SOME ASPECTS OF PROGRESSIVE DEPOSITION IN RESERVOIRS

Abstract

It is hardly necessary to emphasise the need for accurately estimating the progressive sediment deposition in reservoirs. Sedimentation process is a complicated pheno-menon which depends on characteristics ofcatchment such as size; shape; nature of geology and vegetation, climatic conditions such as amount and distribution of rainfall, channel characteristics such as size; shape; slope; dis-charge and sediment charge and reservoir characteristics such as size; shape and operation schedule. Since all these factors are almost always different from reservoir to re-servoir, it has always been a difficult problem for hydro-logists to estimate the probable rate of sedimentation and its location in reservoirs. Because of the unknowns involved, many authors have developed empirical relations taking only a minimum number of variables such as capacity, inflow, catchment area to find out the probable rate of sedimentation. Ana-lysis shows that the estimates based on the available re-lations are far from the observed trend. Moreover, it is always assumed that sediment will first go on occupying dead storage space leaving live storage space unaffected in the initial period. But actual observations show that live storage space may be affected right from the beginning. An attempt has been made in this study to estimate maximum depth of deposition at any time, slope of deposition ii in reservoir, volume of deposition at that time and probable profile of deposition. For this variables such as water in-flow in reservoir, channel slope, shape of reservoir, depth of reservoir at dam, etc. are assumed to be known. These variables for all reservoirs are then related to get genera-lised relation for volume of sediment deposition per unit width of reservoir at any time, in the fo}lowin.g form. v = 1.16 v 0.94 S 0.84 5=l.16 0 Similarly another relation for the peak of deposition is obtained in the following form - 2 .2 Zp/Yd = 0.066 [w5o ...{ii)•] Yd Attempt is also made to relate characteristic time t* of the reservoir with peak of deposition and its upstream slope. For this evaluation, cumulative volume of sediment deposition for the time upto which water inflows are available can be computed from Equation (i) and these can be plotted on log-log paper as Vs/Vo against time t to get t * such that at t = t* , Vs/Vo = 1. This t* is then related to get the following generalised relations for peak of deposition and its upstream and downstream slope at any time. Zp/Yd = tanh-1 r 0.50 (t/t, )0.25 ~. ..•(iii) St /So = tanh 1 0.38 (t/t,~ )-0.066 L -.• (iv) Std/So = a + Log10 (t/t ) ...(iv) Here it is observed that the values of Zp/Yd com-puted from Eqs . (ii) and (iii) for the same year are slightly different but the values obtained from Equation (ii) are in closer conformity to the actual values, hence use of Eq.(ii) is recommended. For prediction of future deposition profiles, water inflow data are necessary which can be generated from the known few years data by carrying out Time Series Analysis. The upstream slope of deposition profile is given by Eq.(iv). The slope of profile downstream of deposition peak car be described by Eq.(v) where values of a, vary from reservoir to reservoir and must be determined from few years data. It is observed that the initial deposition peak occurs at about 0-45 L and/or at where water depth is 0.54 Yd. It's average velocity of propagation towards dam is observed to be of the order of 0.25 kg/year from some reservoirs. The varia-tion of velocity could not be studied with Froude Number or any other parameter. Hence the same may be assumed tentatively,