Vitesses et modalités de recul des falaises crayeuses de Haute-Normandie (France) : méthodologie et variabilité du recul

Letortu, Pauline; Costa, Stéphane; Bensaïd, Abdelkrim; Cador, Jean-Michel; Quénol, Hervé

doi:10.4000/geomorphologie.10588

Cited by 31 publications

(34 citation statements)

References 20 publications

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“…Located at the northwestern end of the sedimentary Paris Basin, Upper Normandy coastal chalk cliffs consist of Upper Cretaceous chalk (from the Cenomanian to Campanian stages) (Figure ). These cliffs recede with an average retreat rate of 0.15 m/yr but with a high spatial variability (0.23 m/yr between Saint‐Valery‐en‐Caux and Dieppe (Costa et al ., , ; Letortu et al ., )). These values are not representative of rock dynamics because, in reality, retreat occurs in jerks (rock falls).…”

Section: Climate and Rock Fall Database Along The Studied Coastlinementioning

confidence: 94%

Statistical and empirical analyses of the triggers of coastal chalk cliff failure

Letortu

Costa

Cador

et al. 2015

Earth Surf Processes Landf

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International audienceThe triggering factors of rock falls remain unknown due to a lack of exhaustive, regular and accurate surveys. Based on an inventory of 331 rock falls collected weekly between 2002 and 2009 from Veules-les-Roses to Le Treport (Upper Normandy), the relationships between coastal chalk cliff rock falls (dates and geomorphological features) and external factors commonly agreed as triggering (rainfall, temperature variations, tide and wind) are studied. The combination of multivariate statistical and empirical analyses indicates that (1) “cold and dry weather” and “high rainfall and high wind” are the conditions most likely to trigger rock falls, (2) the main triggering factors of rock falls are effective rainfall (for rock falls mostly between 200 and 1,400 m3 or larger than 10,000 m3 and coming from the whole cliff face), freeze/thaw cycles (especially for rock falls smaller than 200 m3 and coming from the foot and top of the cliff face) and marine roughness (rock falls mainly smaller than 200 m3 and coming from the cliff foot). However, the contribution of each factor to triggering is difficult to determine because of combinations of factors (85 % of 331 cases), relays of processes and hysteresis phenomena. In view of these first results, it is still presumptuous to predict the location and time of triggering of rock falls. However, the statistical and naturalistic approaches adopted and the observations made in this study are from an original database, and constitute a real starting point for the prediction and prevention of the hazard of coastal chalk cliff rock falls in Upper Normandy

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Section: Climate and Rock Fall Database Along The Studied Coastlinementioning

confidence: 94%

Statistical and empirical analyses of the triggers of coastal chalk cliff failure

Letortu

Costa

Cador

et al. 2015

Earth Surf Processes Landf

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“…The shore platform would have to have developed landwards at an average rate of 17.0 ± 1.2 mm/yr to bevel the carbonate platform to its currently exposed width, assuming that planation spanned the time between the early Pliocene tilting event and the burial of quartz‐bearing sediment into the caves. Such a rate is consistent with modern measurements of 18–35 mm/yr sea cliff retreat rates in similarly poorly consolidated carbonates [e.g., Dornbusch et al ., ; Letortu et al ., ] and 14.3 mm/yr in flysh [ De Lange and Moon , 2005]. Beveling of the volcanoclastic basement could require more time, as reported rates of long‐term cliff retreat in volcanic rocks are generally lower (0.6–1.7 mm/yr [ Smoot , ; Mitchell et al , ]).…”

Section: Knickpoint Initiation and Retreatmentioning

confidence: 99%

Relict landscape resistance to dissection by upstream migrating knickpoints

Brocard

Willenbring

Miller

et al. 2016

J. Geophys. Res. Earth Surf.

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Expanses of subdued topographies are common at high elevation in mountain ranges. They are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds. Preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography. However, relict fragments could also possess intrinsic characteristics that make them resilient to dissection. We document here the propagation of a wave of dissection across an uplifted relict landscape in Puerto Rico. Using 10Be‐26Al burial dating on cave sediments, we show that uplift started 4 Ma and that river knickpoints have since migrated very slowly across the landscape. Modern detrital 10Be erosion rates are consistent with these long‐term rates of knickpoint retreat. Analysis of knickpoint distribution, combined with visual observations along the streambeds, indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat. The studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks. Earlier studies have shown that vegetation over the relict topography efficiently limits erosion, allowing for the formation of a thick saprolite underneath. Such slow erosion reduces streambed load fluxes delivered to the knickpoints, as well as bed load grain size. Both processes limit abrasion. Compounding the effect of slow abrasion, wide joint spacing in the bedrock makes plucking infrequent. Thus, the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream, reducing the celerity at which the relict, subdued landscape is dissected. We conclude that similar top‐down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies.

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“…Thanks to the large tidal range, the wide platform and the dry valley offering an easy access to the foreshore, the entire cliff face can be observed from the foreshore (Figure 1b). According to the surveys of the cliff top evolution on vertical aerial photographs (1966-2008, [25]) and of the cliff face evolution by terrestrial laser scanning (2010-2017, [26]), the erosion value is of 0.36 m/year. Both layers (chalk and Tertiary strata) are prone to erosion, which explains the high annual retreat rate in this area, in comparison with average regional retreat rate (0.15 m/year from Cap d'Antifer to Le Tréport, 1966-2008 [25]).…”

Section: Study Areamentioning

confidence: 99%

“…According to the surveys of the cliff top evolution on vertical aerial photographs (1966-2008, [25]) and of the cliff face evolution by terrestrial laser scanning (2010-2017, [26]), the erosion value is of 0.36 m/year. Both layers (chalk and Tertiary strata) are prone to erosion, which explains the high annual retreat rate in this area, in comparison with average regional retreat rate (0.15 m/year from Cap d'Antifer to Le Tréport, 1966-2008 [25]). The modalities of erosion include rock falls (large-scale movements affecting all or part of the cliff face) and debris falls (very small blocks or flakes (up to a decimeter scale) falling from across the cliff face) [27].…”

Section: Study Areamentioning

confidence: 99%

UAV survey of a coastal cliff face – Selection of the best imaging angle

et al. 2019

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UAVs are relevant for monitoring cliff faces. In this study, several flights are performed with various imaging angles (nadir, 20°, 30° and 40° off-nadir) to assess the impact of the imaging angle on the 3D cliff face reconstruction. Occlusions issues arising with sub-vertical cliffs make nadir surveys nearly irrelevant. The results obtained with 20°, 30° and 40° off-nadir imaging angles are satisfactory regarding texture restitution and accuracy with respectively 5.5 cm, 5.9 cm and 4.9 cm of error, higher tilting angles yielding better reconstructions on sub-vertical or overhanging parts of the cliff. This article also investigates other parameters affecting tiepoint detection on the cliff face, as the effective overlap, the UAV-cliff face distance and the cliff face illumination. Guidelines are provided for UAV survey parameterization, aiming at capturing the whole cliff face with a good trade-off between distance to the cliff, flight height and spatial resolution of the photographs. Highlights Impact of the UAV camera tilting angle on the 3D cliff face reconstruction  Because of occlusions, irrelevance of nadir pointing camera for sub-vertical cliffs  Satisfying accuracy and texture for both 20°, 30° and 40° off-nadir imaging angles  Other parameters impacting tiepoint detection in the reconstruction process

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Vitesses et modalités de recul des falaises crayeuses de Haute-Normandie (France) : méthodologie et variabilité du recul

Cited by 31 publications

References 20 publications

Statistical and empirical analyses of the triggers of coastal chalk cliff failure

Statistical and empirical analyses of the triggers of coastal chalk cliff failure

Relict landscape resistance to dissection by upstream migrating knickpoints

UAV survey of a coastal cliff face – Selection of the best imaging angle

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