Live-Bed Scour at Submerged Weirs

Abstract

Weirs or bed sills are low-head hydraulic structures used for bed stabilization, raising upstream water level, and reducing flow velocity. During high-flow events, the weir is fully submerged in the river and scouring occurs both upstream and downstream of the weir. For a fully submerged weir, the scour mechanism around the weir is dependent on approach flow intensity (clear-water scour conditions or live-bed scour conditions) and flow regimes (surface-flow regime or impinging-jet regime) over the weir. The fast evolution of underwater mobile topographies and propagating bedforms increase the complexities of the scour process and the difficulties for scour measurement at the submerged weir under live-bed scour conditions. This paper develops a measurement and data-processing technique for the study of scour at submerged weirs under extreme measurement environments and investigates the scour process both upstream and downstream of submerged weirs under live-bed scour conditions. The experiments are carried out with uniform sediment in a tilting sediment recirculating flume. Different flow rates and weir heights are used. For all the tests, the flow upstream of the weir is subcritical. Bed elevation changes are measured in the approach flow reach and in the scour zones both upstream and downstream of the weir using a Seatek multiple transducers array (MTA) (SeaTek Instrumentation, Florida). The highly contaminated raw bed-elevation data are filtered. Scour depths and bedform characteristics are extracted in data postprocessing. During live-bed conditions, a scour-and-fill process occurs immediately upstream from the weir in response to periodic approaching bedforms. The influence of the flow regimes on the scour mechanism downstream of the weir is discussed. Based on dimensionless analysis and experimental data, equations for prediction of the scour depth at the weir are proposed.