DEM and GIS analysis of the stream gradient index to evaluate effects of tectonics: The Normandy intraplate area (NW France)

Computer-based geomorphometry using a DEM (Digital Elevation Model) allows the analysis of the three-dimensional properties of landscape. This methodology is particularly useful in an intraplate region like western Europe where the simple visual inspection of the topography cannot resolve the evolutionary trends of landforms. In these domains, the morphologies of the topographic surface may be controlled mainly by climate under a low rate of tectonic deformation. Among the geomorphometric parameters, the stream length index (SL) has been used to characterize fluvial systems in relation to tectonics movements. This work develops an algorithm to derive and map the SL index using a DEM and GIS, to investigate its spatial variations in a broad area. The algorithm is applied to a zone of weak intraplate deformation: the coastal lowlands of Normandy (France). The obtained spatial distributions of SL point to anomalous zones with high SL values. These zones are adjacent to mapped fault scarps and characterized by changes in flow direction. A Kruskal–Wallis test shows that the bedrock lithology has no impact on the SL value. Therefore, the SL variations can be related mainly to a differential uplift due to Quaternary tectonic forcing. Quaternary sea level fluctuations may also be responsible for high SL values in a part of the coastal lowland.