Local Scour at and Around Vertical Hydraulic Structure - A Case Study Around the Piers of Bridges on Rupnarayan River, WB

Local scouring is the lowering of the riverbed level by water erosion due to separation of flow and development of vortices around hydraulic structures. Excavating and carrying away materials from the bed and banks of streams and from around the hydraulic structures cause failure of riverine structures throughout the world especially during the river floods. The present study incorporates the review of the existing literature on scour with emphasis on the understanding of the mechanism of scouring, prediction of scour depth, correlation of local scour and sediment transport capacity and reduction techniques of local scour around vertical piers. Actual scour depth data have been collected from the piers of bridges near Kolaghat region on Rupnarayan River. The expected scour depth is calculated based on different scour controlling parameters for the same. Dou (1997) established the relationship between local scouring and effective sediment transport capacity (STC). Many researchers have employed different techniques to predict the maximum depth and diameter of scour hole. In a few studies, procedures to reduce the scour depth have been used by making use of splitter plates and collars on the piles. Helical wires or cables wrapped spirally around the pile resembling a threaded pile have also been used to reduce the scour. With the use of different special techniques, the scour depth has been reported to reduce by modifying the flow field near the bridge elements. The Lacey-Inglis method (1992, 1944) is used in India for estimation of the design scour depth around hydraulic structures. Prediction of scour depth and protecting it is very difficult due to many uncertainties associated with it and sometimes become very costly. More studies and investigations are still required to predict the scour depth effectively and also to find cost-effective ways to protect the scour.