Characterizing and quantifying the discontinuous bank erosion of a small low energy river using Structure-from-Motion Photogrammetry and erosion pins

River bank erosion is controlled by a complex combination of factors and processes that play a main role in fluvial dynamics. However, the role played by lateral erosion in small low energy rivers functioning still needs to be better understood. This study deals with the characterization and the quantification of the bank erosion of a small low energy river of the Seine Basin (France) in periurban catchment where urbanization had led to an incision and widening of the bed in the past decades, using adapted monitoring surveys: erosion pins and Structure-from-Motion Photogrammetry. The paper provides an opportunity to compare reach-scale flux estimates using both techniques. Spatial and temporal variations in river bank rates were quantified over 16 months and the causes and controls of riverbank erosion were identified. The estimated morphological activity rate ranges from 117 cm/yr to 1 cm/yr, illustrating the very heterogeneous spatio-temporal pattern of bank erosion observed on the Merantaise River. Whereas a number of studies consider subaerial processes as the dominant processes, fluvial erosion was found to be the most significant causal process, and in particular the mean discharge of floods. However, the high local variability of the bank erosion of the studied river appears to be controlled by very local factors such as the bank angle, the presence of a berm at the bank-toe (providing protection against fluvial erosion), lateral supply (outlet of rainwater pipes) and deflected flow in the direction of the bank.