Scour at submerged weirs

Abstract

Weirs or bed sills are river training structures that are used for raising upstream water level, bed stabilization 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. Many empirical scour equations have been proposed for prediction of scour depth downstream of unsubmerged weirs. Research on scour at submerged weirs is very limited. 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. Under livebed scour conditions, the rapid evolution of underwater mobile topographies and propagating bedforms increases the complexities of the scour process and the difficulties for scour measurement at the submerged weir. The aim of this study is to experimentally investigate flow structures and scour process at submerged weirs in sand channels. The experimental programmes were conducted in a tilting recirculating flume. Both clear-water scour and live-bed scour conditions were applied. The experimental results of an investigation of flow structures at the weir show that a large recirculation zone and a flow reattachment region are formed downstream of the submerged weir. Strongly paired cellular secondary flows were observed in the scour hole. The turbulence structures ahead of the recirculation zone govern the dimensions of the scour hole downstream of the weir. In the coarse sand tests, bed elevation changes were measured in the approach flow reach and in the scour zones both upstream and downstream of the weir using a Seatek multiple transducer array. The highly contaminated raw bed-elevation data were filtered using a data processing technique. In the fine sand tests, the scour process was monitored using Logitech HD webcams on both sides of the flume. Image analysis was implemented to obtain bed profiles and scour depths over time. Under clear water scour conditions, the temporal development of maximum scour depth downstream of the weir has three stages: 1) initial fast stage, 2) progressing stage, 3) equilibrium stage. Under live bed scour conditions, the scour occurs both upstream and downstream of the weir, and sediment transport processes are highly influenced by strong turbulence in the vicinity of the submerged weir. A scour-and-fill process occurs immediately upstream from the weir in response to periodic approaching bed-forms. The dimensions of the scour hole downstream of the weir fluctuate due to the unsteady upstream sediment input and the development of secondary flows. The flow regimes over the submerged weir are found to be independent of the sediment size, and the transition flow regime boundary can be expressed as a function of upstream Froude number and the ratio of weir height to tailwater depth. On the basis of the experimental data, equations for prediction of equilibrium scour depths both upstream and downstream of the submerged weir are proposed. A design method is given in this study for estimating the maximum scour depths at the submerged weir.