EFFECT Of SILT FACTOR AND RETROGRESSION ON DESIGN OF BARRAGE

Abstract

A weir/barrage is an important structure used to divert river water through a canal system for irrigation and other beneficial 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". It is designed based on both surface and subsurface flow considerations. Among several others, the silt factor and retrogression form to be important factors governing the design of barrage. Lacey's silt factor `f can be determined from the average size of particle, i.e. X1.76 Mr , where Mr= average size of particle in mm. According to Lacey, the silt factor as a measure of silt grade should be consistent with the flow velocity and hydraulic radius so as to keep the silt in suspension by the vertical components of eddys occurring all along the perimeters. Progressive retrogression or degradation of the downstream river and levels as a result of construction of weir or barrage causes lowering of the downstream river stages which has to be suitably accounted for in the design of downstream cistern. The lowering of river water level due to retrogression downstream causes increased exit gradients. For the design of the weir/barrage on alluvial soil, it is essential to consider retrogression of the D/S river bed as it lowers the D/S water level, and hence, the total energy level. The usual practice to consider retrogression of 0.3 to 0.5 m is employed at high flood discharge for the design of weirs. If the retrogression is not accounted for, the undermining the D/S floor may occur. Thus, as a result of retrogression, low stages of the river are generally affected more than the maximum flood levels. The reduction of river gauge at low stage which governs the design should be carefully evaluated. The retrogression plays an important role in the design of D/S floor level and D/S protection works. A barrage usually consists of the following components: Under Sluices, 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. The design of most of these 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 silt factor and retrogression on the above components of barrage is investigated. In addition, there does not exist any 1 computer. software to facilitate the testing of design of barrage, rather tedious hand computations are usually performed for any change in the governing variables or trial designs. Therefore, a lack of confidence among the designers usually exists largely due to uncertainty in the opted input values. The advent of high speed computers can be exploited to overcome the computational difficulty. This study is thus an attempt to develop computer software for the hydraulic design of a barrage, test the developed software using the example dataset, and investigate the effect of silt factor and retrogression on the various components of barrage in design. The sensitivity of silt factor and retrogression with respect to the available dataset was evaluated in determination of the following elements in the hydraulic design of a barrage: Lowest level of jump formation; d/s floor length; d/s glacis length; total floor length; u/s floor length; u/s sheet pile depth; d/s sheet pile depth; quantity of launching apron; and length of inverted filter, upstream volume of CC blocks, downstream volume of CC blocks, appurtenant works, such as Chute Blocks, Basin Blocks, End Sill. It was found that the silt factor affected significantly the total floor length, u/s & d/s sheet pile depths, quantity of launching apron, upstream volume of CC blocks, downstream volume of CC blocks, appurtenant works, such as Chute Blocks, Basin Blocks, End Sill; quite significantly the u/s floor length and length of inverted filter; and insignificantly the lowest level of jump formation, d/s floor length, and d/s glacis length. On the other hand, the retrogression affected significantly the lowest level of jump formation, d/s floor length, d/s glacis length, u/s & total floor lengths; quite significantly the d/s sheet pile depth, d/s volume of CC blocks; and insignificantly the u/s sheet pile depth, quantity of launching apron, and length of inverted filter, u/s volume of CC blocks, and appurtenant works. It can be seen that both the silt factor and retrogression play their important roles in determination of u/s & total floor length and d/s sheet pile depth. Since the total floor length forms to be one of the most important parts of the barrage, a careful evaluation of both the govering factors, i.e. silt factor and retrogression, is required in the design of a barrage.