Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France

Version 1 : Received: 26 May 2023 / Approved: 29 May 2023 / Online: 29 May 2023 (05:44:18 CEST)

How to cite: Deroin, J. Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France. Preprints 2023, 2023051982. https://doi.org/10.20944/preprints202305.1982.v1 Deroin, J. Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France. Preprints 2023, 2023051982. https://doi.org/10.20944/preprints202305.1982.v1

Abstract

The Mont-Saint-Michel is known worldwide for its unique combination of the natural site and the Medieval abbey at the top of the rocky islet. But the Mont is also located within an estuarine complex, which is considerably silting up. For two decades, large-scale works were planned to prevent the Mont from being surrounded by the expanding salt meadows. The construction of a new dam over the Couesnon River, the digging of two new channels, and the destruction of the causeway were the main operations carried out between 2007 and 2015. The remote sensing approach is fully suitable for evaluating the real impact of the engineering project in both time and space, particularly the expected large-scale hydrosedimentary effects, for reestablishing the maritime landscape around the Mont. Sentinel-2 satellite data have been used for the period from 2015 to 2023. Aster data were used for the previous period covering 2000 to 2017. Aerial photographs and an ALOS scene have been also used. The remote sensing approach is based on time-series images. It allows identifying local or regional consequences and temporary or permanent effects. The migration of the different channels (especially for the new west and east Couesnon river courses) and the erosion-progradation balance of the vegetation through space and time are the main features to study. Between 2007 and 2023, the erosion of the salt meadows is significant to the south-west of the Mont (− 150 ha) but more limited to the south-east (− 65 ha). The erosion effect is limited to the immediate environment because the vegetation fringe of the uppermost tidal flat along the main dike is slightly increasing (+ 35 ha) to the west and to the east (+ 40 ha). During the same period, the sedimentation considerably increased to the north-east of the Bay, between the Bec d’Andaine, the Grouin du Sud and Tombelaine islet, which seems now facing the same silting-up problem. At this stage, the remote-sensing survey indicates mixed results for the engineering project.

Keywords

sedimentation; erosion; Sentinel 2; EOS-Aster

Subject

Environmental and Earth Sciences, Remote Sensing

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