AN OPTIMAL DESIGN OF BARRAGE

Abstract

A barrage is a structure built across a river to divert flow into a man-made channel. It is designed employ of surface and subsurface flow considerations. This study deals with the near optimal design of a barrage, and evaluates the effect of various governing variables on different components of the barrage. A barrage is a costly structure involving an expenditure of several hundred million rupees. Therefore, it poses a considerable challenge to the hydraulic engineer to devise a safe and economical design of barrage to tap the mighty rivers. Since a barrage is designed, to satisfy surface and subsurface flow considerations, there may not easy procedure to fix the basic barrage parameters viz depth of sheet pile/cut-off, length and thickness of the floor etc, in a cost effective manner. The present study is an approach to reduce the cost of a barrage satisfying the design criteria by considering a most optimal range of various parameters;_-:_ an innovative step. A parametric analysis has to be carried out to see the effects of various governing parameters on the optimal design of a barrage. For the design of the barrage there may not exist any generalized free as well as easy to operate, software which facilitate the design and testing of designed components, rather tedious hand computations are performed for any change in the governing variables or trial designs to the field engineers. This study is thus an attempt to develop computer software for the design of barrage, test the developed software using the example data set, and derive the limiting parameters values for optimal or near optimal design of barrage. In this study, employing the data of 4 barrages, the impact of design discharge and silt factor on the cost of the barrage is evaluated with the aid of a Microsoft Excel-based computer program developed. The multiplying factor (X) of scour depth (R) is identified for achieving optimality in hydraulic design of barrage. The cost of barrage is found to significantly depend on design discharge and silt factor, and therefore, require careful and judicious determination. Furthermore, there exists a balancing of costs of barrage between barrage length and its thickness, governed by exit gradient and uplift pressures, respectively. The IS code practice of adopting the multiplying factor of scour depth (X) = ,. 1.25 under normal flow conditions is found not to lead to an economically optimal design, rather a value higher than 1.25 can be considered for optimal design, which being on safer die does not violate the Codal norms.