Coupling fluvial‐hydraulic models to predict gravel transport in spatially variable flows

This study investigated spatial‐temporal variations of shear stress and bed load transport at three gravel bed river reaches of the Williams Fork River, Colorado. A two‐dimensional flow model was used to compute spatial distributions of shear stress ( τ ) for four discharge levels between one third of bankfull ( Q bf ) and Q bf . Results indicate that mean τ values are highly variable among sites. However, the properties of the mean‐normalized distributions of τ are similar across sites for all flows. The distributions of τ are then used with a transport function to compute bed load transport rates of individual grain size fractions. Probability distributions of the instantaneous unit‐width transport rates, q b , indicate that most of the bed load is transported through small portions of the bed with high τ . The mean‐normalized probability distributions of q b are different among sites for all flows except at Q bf , when the distributions overlap. We also find that the grain size distribution (GSD) of the bed load adjusts with discharge to resemble the grain size distribution of the subsurface at Q bf . We extend these results to 13 locations in the basin, using the mean‐normalized distributions of shear stress and measured subsurface grain sizes to compute bed load transport rates at Q bf . We found a remarkably similar shape of the q b distribution among sites highlighting the basin‐wide balance between flow forces and GSD at Q bf and the potential to predict sediment flux at the watershed scale.