The influence of bedrock river morphology and alluvial cover on gravel entrainment. Part 2: Modelling critical shear stress. Issue 14 (6th September 2022)

• Record Type: Journal Article
• Authors: Hodge, Rebecca A.; Buechel, Marcus E. H.
• Is Part Of: Earth surface processes and landforms. Volume 47:Issue 14(2022)
• Page Start: 3348
• Page End: 3360
• Publication Date: 2022-09-06
• Subjects: bedrock river -- critical shear stress -- grain entrainment -- pivot angle -- surface roughness; Geomorphology -- Periodicals; 551.4
• DOI: 10.1002/esp.5462
• Languages: English
• ISSNs: 0197-9337

Abstract

Abstract: The critical shear stress ( τ c ) at which grains are entrained on a bedrock surface is important for determining how bedrock rivers evolve through changes in sediment cover and bedrock erosion. The difference in τ c for grains on bedrock and alluvial surfaces also determines whether a channel may be susceptible to runaway alluviation. Bedrock channel beds can have a wide variety of morphologies, but we do not fully understand how this variation affects τ c . Here we address how bedrock morphology alters the grain entrainment parameters of pivoting angle, grain exposure and roughness height z 0, and thus τ c . In our companion article we used scaled, 3D printed replicas of seven bedrock surfaces to measure grain pivoting angles for four grain sizes. For three surfaces, pivoting angles were also measured with 25–100% sediment cover. In this second article, we combine these pivot angle data with measurements of grain exposure and surface roughness (standard deviation of elevations, σ z ) to predict τ c using a force–balance model. The bedrock topography produces substantial variation in τ c ; for a given grain diameter ( D ), a 3.6× range of σ z across the surfaces without sediment cover produces up to a 5.1× variation in τ c . For comparison, for any single surface, τ c varies by up to 2.5× for a fourfold range in grain size. Comparison to previous models with less representation of grain‐scale geometry shows that in our results grains move at lower values of dimensionless critical shear stress ( τ* c ), and that τ* c decreases more quickly with increasing D / σ z . However, direct comparison is difficult because previous relationships are based on a hydraulic roughness length that cannot be easily predicted without hydraulic data. Our results propose a new relationship between D / σ z and τ* c, but further development and testing require datasets that combine measurements of flow, τ c and grain‐scale geometry. Abstract : Using 3D‐printed replicas of bedrock surfaces, we measure grain pivot angles, exposure and surface roughness, and use these values to parameterise a grain entrainment model. We find that differences in bedrock topography produced greater variation in critical shear stress than changes in grain size. But predictions of critical shear stress are sensitive to how the roughness height is parameterised, and improved predictions require better understanding of how bedrock topography affects near‐bed hydraulics.