Coastal Retreat Rates Research Articles

Temporal and spatial variations in recession rates and sediment release from soft rock cliffs, Suffolk coast, UK

High rates of coastal retreat characterise the weakly cemented Plio-Pleistocene rocks and sediments which form much of the cliffed coastline of East Anglia, southern North Sea. The accurate establishment of sediment losses from these cliffs has a regional significance as these sediments are important in maintaining beaches and nearshore bank systems and in feeding nearshore sediment transport pathways. However, the high spatial and temporal variability of cliff failure processes in such materials necessitates fine-scale integration of alongshore variations in cliff retreat over a series of well-established time periods to accurately define cliffline recession rates and sediment volume inputs to the nearshore system. This study applied the DSAS (Digital Shoreline Analysis System) within the GIS software package ArcMap to digitised, georeferenced positions of former shorelines, obtained from historic maps and aerial photographs (after 1992), for the sections of Benacre–Southwold and Dunwich–Minsmere on the Suffolk coast of East Anglia, UK; transects were cast every 10 m alongshore, producing very high spatial resolution upon which to assess shoreline retreat (over 1000 transects along 11 km of shoreline). Long-term (1883–2008) mean shoreline retreat rates varied between 2.3–3.5 m a − 1 (Benacre–Southwold) and 0.9 m a − 1 (Dunwich–Minsmere). For six cliffed subunits within these larger coastal sections, spatial variations in cliffline recession rates for shorter time intervals (at ca. 20-year intervals) within this longer (125 year) period were established. The combination of recession rates with photogrammetric methods of obtaining cliff top elevation at the same spatial resolution, available using aerial photographs and digital terrain models, along with cliff sediment composition, allowed the calculation of sediment volumetric inputs from cliff retreat in the period 1992–2008. Re-assessment of the magnitude and location of sediment inputs into the nearshore zone, their interaction with regional sediment transport and the growth of inshore bank systems, as well as the implications for contemporary and near-future coastal management strategies are discussed with reference to this section of the Suffolk coast.

The Influence of Cryogenic Processes on the Erosional Arctic Shoreface

Coastal dynamics and shoreface relief in ice-free seas are a function of hydrodynamic interactions between the sea and bottom sediments. In the Arctic, additional, cryogenic factors such as permafrost and the action of sea ice influence coastal processes. The goal of our paper is to assess this influence, mainly on the profile shape. Mathematical analyses of the shape of 63 shoreface profiles from the Laptev, Beaufort, and Chukchi Seas were carried out. The shapes of Arctic shoreface profiles and those of ice-free seas are compared. We found that large ice and silt content in perennially frozen sediments composing Arctic coasts favor their erosion. Sea ice plays an important role in sediment transport on the shoreface and correspondingly changes shoreface relief significantly. Some effects of ice intensify coastal erosion considerably, but others play a protective role. The overall influence of cryogenic processes on Arctic coasts composed of loose sediments is seen in that the average rate of coastal retreat is larger than in the temperate environments, even though Arctic coasts are protected by a continuous ice cover most of the year. The shape of the shoreface profile in the Arctic does not differ from that in ice-free seas, and is satisfactorily described by the Bruun/Dean equilibrium profile equation. The explanation of this fact is that all changes of the profile shape, caused by cryogenic processes, are short lived and quickly eliminated by wave action.

Fifty years of coastal erosion and retrogressive thaw slump activity on Herschel Island, southern Beaufort Sea, Yukon Territory, Canada

Patterns of coastal erosion in the Arctic differ dramatically from those coasts in more temperate environments. Thick sea ice and shore-fast ice limit wave-based erosional processes to a brief open water season, however despite this, permafrost coasts containing massive ice, ice wedges and ice-bonded sediments tend to experience high rates of erosion. These high rates of erosion reflect the combined thermal–mechanical processes of thawing permafrost, melting ground ice, and wave action. Climate change in the Arctic is expected to result in increased rates of coastal erosion due to warming permafrost, increasing active layer depths and thermokarst, rising sea levels, reduction in sea ice extent and duration, and increasing storm impacts. With the most ice-rich permafrost in the Canadian Arctic, the southern Beaufort Sea coast between the Tuktoyaktuk Peninsula and the Alaskan border is subject to high rates of erosion and retrogressive thaw slump activity. Under many climate change scenarios this area is also predicted to experience the greatest warming in the Canadian Arctic. This paper presents results of a remote sensing study on the long-term patterns of coastal erosion and retrogressive thaw slump activity for Herschel Island in the northern Yukon Territory. Using orthorectified airphotos from 1952 and 1970 and an Ikonos image from 2000 corrected with control points collected by kinematic differential global positioning system and processed using softcopy photogrammetric tools, mean coastal retreat rates of 0.61 m/yr and 0.45 m/yr were calculated for the periods 1952–1970 and 1970–2000, respectively. The highest coastal retreat rates are on north–west facing shorelines which correspond to the main direction of storm-related wave attack. During the period 1970–2000 coastal retreat rates for south to south–east facing shorelines displayed a distinct increase even though these are the most sheltered orientations. However, south to south–east facing shorelines correspond to the orientations where the highest densities of retrogressive thaw slumps are observed. Differences in rates of headwall retreat of retrogressive thaw slumps and coastal erosion results in the formation of larger thermokarst scars and the development of polycyclic thaw slumps on south to south–east exposures. The number and the total area of retrogressive thaw slumps increased by 125% and 160%, respectively, between 1952 and 2000. As well, the proportion of active retrogressive thaw slumps increased dramatically. Polycyclic retrogressive thaw slumps appear to develop in a periodic fashion, related to retrogressive thaw slump stage and maximum inland extent.

Coastal flooding hazard related to storms and coastal evolution in Valdelagrana spit (Cadiz Bay Natural Park, SW Spain)

Mapping of coastal inundation hazard related to storms requires the combination of multiple sources of information regarding meteorological, morphological and dynamic characteristics of both the area at risk and the studied phenomena. Variables such as beach slope, storm wave height or wind speed have traditionally been used, but detailed geomorphological features of the area as well as long-term shoreline evolution trends must also be taken into account in order to achieve more realistic results. This work presents an evaluation of storm flooding hazard in Valdelagrana spit and marshes (SW Spain), considering two types of storm that are characteristic of the area: a modal storm with 1 year of recurrence interval (maximum wave height of 3.3 m), and an extreme storm with 6–10 years of recurrence interval (maximum wave height of 10.6 m), both approaching the coast perpendicularly. After calculating theoretical storm surge elevation, a digital terrain model was made by adjusting topographic data to field work and detailed geomorphological analysis. A model of flooding extent was subsequently developed for each storm type, and then corrected according to the rates of shoreline change in the last decades, which were assessed by means of aerial photographs taking the dune toe as shoreline indicator. Results show that long-term coastline trend represents an important factor in the prediction of flooding extent, since shoreline retreat causes the deterioration of natural coastal defences as dune ridges, thus increasing coastal exposure to high-energy waves. This way, it has been stated that the lack of sedimentary supply plays an important role in spatial variability of inundation extent in Valdelagrana spit. Finally, a hazard map is presented, where calculated coastal retreat rates are employed in order to predict the areas that could be affected by future inundation events.

Human impacts on coastal stability in the Pechora Sea

The geoecological situation in the regions of intense industrial exploitation on the Pechora Sea coast, particularly in the Varandei area, is dangerous. Human factors intensify eolian and slope processes and thermoerosion. Coastal stability decreases and coastal retreat rates are twice as high as in regions unaffected by human activity. Industrial exploitation results in the destruction of natural environments and considerable material losses. Several housing estates and industrial constructions have already been destroyed because of coastal erosion. Damage increases each year as the cliff retreats towards the center of the Varandei settlement. The oil terminal, airport and other industrial objects are also endangered.

Holocene Evolution of the East Texas Coast and Inner Continental Shelf: Along-Strike Variability in Coastal Retreat Rates

ABSTRACT The low gradient east Texas coast and inner-continental shelf, from Sabine Lake at the Texas-Louisiana border to the western end of Galveston Island, experienced extreme along-strike variations in rates of Holocene coastal retreat. Around 7.7 ka a barrier shoreline was located approximately 55 km offshore. Toward the western end of Galveston Island, the shoreline retreated 55 km, occupying a position on the lagoon side of the Island by about 5.3 ka. Toward the Texas-Louisiana border, the shoreline retreated more gradually, occupying a position seaward of Sabine Bank by 5.3 ka. Between 4.7 ka and 2.8 ka the shoreline at Sabine Bank retreated 30 km, while Galveston Island prograded seaward. Bolivar Peninsula began to accrete around 1.5 ka. Heald and Sabine banks, located on the inner continental shelf above terraced fluvial deposits of the Trinity-Sabine incised valley, are the only preserved remnants of these former shoreline positions. Fluctuating rates of sea-level rise were not the forcing mechanism behind episodes of rapid shoreline retreat because these events were localized. Rather, along-strike variations in the rate of transgression were caused by the variable inner-shelf gradients, which increase towards the west, and the orientation of the Sabine incised valley and associated terraced fluvial deposits, which trend northeast-southwest (parallel to shore). As shorelines retreated over fluvial deposits, these served as local sand sources that enabled barrier islands to persist Figure 1. Map of the east Texas inner shelf showing locations of Sabine, Heald, and Shepard banks. Paleotopographic map of the Pleistocene surface (thin contours; Siringan 1993) delineates the Trinity-Sabine incised valley, which formed during the last lowstand of sea level (Thomas and Anderson 1994). Bathymetric contours are shown as thick lines with bold numbers. Dots are core locations. End_Page 405------------------------ offshore, out of equilibrium with sea level. Once these sand sources became depleted, and/or sea level reached some critical threshold, barrier shorelines became stranded offshore as banks, and new shorelines formed landward. The geologic setting of coastal areas, specifically antecedent topography, plays a primary role in controlling coastal evolution. To accurately forecast long-term (centennial to millennial) coastal evolution, it is essential that impacts associated with variations in the underlying geology of coastal areas be incorporated into coastal forecasting models.

Micro- to mesoscale evolution of beaches in response to climatic shift: observation and conceptual modelling (Brittany, France)

In the framework of the European project 'Climate change and coastal evolution in Europe', the microscale (< 1 year) morphological evolution of beaches in Brittany (W France) has been monitored for some years and linked to immediate forcings, the first one being tides (with a 13-m tidal range), and the second storm intensity. For the same sites, a mesoscale (decades) evolution was reconstructed from air photographs and historical data. The evolution is forced first of all by human activities, and secondly by climatic shift: wind direction changes and average wind velocity evolution. These results were used to build and calibrate a model of coastal behaviour. The model is based on a simple sediment cell (input – transit – output) and simulates the movements of volumes of sands under successive morphodynamic processes. Each of these processes is modelled (in terms of volume exchanges) and parameterized by the relevant atmospheric element. Thus, the model is not deterministic but simulates different probabilities for atmospheric events and morphological response. The model runs for one year and produces realistic sediment accumulation values and coastal retreat rates. When run for half a century, results are also quite comparable to the field data. Storm frequency changes appear to be an important element for the evolution, but the main and first forcing is the availability of sediment.

Temperature changes and thaw of permafrost adjacent to Richards Island, Mackenzie Delta, N.W.T.

Thawing of ice-bonded sediment below the sea floor accompanies sea-level rise and shoreline retreat along the Beaufort Sea coast. The rate of thaw is primarily controlled by the warm spring and summer discharge of the Mackenzie River, which affects sea-bottom temperatures to water depths of 10–20 m. The intensity of the warming at any one location decreases with depth until continuous sub-0 °C temperatures are encountered. Sea-level rise eventually brings the sea floor into the cold sea-water zone and thawing stops. Therefore thawing is also controlled by the residence time in the warm zone. Sedimentation or scouring can, respectively, slow or accelerate the increase in water depth and thereby modify the rate of thaw progression. This, combined with changes in the rate of coastal retreat, can produce a subbottom frost table with considerable relief. In the extreme, ice bonding within a few metres of the sea floor can be explained by accelerated submergence.

Morphology of the Canadian Beaufort Sea coast

A study of the Canadian Beaufort Sea coast (Alaskan-Yukon border to Baillie Island) was conducted to provide a regional summary of (a) coastal morphology, (b) coastal stability and (c) coastal processes. A total of 2077 km of coastline was mapped using aerial photographs, low-altitude and oblique aerial-video imagery and information from previous field studies. The coast can be adequately characterized by six repetitive coastal types, four of which are erosional landforms and comprise 60% of the coast (ice-rich cliffs, ice-poor cliffs, low-tundra cliffs and inundated tundra). Two repetitive coastal types are accretional landforms and comprise 40% of the coast (barrier islands and spits, and deltas). The deltas in the study area are retreating rapidly, and therefore over 80% of the coast is considered erosional and less than 20% accretional. Over 1000 measurements of coastal stability indicate wide-scale coastal retreat. Most sections of the coast show mean coastal retreat rates in excess of 1 m yr −1 despite very low annual wave exposure levels. Near active areas of the Mackenzie River delta, mean retreat rates are in excess of 2 m yr −1 and a maximum retreat rate of 18 m yr −1 was documented near Shallow Bay. The presence of significant quantities of ice in the coastal sediments, both in the form of pore ice and massive ice beds, appears to be the primary cause of the wide-scale regional retreat of the coast, although a relative sea level rise in the region may also be important.

Thermal abrasion of sea coasts (part I)

Abstract Along arctic coasts consisting of unconsolidated sediments in permafrost, and especially where these sediments are ice‐rich, the thermal energy of the sea water is added to the normal erosive impact of the waves. The resulting process of erosion due to the combined action of the mechanical and thermal energy of the sea is termed thermal abrasion. This study concentrates on almost every aspect of the retreat of coastal cliffs and changes in the sea bed resulting from this process. The author's fieldwork on Muostakh Island near Tiksi forms the core of the study. Attention is focused particularly on the shape and height of the resulting cliffs, coastal retreat rates and subsequent lowering of the sea bed. In addition attempts are made at calculating rates of thermal abrasion based on water temperature and period of wave action, and of thermal denudation of the subaerial cliff based on air temperatures, solar radiation and the ice content of the sediments. The rate of thermal abrasion of ice‐rich coa...

Coastal processes along the west coast of Yamal

Abstract The section of coast under consideration is developed on generally fine‐grained sediments with substantial ice contents and with polygonal ice wedges in some areas. Thermal abrasion is producing a range of typical coastal types. These include “cornice coasts,”; where a layer of surface peat has been left overhanging by the retreat of a body of massive ground ice, and “thermokarst slump”; coasts where the landward retreat of a slump‐scar in ice‐rich sediments and massive ground ice has outrun the zone of wave action and thermal abrasion which initiated it. Maximum rates of coastal retreat due to thermal abrasion in this area are 7–10 m per year; mean rates of erosion do not exceed 5–6 m per year.

Thermoerosion of Frozen Sediment Under Wave Action

Ice‐bonded sediment is present in tundra cliffs and gravel structures in the southern Beaufort Sea. Thermoerosion of the frozen sediment under wave action can be important in determining the rate of coastal retreat and the severity of erosion of a gravel structure. The thermal processes involved in the thermoerosion of the frozen sediment are examined assuming the transport capacity of the wave action exceeds its melting capacity. The location of the melting surface of the frozen sediment is determined by solving the heat conduction equation in the frozen sediment exposed to the wave action, which causes the convective heat transfer between the sea water and the frozen sediment. Analytical solutions are presented for simplified one‐dimensional problems. A two‐dimensional finite element method based on a fixed domain approximation is proposed to facilitate the numerical computation. The example computation made for a frozen gravel causeway indicates that the melting of the frozen gravel will occur rapidly ...

Preservation potential of transgressive coastal lithosomes on the U.S. Atlantic shelf

Migration of coastal lithosomes across the continental shelf is a response to the latest Quaternary rise in sea level. Variable fractions of the transgressive sequence may be preserved, depending on pre-existing topography, depth of erosion, wave energy, sediment supply, erosion resistance, tidal range, and rate of relative sea-level change. Materials at greater depth in the stratigraphic column are more likely to be preserved. Sediment samples, seismic profiles, and bathymetric observations, indicate better preservation of shoreline elements on the outer shelf and more reworking on the inner shelf. This is hypothesized to be due primarily to the rate of sea-level rise. A relative sea-level curve for Delaware, obtained from radiocarbon dates on basal peats, rises smoothly from 25 m below present sea level 10,000 years B.P. The rate of rise decreases with time. Sea-level rise resulted in rates of coastal retreat of 20 m/yr 10,000 years B.P., 5 m/yr 5000 years B.P., and 1.5 m/yr at present. The long-term average rate of coastal retreat throughout the Holocene was 10 m/yr. In a conceptual model of constant volume of net erosion per unit length of coast, smaller depths of erosion are envisioned during the period of rapid coastal retreat early in the transgression, allowing a greater preservation potential. During the present slower rate of sea-level rise and coastal retreat, depths of erosion are greater, averaging 10 m off Delaware. In this situation there is a decreased potential for preservation of the coastal environmental lithosomes in a continuing transgression. Changes in factors such as wave climate, sediment supply, tidal range, and tectonics affect this simple model.

Fractional Preservation of Transgressive Coastal Lithosomes on Atlantic Continental Shelf: ABSTRACT

Migration of coastal environmental lithosomes across the continental shelf is a response to the latest Quaternary rise of the sea. Preservation of fractions of the transgressive sequence is dependent on depth of erosion, which is a function of impinging wave energy, sediment supply, resistance to erosion, and rate of relative sea-level change. Materials deeper in the column have a greater potential for preservation. The relative sea-level curve for Delaware, based on C14-dated basal peats, rises smoothly from 25 m below present from 10,000 years B.P. to the present at a decreasing rate with time. Shells and peats 9,000 to 10,000 years old on the shelf are 40 m deeper, suggesting an east-southeast shelf tilt, tectonically or hydro-isostatically induced. Sea-leve rise results in rates of coastal retreat of 102 m/year for 10,000 years B.P., 101 m/year for 5,000 years B.P., and 100 m/year at present. In a model of constant volume of net erosion per unit length of coast, a much smaller depth of erosion applies early in the transgression, allowing a greater preservation potential. Changes in wave climate, sediment supply, and downwarping across the shelf also apply. Recovered sediments, seismic profiles, and recognized morphic features indicate better preservation of shoreline elements on the outer shelf, and more planing off and reworking on the inner shelf. Similar analysis of Delaware Bay indicates that it too follows such a model, in changing from a dendritic fluvial system to a broad estuary. End_of_Article - Last_Page 675------------