Stéphane Costa

Retreat of Chalk cliffs in the eastern English Channel during the last century

Abstract Please click here to download the map associated with this article. The retreat of chalk cliffs fringing the eastern English Channel contributes shingle to the beaches which helps to protect the cliffs and slow down erosion. Conversely, cliff retreat endangers settlements and infrastructure on the clifftop. Rates of retreat have been calculated by a variety of methods over the past century, but no attempt has been made to provide a complete coverage that allows for a true comparison of retreat rates over the entire coastline. Using historic maps and recent orthophotos, cliff retreat rates have been calculated for consecutive 50 m sections of chalk cliff along the English side of the entire eastern English Channel for a period of ∼ 125 years. The chalk cliffs of East Sussex erode at an average rate of 0.25—0.3 m y−1 while those in Kent at a rate of ∼ 0.1 m y−1.

Fronts froids et submersions de tempête dans le nord-ouest de la France Le cas des inondations par la mer entre l'estuaire de la Seine et la baie de Somme

Cold fronts and storm floods in the north-west of France In the north-west of France, particularly along the coasts of upper Normandy and Picardy, storm floods result mainly from a phenomenon of storm tide (astronomical tide plus storm surge) combined with very strong inshore wind, when pebble beaches which naturally protect low urban coastal areas have shrunk. It is generally observed that flooding occurs shortly after the passage of a vigorous cold front, which brings in its wake strong westerly or northwesterly winds. The sea rises rapidly to flood the threatened areas. This article discusses the general causes of these events and highlights the role of cold fronts in the origin of sea flooding.

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.

On the Investigation of the Sea-Level Variability in Coastal Zones Using SWOT Satellite Mission: Example of the Eastern English Channel (Western France)

The future mission of surface water and ocean topography (SWOT), launched in 2020 over a period of 3-5 years, will be designated to address the issue of combining surface water hydrology with physical oceanography aiming to present new perspectives of applications for coastal areas. The extent to which the synthetic SWOT measurements can reproduce the temporal variability of the sea level was investigated. The eastern English Channel (NW France) was considered as a case of application. The hourly sea-level records were filtered from the aliased harmonic tides by classic harmonic analyses to obtain the nontidal residual. This residual was used to simulate synthetically the satellite samples based on the number of overpasses per repeat cycle at each geographical station. Both real and synthetic SWOT measurements were compared by the use of different approaches of inference statistics and wavelets. The statistical behavior, deduced from the functions of probability density (pdf) and cumulative distribution (cdf), shows correlations between 65% and 75% for hourly measurements, which increase to 85% for monthly average ones. The frequency of positive and negative extreme values is under-estimated with an order less than 25%. The potential use of SWOT depends on the number of measurements and the sampling interval between SWOT overpasses per repeat orbit. In the time- frequency domain, the wavelet multiresolution analysis of the nontidal sea level displays four components: 1) 1 year; 2) ~4-7 month; 3) ~2-3 month; and 4) <;2 month bands. Such modes seem to be well manifested by SWOT samples with a mean explained variance more than 75%. The aliasing frequency of the altimeter generates a dispersion and an overexpression of the energy spectrum, which increases with the number of overpasses per repeat orbit and the high frequency (~2-3 and <;2 month bands). The reconstructed wavelet components evidence the capacity of SWOT to estimate the annual and the seasonal variability of the nontidal sea level. In particular, SWOT is able to reproduce the most of extreme storm surges in the English Channel. The main finding of this research clearly shows the utility of SWOT satellite altimetry in observing and understanding the sea-level variability and storm surges, complementing tide-gauge observations for the validation and improvement of coastal models.

The High Normandy Chalk Cliffs: An Inspiring Geomorphosite for Painters and Novelists

The High Normandy coast specificity relies on the existence of remarkable landforms, such as white chalk cliffs and pebble beaches. Due to the low mechanical strength of chalk, these cliffs are very susceptible to erosion, and the rapid cliff retreat (~20 cm/year) threatens homes located inconveniently at the top of the cliff. Because of cumulative factors (exposure to strong west wind, beach sedimentary crisis, low altimetry of valleys), this study area of the eastern English Channel is also particularly sensitive to storm flooding. Moreover, the High Normandy cliffs, with their imposing verticality, their whiteness varying in tone with the ever-changing light and tide and their ghostly shapes, exerted also a powerful attraction to painters and novelists. One of the most famous chalk cliffs known in High Normandy, and probably in France, is Etretat. This site is remarkable because of its geological features and its very low rates of cliff erosion at a historical scale (few cm/year). The touristic site of Etretat, which has been affected by the fashion of sea bathing of the nineteenth century, has inspired painters. French writers, such as Guy de Maupassant and Andre Gide, also fell in love with the site, not to mention Maurice Leblanc, father of the “gentleman burglar”, Arsene Lupin, who hid the treasure of the kings of France in the “hollow stack”.

Examining high-resolution survey methods for monitoring cliff erosion at an operational scale

This paper aims to compare models from terrestrial laser scanning (TLS), terrestrial photogrammetry (TP), and unmanned aerial vehicle photogrammetry (UAVP) surveys to evaluate their potential in cliff erosion monitoring. TLS has commonly been used to monitor cliff-face erosion (monitoring since 2010 in Normandy) because it guarantees results of high precision. Due to some uncertainties and limitations of TLS, TP and UAVP can be seen as alternative methods. First, the texture quality of the photogrammetry models is better than that of TLS which could be useful for analysis and interpretation. Second, a comparison between the TLS model and UAV or TP models shows that the mean error value is mainly from 0.013 to 0.03 m, which meets the precision requirements for monitoring cliff erosion by rock falls and debris falls. However, TP is more sensitive to roughness than UAVP, which increases the data standard deviation. Thus, UAVP appears to be more reliable in our study and provides a larger spatial coverage, enabling a larger cliff-face section to be monitored with a regular resolution. Nevertheless, the method remains dependent on the weather conditions and the number of operators is not reduced. Third, even though UAVP has more advantages than TP, the methods could be interchangeable when no pilot is available, when weather conditions are bad or when high reactivity is needed.

Current inputs of continental sediment to the English Channel and its beaches: A case study of the cliffs and littoral rivers of the Western Paris Basin

A qualitative and quantitative understanding of the transport of sediment from the continents to the oceans is essential for environmental and natural resource management, particularly when it is associated with rock falls, slumping, landslides, contaminant transport, or flooding. The study presented here quantifies and characterizes sediment transport from the northwest Paris Basin to the English Channel by analyzing the two processes providing the solid material: the retreat of coastal cliffs and the erosion of coastal watersheds. For the analysis, we combine two approaches, the photogrammetric analysis of retreating cliff faces and the high-frequency measurement of river discharge and suspended sediment concentration, complemented by a grain-size analysis of the sediment sources. The results indicate that the sediment contribution from cliff erosion (1 million m3/yr producing 2 million t/yr of sediment) greatly exceeds that from the coastal rivers (43 000 t/yr). The cliff erosion input exceeds the fluvial input at the event scale as well, with cliff erosion contributing, for a single event, 200 to 2000 t/day (100 to 1000 m3) (for the most frequent collapse), and rivers contributing 4 to 6 t/day (for a flood). The influx from rivers, however, should not be overlooked, as the 4 to 6 t/day contributed is carried by 7000 to 160 000 m3/day of water, sometimes causing natural disasters such as floods and mudflows. The solids resulting from cliff erosion are composed of flint (462 500 t) and fine silicates (mostly silt and clay) derived from the cliff-forming chalk as well as from overlying formations (clay-with-flints and loess), whereas the fluvial inputs consist almost entirely of silt.

Stages of sedimentary infilling in a hypertidal bay using a combination of sedimentological, morphological and dynamic criteria (Bay of Somme, France)

ABSTRACT In the context of rising sea level, many estuaries and bays show an overall trend to sedimentary infilling. Among these coastal environments, the Bay of Somme is a hypertidal tide and wave-dominated estuary, filled in by marine sands, with a superficy of 70 km2. This study proposes a spatial zonation of the intertidal area of the bay based on the combination of information on sediments, seabed morphology and dynamics. Data come from a surficial sediment sampling campaign and six airborne LiDAR topographic surveys, acquired over the period 2011–2013, providing information on grain size, carbonate content, bedform occurrence and seabed dynamics. The Main Map shows the morpho-sedimentary and -dynamic zonation used as a basis to describe infilling stages in the bay.

The Hydro-sedimentary System of the Upper-Normandy Coast: Synthesis

The aim of this paper is to examine the sediment budget along the coast of Upper-Normandy, including the gravel beach (stock, input and output). The cliff coast and pebble/gravel beach, such along Upper-Normandy coast, is also considered as a system because cliff retreat induces widening of shore platforms and supplies the pebble/gravel beach with flint. In return, shore platform and pebble/gravel beach contribute to reduce intensity and action time of the waves at the foot of chalk cliff, and therefore cliff instability. However, for this type of coast, sedimentary exchanges with shoreface and offshore are rarely taken into account because of the low crosshore mobility of pebbles/gravels. However, the internal structure of pebble beaches is very often composed of a fraction proportion which can exceed 50 %. The sand fraction, presents also on rock shore platform and creating a sandy low tide terrace, seems to participate in the adjustment of these gravel beaches with wave. Theses interactions between two domains (coarse grained beach/sandy foreshore and shoreface) require to extend seaward the limits of the hydro-sedimentary system coast of Upper-Normandy.

Spits on the French Atlantic and Channel Coasts: Morphological Behaviour and Present Management Policies

Spits are present all along the Channel and Atlantic coasts of France. They belong to different types, depending on their composition and genesis. Some of them were initiated when RSL stabilized about its present level, 2,500 years ago. Many others are much more recent and were created when major land-use changes took place in mainland watersheds. Soil erosion increased the fluvial sediment supply and many river mouths were stabilized. All these spits continue to evolve under variable degrees of human control (mainly on sediment delivery). This paper present three examples (Arcay, Talbert, Cayeux) which illustrate the main types of issues faced by coastal planners dealing with spits in western France.