EXPERIMENTAL STUDIES ON PERFORMANCE OF LAUNCHING APRON AROUND A GUIDE BANK IN ALLUVIAL STREAMS

Abstract

Indian rivers in plains areas are mostly shallow and flow in a wide alluvial belt, with meandering and/or braiding characteristics. To construct engineering structure across these rivers like weirs, barrages, bridges, aqueducts etc, it becomes necessary to narrow down the rivers section and restrict its course of flow centrally through the structure built over it. This is achieved by construction of heavy embankments, called guide bunds. The flow gets modified in the presence of guide bund as a result scour develops around the guide banks. Design of guide bank requires correct estimation of this scour depth and measures to ensure its safety against accompanying scour. Generally a launching apron consisting of suitable size of coarse material (stone) and thickness is provided around guide bank to provide a desired safety against scour. In present literature considerable information is available for estimation of scour depth and for the design of launching apron. However, very scanty information is available about the performance of launching apron as far as its behavior under increased shear stress is concerned. Modification in the flow and bed geometry that it is likely to produce around the guide bank after occurrence of scouring and launching of the apron are not studied as yet in the field and through laboratory experimentations. Therefore present study was taken up to fulfill the above gap in knowledge. Well-controlled experiments have been conducted in the laboratory for studying the variation in scour depth around the guide bund and control of the scour by the launching apron. Launching of the apron and variation of scour depth with the pertinent hydraulic variables is also studied.