Bed and Bank Stress Partitioning in Bedrock Rivers

Approximation of bed and wall stresses in confined, narrow bedrock rivers is key to accurately assessing hydraulic roughness, sediment transport, bedrock erosion and the morphodynamics of bedrock channels. Here, we partition bed and wall stresses using the ray-isovel model (RIM) and field observations. We used the RIM to calculate the distribution of shear stress across an idealized trapezoidal channel and found that the ratio of wall to bed stress () grows slightly with increasing bank angles, but exponentially declines with increasing width-to-depth ratio. We applied the RIM to 26 of the Fraser Canyons and found that RIM predicts 0.60 0.9. We used field observations of bed and total stress to calculate the wall stress in each canyon. The total stress was calculated from the 1D momentum balance (depth-slope product). The distribution of bed stress was calculated from the near-bed velocity profiles and showed that bed stress spikes as water enters constriction-pool-widening (CPW) sequences. For the majority of the Fraser Canyons, the observed wall stress is larger than the total stress and the observed bed stress. The maximum observed bed stress through a CPW sequence is ~7.5 times the mean bed stress and ~4.9 times the total stress. Compared with the observed stresses, the model systematically over-predicts the observed bed stress and under-predicts the observed wall stress by ~55%. Our results reveal that the complex flow structure in bedrock canyons influences the distribution of bed and wall stresses and that bedrock walls contribute more hydraulic roughness than predicted with RIM.