Cliff Retreat Rates Research Articles

Documenting a Century of Coastline Change along Central California and Associated Challenges: From the Qualitative to the Quantitative

Wave erosion has moved coastal cliffs and bluffs landward over the centuries. Now climate change-induced sea-level rise (SLR) and the changes in wave action are accelerating coastline retreat around the world. Documenting the erosion of cliffed coasts and projecting the rate of coastline retreat under future SLR scenarios are more challenging than historical and future shoreline change studies along low-lying sandy beaches. The objective of this research was to study coastal erosion of the West Cliff Drive area in Santa Cruz along the Central California Coast and identify the challenges in coastline change analysis. We investigated the geological history, geomorphic differences, and documented cliff retreat to assess coastal erosion qualitatively. We also conducted a quantitative assessment of cliff retreat through extracting and analyzing the coastline position at three different times (1953, 1975, and 2018). The results showed that the total retreat of the West Cliff Drive coastline over 65 years ranges from 0.3 to 32 m, and the maximum cliff retreat rate was 0.5 m/year. Geometric errors, the complex profiles of coastal cliffs, and irregularities in the processes of coastal erosion, including the undercutting of the base of the cliff and formation of caves, were some of the identified challenges in documenting historical coastline retreat. These can each increase the uncertainty of calculated retreat rates. Reducing the uncertainties in retreat rates is an essential initial step in projecting cliff and bluff retreat under future SLR more accurately and in developing a practical adaptive management plan to cope with the impacts of coastline change along this highly populated edge.

Sedimentary Coastal Cliffs of Normandy: Modalities and Quantification of Retreat

Costa, S.; Maquaire, O.; Letortu, P.; Thirard, G.; Compain, V.; Roulland, T.; Medjkane, M.; Davidson, R.; Graff, K.; Lissak, C.; Delacourt, C.; Duguet, T.; Fauchard, C., and Antoine, R., 2019. Sedimentary coastal cliffs of Normandy: Modalities and quantification of retreat. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 46–60. Coconut Creek (Florida), ISSN 0749-0208.This paper examines the spatial and temporal variations of cliff retreat rates over multi-temporal data of the Normandy cliffs. Data are derived from historical maps and aerial photographs and from recent lasergrammetry and photogrammetry monitoring. The diachronic analysis of all these data gives retreat rates of -0.1 to -0.5 m/yr. in line with the international literature. The spatial variations of the cliff retreat rates, at the Normandy scale, can be explained by geological structure, especially at the cliff foot, but also by the influence of cliff collapses or anthropogenic obstacles that disrupt the longshore drift. Multi-temporal data shows that the evolution of the cliffs occurs on scales from 10 to 70 years according to the lithology. The high resolution and frequency monitoring also provide information about the factors responsible for triggering gravitational landslides (rockfalls, slides, debris falls). The study proposes a regional warning threshold for the cliff characterized by landslides, under the dominating influence of rainfall and groundwater level evolution. In this respect the monitoring is inconclusive for chalk and limestone cliffs, because the origin of evolutions is more multifactorial (combination of more continental and marine processes).

Multi-Temporal Cliff Erosion Analysis Using Airborne Laser Scanning Surveys

Rock cliffs are a significant component of world coastal zones. However, rocky coasts and factors contributing to their erosion have not received as much attention as soft cliffs. In this study, two rocky-cliff systems in the southern Baltic Sea were analyzed with Airborne Laser Scanners (ALS) to track changes in cliff morphology. The present contribution aimed to study the volumetric changes in cliff profiles, spatial distribution of erosion, and rate of cliff retreat corresponding to the cliff exposure and rock resistance of the Jasmund National Park chalk cliffs in Rugen, Germany. The study combined multi-temporal Light Detection and Ranging (LiDAR) data analyses, rock sampling, laboratory analyses of chemical and mechanical resistance, and along-shore wave power flux estimation. The spatial distribution of the active erosion areas appear to follow the cliff exposure variations; however, that trend is weaker for the sections of the coastline in which structural changes occurred. The rate of retreat for each cliff–beach profile, including the cliff crest, vertical cliff base, and cliff base with talus material, indicates that wave action is the dominant erosive force in areas in which the cliff was eroded quickly at equal rates along the cliff profile. However, the erosion proceeded with different rates in favor of cliff toe erosion. The effects of chemical and mechanical rock resistance are shown to be less prominent than the wave action owing to very small differences in the measured values, which proves the homogeneous structure of the cliff. The rock resistance did not follow the trends of cliff erosion revealed by volume changes during the period of analysis.

Dip, layer spacing, and incision rate controls on the formation of strike valleys, cuestas, and cliffbands in heterogeneous stratigraphy

AbstractLandscapes developed over heterogeneous stratigraphy exhibit a spectrum of landforms from dramatic cliffbands to hogbacks, depending on the dip and spacing of the layers. In deeply incised landscapes, a single cliffband may consist of multiple resistant layers, whereas similar stratigraphy elsewhere is separated by strike valleys into individual cuesta benches or hogbacks. This paper presents a geometric analysis, informed by a numerical landscape model, to explain the conditions for development of a strike valley floored by erodible rocks. The results define a threshold incision rate below which strike valleys are more likely to form; this threshold incision rate is proportional to the stratigraphic spacing of cliff-forming layers and a trigonometric function of dip angle. The analysis also yields a time scale for the adjustment of structural landforms to changes in regional incision rate, which is a function of dip angle and the coupling between cliff retreat rate and escarpment height. In example landscapes of the Colorado Plateau, this time scale is likely much longer than that of documented variations of incision rates due to late Quaternary climate and land-use changes. The transitional state of escarpments in layered rock may therefore contain information about regional downcutting rates over time scales different from those recorded by the fluvial network. The utility of such features will require better understanding of the coupling between incision of a foot slope and the retreat rate of the cliff above in different kinds of rocks.

Global distribution of coastal cliffs

AbstractPrevious studies have estimated that coastal cliffs exist on about 80% of the global shoreline, but have not been validated on a global scale. This study uses two approaches to capture information on the worldwide existence and erosion of coastal cliffs: a detailed literature survey and imagery search, and a GIS‐based global mapping analysis. The literature and imagery review show coastal cliffs exist in 93% of the combined recognized independent coastal states and non‐independent coastal regions worldwide (total of 213 geographic units). Additionally, cliff retreat rates have been quantified in at least one location within 33% of independent coastal states and 15% of non‐independent regions. The GIS‐based mapping used the near‐global Shuttle Radar Topography Mission 3 arc second digital elevation model and Arctic Coastal Dynamics Database to obtain near‐global backshore coastal elevations at 1 km alongshore intervals comprising about 1,340,000 locations (81% of the world vector shoreline). Backshore coastal elevations were compared with the mapped distribution of European coastal cliffs to produce a model training set, and this relationship was extended globally to map the likelihood of coastal cliff locations. About 21% of the transects (17% of the world vector shoreline) were identified as mangroves and eliminated as potential cliff locations. The results were combined with estimates of cliff percentages for Greenland and Antarctica from the literature, extending the global coverage to estimate cliff occurrence across 89% of the world vector shoreline. The results suggest coastal cliffs likely exist on about 52% of the global shoreline. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Overestimation of short‐term coastal cliff retreat rates in the eastern Mediterranean resolved with a sediment budget approach

AbstractOur understanding of sea‐cliff erosion processes and their response to recent and/or projected environmental changes such as sea‐level rise, climate change and anthropogenic development hinges on our ability to quantify sea‐cliff retreat rates and their variability through time. Here, we focus on Israel's Mediterranean ‘Sharon’ sea‐cliff as a case study for examining the significance of recent short‐term (i.e. annual to decadal) cliff‐top retreat rates that appear to exceed longer‐term rates of ‘background’ (i.e. centennial to millennial) retreat by 1–2 orders of magnitude. We demonstrate that an inherent sampling bias in rate estimates inferred from observation intervals shorter than process episodicity can also explain such a pattern. This potential ambiguity leads to a striking paradox where despite highly accurate and robust documentation of recent cliff‐top retreat, such as that obtained from aerial photographs and/or instrumental surveys, the short‐term retreat rates of episodically retreating sea cliffs remain poorly constrained. To address this key data gap along the Sharon sea cliff we employed a sediment budget approach that focuses on quantifying the continuous wave scouring of cliff‐collapsed material from the shore platform as a rate‐limiting process for episodic retreat of the cliff above. We used four high‐resolution (0.5 m/pixel) airborne LiDAR data sets acquired between 2006 and 2015 to determine short‐term maximum retreat rates of up to ~0.08 m/yr during this nine‐year period. These modern retreat rates compare to the cliff's background retreat rate of 0.03 to 0.09 m/yr since the mid‐Holocene, as determined herein from multiple geologic and archeological observations. Our results demonstrate that previously reported twentieth century cliff‐top retreat rates for this sea cliff, which range up to values of several meters per year, are biased and that sea‐cliff erosion rates have not yet been significantly impacted by recent environmental changes in the eastern Mediterranean basin, such as the restriction of sediment supply following emplacement of the Nile's Aswan dam system. © 2018 John Wiley & Sons, Ltd.

Geo-archaeological markers reveal magnitude and rates of Israeli coastal cliff erosion and retreat

Geo-archaeological studies along the Mediterranean coast of Israel and its seabed have revealed shipwrecks, anchorages, coastal installations and natural features that can act as markers to estimate the formation date and retreat rates of the coastal cliff of central Israel. The Sharon coastal ridge consists of alternating layers of kurkar (local term for aeolian carbonate-cemented, quartz sandstone) and poorly consolidated palaeosol deposits. The ridge was formed during the Late Pleistocene (about 70,000 to 10,000 yr. BP). At about 7,500 yr. BP, sea level reached the western edge of the present coastal ridge, currently located about 8 m below the present sea level, and a coastal cliff developed. Since then the cliff has continuously been eroded and retreated eastward by natural processes, as well as by anthropogenic impact. This article is an interdisciplinary geo-archaeological study of the extent and rates of retreat of the coastal cliff over the last 7,500 years. The findings suggest that overall the cliff has retreated about 730 m in this period, at an average rate of 9.7 cm/yr. However, the study shows that a considerably higher rate of cliff retreat occurred between about 7,500 and 3,900 yr. BP (about 650 m in about 3,600 years, at about 18 cm/yr). Sea level reached its present level at about 4,000 yr. BP (Middle Bronze Age) and has not changed significantly since. Since the Middle Bronze Age, the cliff has retreated about 80 m in 3,900 years (at about 2 cm/yr). Human activity and sea level rise during the last 100 years have significantly accelerated coastal erosion and cliff retreat.

GlobR2C2 (Global Recession Rates of Coastal Cliffs): a global relational database to investigate coastal rocky cliff erosion rate variations

Abstract. Rocky coast erosion (i.e., cliff retreat) is caused by a complex interaction of various forcings that can be marine, subaerial or due to rock mass properties. From Sunamura's seminal work in 1992, it is known that cliff retreat rates are highly variable over at least four orders of magnitude, from 1 to 10 mm yr−1. While numerous local studies exist and explain erosion processes at specific sites, there is a lack of knowledge at the global scale. In order to quantify and rank the various parameters influencing erosion rates, we compiled existing local studies into a global database called GlobR2C2 (which stands for Global Recession Rates of Coastal Cliffs). This database reports erosion rates from publications, cliff setting and measurement specifications; it is compiled from peer-reviewed articles and national databases. In order to be homogeneous, marine and climatic forcings were recorded from global models and reanalyses. Currently, GlobR2C2 contains 58 publications that represent 1530 studied cliffs and more than 1680 estimated erosion rate. A statistical analysis was conducted on this database to explore the links between erosion rates and forcings at a global scale. Rock resistance, inferred using the criterion of Hoek and Brown (1997), is the strongest signal explaining variation in erosion rate. Median erosion rates are 2.9 cm yr−1 for hard rocks, 10 cm yr−1 for medium rocks and 23 cm yr−1 for weak rocks. Concerning climate, only the number of frost days (number of day per year below 0 ∘C) for weak rocks shows a significant, positive, trend with erosion rate. The other climatic and marine forcings do not show any clear or significant relationship with cliff retreat rate. In this first version, GlobR2C2, with its current encompassing vision, has broad implications. Critical knowledge gaps have come to light and prompt a new coastal rocky shore research agenda. Further study of these questions is paramount if we one day hope to answer questions such as what the coastal rocky shore response to sea-level rise or increased storminess may be.

Quantification of cliff retreat in coastal Quaternary sediments using anatomical changes in exposed tree roots

AbstractSea cliffs represent 80% of the world's coasts and can be found virtually in all types of morphogenetic environments. Most studies on rocky environments focused on the impacts of modern sea level rise on cliff stability derived from sequential surveys, direct measurements or erosional features in anthropogenic structures. In this study, we explore the potential of dendrogeomorphic techniques to quantify multidecadal changes in coastal environments on Porquerolles Island (France). We sampled a total of 56 cross‐sections from 16 Pinus halepensis Mill. roots growing on former alluvial deposits and on sandy‐gravelly cliffs to quantify mean annual cliff retreat rates as well as changes in cliff geometry. Anatomical changes in roots have been used successfully in the past to quantify continuous denudation rates on slopes, channel incision and gullying processes but the approach has not been used so far in a coastal cliff context. At Porquerolles Island, reconstructed rates of cliff retreat cover 30–40 years and show average erosion rates between 0.6 and 3.9 cm yr−1 (average: 2.1 cm yr−1). Highest rates are observed at Pointe de la Tufière (2.6–3.9 cm yr−1), a small rock promontory that is more exposed to wave and storm surges than the remainder of the study area. By contrast, lower erosion rates are recorded at cliffs protected by the La Courtade pocket beach (0.6–1.9 cm yr−1). This contribution demonstrates that dendrogeomorphic analyses of roots clearly have a significant potential and that they are a powerful tool for the quantification of multidecadal rates of cliff retreat in areas where measurements of past erosion are lacking. More specifically, the approach also has clear advantages over the shorter time series obtained with repeat monitoring (e.g. terrestrial laser scanning, sensors, erosion pins) or over longer, but more coarsely resolved records obtained from aerial photographs or radio‐nuclides. © 2018 John Wiley & Sons, Ltd.

A Model Ensemble for Projecting Multidecadal Coastal Cliff Retreat During the 21st Century

AbstractSea cliff retreat rates are expected to accelerate with rising sea levels during the 21st century. Here we develop an approach for a multimodel ensemble that efficiently projects time‐averaged sea cliff retreat over multidecadal time scales and large (>50 km) spatial scales. The ensemble consists of five simple 1‐D models adapted from the literature that relate sea cliff retreat to wave impacts, sea level rise (SLR), historical cliff behavior, and cross‐shore profile geometry. Ensemble predictions are based on Monte Carlo simulations of each individual model, which account for the uncertainty of model parameters. The consensus of the individual models also weights uncertainty, such that uncertainty is greater when predictions from different models do not agree. A calibrated, but unvalidated, ensemble was applied to the 475‐km‐long coastline of Southern California (USA), with four SLR scenarios of 0.5, 0.93, 1.5, and 2 m by 2100. Results suggest that future retreat rates could increase relative to mean historical rates by more than twofold for the higher SLR scenarios, causing an average total land loss of 19–41 m by 2100. However, model uncertainty ranges from ±5 to 15 m, reflecting the inherent difficulties of projecting cliff retreat over multiple decades. To enhance ensemble performance, future work could include weighting each model by its skill in matching observations in different morphological settings.

Long‐term retreat rates of Israel's Mediterranean sea cliffs inferred from reconstruction of eroded archaeological sites

AbstractErosion and inland retreat of coastal cliffs present one of the most dynamic earth‐surface processes presently challenging coastlines. The rate of cliff retreat is central to coastal planning and protection of shoreline infrastructure. Until now, the majority of retreat rate estimates have been based on aerial photos from the past century, and therefore do not provide multicentury estimates of retreat rates. Here, we studied Bronze Age to Crusader archeological sites (∼3700 years) located on Israel's coastal cliff and used their spatial relation to the cliff to estimate the long‐term centurial‐millennial retreat rates of the cliff. To accomplish this, original layouts of partially eroded archaeological structures were reconstructed and compared to the modern coastal cliff line. The eroded parts of the studied structures and their original age constrained the maximum timing of the retreat. The resulting retreat rates are significantly lower than those previously calculated using observations of around 100 years. The archeological data also record the episodicity of the cliff failure events. The research highlights both the issue of the loss of valuable archaeological cultural resources, and simultaneously the usefulness of eroding coastal archaeological features to resolve questions of modern significance.

Zooming in and out: Scale dependence of extrinsic and intrinsic factors affecting salt marsh erosion

AbstractSalt marshes are valuable ecosystems that provide important ecosystem services. Given the global scale of marsh loss due to climate change and coastal squeeze, there is a pressing need to identify the critical extrinsic (wind exposure and foreshore morphology) and intrinsic factors (soil and vegetation properties) affecting the erosion of salt marsh edges. In this study, we quantified rates of cliff lateral retreat (i.e., the eroding edge of a salt marsh plateau) using a time series of aerial photographs taken over four salt marsh sites in the Westerschelde estuary, the Netherlands. In addition, we experimentally quantified the erodibility of sediment cores collected from the marsh edge of these four marshes using wave tanks. Our results revealed the following: (i) at the large scale, wind exposure and the presence of pioneer vegetation in front of the cliff were the key factors governing cliff retreat rates; (ii) at the intermediate scale, foreshore morphology was partially related to cliff retreat; (iii) at the local scale, the erodibility of the sediment itself at the marsh edge played a large role in determining the cliff retreat rate; and (iv) at the mesocosm scale, cliff erodibility was determined by soil properties and belowground root biomass. Thus, both extrinsic and intrinsic factors determined the fate of the salt marsh but at different scales. Our study highlights the importance of understanding the scale dependence of the factors driving the evolution of salt marsh landscapes.

Analysis of shoreline changes and cliff retreat to support Marine Spatial Planning in Shabla Municipality, Northeast Bulgaria

Measuring historical shoreline change and cliff retreat is an essential aspect of understanding the long-term geomorphic evolution of any coastal system. These data are also critical for predicting future changes and incorporating that data into proper coastal management. In recent years, Marine Spatial Planning (MSP) has increasingly been recognized as an important tool in the sustainable management of marine space across Europe and worldwide, including also issues of coastal zone management and land-sea interactions (Directive 2014/89/EU for MSP). This paper deals with investigation of the shoreline changes and cliff retreat along the 34 km section of the Black Sea coast at Shabla Municipality (Northeast Bulgaria). The study area has a low density of development and comprises large sand beaches, dunes and rapidly retreating loess cliffs. The present research was formulated under the European Union (EU) Directive for MSP in the Black Sea Basin (MARSPLAN-BS) Project. The goal was to provide reliable data and useful information in support of the development of a pilot marine spatial plan for Shabla Municipality. The study was focused on the analysis of shoreline movement (at sand beaches – erosion/accretion) and cliff retreat (at rocky coasts) utilizing a Geographic Information System (GIS). Primary data sources include topographic maps in scale 1:5,000, tachometry survey and modern Very High Resolution (VHR) orthophoto images. Change rates were determined using the Digital Shoreline Analysis System (DSAS), an ArcGIS extension for calculating shoreline change (Version 4.3), and rate-of-change statistical method. Management of coastal erosion, hard engineering methods and lack of required setbacks were discussed. Results for rates of shoreline change and cliff retreat were verified with preliminary field measurement studies and cross-checked with existing hydrodynamics, geology and geomorphology of the coast of Shabla Municipality.

Controls on the distribution of cosmogenic <sup>10</sup>Be across shore platforms

Abstract. Quantifying rates of erosion on cliffed coasts across a range of timescales is vital for understanding the drivers and processes of coastal change and for assessing risks posed by future cliff retreat. Historical records cover at best the last 150 years; cosmogenic isotopes, such as 10Be could allow us to look further into the past to assess coastal change on millennial timescales. Cosmogenic isotopes accumulate in situ near the Earth surface and have been used extensively to quantify erosion rates, burial dates and surface exposure ages in terrestrial landscapes over the last 3 decades. More recently, applications in rocky coast settings have quantified the timing of mass wasting events, determined long-term averaged rates of cliff retreat and revealed the exposure history of shore platforms. In this contribution, we develop and explore a numerical model for the accumulation of 10Be on eroding shore platforms. In a series of numerical experiments, we investigated the influence of topographic and water shielding, dynamic platform erosion processes, the presence and variation in beach cover, and heterogeneous distribution of erosion on the distribution of 10Be across shore platforms. Results demonstrate that, taking into account relative sea level change and tides, the concentration of 10Be is sensitive to rates of cliff retreat. Factors such as topographic shielding and beach cover act to reduce 10Be concentrations on the platform and may result in overestimation of cliff retreat rates if not accounted for. The shape of the distribution of 10Be across a shore platform can potentially reveal whether cliff retreat rates are declining or accelerating through time. Measurement of 10Be in shore platforms has great potential to allow us to quantify long-term rates of cliff retreat and platform erosion.

Recent acceleration in coastal cliff retreat rates on the south coast of Great Britain

Rising sea levels and increased storminess are expected to accelerate the erosion of soft-cliff coastlines, threatening coastal infrastructure and livelihoods. To develop predictive models of future coastal change we need fundamentally to know how rapidly coasts have been eroding in the past, and to understand the driving mechanisms of coastal change. Direct observations of cliff retreat rarely extend beyond 150 y, during which humans have significantly modified the coastal system. Cliff retreat rates are unknown in prior centuries and millennia. In this study, we derived retreat rates of chalk cliffs on the south coast of Great Britain over millennial time scales by coupling high-precision cosmogenic radionuclide geochronology and rigorous numerical modeling. Measured 10Be concentrations on rocky coastal platforms were compared with simulations of coastal evolution using a Monte Carlo approach to determine the most likely history of cliff retreat. The 10Be concentrations are consistent with retreat rates of chalk cliffs that were relatively slow (2-6 cm⋅y-1) until a few hundred years ago. Historical observations reveal that retreat rates have subsequently accelerated by an order of magnitude (22-32 cm⋅y-1). We suggest that acceleration is the result of thinning of cliff-front beaches, exacerbated by regional storminess and anthropogenic modification of the coast.

A regional approach for modeling cliff retreat rate: The Makhteshim Country, Israel

Cliff retreat rate significantly affect the evolution of landforms and cliff stability. Cliff retreat studies also provide intriguing clues regarding past geomorphic conditions and environmental changes. We hereby present a model to calculate cliff retreat rate based on spatial data of cliff structure and morphology. The model is applied to numerous cliffs in the arid Makhteshim Country, Israel, and results are calibrated using published rates of two local cliffs. The calculated retreat rates confirm previous assertions that the crater cliffs are receding very slowly, but reveal that the rates vary significantly along the cliffs (1–18cmky−1). Results also provide first estimates of retreat rates of other major cliffs in the region including fast retreat rates at the Sede Zin cliff (300–600cmky−1). The proposed model provides a robust analysis to account for local cliff–talus morphology and yields rate estimates representative of current conditions rather than a long-term (geologic) average rate. Results presented constitute important new insights into regional geomorphic processes and on the stability of specific cliff sections within the study area.

Spatial and temporal variations in rockfall determined from TLS measurements in a deglaciated valley, Switzerland

AbstractSteep U‐shaped valleys produced by glaciation are commonly eroded by rockfall. In this study we investigate modern and long‐term (power law predicted) rates of rock wall retreat in a deglaciated valley. Our emphasis is on the 5.2 km2 calcareous cliffs covering in the Lauterbrunnen Valley, Switzerland. Terrestrial laser scans were collected during nine field campaigns over 18 months to provide a continuous coverage of most of the valley walls. Results indicate a total of 122 rockfalls with volumes ranging between 0.06 ± 0.01 and 119.34 ± 1.07 m3. Different size groups of rockfall events were correlated with environmental factors (e.g., freeze‐thaw cycles/temperature, precipitation, and seismicity) using a linear regression with variable lag times of 0–6 months. The highest correlation factor (r = 0.6, P = 0.08, 90% significance level) is observed for freeze‐thaw cycles and rockfall events smaller than 1 m3 with a 2 month delay between temperature extremes and rockfall. Frequency‐magnitude relationships for rockfall events were calculated to predict less frequent larger rockfall events that did not occur during the observation period. A Monte Carlo analysis was applied to the frequency‐magnitude relationship to evaluate the sensitivity of results to their stochastic nature. An average power law exponent of 1.71 ± 0.09 was calculated using linear regression. The calculated exponents are similar to other studies in calcareous rock settings conducted over different observation durations. Finally, we determine an average total eroded volume of m3/yr and a corresponding long‐term (power law predicted) cliff retreat rate of .

Retreat rates, modalities and agents responsible for erosion along the coastal chalk cliffs of Upper Normandy: The contribution of terrestrial laser scanning

In order to follow all the changes affecting the coastal chalk cliff face in Upper Normandy and improve knowledge about cliff erosion, repeated terrestrial laser scanning (TLS) surveys were carried out frequently between 2010 and 2013 (every 4–5 months). They were conducted at two sites with similar lithostratigraphic characteristics but different exposures to marine actions (the former being an abandoned cliff and the latter an active cliff). They provide a quantification of the production of debris with centimeter precision (from ±0.01 to 0.04m). These surveys provided three major outcomes: 1) cliff retreat rates were measured at high spatial resolution with retreat values, unsurprisingly, 3–4 times higher for an active cliff than for an abandoned cliff. This result highlights that marine actions should be seen as not only a transport agent but also a particularly effective erosion agent; 2) a significant proportion of debris fall production (about 25%) in the total active cliff retreat was identified; and 3) one of the modalities of retreat was visualized as the creation of a basal notch, which propagates instability towards the upper part of the cliff face. Later, this instability generates rock falls coming from the whole cliff face.

MORPHOLOGY OF THE BLACK SEA COAST BETWEEN LAGERNAYA SPIT AND THE OCHAKOVSKIY CAPE

The studied sea coast settle down between Berezansky strait and Ochakovskiy Cape with long is 16 km. The coast represented the abrasion site (active clayey cliff) and the accumulative site (the sandy Lagernaya spit). In nature the cliff height is about 25 m, and average rate of abrasion cliff retreat 2.5 m/year. The Lagernaya spit has length about 6 km, height to 2 m and width to 150-200 m. It is linked with active cliff and displaced towards liman. Nowadays Lagernaya spit is in a degradation stage by impact of anthropogenous factor. Beaches are disappear gradually. The coast is built strongly up. The submarine slope is shallow up to 0.001-0.005. As a result, the wave factor is low, and at the same time impact of storm-surges is heightened. Artificial relief forms wide distributed as a hydrotechnical constructions that are sea-walls, jetties, moorings, and bearms, groins, sea-port dippers, navigate canals too located. All of them are blocking abrasive sources of shore sediments in coastal zone, and its are aggravating the sediment deficit and increase destructive tendencies of nature evolution. Around of Kinburn Strait typical is relative the sea level rise. During past 15 years the average rise value is up to 9,8 mm/year, and as a result this process lead to submerging of Lagernaya Spit, Ochakovskiy Cape terrace and Kinburn Peninsula. Also, rising of underground water, activization of landslides, salting waters in Dnepr and Southern Bug mouths, reinforcement of stormy waves development take place in investigated coastal region.

Sediment input from fluvial sources and cliff erosion to the continental shelf of Argentina

The coasts of southern Buenos Aires, Patagonia and Tierra del Fuego are dominated by cliff erosion. Mean rates of cliff retreat are estimated to be about 0.5-0.6 m/year by comparing old photographs with modern satellite images. Considering the height of the Patagonian and Fueguian cliffs (70 to 120 m), the volume of sediment eroded from these cliffs exceeded the volumes provided by the erosion of the cliffs of Buenos Aires (10 to 20 m height). These erosion rates support an estimated delivery of 217 million tons of sediment per year to the continental shelf, exceeding significantly the 22 millions of tons/year transported by the larger Patagonian rivers Negro and Colorado. However, the contribution of these rivers has decreased since the Late Pleistocene changes in the direction of transport of some watersheds. The Chubut and Chico de Santa Cruz rivers suffered reductions of 21-24% in their watershed areas, resulting in reductions of about 33-34% in the volume of water transported to the Atlantic Ocean per year. As the amount of sediment delivered to the Argentine continental shelf by cliff erosion is higher than the fluvial transport, it should be also considered in the balance of beaches fed by longshore transport.