capacity, but on the other embodies and amplifies property rights. Property discourses are embedding a dominant property rights path dependency for coastal climate adaptation. This path dependency entrenches an incremental approach to coastal governance, with normative ideas on private property rights and the path dependencies they create, underpinning managed retreat theory and action. Lessons from case studies in coastal locations in Australia provide important insights into future behaviours of property owners. These insights are useful as coastal management strategies seek to embrace climate change adaptation, and as the financial sector reassesses climate risk exposure.

Numerical assessment of potential sea level rise impacts on coastal retreat along the Nigerian Mahin mud coast

Numerical assessment of potential sea level rise impacts on coastal retreat along the Nigerian Mahin mud coast

Evolution of the French Guiana coast from Late Pleistocene to Holocene based on chenier and beach sand dating

The 1500-km-long Guianas coast between the Amazon delta in Brazil and the Orinoco delta in Venezuela is characterized by alternations of muddy shoreline advance and retreat caused by large mud banks migrating alongshore from the mouths of the Amazon. In this dominantly muddy environment, wave reworking of sand and shells results in the formation of beaches, termed ‘cheniers’, that provide valuable information on coastal evolution, especially on past erosional phases. Twenty-eight depositional ages showing the long-term patterns of shoreline mobility in French Guiana were obtained from optically stimulated luminescence. Twenty-one ages younger than 7 ka define three clusters centred on 4.5 ka, 1.0–1.3 ka, and 0.30 ka. They indicate that chenier formation was relatively synchronous and significantly affected by alongshore diversions of river mouths and changes in river-mouth position over time under the influence of muddy shoreline advance. A prominent cluster at 1.0–1.3 ka reveals a clear hiatus after the 4.5 ka cluster, indicating that the present muddy coastal plain of French Guiana was largely formed and preserved after 1.0–1.3 ka. This cluster also implies either an episode of coastal retreat or no coastal advance around 1.0–1.3 ka. The remaining seven samples were derived from Late Pleistocene deposits of headland-bound beaches and probable cheniers capped by aeolian sand, suggesting similar conditions of coastal Amazon mud-dominated sedimentation. By informing on past patterns of shoreline mobility, these results have broader implications for coastal land-use planning and shoreline management between the mouths of the Amazon and Orinoco Rivers.