EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The influence of bedrock topography on grain entrainment in bedrock-alluvial channels

Rebecca Hodge1, Marcus Buechel2, and Sophie Kenmare1
Rebecca Hodge et al.
  • 1Durham University, Geography, Durham, UK (
  • 2University of Oxford, School of Geography and the Environment, Oxford, UK

Sediment grains in bedrock-alluvial channels can be entrained from bedrock surfaces or from alluvial patches. Field tracer data has shown that grains entrained from different surfaces can have very different critical shear stresses, which will affect bedload transport rates, the stability of sediment cover and bedrock incision. We hypothesise that the topography of the bedrock surface affects the critical shear stress of a sediment grain in at least three ways: the pivot angle through which the grain must move to be mobilised; the extent to which the grain is sheltered by upstream bedrock protrusions; and the impact on the flow profile via the roughness length z0. Here we quantify how bedrock topography affects these three different components, and their overall impact on critical shear stress.

Our analysis is based around six samples of bedrock river topography, from rivers with different degrees of roughness and structural characteristics. Each surface was 3D printed at a reduced scale, and pivot angles were measured by dropping grains of different sizes at different locations, and tilting the surface until the grain moved. For the surface with bedrock ribs, experiments were repeated with the ribs parallel and perpendicular to the downslope direction. Further experiments were performed after incrementally covering 25% through to 100% of the surface with fixed sediment cover. Bedrock sheltering and z0 were estimated from analysis of surface topography.

Overall, we find that measured pivot angles decrease with increasing surface roughness, similar to previous relationships from alluvial channels. However, we find that the pivot angle for a grain at any particular location cannot be predicted from the local surface topography, because of the complex interaction between grain shape and the different scales of roughness present on the surface. Rib direction also has a significant influence on mean pivot angle. The impact of sediment cover depends on the relative roughness of the cover and the bedrock surface.

We calculate critical shear stress using Kirchner’s force balance model, parameterised using our measurements of pivot angle, sheltering and z0. We find that z0 has the largest impact on the predicted median values of critical shear stress. Including the measured pivot angles reduces the lowest values of critical shear stress, with implications for the onset of sediment transport. Overall, our data represent the first attempt to quantify fully how bedrock topography influences the critical shear stress of sediment grains in bedrock-alluvial channels.

How to cite: Hodge, R., Buechel, M., and Kenmare, S.: The influence of bedrock topography on grain entrainment in bedrock-alluvial channels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15504,, 2020


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