A mechanistic model for lateral erosion of bedrock channel banks by bedload particle impacts
Abstract
Bedrock rivers get wider by lateral erosion. The processes of lateral erosion are not well understood, but lateral erosion is widely thought to occur when the bed is covered by alluvium, which deflects the downstream transport of bedload particles into bedrock walls. Here we develop a model for lateral bedrock erosion by abrasion caused by impacts of deflected bedload particles. The lateral erosion rate is the product of the volume eroded per particle impact and the impact rate. The volume eroded per particle impact is modelled numerically, by determining the initial particle velocity before collision from well-established empirical relations, estimating the momentum transfer during collision from a simplified reflection methodology and tracking the motion of bedload particles from collision with roughness elements to impacts on the wall. The impact rate is predicted from excess shear stress, sediment supply rate, and the size and spacing of roughness elements on the bed. The numerical model successfully predicts the undercut wall shape and the cross-sectional and alongstream variations of lateral erosion rate observed in previously published physical experiments by incorporating the movement of spherical particles in three dimensions and the co-evolution of wall morphology, shear stress and erosion rate over time. A non-dimensional form of the lateral erosion model is used to explore how lateral erosion varies under various relative sediment supply (defined as the ratio of sediment supply to transport capacity) and transport stage conditions. Maximum lateral erosion rates occur at the threshold of full cover and moderate transport stages. The competition between lateral and vertical erosion is investigated by coupling the saltation-abrasion vertical erosion model with our lateral erosion model. The results suggest that vertical erosion dominates under nearly 85% of relative sediment supply and transport stage conditions. Lateral erosion dominates when the relative sediment supply is at the threshold for full bed coverage and at transport stages near the threshold for suspension.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP52B..06L
- Keywords:
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- 1625 Geomorphology and weathering;
- GLOBAL CHANGE;
- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1856 River channels;
- HYDROLOGY