Surface grain-size mapping of braided channels from SfM photogrammetry

Abstract. Braided channels are known as fluvial systems with a high heterogeneity of physical conditions, resulting from particularly active interacting processes of coarse sediment sorting and transport. This in turn generates a complex mosaic of terrestrial and aquatic habitats, supporting an exceptional biodiversity. However, documenting this physical heterogeneity is challenging, and notably the textural variability of these rivers, which is particularly strong. Distributed and continuous grain-size maps of braided channels are notably of great interest in this regard. In this study, high-resolution imagery obtained from UAV equipped for direct georeferencing were used to produce 3D point clouds (Structure from Motion photogrammetry), from which surface grain-size has been inferred. A set of 12 braided river reaches located in SE of France were used to calibrate a roughness-based grain-size proxy, and this proxy was used for the production of distributed grain-size maps. The calibration curve can be used to determine the surface median grain-size with an independent error of 5 mm (14 % of relative error). Resampling procedure shows a good transferability of the calibration, with a residual prediction error ranging from 5 to 17.5 %. Reach-averaged median grain-sizes extracted from roughness-based grain-size maps were in very good agreement with values collected in the field from intensive grain-size samplings (differences of less than 5 %). Some examples of morpho-sedimentary signatures derived from these maps are provided. They notably show a systematic altimetric gradient of the maximum grain-size of bars, that is interpreted as an hydrological imprint, that should be better integrated into conceptual models of grain-size patchiness developed for these rivers.