Transition Flow in Step-Pool Systems: Pressure Distributions and Drag Forces

Step-pool systems occur naturally in steep mountain streams and as man-made structures in steep channel sections where they serve as energy dissipating structures. Three different flow regimes may occur. The dominant flow regime is the cascading "nappe flow regime." At higher discharges the "skimming flow regime" develops with an almost plane water surface. In between these flow regimes a "transition flow regime" occurs. A physical model was used to measure the pressure distribution around a single step for a particular step-pool geometry and for different flow regimes (nappe/transition/skimming). To achieve this, 14 simultaneously recording piezoresistive pressure transducers were distributed around the step. The results showed that the regime change from nappe to transition flow is associated with a discontinuity of the average Froude number of a step-pool unit as well as drops in pressures and water depth. Relative instantaneous drag forces reached a maximum for this regime change. This implies that the regime change from nappe to transition flow is a critical loading case with respect to the stability of the steps. On the other hand, the mean drag forces showed little systematic variation as the flow rate changed with a large spread in results for measurements repeated at the same flow. Thus, for the nappe and transition flow regimes, the mean drag force is not a good criterion to use to predict step stability. The occurrence of the different flow regimes could be related to the average Froude number of a step-pool unit. The results have practical implications for the design of step-pool systems.