Cliff Retreat Rates Research Articles

Sea stack formation and the role of abrasion on beach‐mantled headlands

AbstractSea stacks are common and striking coastal landforms, but few details are known about how, how quickly, and under what conditions they form. We present numerical and analytical models of sea stack formation due to preferential erosion along a pre‐existing headland to address these basic questions. On sediment‐rich rocky coasts, as sea cliffs erode and retreat, they produce beach sediment that is distributed by alongshore sediment transport and controls future sea cliff retreat rates. Depending on their width, beaches can encourage or discourage sea cliff erosion by acting either as an abrasive tool or a protective cover that dissipates wave energy seaward of the cliff. Along the flanks of rocky headlands where pocket beaches are often curved and narrow due to wave field variability, abrasion can accelerate alongshore‐directed sea cliff erosion. Eventually, abrasion‐induced preferential erosion can cut a channel through a headland, separating it from the mainland to become a sea stack. Under a symmetrical wave climate (i.e. equal influence of waves approaching the coastline from the right and from the left), numerical and analytical model results suggest that sea stack formation time and plan‐view size are proportional to preferential erosion intensity (caused by, for example, abrasion and/or local rock weakness from joints, faults, or fractures) and initial headland aspect ratio, and that sea stack formation is discouraged when the sediment input from sea cliff retreat is too high (i.e. sea cliffs retreat quickly or are sand‐rich). When initial headland aspect ratio is too small, and the headland is ‘rounded’ (much wider in the alongshore direction at its base than at its seaward apex), the headland is less conducive to sea stack formation. On top of these geomorphic and morphologic controls, a highly asymmetrical wave climate decreases sea stack size and discourages stack formation through rock–sediment interactions. Copyright © 2014 John Wiley & Sons, Ltd.

Measuring and Modeling the Retreat of the Summit Ice Fields on Kilimanjaro, East Africa

Terrestrial laser-scanning surveys of the south-facing cliff of the northern Ice Field on the summit crater of Kilimanjaro were taken on three occasions, in September 2004, January 2006, and August 2008. By comparing the three scans, the rates of lateral cliff retreat and surface lowering can be assessed. During 2004–2006, the mean lateral retreat was 1.39 m yr-1, falling to 0.89 m yr-1 during 2006–2008. These rates are broadly comparable with previous work using ablation stakes. Surface lowering is much less rapid, at 0.65 and 0.25 m yr-1, respectively. Analysis of seasonal forcings (radiation on a south-facing cliff, radiation on a flat surface, surface vapor pressure, and relative humidity) shows that most of the lateral retreat occurs during the austral summer, when direct radiative input is considerably higher on the south-facing ice cliff. On the ice surface, however, high-sun periods around the equinoxes dominate the surface lowering. Lowering is more during the wet than the dry seasons, which suggests that the current moisture availability on Kilimanjaro is not frequent enough to prevent lowering year round.

Chapter 6 The rock coast of continental Europe in the Atlantic

Abstract Rocky coasts occur along more than one-third (37%) of the Atlantic continental European coastline, approximately 3666 km, often forming vertical cliffs and characteristically gently sloping shore platforms. The continental European Atlantic rocky coasts show a great variability of rock types and structural contexts, as well as different wave climates and tidal ranges. Through a review of previously published data on cliff retreat rates and shore platform erosion measured on monthly, seasonal, annual and decadal timescales, this paper highlights the different processes and agents, their magnitude and frequency in shaping rocky coasts. In particular, the links between cliff retreat, shore platform evolution, present dynamics and inheritance (understood as whether platform and other rock coast features were shaped by a higher sea level than the present) comprise one of the major contributions from continental European Atlantic rocky coasts to a global understanding of rock coast coastal geomorphology.

The unsteady nature of sea cliff retreat due to mechanical abrasion, failure and comminution feedbacks

Sea cliff retreat is often linked to large waves, heavy precipitation and seismic events, but the specific operative mechanics have not been well constrained. In particular, what is the role of mechanical abrasion by beach sediments in cliff/platform evolution and how does it relate to the episodic nature of cliff retreat observed at certain locations? Here we present a simple, numerical model of sea cliff retreat that incorporates mechanical abrasion of a basal notch, threshold-controlled failure of the cantilevered block, and a feedback mechanism wherein retreat is dependent on the rate of sediment comminution within the surf zone. Using shore platform and cliff characteristics found in two coastal settings (the central California coast and the English North Sea coast), the model produces retreat rates comparable to those observed via field measurements. The highest retreat rates coincide with the steepest shore platforms and increasing wave height. Steeper platforms promote wave access to the cliff toe and, correspondingly, the receding cliff face produces additional accommodation space for the platform beach, preserving the erosive efficacy of the beach sediments. When exposed to energetic wave forcing, the slope of the inner platform segment controls retreat rates for concave platforms, whereas the slope of the outer platform segment exerts greater control for convex platforms. Platform beaches approached a long-term dynamic equilibrium on the concave profiles, leading to more consistent and steady retreat. Platform beaches were ephemeral on convex profiles, mirroring observed sand wave (Ord) migration on the Holderness coast, UK. These findings agree with previous field observations and support mechanical abrasion as a viable cause of temporal heterogeneity in cliff retreat rate for both coastlines.

Evolution and degradation of flat‐top mesas in the hyper‐arid Negev, Israel revealed from 10Be cosmogenic nuclides

ABSTRACTMesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat‐top plateau due to cliff retreat but ignores possible denudation of the mesa flat‐top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200–230 m) flat‐top and a thick caprock whereas the other two are characterized by a much narrower remnant flat‐top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide 10Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The 10Be data, combined with field observations, suggest a correlation between the width of flat‐top mesa and the denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat‐top mesa; (b) vertical denudation rates increase with decreasing width of the flat‐top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat‐top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical denudation rate which can persist for a very long time (>106 Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of denudation increases dramatically and mesa degradation is completed in a short time. Copyright © 2014 John Wiley & Sons, Ltd.

Unraveling the dynamics that scale cross‐shore headland relief on rocky coastlines: 2. Model predictions and initial tests

AbstractWe explore the behavior of a theoretical model of cross‐shore headland relief caused by alongshore differences in lithology and rock strength on rocky coastlines. Results address the question of why some rocky coasts exhibit frequent headland and embayment sequences while others evolve to a flat, smooth, and sandy configuration. Main model predictions are that cross‐shore headland amplitude is inversely proportional to beach sediment supply and the strength of wave energy convergence and divergence along the headland and bay, and proportional to the alongshore embayment length (or distance between headlands) and the difference between headland and bay rock strength. The coastline's initial physical properties (sea cliff height, composition, etc.) largely determine whether headlands will be persistent or transient landscape features. Model timescales over which the headland and bay reach steady state amplitude, or disappear to a flat coastline, range from 120 to 175,000 years depending on how close the initial amplitude is to steady state. In many cases, the coastline must evolve over several sea level highstands in order to reach equilibrium. A characteristic timescale (independent of initial conditions) shows that the coastline evolves most rapidly when: wave focusing is stronger; sea cliff rock is weaker or retreats faster in a given wave climate; the sea cliff retreat rate decreases rapidly as a function of beach width (i.e., the beach is very effective at dampening wave energy); and the coastline is sediment rich. Comparisons to nature suggest that our model is qualitatively capturing general rocky coastline dynamics and that modeled headland amplitudes are consistent with observed amplitudes.

Identification of hazardous zones combining cliff retreat rates with landslide susceptibility assessment

Epifânio, B., Zêzere, J.L., Neves, M., 2013. Identification of hazardous zones combining cliff retreat with landslide susceptibility assessment.Landslides are one of the main natural hazards which affect the coastal cliffs of Lourinhã council, located in Central Portugal. In this work, a methodology for hazard evaluation on coastal cliffs is presented, which combines the calculation of cliff retreat rates with landslide susceptibility assessment essential because landslide usually do not affect the cliff top. A landslide inventory was produced by aerial photo interpretation and a systematic field survey. The entire coastal stretch was split into 50m-long areas, resulting in 261 terrain mapping units. For each unit, 9 landslide predisposing factors were derived and classified. A predictive susceptibility model was developed using the Information Value method and the respective degree of fit was assessed using standard Receiver Operator Curves (ROC). Cliff retreat rates were calculated for a 60-year period using the ESRI′s Digital Shoreline Analysis System ArcGIS extension on a set of aerial photographs (which were manually geo-referenced and ortho-rectified) and a digital orthophotomap. Subsequently, predicted susceptibility data were integrated with the computed cliff retreat rates in a hazard matrix in order to create a map of the coastal cliff hazard. Cliff retreat rates ranged from 0.013 to 0.130m/yr and landslide susceptibility ranged from very high (4.2% of terrain unit) to low (0.8 % of terrain units). Approximately half of the study rocky coast is identified as being in a high hazard class. Thus, it is essential that hazard zones are considered for land use planning and management in order to reduce the probability of future incidents related to slope instability.

Coastal chalk platform erosion modulated by step erosion and debris shielding: example from Normandy and Picardy (northern France)

ABSTRACT Regard, V., Dewez, T., Cnudde, C. and Hourizadeh, N., 2013. Coastal chalk platform erosion modulated by step erosion and debris shielding: example from Normandy and Picardy (northern France) Coastal platforms and cliffs compose an inter-dependant dynamic system. To understand how cliffed coast evolve, it is therefore crucial to quantify the magnitude and dependence of their processes. Here we question the role of erosion by waves at platform steps, often linked to lithological discontinuities. Along the French chalky coast of the English Channel, we determine, by comparison of aerial photographs at 23 yr interval that the mesoscale steps are retreating at a rate of ~2 cm/yr, ca. 10% cliff retreat rate. This corresponds to an average erosion rate of 0.32 ± 0.08 mm/a, quite uniform across the study area, suggesting that the geology has more of a control on erosion processes than the position in the tidal frame. In reality, erosion occurs only where platform surface is not shielded by debris fallen ...

Late Holocene seacliff retreat recorded by 10Be profiles across a coastal platform: Theory and example from the English Channel

We explore a new method for documenting the long-term retreat rate of seacliffs based on measurements and modeling of 10Be concentration transects across present-day shore platforms. The proposed forward numerical model relies on a scenario of sea-level rise since the last deglaciation, and predicts the shape of 10Be concentration transects as a function of prescribed cliff recession and vertical coastal platform downwearing rates. Two independent transect features allow fitting the long-term recession rate model to field observations: a sharp 10Be concentration drop predicted at the former stationary location of the cliff during the last glacial period ∼100ka ago, and a characteristic dome shape whose magnitude is directly related to the recession rate of the cliff. A retreating chalk cliff site from the English Channel coast of France, at Mesnil-Val, where the 7m tidal range broadly exposes the shore platform, was selected to test the proposed method. Although retreat rates were too high to pinpoint the predicted 10Be concentration drop at the last glacial cliff position, the 10Be concentration of the flints sampled across the shore platform is consistent with the expected dome shape. When modeled using the proper tidal range and proposed Holocene sea-level rise history, the 10Be data yield a cliff retreat rate since the mid-Holocene of 11–13cm/yr. This is consistent with a 30-year-long measurement record, strongly supporting the utility of the 10Be method.

Beach and sea-cliff dynamics as a driver of long-term rocky coastline evolution and stability

We investigate rocky coastline evolution over millennial time scales using exploratory analytical and numerical models based on interactions between beaches and sea cliffs. In the models, wave-driven sea-cliff retreat is a nonlinear function of beach width, where cliff retreat is maximized by sediment abrasion and minimized by either a lack of beach sediment or too much sediment (which prevents waves from reaching the sea cliff). As sea cliffs retreat, beach sediment is produced and distributed alongshore by wave-driven sediment transport, and local beach widths determine future cliff retreat rates. Numerical experiments indicate that through such interactions, rocky coastlines can reach an equilibrium configuration where headlands and embayments remain stable through time, even in the absence of alongshore variations in sea-cliff lithology. Furthermore, the equilibrium coastline configuration, or the alongshore proportion of rocky headland to cliff-backed pocket beach, can be predicted analytically. Initial tests suggest that predictions match well qualitatively with actual landscapes.

Development of an automated method for continuous detection and quantification of cliff erosion events

AbstractThree intrusive systems of detection and quantification of coastal erosion events (using thermocouples and thermal pins) were developed and tested from 2005 to 2008 in different regions of the Gulf and maritime estuary of the St Lawrence (Quebec, Canada). The 3‐m‐long thermal pins inserted inside unconsolidated deposits allow the monitoring of erosion for a time period sometimes extending over several seasons. The thermocouple or thermocable method allows not only the instrumentation of unconsolidated deposits but also of rocky and cohesive substrate to a depth of 85 cm. An autonomous microclimatic station located near the experimental sites simultaneously samples temperature parameters, precipitation, snow cover, wind speed and direction as well as global radiation.The differential analysis of cliff thermal regime performed simultaneously with an analysis of air temperature makes it possible to determine the activation periods of coastal erosion processes. The results also make it possible to establish with precision the actual influence of rapid variations of certain climatic and microclimatic parameters (radiation, presence of snow cover, precipitation, etc.) on the physical state of surfaces and also on the activation of certain physical processes connected to coastal erosion events. The automated thermal erosion pin system (ATEPS) allows high temporal resolution (i.e. continuous) monitoring, enabling a real coupling of coastal erosion rates and climatic parameters. Preliminary results with the ATEPS system indicate that mild winter temperature and direct solar radiation are significant factors controlling cliff retreat rates. Moreover, the melting of segregation ice during the spring thaw contributed for more than 70% of cliff retreat against only 30% for frost shattering. Copyright © 2010 John Wiley & Sons, Ltd.

Sediment dynamics below retreating cliffs

ABSTRACTThe retreat of cliffs may constitute the dominant erosional response to base‐level fall in arid settings underlain by horizontally‐bedded sedimentary rock. These vertical cliffs typically loom above a relatively straight bedrock slope (‘plinth’) that is mantled with a thin layer of sediment and perched near the angle of repose. In detail, a plinth consists of a system of quasi‐parallel ridges and channels. We ask how the sediment supplied from a retreating cliff influences the erosion of the plinth hillslopes and channels, and how this affects the rate of cliff retreat. Motivated by field observations and high‐resolution topographic data from two sites in western Colorado, we develop a two‐dimensional (2D), rules‐based numerical model to simulate the erosion of channels draining a plinth and diffusive erosion of the intervening interfluves. In this model, retreat of a cliffband occurs when the height of the vertical cliff exceeds a threshold due to incision by channels on the plinth below. Debris derived from cliff retreat is distributed over the model plinth according to the local topography and distance from the source. This debris then weathers in place, and importantly can act to reduce local bedrock erosion rates, protecting both the plinth and ultimately the cliff from erosion. In this paper, we focus on two sets of numerical model experiments. In one suite, we regulate the rate of rockfall to limit the cliff retreat rate; in most cases, this results in complete loss of the plinth by erosion. In a second suite, we do not impose a limit on the cliff retreat rate, but instead vary the weathering rate of the rockfall debris. These runs result in temporally steady cliff‐plinth forms and retreat rates; both depend on the weathering rate of the debris. Copyright © 2011 John Wiley & Sons, Ltd.

Quaternary-scale evolution of sequences of talus flatirons in the hyperarid Negev

Talus flatiron sequences are ubiquitous landforms in arid and semiarid regions characterized by horizontal erodible rocks capped by more resistant rocks. Alternating phases of deposition and erosion lead to the formation of generations of talus flatirons in which ancient ones are located farther from the source cliff. The existing conceptual model of this systematic spatial distribution of talus flatirons is related to glacial–interglacial climatic cycles and to relatively high rates of cliff retreat. Three groups of talus flatirons were analyzed in the northeastern hyperarid Negev desert. The analyses include mapping, age determination using optically stimulated luminescence and cosmogenic nuclide exposure dating, electrical resistivity tomography and sedimentological and soil analyses. All talus flatirons contain gypsic-salic soil catena typical to hyperarid climate (< 80 mm yr − 1 ). No pedogenic indicators of past wetter environments such as buried calcic soil horizons, evidence of intense biogenic activity, or buried organic material were observed. 10Be exposure ages of the talus flatirons suggest that they were deposited during the middle Pleistocene; ~ 610 ka and ~ 170 ka for the oldest and intermediate-aged talus flatiron groups, respectively. These ages, combined with the present location of the talus flatirons relative to the source cliff yield retreat rates of 6–12 and ~ 200 m Ma − 1 for the bedrock cliff and talus flatiron apex, respectively, and in opposite directions. Our results show that (a) climatic changes at glacial–interglacial time scales are not the main controls over the formation of talus flatiron generation, and (b) that significant cliff retreat is not the main cause for the systematic spatial distribution of talus flatiron generations relative to the cliff. Our results indicate that the process of talus flatiron formation in the Negev desert must be associated with the balance between production, deposition, and removal of clasts under hyperarid conditions with only a minor cliff retreat.

East Australian marine abrasion surface

Abstract Almost one-third of the seabed off the coastline north and south of Sydney comprises a planated bedrock surface, evident from sidescan surveys over the inner continental shelf. In seismic records, this rock surface extends up to 23 km offshore from the sea cliffs along 300 km of the coast. The rock surface dips offshore to as much as 180 m below sea level, where it merges with a major unconformity in the shelf sediment wedge. The surface is eroded into Mesozoic and Palaeozic rocks and is heavily dissected by sediment-filled, palaeo-valley incision and structural jointing. The sediment-fills comprise sand wedges that thicken landwards to form beaches and estuarine flood-tide deltas, respectively, in smaller and larger palaeo-valleys incised to below present sea level. At the base of the cliffs, the planated surface is buried by shelf sand bodies up to 30 m thick in places. The seaward edge of the surface is everywhere buried by the onlapping continental-shelf sediment wedge. The contiguity of the abrasion surface with the unconformity in the shelf sediment wedge suggests that marine planation began in the Mid-Oligocene, indicating time-average rates of gross cliff retreat at about 1 mm a −1 .

Rates of slope decline, talus growth and cliff retreat along the shomyo river in central japan: a space‐time substitution approach

.On the basis of a space–time substitution, rates of cliff retreat and talus development overthe pasttens ofthousands of years were quantified for valley‐side slopes along the Shomyo River with a known rate of waterfall recession. Detailed profiles of the valley‐side slopes were obtained at 19 sites along the river by map reading and field survey, and the morphologic characteristics of the slopes were then measured. By combining the locations of the slopes with the recession rate of Shomyo Falls, it is estimated that the growth rate of the talus slope is 3.9–7.2 mm/yr, the rate of decline of the cliff is 2.0–4.0 × 10−4 deg/yr, and the retreat rate of the cliff top is 6.2–11.6 mm/yr.

Effects of Holocene climate and sea‐level changes on coastal gully evolution: insights from numerical modelling

AbstractGullies are sensitive to a range of environmental disturbances so they can provide insight into the environmental history of surrounding landscapes. Coastal gullies are of particular interest as they are influenced by terrestrial and marine processes. For example, the coastal gullies found on the Isle of Wight, UK, are highly dynamic, with episodes of sea cliff erosion causing rejuvenation of the channel network. Consequently a key factor in their long‐term evolution is the relative balance between rates of cliff retreat (driven primarily by Holocene sea‐level rise) and headwards incision caused by knickpoint migration (driven primarily by Holocene climate via its impact on runoff). In this paper we explore the Holocene erosional history of these gullies using a numerical landscape evolution model modified to include a cliff recession function. Knickpoint recession rates are simulated using a detachment‐limited erosion law wherein erosion rate is a power function of drainage area and stream gradient with model parameters defined using empirically‐derived data. Hindcast simulations, from 12 000 cal. years BP to present, are undertaken for a range of scenarios of Holocene climate change and sea‐level rise. Plausible erosional histories are extracted from scenarios in which simulated and observed gully forms match. The results suggest that rate of sea‐level rise is the key control on gully formation and that it is only in the late Holocene period, and specifically in the last 2000 years, that sea‐level rise has slowed sufficiently for knickpoint recession rates to exceed cliff recession rates and create sustainable gully networks. The simulations also indicate that the contemporary gully systems are close to a critical threshold, suggesting that future gully evolution is likely to be sensitive to small changes in rates of effective precipitation and/or sea‐level rise. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Erosion risk assessment of active coastal cliffs in temperate environments

The potentially negative consequences resulting from cliff recession are a matter of serious concern in many coastal areas worldwide. The assessment of such processes has traditionally been performed by calculating average cliff recession rates and projecting them into the future, without taking into consideration the diverse factors affecting cliff dynamics and stability. In this work a new, practical method is presented to evaluate cliff erosion risk in temperate environments, by analysing the main factors responsible for both the physical and the socioeconomic aspects of erosion, representing cliff loss potential and damage potential, respectively. For this purpose an integration of 11 physical variables (such as cliff lithology, beach characteristics or rainfall regime) and 6 socioeconomic variables (such as land use type or population density) is proposed. These variables are weighted and combined into a Hazard Index and an Impact Index, which in turn are merged into a composite Risk Index where the resulting values are normalized and expressed as a percentage of the maximum theoretical risk. The method is tested and validated by using data about cliff retreat rates and mass movement processes in the coast of Cádiz province (SW Spain). The proposed approach allows the zoning of coastal cliffs according to the risk, hazard and/or impact levels, including the recognition of critical areas where specific intervention strategies should be adopted. The method presented in this work is deemed both practical and scientifically valid, without requiring extensive and detailed surveys of the area where it is to be applied. This way, it constitutes an easy to use, valuable tool for decision-making regarding land use planning and management strategies for active coastal cliffs.

Rates and Trends of Coastal Change in California and the Regional Behavior of the Beach and Cliff System

The U.S. Geological Survey (USGS) recently completed an analysis of shoreline change and cliff retreat along the California coast. This is the first regional, systematic measurement of coastal change conducted for the West Coast. Long-term (∼120 y) and short-term (∼25 y) shoreline change rates were calculated for more than 750 km of coastline, and 70 year cliff-retreat rates were generated for 350 km of coast. Results show that 40% of California's beaches were eroding in the long term. This number increased to 66% in the short term, indicating that many beaches have shifted toward a state of chronic erosion. The statewide average net shoreline change rates for the long and short term were 0.2 m/y and −0.2 m/y, respectively. The long-term accretional signal is likely related to large coastal engineering projects in some parts of the state and to large fluxes of sediment from rivers in other areas. The cliff-retreat assessment yielded a statewide average of −0.3 m/y. It was found that Northern California has the highest overall retreat rates, which are influenced by erosion hot spots associated with large coastal landslides and slumps. The databases established as part of the shoreline change and cliff-retreat analyses were further investigated to examine the dynamics of the beach/cliff system. A correlation analysis identified a strong relationship between the geomorphology of the coast and the behavior of the beach/cliff system. Areas of high-relief coast show negative correlations, indicating that higher rates of cliff retreat correlate with lower rates of shoreline erosion. In contrast, low-to moderate-relief coasts show strong positive correlations, wherein areas of high shoreline change correspond to areas of high cliff retreat.

Evaluation of Cliff Retreat and Beach Nourishment in Southern Portugal Using Photogrammetric Techniques

Abstract This paper develops an evolutionary model of shoreline retreat along the cliffed coast of Forte Novo–Garrao (Algarve, Portugal) and investigates the relationship of the retreat pattern to coastal protection works, to the Quarteira fishing harbor, and to artificial beach nourishment at Vale do Lobo. The amounts and rates of cliff retreat for six coastal cliff sectors were measured using high-precision digital photogrammetric techniques utilizing data from three aerial photograph surveys conducted in 1991, 1999, and 2001. A geographic information system was used to integrate photogrammetric data and to quantify the temporal and spatial patterns of retreat. Forte Novo was the cliff sector with the highest amount of retreat, 22.7 m between 1991 and 2001. The other coastal sectors recorded retreats of less than 10 m for the same period. Retreat rates ranged from 2.27 m/y−1 at Forte Novo to a minimum retreat rate of 0.14 m/y−1 at Garrao. The replenishment measures at the Vale do Lobo beachfront have be...

Temporal and spatial variations of chalk cliff retreat in East Sussex, 1873 to 2001

Spatial and temporal variations of chalk cliff retreat rates over a 130 yr period are examined for 22 km of the East Sussex chalk coastline between Brighton and Belle TouteTout lighthouse, England. Data are derived from maps surveyed between 1873 and 1925/28 and air photographs dating from 1973 and 2001. The long-term average cliff retreat rate over the period 1873 to 1973 is 0.36 m y − 1 and from 1873 to 2001 is 0.35 m y − 1 . Over the whole 23 km study area, rates calculated over shorter time slices indicate a decline in cliff retreat: 0.37 m y − 1 between 1873 and 1925/28; 0.35 m y − 1 between 1925/28 and 1973; 0.27 m y − 1 between 1973 and 2001. Temporal variations are unlikely to be due to variations in precipitation and frost days though the relationship remains uncertain due to the resolution of available cliff retreat and climatic data. Changes in wave climate are small and cannot be responsible for the decline in cliff retreat. The study suggests that the most important factors causing the measured decline in cliff retreat rates are firstly, the loss of cliff toe beaches over time and secondly, widening of the rocky shore platforms that front the cliffs. Some of the spatial variability in retreat rates can be explained by local variations in lithology.