A physically based model to predict hydraulic erosion of fine-grained riverbanks: The role of form roughness in limiting erosion

Hydraulic erosion of bank toe materials is the dominant factor controlling the long-term rate of riverbank retreat. In principle, hydraulic bank erosion can be quantified using an excess shear stress model, but difficulties in estimating input parameters seriously inhibit the predictive accuracy of this approach. Herein a combination of analytical modeling and novel field measurement techniques is employed to improve the parameterization of an excess shear stress model as applied to the Lower Mekong River. Boundary shear stress is estimated using a model (Kean and Smith, 2006a, 2006b) for flow over the irregular bank topography that is characteristic of fine-grained riverbanks. Bank erodibility parameters were obtained using a cohesive strength meter (Tolhurst et al., 1999). The new model was used to estimate annual bank erosion rates via integration across the Mekong's annual flow regime. Importantly, the simulations represent the first predictions of hydraulic bank erosion that do not require recourse to calibration, thereby providing a stronger physical basis for the simulation of bank erosion. Model predictions, as evaluated by comparing simulated annual rates of bank toe retreat with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery, indicate a tendency to overpredict erosion (root-mean-square error equals +/- 0.53 m/yr). Form roughness induced by bank topographic features is shown to be a major component (61%-85%) of the spatially averaged total shear stress, and as such it can be viewed as an important factor that self-limits bank erosion.