Flow pattern formed over steps at a low-head dam: Salmon River Dam

Abstract

Building Stepping way over the low-head dams has been proposed to reduce the risk of drowning in submerged flow conditions. In the Salmon Barrier, adding two steps downstream of weir was previously suggested as an effective method for reducing the risk of drowning downstream of this dam. Given the effect of steps on the flow pattern downstream of the weir, the occurrence of dangerous submerged flows at different tailwaters was predicted, and the flow pattern expected at variable tailwaters was studied in comparison with the flow pattern passing over the sharp crested weir. According to the results, the submerged flow pattern downstream of the dam is a combination of the submerged flow pattern over the abrupt drop at low tailwaters and that over the sharp crested weir at high water levels. Consequently, similar correlations for predicting the flow regime downstream of these structures can be presented to predict the flow pattern at different tailwaters. The flow pattern over the step was compared with that over the abrupt drop, and a few diagrams were presented for predicting the occurrence of different flow patterns based on the submergence ratio of steps. Given the similarity of the flow pattern with those in the literature for sharp crested weirs, the flow pattern was also investigated at higher tailwaters. The comparison of correlations developed in this study indicated that the flow pattern approaches that reported in the literature for sharp crested weirs with increasing discharge.