Dynamic Feedbacks Between Flow, Erosion and Evolving River Bank Roughness Revealed Through Repeat High-Resolution Topographic Surveys

Bank erosion is a key process in fluvial dynamics, with significant fractions of the total sediment load being sourced from river banks. Studies have shown that hydraulic erosion of the bank toe is a driving factor of long term rates of bank retreat. Fluvial bank erosion rates are often quantified using an excess shear stress model where the erosion rate is a function of the boundary shear stress applied by the flow above a critical threshold. Research has shown that the form roughness induced by natural topographic bank features such as slumps, spurs and embayments, is a major component of the spatially-averaged total shear stress. The skin friction component of this shear stress is typically an order of magnitude less than the total, meaning that the form roughness provides an important control on bank erosion rates. However, measuring the relative components of the total shear stress for a natural system is not straightforward. In this research we apply the method of Kean and Smith [2006, J. Geophys. Res., 111(4), F04009, doi:10.1029/2006JF000467] to partition the form and skin drag components of river bank roughness for an eroding bank of the Cecina River in central Italy. This method approximates the form drag component of the roughness along a longitudinal bank profile as a series of user defined Gaussian curves, with the skin friction component estimated through analysis of the deviations of the data from the fitted curves. For our site, a temporal sequence (2003 - 2011) of high-resolution topographic surveys has been collected through a combination of photogrammetry and Terrestrial Laser Scanning. For each survey five vertically equidistant profiles are extracted and analysed alongside DEMs of difference and associated flow data modelled using the distributed hydrological model MOBIDIC. The data are used to explore the dynamic feedbacks that exist between river discharge, bank erosion processes and bank form roughness, revealing insights into the self-limiting nature of erosion rates.