Shoreline Retreat Research Articles

Exceptional rates and mechanisms of muddy shoreline retreat following mangrove removal

AbstractProbably the largest regular shoreline fluctuations on Earth occur along the 1500 km‐long wave‐exposed Guianas coast of South America between the mouths of the Amazon and Orinoco Rivers, the world's longest muddy coast. The Guianas coast is influenced by a succession of mud banks migrating northwestward from the Amazon. Migrating mud banks dissipate waves, partially weld onshore, and lead to coastal progradation, aided by large‐scale colonization by mangroves, whereas mangrove‐colonized areas between banks (inter‐bank areas) are exposed to strong wave action and undergo erosion. On large tracts of this coast, urbanization and farming have led to fragmentation and removal of mangroves, resulting in aggravated shoreline retreat. To highlight this situation, we determined, in a setting where mangroves and backshore freshwater marshes have been converted into rice polders in French Guiana, shoreline change over 38 years (1976‐2014) from satellite images and aerial orthophotographs. We also conducted four field experiments between October 2013 and October 2014, comprising topographic and hydrodynamic measurements, to determine mechanisms of retreat. The polder showed persistent retreat, at peak rates of up to ‐200 m/yr, and no recovery over the 38‐year period of monitored change. Notwithstanding high erosion rates, mangrove shorelines show strong resilience, with recovery characterized by massive accretion. Retreat of the polder results in a steep wave‐reworked shoreface with a lowered capacity for bank welding onshore and mangrove establishment. Persistent polder erosion is accompanied by the formation of a sandy chenier that retreats landwards at rates largely exceeding those in inter‐bank situations. These results show that anthropogenic mangrove removal can durably modify the morphodynamics of muddy shorefaces. This limits the capacity for shoreline recovery and mangrove re‐establishment even when there is no sustained long‐term deficit in mud supply, as in the case of the Amazon‐influenced Guianas coast. © 2019 John Wiley & Sons, Ltd.

Influence of Underwater Bar Location on Cross-Shore Sediment Transport in the Coastal Zone

The effect of the underwater bar position on a sandy beach profile was studied on a timescale of one storm, using the XBeach numerical model. The largest shoreline regress occurred in the first hour of storm. For the chosen wave regime an underwater profile close to the theoretical Dean’s equilibrium profile is formed after 6 h. The position of the underwater bar affects the shoreline retreat rate. The lowest shore retreat occurs when the bar crest is located at a distance equal to 0.70–0.82 of the deep-water wavelength, corresponding to the period of the wave spectrum peak. The maximal shoreline retreat occurs when the bar is located at a distance that is close to a half wavelength. The shoreline recession depends on the heights of low-frequency waves. The smaller the mean wave period and the higher low-frequency waves’ height near the coast, the smaller the retreat of the shoreline. The distance of seaward sediment transfer is directly proportional to the significant wave height near shore.

Masked Shoreline Erosion at Large Spatial Scales as a Collective Effect of Beach Nourishment

AbstractSea‐level rise along low‐lying coasts of the world's passive continental margins should, on average, drive net shoreline retreat over large spatial scales (>102 km). A variety of natural physical factors can influence trends of shoreline erosion and accretion, but trends in recent rates of shoreline change along the U.S. Atlantic Coast reflect an especially puzzling increase in accretion, not erosion. A plausible explanation for the apparent disconnect between environmental forcing and shoreline response along the U.S. Atlantic Coast is the application, since the 1960s, of beach nourishment as the predominant form of mitigation against chronic coastal erosion. Using U.S. Geological Survey shoreline records from 1830–2007 spanning more than 2,500 km of the U.S. Atlantic Coast, we calculate a mean rate of shoreline change, prior to 1960, of −55 cm/year (a negative rate denotes erosion). After 1960, the mean rate reverses to approximately +5 cm/year, indicating widespread apparent accretion despite steady (and, in some places, accelerated) sea‐level rise over the same period. Cumulative sediment input from decades of beach‐nourishment projects may have sufficiently altered shoreline position to mask “true” rates of shoreline change. Our analysis suggests that long‐term rates of shoreline change typically used to assess coastal hazard may be systematically underestimated. We also suggest that the overall effect of beach nourishment along of the U.S. Atlantic Coast is extensive enough to constitute a quantitative signature of coastal geoengineering and may serve as a bellwether for nourishment‐dominated shorelines elsewhere in the world.

UAS based 3D shoreline change detection of Jupiter Inlet Lighthouse ONA after Hurricane Irma

ABSTRACTThe shoreline along the Jupiter Inlet Lighthouse Outstanding Natural Area (ONA) located in Jupiter, FL, USA, has been impacted by a variety of natural and anthropogenic factors over the years. This has resulted in significant erosion of the ONA’s shoreline, with an estimated retreat of up to 23 m over the last 50 years. Storm events, such as recent hurricanes Matthew (2016) and Irma (2017) contribute to the increase in the rate of shoreline retreat. The higher wind speed and storm surge affected the beach topography by transporting significant amounts of sand into the waterways and away from the site. The purpose of this research is to quantify the shoreline retreat at the Jupiter Inlet Lighthouse ONA caused by Hurricane Irma using Unmanned Aerial Systems (UAS)-based photogrammetry. The historical coastlines represented on aerial photography dating back to 1953 were used to analyse the rate of long-term change and isolate the impact due to Hurricane Irma. Short-term change rate of shoreline and volume change of beach topography was performed using transect-based spatial analysis of pre- and post-hurricane UAS data. The case study reveals that recent hurricane events increased the annual shoreline change rate to more than three times. Further, the volume of sand that has eroded due to Hurricane Irma was determined to be 2.8 hectares.

Assessing Beach and Dune Erosion and Vulnerability Under Sea Level Rise: A Case Study in the Mediterranean Sea

In this study, we estimate the shoreline retreat, the vulnerability and the erosion rates of an open beach-dune system under projected sea level rise and the action of wind-waves (separately and in combination). The methodology is based on the combination of two state-of-the-art numerical models (XBeach and Q2D-morfo) applied in a probabilistic framework and it is implemented in an open sandy beach in Menorca Island (Western Mediterranean). We compute the shoreline response to sea level rise during the 21st century and we assess the changing impacts of storm waves on the aerial beach-dune system. Results demonstrate the relevant role that the beach backshore features, such as the berm, play as coastal defence, reducing the shoreline retreat and dune vulnerability rates in the near-term (a few decades ahead) and highlighting the importance of simulating the beach morphodynamic processes in coastal impacts assessments. Our findings point at sea level rise as the major driver of the projected impacts over the beach-dune system, leading to an increase of ~25% of the volume eroded due to storm waves by the end of the century with respect to present-day conditions.

GIS-based vulnerability mapping of the coastal stretch from Puri to Konark in Odisha using analytical hierarchy process

The 485-km-long coastline of Odisha, a state in the northeastern part of the Indian peninsula, is potentially vulnerable to several disaster events that take place frequently. In addition to threats due to natural hazards, these coastal regions also face immense population and developmental pressures. The increase in the intensity and frequency of cyclones and accelerated sea level rise related to increased sea surface temperature have led to flooding, coastal erosion and shoreline retreat causing damage to coastal ecosystems and resources in these regions. In recognition of these risks, the present work demonstrates a GIS-based approach to assess the vulnerability of the 187-km stretch from Puri to Konark out of the total 485-km coastline using analytical hierarchy process (AHP). The present study focuses on computation of integrated coastal vulnerability index which is an integration of physical vulnerability index, geotechnical vulnerability index and social vulnerability index using AHP taking nine risk variables into consideration. An attempt has been made to demonstrate the state-of-the-art microzonation of the coastal stretch between Puri and Konark based on the vulnerability indices using geographical information system.

РАЗВИТИЕ БЕРЕГОВЫХ ПРОЦЕССОВ ПРИ ПОВЫШЕНИИ УРОВНЯ ВОДОЕМА (НА ПРИМЕРЕ ОЗЕРА БАЙКАЛ)

The article provides estimations of the impact from abrupt increase in water level in Lake Baikal on coastal processes of bar Yarki (north part of Lake Baikal). The author has made a forecast of coastal processes of island bar under the current conditions and created a digital elevation model and profiles of the nearshore. Relationship between sea level rise and shoreline retreat are considered. Numerical simulation of evolution the beach profile under different conditions was carried out. The construction of hydroelectric power station in Irkutsk is the main reason that caused the change in morphological parameters of the bar. At the level of Lake Baikal at the time of expeditionary works in 2013, the shore profile showed relative stability even with maximum storms. When the lake level rose by 1 metre, the profile retreated “back and up” in compliance to the Dean-Maurmeier model (Dean, Maurmeyer, 1981) for coastal bars.

SEA-LEVEL EFFECT ON ANNUAL CYCLIC BEACH MORPHOLOGY IN SWASH ZONE AT HASAKI COAST

depend on the accurate knowledge of the beach response to sea level regime on multi-time scale. For the long-term beach response, Bruun (1962) suggested that the equilibrium beach profile would move to new equilibrium profile in response to a rising sea level. In this concept called as Bruun rule, the upper part of the beach profile is eroded due to the sea level rise, resulting in the shoreline retreat. It is widely used for the future shoreline prediction. However, the Bruun rule predicts just only the final beach state with a constant wave impinging for an infinite period after sea level rise. On the other hand, simultaneous function of wave and sea level is more important on interannual to decadal-scale beach response. El niño in 2015 and 2016 increased wave energy and sea level, corresponding to large beach erosion across the US west coast (Barnard et al., 2017). Sea level influences the response sensitivity to the wave forcing as a subordinate factor on the morphological change. High water level anomalies made the beach more eroded even if the wave condition was equal. Beach morphology in the swash zone often changes on a 1-year cycle due to seasonal wave conditions. The effect of sea level on the annual cyclic beach morphology in swash zone is still unclear because long-term beach observation data required for the analysis are difficult to obtain. In this study, we investigated the simultaneous effects of the wave and sea level on annual cyclic beach morphology in the swash zone with spectrum analysis for 25-year Hasaki beach observation data.

BEACH RESPONSE TO EXPOSED RIVERINE SEDIMENT AND BEACH NOURISHMENT

Studying the process of riverine sediment at mouths and continental shelves is a critical subject for many engineering applications, such as dredging, navigation, dispersal and remobilization of contaminants. Sediment deposits also determine seabed properties, coastal geomorphology, and the health of coastal habitat/ecology. During extreme conditions, episodic river discharge triggered by large rainfall due to tropical cyclones may contribute significant amount of riverine sediment into the ocean. In the past decade, evidence of severe seabed erosion (up to 1m/year) along the sandy coast of Yunlin County has raised concerns regarding the sustainability of coastal structures. The exposed riverine sediment from the Jhuoshuei River is considered as one of major sources for sediment supply in this region. Bottle samples collected from bridge station in the Jhuoshuei River during the passage of tropical cyclones suggest sediment concentration can exceed 40 g/l for the major duration of the storm (Milliman et al. 2007). To mitigate the damage caused by shoreline retreat, 600,000 cubic meters per month of sand has been placed in two specific locations near the offshore industry park. The overarching goal of this study is to clarify the contribution of exposed riverine sediment and beach nourishment to enhance our understanding on the observed sediment transport and morphological evolution.

LARGE SCALE SHORELINE PROTECTION WITH MINIMIZED DOWNDRIFT IMPACT, COTONOU, WEST AFRICA

Benin is a country in West Africa with a little more than 120 kilometers of coastline. The main port at Cotonou serving the country of Benin was completed in 1963. According to Dibajnia and Nairn (2004), Benin’s coastline experiences incessant wave attack causing a longshore sand transport rate in the order of 1 million m3 per year (moving from west to east). The port facility initially blocked the full amount of sand transported along the shore, resulting in accumulation of a large beach on the updrift (west) side of the port and corresponding dramatic erosion of the downdrift (east) shoreline consisting of over 400 m of shoreline retreat by 2003 (i.e. over approximately 40 years). Additionally, since 1998 sand started to bypass around the harbor breakwater into the entrance channel. Harbor entrance channel dredging requirements was escalating with almost 1.5 million m3 dredged between 1998 and 2002. At the time of port construction, the downdrift erosion was not a concern as the area was largely undeveloped. However, with the inevitable expansion of the city of Cotonou towards the east, the ongoing erosion started to destroy newly developed residential and commercial areas.

FIELD STUDY OF CONTRASTING BEACH RECOVERY PROCESSES OBSERVED IN NAMI-ITA and KIRIKIRI COASTS AFTER THE 2011 TOHOKU EARTHQUAKE

The 2011 Tohoku Earthquake caused significant shoreline retreat along the northern Pacific coast of Japan and most of the affected beaches have not yet fully recovered even six years after the event. This study investigates overall characteristics of the shoreline change after the event and also investigates contrasting beach recovery processes observed in Nami-ita and Kirikiri coasts.

CHARACTERIZATION OF HYDRODYNAMIC PROCESSES DRIVING TIDAL RIVER ISLAND SHORELINE CHANGE

Bird/Long Island is a dredge-spoil island located between the north and south channels near the inlet of the Savannah River at the border of Georgia and South Carolina. The island is in a tidally dominant environment and contains cultural and natural resources, including remnants of a Civil War era artillery battery. As a wetland mitigation bank, it is particularly important to the state of Georgia. However, these resources are under threat from documented and ongoing sea level rise, shoreline change (i.e., erosion and accretion) from natural and anthropogenic causes, and land subsidence. In addition to substantial tidal and freshwater flows, the island is subject to locally-generated wind waves primarily from northeast winds, as well as wake from the large container ships transiting to and from the Port of Savannah. A previous study examined the effects of wind and vessel-generated waves on shoreline retreat for the Fort Pulaski National Monument on nearby Cockspur Island (Houser, 2010). The study concluded that while the vessel-generated waves account for nearly 25% of the energy, the wind waves during storm events with increased water levels accounted for the majority of the marsh retreat. Although the proximity of this previous study site to Bird/Long Island is relevant, the different orientations of the islands and the narrowing of the channel create a different hydrodynamic environment. The present work uses field data to characterize the hydrodynamic processes affecting Bird/Long Island, which will improve modeling of its shoreline change.

The Effects of Tropical Cyclone-Generated Deposition on the Sustainability of the Pearl River Marsh, Louisiana: The Importance of the Geologic Framework

Shoreline retreat is a tremendously important issue along the coast of the northern Gulf of Mexico, especially in Louisiana. Although this marine transgression results from a variety of causes, the crucial factor is the difference between marsh surface elevation and rising sea levels. In most cases, the primary cause of a marsh’s inability to keep up with sea level is the lack of input of inorganic material. Although tropical cyclones provide an important source of such sediment, little effort has been made to determine the point of origin of the deposited material. In this study we use sedimentary, geochemical and biogeochemical data to identify the bed of the Pearl River and/or Lake Borgne as the source of a ~5 cm thick clastic layer deposited on the surface of the Pearl River marsh on the Louisiana/Mississippi border. Radiochemical chronologies and sedimentary evidence indicate that this layer was associated with the passage of Hurricane Katrina in 2005. As this material would otherwise have been lost to the system, this deposition indicates a net gain to marsh surface elevation. Accretion rates, determined from 137Cs and 14C profiles and the use of the Katrina layer as a stratigraphic marker, indicate that short-term (~50 years) rates are as much as an order of magnitude higher than the long- term (1000s of years) rates. We suggest that the marsh’s geologic setting in an incised river valley with steep vertical constraints and a large fluvial discharge, promotes rapid accretion rates, with rates accelerating as the sea moves inland, due to extended hydroperiods and the input of clastic material from both the marine and terrestrial sides. These rates are especially large when compared to accretion occurring in the more common open marshes fringing the Gulf that lack fluvial input. The difference is particularly large when related to marsh recovery/regrowth following the deposition of thick hurricane-generated clastic layers. Given the number of similar incised river valleys along the Gulf Coast, we believe that understanding the processes controlling marsh accretion in such environments is essential in evaluating marsh sustainability on a regional basis.

Tracking shoreline erosion of “at risk” coastal archaeology: the example of ancient Siraf (Iran, Persian Gulf)

Successful heritage management requires a robust comprehension of the threats facing archaeological sites, at both current and future timescales. Siraf, on the Persian Gulf of Iran, is a site of national and international importance whose history stretches back to the Sassanid period (224–652 AD). In the present context of global change (drought and relative sea-level rise) and anthropogenic impacts (coastal artificialization and reduced sediment supply), the city's waterfront archaeology is undergoing significant erosion. Nonetheless, at present, the processes leading to the loss of Siraf's in situ archaeological remains are still poorly understood, including the rates, timing and drivers of coastal erosion. Here we use Landsat images to monitor shoreline changes along the Siraf coastline between 1973 and 2016. We spatially quantify the causes and impacts of surface changes along 244 transects. The results demonstrate that coastal erosion is responsible for widespread and archaeologically significant damage, with 48% of the studied transects showing erosion during the period 1973–2016. We elucidate significant temporal variations in the data and, most notably, demonstrate that continued drought since the early 2000s has impacted upon sediment supply to coastal areas, severely accentuating erosion. For instance, between 2003 and 2016, 70% of transects recorded shoreline retreat with worrying implications for the waterfront archaeology of Siraf. These quantitative results provide invaluable spatial information regarding the causes and impacts of erosion upon Siraf's waterfront heritage, in addition to furnishing a template for the protection of the city's internationally important cultural heritage.

Evolution of the coast line and associated impacts in coastal Ceará State, Northeast, Brazil

ABSTRACTHigh-energy events are increasingly frequent in Northeast Brazil. Of these we mention undertow between December and April, causing several impacts on coastal areas. Therefore, the aim of this chapter is to establish the evolution of the Aquiraz city's coast line in the period 1958 to 2014 and from 2009 to 2014 to identify erosion and silting trends, the natural and anthropogenic tensioners associated with these procedures. In the methodology, the maximum flood line was chosen as an indicator of advance or retreat of shoreline, identified in QuickBird and Landsat satellite images and aerial photographs. The program DSAS was applied to evolution of the coastline in the last 56 years. It was found that the coast is in progressive erosive phase. Between 1958 - 2014, Japan's AEPR from -0.07 to -1.29 m / year and Porto das Dunas from -0.29 to -2.61 m / year. The reflection of this human action coincides with the period of greatest retreat of the coastline throughout the area. The frontal dunes are arranged discontinuously, are absent in some places due to the occupation of tents and fixed constructions. Thus this coastal line study was an essential tool to deepen regarding the local dynamics in order to suggest further mitigation measures by generating new data considering the occupation advancement and climate and oceanographic changes.

Hurricanes as a Major Driver of Coastal Erosion in the Mississippi River Delta: A Multi-Decadal Analysis of Shoreline Retreat Rates at Bay Champagne, Louisiana (USA)

The Louisiana shoreline is rapidly retreating as a result of factors such as sea-level rise and land subsidence. The northern Gulf of Mexico coast is also a hotspot for hurricane landfalls, and several major storms have impacted this region in the past few decades. A section of the Louisiana (USA) coast that has one of the highest rates of shoreline retreat in North America is the Caminada-Moreau headland, located south of New Orleans. Bay Champagne is a coastal lake within the headland that provides a unique opportunity to investigate shoreline retreat and the coastal effects of hurricanes. In order to examine the influence of hurricanes on the rate of shoreline retreat, 35 years (1983–2018) of Landsat imagery was analyzed. During that period of time, the shoreline has retreated 292 m. The overall rate of shoreline retreat, prior to a beach re-nourishment project completed in 2014, was over 12 m per year. A period of high hurricane frequency (1998–2013) corresponds to an increased average shoreline retreat rate of >21 m per year. Coastal features created by multiple hurricanes that have impacted this site have persisted for several years. Bay Champagne has lost 48% of its surface area over the last 35 years as a result of long-term shoreline retreat. If shoreline retreat continues at the average rate, it is expected that Bay Champagne will disappear completely within the next 40 years.

High human influence on beach response to tropical cyclones in small islands: Saint-Martin Island, Lesser Antilles

Using multi-date satellite imagery and field observations, this paper assesses the inferred impacts of September 2017 cyclones on the beaches of Saint-Martin Island. Twenty-two beaches out of 30 predominantly exhibited shoreline retreat, with the highest retreat value (−166.45 m) recorded on the north-eastern coast. While erosion predominated on beaches and at the sand dune front, inner areas generally exhibited accretion, with sand sheets (up to 135 m from the pre-cyclone vegetation line) indicating landward sediment transfer. Natural back-reef beaches exhibited the formation of new beach ridges, marked (up to 2 m) upward growth and alongshore beach extension. The high spatial variability of inferred impacts is attributed to the cyclone's track, coast exposure, beach configuration and, importantly, human-driven environmental change. Whereas vegetation removal exacerbated marine inundation and inhibited the vertical accretion of beaches, shoreline hardening aggravated wave-induced sediment loss while also inhibiting sediment deposition. Four beach response modes are distinguished. Based on findings, we identified three major areas of action for risk reduction and adaptation to climate change. Depending on beach response and site specificities, relocation and the determination of set-back lines, coastal buffer restoration, or engineered structures' upgrading should be prioritized.

Analysis of shoreline change in Cua Dai beach by using Empirical Orthogonal Function

ABSTRACT Cua Dai beach, located in Hoi An town – Quang Nam province, has experienced serious erosion for the past few years with extremely high speed of shoreline retreat. Despite coastal erosion itself may not be that much problematic, it turns out to be serious problem since there is a large residential area right behind and a huge investment in tourism has been done. Generally, causes of erosion are among unbalance of sediment transport, storm surges and sea level rise. However, the rate of shoreline retreat in Cua Dai beach within the past decade is so aberrant and not likely to be explained by the above phenomenon alone. The goal of this study is to consider effective factors of this coastal erosion based on analysis of the recent progress of erosion using Empirical Orthogonal Function. As a result, the dominant factor for the long-term erosion is concerned to be the change of longshore sediment transport, which was caused by the decrease in sediment supply from Thu Bon River. Therefore, it is recommended to collect more data on sediment transport in the river system and study the relationship between coastal erosion and the change of sediment supply.

Quantification of Swell Energy and Its Impact on Wetlands in a Deltaic Estuary

The extensive coastal wetlands in Mississippi River Delta represent the seventh largest deltaic floodplain in the world, contributing to many services that sustain the economies of the region. Subsidence, sea level rise, saltwater intrusion, wave action from storms, and sediment depletion have contributed to chronic wetland losses, converting vegetated lands into open waters and increasing wind fetch. Among these factors listed, wave energy plays the largest role in marsh edge erosion in an open bay environment. Degrading barrier islands along the shoreline of this delta allow swell energy to enter protected bay areas, contributing to marsh edge erosion. Locally generated wind waves within enlarged bays also contribute to wetland loss. Quantifying the roles of swell and wind waves in marsh edge erosion is essential to any ecosystem restoration design. In this study, a numerical model is implemented to describe the wave climate of combined swell and wind waves in a deltaic estuary. Terrebonne Bay was chosen as the study area because it has experienced one of the largest reductions in barrier islands and wetland loss rates among Louisiana estuaries. A continuous wave measurement in upper Terrebonne Bay was obtained over the course of a year. A spectral wave model is used to hindcast the wave climate in the estuary. The model results are compared against the in situ wave measurement. The wave power is partitioned into swell and wind sea at different locations in Terrebonne Bay using the model results. An extensive analysis on a valid effective wave power range that directly impacts the marsh edge is performed and presented. Insight into the temporal and spatial variability of wave power is gained. Through differentiating swell and wind sea energies around the bay, improvements of long-term wave power computation for shoreline retreat prediction are made. It is found that the swell energy becomes the primary driver of marsh edge retreat in the southwest part of Terrebonne Bay as the barrier islands are degrading.

Sandy Beach Restoration Using Beach Nourishment Method: A Case Study of Nha Trang Beach, Vietnam

ABSTRACT Thuy, M.T.T; Nagasawa, T.; Tanaka, H., and Viet, N.T., 2018. Sandy beach restoration using beach nourishment method: A case study of Nha Trang Beach, Vietnam. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal of Coastal Research, Special Issue No. 81, pp 57–66. Coconut Creek (Florida), ISSN 0749-0208. Nha Trang Beach, located in the Northern part of Cam Ranh Bay, is famous as a tourist destination for its beautiful sandy beach. Since 1975, the Southern part of the beach has been considered a tourist hotspot where accommodation and business infrastructure are concentrated. However, in the past decade, severe erosion and shoreline retreat has occurred in the area. Human intervention in terms of house construction and other tourism infrastructure building is supposed to be one of main causes for these erosion problems. On the Northern beach, tourism demand is low despite its considerable potential. Between 2004 and 2005, a road was bu...