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Taux d’ablation des falaises crayeuses haut-normandes : l’apport du scanner laser terrestre

Pauline Letortu 1 Stéphane Costa 1 Abdelkrim Bensaïd 1 Jean-Michel Cador 1 Hervé Quénol 2 
1 LETG - Caen - Littoral, Environnement, Télédétection, Géomatique
LETG - Littoral, Environnement, Télédétection, Géomatique UMR 6554
2 LETG - Rennes - Littoral, Environnement, Télédétection, Géomatique
LETG - Littoral, Environnement, Télédétection, Géomatique UMR 6554
Abstract : The cliffs of Upper Normandy have high erosion dynamics which have mostly been studied by monitoring the cliff top. By integrating the orthophotograph of 2008 (BD Ortho®, IGN) to the initial photogrammetric analysis (1966-1995, Costa, 2000), this work provides results over a longer time period (1966-2008, so 42 years). To determine the rate of cliff top retreat, two calculation methods on fine scale (every 50 m) were tested: the “baseline approach” and the “area-based approach”. These retreat rates were calculated every 50 m in order to highlight the fine scale phenomena (especially the influence of shingle transit obstacles as major falls or port piers leading to multiplication by 3 to 4 of ablation rates). Beyond the traditional approach of estimating retreat rates, a work about retreat modalities was conducted between Etretat and Le Treport thanks to the inventory of falls on vertical aerial photographs of National Geographic Institute (IGN) (1939-1995), previously georeferenced, and orthophotographs (2000 and 2008). An inventory of falls from 2002 to 2009 (ESTRAN non-profit organization) along 37.5 km of coast provides additional information. Associated to rates, these values bring information about evolution modalities, two areas with different dynamics are observed. The former area (Antifer cape/Paluel and Dieppe/Le Treport) has low to moderate retreat rates (from 0.09 to 0.18 m/y) and is characterized by rare but large falls (median of departure area of falls of 700 m² for Fecamp/Eletot). The latter sector (Saint-Valery-en-Caux/Dieppe) has greater retreat rate (0.23 m/y) and falls are more frequent but not massive (median of departure area of falls of 200 m² for Quiberville/Dieppe). However, thanks to the Service Littoral (ESTRAN non-profit organization), weekly fall inventory from Veules-les-Roses to Le Treport (2002-2009) statistically provides some answers about the occurrence frequency of falls and their return periods (fitting to the power law). This information corroborates retreat results obtained by photo-interpretation because, again, the area of Ailly cape appears as the most dynamic with a fall over 1,000 m3 per linear km expected every 6 month whereas it is from 2 to 3 years for other sectors (Veules-les-Roses/Le Treport, Hautot/Bracquemont), with a retreat rhythm (elapsed time between two successive falls at the same place) estimated at 7 years. The obtained information is of great interest for mapping coastal erosion
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Submitted on : Tuesday, December 15, 2020 - 2:50:09 PM
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Pauline Letortu, Stéphane Costa, Abdelkrim Bensaïd, Jean-Michel Cador, Hervé Quénol. Taux d’ablation des falaises crayeuses haut-normandes : l’apport du scanner laser terrestre. Géomorphologie : relief, processus, environnement, Groupe français de géomorphologie (GFG), 2014, Actes des 14e Journées des Jeunes Géomorphologues, 20 (2), pp.133-144. ⟨10.4000/geomorphologie.10588⟩. ⟨hal-01121100⟩

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