Flow regimes, bed morphology, and flow resistance in self-formed step-pool channels

We used a mobile bed flume with scaled grain size distribution, channel geometry, and flow to examine morphology and hydraulics of stepped channels. We hypothesized that (1) step geometry and flow resistance differs significantly as a function of the range of grain sizes present, (2) a transition from nappe to skimming flow occurs in stepped channels with mobile beds for conditions similar to stepped spillways, and (3) the partitioning of flow resistance changes significantly when flow passes from nappe to skimming conditions. Results support each of these hypotheses and help to illuminate the complexity of V-Q relationships in stepped channels, in which a dramatic decrease in flow resistance and increase in velocity accompany the transition from nappe to skimming flow near step-forming events. Therefore, a single flow resistance equation applicable to both ordinary and large floods may not be ideal in stepped channels. Nonetheless, models based on dimensionless velocity and unit discharge appear more robust compared to those based on the Darcy-Weisbach friction factor.