Modelling interactions between fluvial erosion and mass failure in cohesive riverbanks

In recent years progress has been made in understanding mass failure mechanisms, while less attention has been paid to the process of fluvial erosion, and particularly to the interaction between the two processes. The aims of this research are: (a) to investigate the interaction between fluvial erosion and mass failure at the scale of single flow events; and (b) to quantify the main effects of riparian vegetation on bank stability. The study reach is located along the Cecina River (Italy). Monitoring was carried out to collect geotechnical and hydrological data and to characterise riparian vegetation properties. A modelling approach was used for a single, representative bank profile, involving the following components: a) hydraulic calculation of the near-bank shear stresses, fluvial erosion, and consequent changes in bank geometry; b) finite element seepage analysis; c) stability analysis using the limit equilibrium method. Four significant flow events monitored during the period 2003-2004, representative of the typical range of annual hydrographs, were simulated. Results show that the occurrence of bank processes (fluvial erosion, slide, cantilever) and their relative dominance differ significantly between each event, depending on seasonal hydrological conditions and flow hydrograph. Some simulations including the main effects of vegetation on bank stability were also performed. Preliminary results show that the presence of a single tree located on the bank edge, and with a root depth significantly lower than the thickness of the cohesive portion of the bank, does not have a substantial effect on bank stability with respect to mass-failure.