Experiments on the morphological controls of velocity inversions in bedrock canyons

A recent investigation of flow through bedrock canyons of the Fraser River revealed that plunging flows occur where the canyons are laterally constrained and have low width-to-depth ratios. An experimental investigation was undertaken to reproduce the plunging flow fields observed in the Fraser canyons and to explore the influence of morphological controls on the occurrence and relative strength of plunging flow in bedrock canyons. Observations show that the plunging flow structure can be produced by accelerating the flow at the canyon entrance either over submerged sills or through lateral constrictions at the top of a scour pool entrance slope. The occurrence and strength of plunging flow into a scour pool can be enhanced by sill height, amount of lateral constriction, pool entrance slope, discharge, and reduced width-to-depth ratio. Plunging flow greatly enhances the potential for incision to occur along the channel bed and is an extreme departure from the assumptions of steady, uniform flow in bedrock incision models, highlighting the need for improved formulations that account for fluid flow.