Bed Stability of Step-Pool Channels With Macrorough Sidewalls

A step-pool bed morphology often develops in steep mountain streams. The main controlling parameters of step formation and stability are hydraulic forcing, sediment transport, and granular effects (i.e., grain-grain interlocking, jamming). Previous studies found macrorough sidewalls to further increase bed stability due to grain-sidewall interlocking but the effect of the reduced bed shear stress caused by sidewall friction was not explicitly considered. The present study aims at untangling hydraulic effects induced by the macrorough sidewalls and granular effects, that is, grain-sidewall interaction. Flume experiments with five types of macrorough sidewalls were conducted under clear-water conditions for bed slopes ranging between 4% and 10% and jamming ratios 5.3 < W-m/d(84) < 11.2, where W-m is the mean channel width and d(84) is a characteristic grain size of the sediment mixture. A modified sidewall correction procedure accounting for sidewall friction was applied to estimate bed shear stress. Bed stability considerably increased with increasing sidewall roughness. The dimensionless bed shear stress theta(Rb) controlled step occurrence as the system evolved toward a state of a maximum number of steps for critical Shields ratios theta(Rb)/theta(c) approximate to 1. Contrary to the expectations, fewer steps formed in the roughest sidewall experiments as few very stable steps inhibited sediment relocation and therewith step formation. Steps were more frequently located in the constrictions caused by the macrorough elements and sustained higher bed shear stress increments. Overall, hydraulic effects fully explained the increase in bed stability in moderately rough sidewall experiments whereas granular effects became important in channels with strong local constrictions.