EFFECT OF AFFLUX AND CONCENTRATION FACTOR ON DESIGN OF BARRAGE

Abstract

Afflux is the rise in the water level on the u/s of the structure after the construction of barrage. The high flood level on the u/s is higher than that at the downstream. The area of submergence and the design of guide banks depend on the afflux. The location of the hydraulic jump on the d/s glacis also depends on upstream total energy line which is a function of afflux. If afflux is large, from weir formula, the length of the weir will be small for a given design discharge and, in turn, the discharge intensity (q) over the crest will increase. Thus, the length of barrage (or waterway) is governed by the magnitude of afflux. It is also important for the design of d/s cistern flood protection and river training works, u/s and d/s loose protection works and u/s and d/s cut-offs. IS 6933-1973 recommends an afflux of 1 m for the alluvial rivers in the upper and middle reaches of the river and of 0.3 m in the lower reaches. The actual discharge intensity (q) over the entire length of the weir is generally not uniform. Because of the possibility of nonuniform flow over the crest, the actual discharge intensity in some portion of the barrage may be greater than the average discharge intensity (q). The concentration factor is the ratio of the actual (larger) discharge intensity to the average (smaller) discharge intensity, expressed as percentage. For the design of impervious floor, cistern and cut-off wall, concentration factor of 20% is generally taken, i.e. the actual discharge intensity is increased by 20%. A barrage usually consists of the following components: Under Sluices, barrage bay, canal head regulator, divide wall, fish ladder, piers and abutments, protection works, river training works, gates and hoisting arrangements and working platform. The design of most of these components is primarily governed by the magnitude of river discharge, silt factor, retrogression, afflux, concentration factor, discharge intensity, waterway. In this study, however, the effect of afflux and concentration factor on the above components of barrage is investigated. In addition to the design of the barrage there does not exist any generalized software to facilitate the testing of designed components, rather 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 barrage, test the developed software using the example dataset, and investigate 1 the effect of afflux and concentration factor on various components in the design of barrage. The sensitivity of afflux and concentration with respect to an example dataset was evaluated in determination of the following barrage elements in the hydraulic design: Lowest level of jump formation; total length of waterway; total floor length; d/s floor length; d/s glacis length; u/s floor length; u/s sheet pile depth; d/s sheet pile depth; quantity of block protection work u/s and d/s; appurtenances and the length of back water curve. It is found that the following parameters, the lowest level of jump formation, the total length of waterway, total floor length, d/s floor length, d/s sheet pile depth, length of back water curve are more sensitive to afflux. Similarly the parameters, the u/s floor length, u/s sheet pile depth, block protection works are marginally sensitive to afflux. Like wise in the case of Concentration the parameters, the lowest level of jump formation, total floor length, d/s floor length, u/s floor length, d/s sheet pile depth, d/s block protection works are sensitive to the concentration of flow. However the total length of waterway, u/s sheet pile depth, u/s block protection works are insensitive to the concentration.