Scour pool incision in bedrock canyons

A flume experiment was conducted to investigate the effect of lateral constrictions on the initiation and evolution of scour pools in bedrock canyons. Results show that lateral constriction can initiate the formation of a scour pool. Deceleration of the flow upstream of the constricted canyon promotes alluviation, while flow acceleration through the canyon prevents a permanent sediment cover from developing. The elevation difference upstream and through the canyon causes flow and sediment to plunge towards the bed, enhancing scouring. The erosion rate was controlled by sediment size and presence of alluvial cover. Scour pools reach equilibrium morphology for a given constant discharge and sediment supply by cutting a slot, which then gets deep enough to maintain a permanent alluvial cover, protecting the bed from further vertical erosion and promoting lateral erosion. Shear stress calculated from the near-bed velocity gradient and Reynolds shear stresses are counterintuitively large in alluviated areas and low in places where the bed is clear of sediment. This highlights a general problem with using shear stress as a predictor of alluviation and rock erosion patterns in highly non-uniform flows. However, changes in near-bed velocity had strong correlations with alluviation patterns and erosion rate, suggesting near-bed velocity may be a more practical way to calculate rock erosion rates in non-uniform flows.