STUDY OF SILTATION OF RESERVOIRS AND ITS CONTROL WITH SPECIAL REFERENCE TO NIZAM SAGAR PROJECT IN ANDHRA PRADESH

Abstract

Impounding reservoirs are the most important part of all water resources development projects. They serve the purpose of balancing the utilisation of water by storing large flows during monsoons for release during periods of drought and low runoff, and are primarily built for water supply, irrigation, generation of power and flood control etc. But all streams transport some sediment, the quanti-ty depending on the stream characteristics, and hence any reservoir fed by stream flow undergoes siltation to varying extents from deposition of stream borne soil waste or sedi-ment due to sudden reduction of transporting power of stream while flowing into the quiet impounded :waters.s of the reser-voir.flt This eventually reduces the usefulness of the reservoir during the course of its operation. Thus when-ever a new dam is contemplated, the first question which has to be satisfactorily answered is, "How quickly the reservoir would silt and what would be its useful life?* Only a couple of decades ago* there was neither enough data nor rational approach to this subject, to guide the designer and the out come was left to "Chance*". Some reservoirs served usefully for several generations, while others silted rapidly. Such an important question as the life of a reservoir cannot be left to chance and serious thought is being given in recent years to estimate the "useful life" on systematic observations of silt load in the rivers,,, streams and reservoirs. Siltation of reservoir is a serious problem not just because the sediment is deposited but because it is deposited in such quantities as to cause serious economic and social losses to the present and future generations, due to its encroachment upon useful capacity of the reservoir meant for conservation and flood control* A comprehensive study of the sediment load of the stream and preventive measures to control it must thus be made in advance, before taking up any reservoir scheme, so that costly storage scheme may have proper life. A fairly prominent example that is arousing consi-derable interest and provoking a great deal of discussion amongst Engineers and specialists in A.P. as regards silting is Nizamsagar project. Nizamsagar reservoir was constructed on the Manzira River during 1923 to 1930. It had a useful capacity of . '; 72.491 thousand hectare meters (25,60 T.M.C. ) which is a fraction of total inflow 3218.05 McM(1, 3,3, 712 Mcft) No allowance for loss of capacity from siltation was made. The lose in gross capacity of the Nizamsagar based on the capacity surveys done by Andhra Pradesh Engineering Research Laboratories in 1967 has however been estimated to be about 52%, The rate of deposition for 100 sq.miles of catchment works out to 134-3 acre feet (6.37 hectare meters per 100 sq.km. of C.A. ) This dissertation deals with the general subject of' erosion and sediment yields as related to the processes and rates of reservoir sedimentation, and describes guide lines for control measures. The problem of sedimentation of Nizemsagar has been brought out in particular. Finally the discussions have been summar ,zed,, and the recommendations have been made outlining the areas where further study is indicated.