SEDIMENT YIELD AND DEPOSITION PROFILES IN RESERVOIRS

Abstract

INTROIUCTION Reservoirs are constructed for the purpose of storing large quantity of water during monsoons and releasing it during different seasons for different purposes such as generation of power, irrigation, water supply etc. Whenever a dam is to be built, the first question which has to be answered is: what would be its life? It has been always difficult for engineers to esti mate the life of reservoir at the design stage. This is because the sedimentation process is a complicated phenomenon which is governed by several variables related to the hydrology of the catchment, catchment, the stream, the reservoir and method of reservoir operation. To answer the above question the problem posed for investigation is to analyse the data from existing Indian reservoirs and to develop a method for predicting the amount of sediment eroded in the catchment area and the pattern of deposition profiles on the upstream of the dam with the passage of time. ANilYSIS The data for Indian reservoirs have shown that the absolute volume of sediment coming from the catchment Vq/rgrj can be estimated by any one of the following Equations. > VgAB0 =1.182X10-6 ^.02^1.28^0.28730.07530.398^2.422^^ ii VSAB0 - 1.067X10"6 ,1.292^.38430.129,0.597,2.51 ... n YQmn - 2.41X10"6 A1'154 p1.071s0.06p1.893 _ m bABU C o where A is the catchment area in Ian , P is the annual rainfall in cm, Q is the annual runoff in Mm , S is the slope of the catchment, D^ is the drainage density in the catchment in km"1 and F is the vegetal cover factor of the catchment. If T^ is defined as the time in years when cumulative sediment deposition is equal to the capacity of reservoir, the relation between T* and other parameters in the catchment area has been found as 7 A0.886 p 2.869 T = 3.789x10"' & „*ni ,, ... IV 01.771 .1.81% 8.678 where P is the mean annual rainfall in cm and G is the initial capacity of reservoir in Mnr. The mean curve for trap efficiency and capacity inflow ratio which is introduced by Brune has been expressed in the form of equations to be used in this analysis for predicting the absolute sediment deposited in the reservoir from the known absolute volume produced in the catchment area. The observed data of unit weight and fraction of clay, silt and sand in the mixture for 415 samples for some Indian iii reservoirs have been analysed and the average initial unit weight for clay, silt and sand for Indian reservoirs have been found as 640 kg/m5, 760 kg/m5 and 1540 kg/m* to be used in determing the average unit weight of deposited material after (I) years of consolidation. Autoregressive model is used to generate the annual rainfall for desired number of years. Knowing the historical annual rainfall P and annual runoff R in cm the relation of the type is developed. r = K(P - gS- ) and is used for obtaining runoff for given P. The average surface width of reservoir B in terms of water s depth Y near the dam is expressed as Bg = K^ ym The equations for predicting upstream slope, downstream slope and the maximum peak of sediment deposition profiles with the passage of time have been developed to be used in the computer programme. From the study of observed profiles it has been found that initial maximum peak of deposition profile occurs at about 0.424Lr upstream of dam where the water depth is about 0.513 YD. The peak moves toward the dam at a velocity of 0.2 km per year. A computer programme for finding the deposition profiles for various years and the life of reservoir has been written and tested for Bhakra reservoir which is in operation stage and for Tehri and Lakhwar reservoirs which are to be constructed.