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Evolution of roughness on eroding riverbanks

Evolution of roughness on eroding riverbanks

In collaboration with Steve Darby (University of Southampton), Massimo Rinaldi (University of Florence) and Liliana Teruggi (also Florence) we have been monitoring a site on the Cecina River in Tuscany, Italy for a number of years. The dataset consists of high resolution topographic bank surveys undertaken using photogrammetry (2003 – 2007) and thereafter the Geography and Environmnet Terrestrial Laser Scanner. The British Society for Geomorphology awarded me £1.3K to re-visit the site in July 2011 to undertake another TLS survey.

Recent progress in modelling rates of hydraulic river bank erosion [Darby et al., 2010] has indicated that the form roughness induced by natural topographic bank features (slumps, embayments, etc.) is a major component of the spatially-averaged total shear stress. The skin friction component is found to be typically an order of magnitude less than the total stress, such that the form roughness provides an important control on self-limiting bank erosion rates. However, given that the form roughness is induced by topographic forms that are themselves created by hydraulic bank erosion, it remains unclear whether and how bank roughness co-evolves with the erosion process.

In an attempt to address this issue we are evaluating the temporal evolution of bank roughness parameterson the Cecina River. In our study bank ‘roughness profiles’ are extracted from the annual series of high-resolution DEMs of the river bank. The DEMs are used to quantify accurately the spatial trends and amounts of annual bank erosion observed in relation to the hydrological regime of the river, and are compared to the temporal variations in river bank form and skin roughness components as the bank erodes. The data are being used to evaluate the extent to which there is a dynamic feedback between the bank erosion process and bank form roughness and a paper for submission to Earth Surface Processes and Landforms is in preparation.