The influence of bedrock river morphology and alluvial cover on gravel entrainment. Part 2: Modelling critical shear stress

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 3.6Â range of σ z across the surfaces without sediment cover produces up to a 5.1Â variation in τ c . For comparison, for any single surface, τ c varies by up to 2.5Â 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.

Introduction to the Women in Geomorphology special issue

“…• numerical modelling of critical shear stress for grain entrainment based on imagery of bed-rock channels (Hodge & Buechel, 2022);…”

“…Remote sensing ( 26) Agostini et al, 2022;Atwood & West, 2022;Davies et al, 2022;Davis et al, 2021;Fadul et al, 2022;Gervasi et al, 2021;Hendrickx et al, 2022;Hodge & Buechel, 2022;Hooke, 2022;Hovenga et al, 2021;Konlechner & Hilton, 2022;Lefebvre et al, 2022;Marchetti et al, 2022;McKenzie et al, 2022;Mol & Grenfell, 2022;Opalka et al, 2022;Rabanaque et al, 2022;Regalla et al, 2022;Reitman et al, 2022;Sampietro-Vattuone et al, 2021;Schoch-Baumann et al, 2022;Thompson et al, 2022;Vah et al, 2022;Valentine et al, 2022;Zhang et al, 2022 Physical experiments (8) Bako et al, 2022;Bywater-Reyes et al, 2022;Demiral et al, 2022;Friedrich et al, 2022;Negreiros et al, 2022;Rachelly et al, 2022;Roelofs et al, 2022;Vah et al, 2022 Numerical models ( 11) Agostini et al, 2022;Bywater-Reyes et al, 2022;Cullen et al, 2022;Glover et al, 2022;Hodge & Buechel, 2022;Kooijma...…”

“…Many of the major research directions in contemporary geomorphology are reflected in this collection of papers. With respect to fluvial environments, including alluvial fans, individual papers examine use of images to detect sediment motion during physical experiments (Vah et al, 2022); comparison of grain‐size distributions from field measurements and remotely acquired images (Marchetti et al, 2022); physical experiments on bedload saltation under differing conditions (Demiral et al, 2022); numerical modelling of critical shear stress for grain entrainment based on imagery of bed‐rock channels (Hodge & Buechel, 2022); biogeomorphology (Bywater‐Reyes et al, 2022; Gurnell et al, 2022); channel morphology and classification (Lane et al, 2022), hydromorphology of ephemeral streams (Rabanaque et al, 2022) and interpretation of geomorphic unit assemblages (Fryirs & Brierley, 2022); flood effectiveness (Gervasi et al, 2021); channel evolution, terraces and sediment sources (Guyez et al, 2022) and the influence of sediment supply on gravel‐bed river widening (Rachelly et al, 2022); hydrological variation and meander morphodynamics (Hooke, 2022); the dynamics of wood in river corridors (Guiney & Lininger, 2022; Gurnell et al, 2022; Wohl, 2022) and numerical modelling of wood‐related processes (Friedrich et al, 2022); numerical model validation (Negreiros et al, 2022) and numerical modelling of groundwater‐driven network evolution (Cullen et al, 2022); base‐level controls and landscape evolution (Fadul et al, 2022); long‐term dryland channel response to urbanization (Chin, 2022); watershed restoration and management, including human alterations of process and form (Atkins et al, 2023; Davis et al, 2021; Goodman et al, in press; Gurnell et al, 2022; Hinshaw et al, 2022; Mossa & Chen, 2022; Sampietro‐Vattuone et al, 2021; Valentine et al, 2022); and alluvial fan dynamics (Opalka et al, 2022; Quock et al, 2022; Regalla et al, 2022; Schoch‐Baumann et al, 2022). Papers addressing process and form in coastal environments include examinations of dune morphodynamics and evolution (Hovenga et al,...…”

Patterns of Alluviation in Mixed Bedrock‐Alluvial Channels: 1. Numerical Model

“…(2016) performed experiments showing that the entrainment threshold for sediment motion is influenced by the presence or absence of a surrounding patch of sediment, and that the shape and distribution of sediment patch changes with changes in discharge. Hodge and Buechel (2022) used 3D printed replicas of bedrock surfaces to show that bedrock topography produces substantial variation in the critical shear stress. Hodge and Hoey (2016a, 2016b) performed experiments in a 3D printed scale model of a jointed limestone bedrock river in which the patterns of sediment deposits at different sediment supply rates were documented.…”

The influence of bedrock river morphology and alluvial cover on gravel entrainment: Part 1. Pivot angles and surface roughness

“…This allows us to reduce aleatory uncertainties generated by hydraulic processes in the field (e.g., turbulent sweeps and instantaneous pressure gradients in the water column; Schmeeckle et al, 2007; Vollmer & Kleinhans, 2007). In the companion article (Hodge & Buechel, 2022), we calculate critical shear stress by using the pivot angles and roughness values to parameterize the grain entrainment model of Kirchner et al (1990), and we compare our results to entrainment models for bedrock rivers developed by Inoue et al (2014) and Johnson (2014).…”