Coastal chalk platform erosion modulated by step erosion and debris shielding: example from Normandy and Picardy (northern France)
Abstract
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 from the cliff. We study their size distribution (16% of the platform) and location and draw the conclusion that small falls are quickly erased by wave action while far reaching fans may cover the entire platform width. The peak shielding by fan debris occurs at a distance of ~40 m from the cliff. Step erosion by block quarrying is the process likely responsible for 20% to 100% of platform erosion.