Spatial coexistence and temporal logout between mainland and marine hazards in Upper Normandy (NW France)

Abstract

This paper focuses on the spatial coexistence between different hazards (strong west winds, beach sedimentary crisis and flash floods) in outlets of small catchments in a sedimentary basin. This study was carried out in the department of Upper Normandy located in north-western France. To anticipate possible future hazards and damages without depending on meteorological triggers (neither intense rainfall nor synoptic conditions), we combine previous knowledge accumulated on local phenomena to highlight the pre-conditioning factors and identify those areas characterised by the highest susceptibility to risk due to the accumulation of numerous hazards. This approach enables both space and time distribution patterns of natural hazards to be revealed. In space, the risk susceptibility strongly depends on morphology. Populations located at the outlet of large wet valleys, at the final outlets of small basins at a short distance from the plateaus and on the coastal fringe appear to be the most endangered. Even if hazards never occur at the same time, these areas can be affected by different hazards during one year (summer flash floods, floods, winter storm surges). This study identifies 14 sensitive areas along hollow shapes of the study area. Finally, our methodological investigations question the creation of a warning system combining mainland and marine hazards in these 14 outlets.