EFFECT OF DESIGN FLOOD ON DESIGN OF BARRAGE

Abstract

A barrage is a structure built across a river to divert flow into a man-made channel. It is designed based on surface and subsurface flow considerations. It forms to be an important structure to divert river water through a canal system for irrigation and other useful purposes. If the difference between the pond level and crest level is within 1.5m the pond level can be maintained by means of falling shutters however, if the difference is more than 1.5m a gate controlled weir is necessary which is called a "Barrage". A barrage usually consist of the following components: undersluices, barrage bay, canal head regulator, divide wall or groyne, fish ladder, piers and abutments, protection works, river training works, gates and hoisting arrangements and working platform. This study is to provide a computer aided design of barrage, test it with the available example data set, and to see the effect of design flood on different components of the barrage. In the design of a barrage, a number of input variables are required. Among these, design flood forms to be a major governing factor. Design flood is the flood discharge adopted for the design of a hydraulic structure and is taken as PMF or SPF, i.e. a severe most flood that is physically possible as a result of severe most combinations of all the meteorological and hydrological conditions which are possible. Depending on the degree of protection, various organisations have suggested different criteria for fixing the design flood discharge by using different methods, viz. part flood marks method, empirical method, unit hydrograph method, and flood frequency method. When the discharge in the river is less than the design flood there will be shoal formation and meandering and may cause encroachment on the free board of river training works. For purposes of design of barrage segments, a design flood of 50 year frequency may normally suffice and for design of free board a minimum of 500 years frequency flood or SPF may be desirable. At design flood the reduction gauges due to retrogression may be considered to vary from 0.3 to 0.5 m depending on whether the river is shallow or it is confined during floods. Under surface flow considerations, the design flood discharge governs the length of waterway, discharge intensity, scour depth. Width of upstream is responsible for designing the depth of cut off and upstream and downstream protection works. The design of most of the barrage components is primarily governed by the magnitude of river discharge, silt factor, retrogression, afflux, concentration factor, discharge intensity, and waterway. In this study, the effect of design flood on the above components of barrage is xv investigated with the help of computer based software developed for its hydraulic design. In practice, tedious hand computations are performed for any change in the governing variables or for trial designs. This study is thus an attempt to develop computer software for the design of a barrage, test the developed software using the example data set, and investigate the effect of design flood on various components of barrage. This study provides a systematic evaluation of the impact of design flood on various design components of the barrage, viz, lowest level of jump formation, length of waterway, total floor length, downstream floor length, downstream glacis length, upstream floor length, depth of sheet piles, length of cement concrete blocks, length of launching apron, and appurtenant works. The effect of discharge on these elements was investigated employing three datasets. Trial runs with varying design flood magnitudes were taken using a Microsoft-Excel-based computer program developed. It was found all these elements were affected either significantly or quite significantly with an increase or decrease in discharge. Thus, the design discharge plays a vital role in the design of barrage and should be evaluated critically considering both economic and safety aspects.