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

Abstract

[1] 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.Citation: Darby, S. E., H. Q. Trieu, P. A. Carling, J. Sarkkula, J. Koponen, M. Kummu, I. Conlan, and J. Leyland (2010), A physically based model to predict hydraulic erosion of fine-grained riverbanks: The role of form roughness in limiting erosion,

Spatial variability in bank resistance to erosion on a large meandering, mixed bedrock-alluvial river

“…(Table 1). In contrast to common excess shear stress modeling approaches, where the erosion coefficient k is determined through a best fit to observed rates of bank erosion (Constantine et al, 2009), the approach adopted herein relies on direct estimates of k without recourse to statistical calibration and therefore provides a more physically based model of bank erosion (Darby et al, 2010). However, the erosion-resistance tests and regression analyses based on data derived from these tests are only valid for cohesive bank materials that have substantial amounts of clay.…”

Streambank erosion rates and loads within a single watershed: Bridging the gap between temporal and spatial scales

“…Hydraulic erosion removes bank materials from the direct erosive action of the flow (Darby et al, 2010). During high flow events in incised Walnut Creek, streamflow is confined within the channel and scours the streambanks over the entire bank height.…”

“…We have observed exceptionally flashy streamflow in Walnut Creek in response to precipitation when stage increased to the top edge of the channel and then decreased approximately 2.5 m within 8 h (Schilling et al, 2006). The evaluation of specific processes associated with bank erosion is the subject of considerable investigation (e.g., Rinaldi and Darby, 2007;Darby et al, 2010) with many studies focusing on quantifying variations in shear stress. While our study was not focused on evaluating bank erosion processes, our data suggest a close association between stormflow discharge and increased bank erosion.…”

Formation of a cohesive floodplain in a dynamic experimental meandering river

“…First, banks are undercut by fluvial erosion at the base and lower portion of the banks; second, bank retreat occurs by mass failure of the bank (Darby et al, , 2007Simon et al, 2000;Simon and Collinson, 2002). Then, the bank sediment settles at the bank toe and armoring protects the bank against fast erosion (Thorne, 1982;Parker et al, 2011) and shifts the locus of the high flow velocity, which reduces the shear stress acting on the bank (Kean and Smith, 2006a,b;Darby et al, 2010). Several studies attempted to predict bank erosion rates by calculating the bank erosion processes (e.g.…”