Consideration of bank storage when determining the power output of a hydroelectric station

Abstract

The development of the power industry necessit@tes the creation of cascades of hydroelectric stations with reservoirs regulating the streamflow, which are an integral part of power and water-management systems.Streamflow regulation by reservoirs, among which carryover reservoirs are gaining ever greater influence, leads to an increase of the range of variations of the water level in reservoirs during drawdown and filling.During fluctuations of the level between the normal pool (NPL) and dead storage level (DSL), periodic storage and release of groundwaters occur, i.e., the "subsurface" reservoir also participates along with the main reservoir in streamflow regulation.In water-management and hydropower calculations a quantitative evaluation of the interaction of subsurface and surface waters, as a rule, is not taken into account.Qualitatively, the indicated interaction of subsurface and surface waters during fluctuations of the water level in a reservoir related to its drawdown in the low-flow period and filling in the flood period is expressed in the following.During a rise of the water level additional seepage into the bank zone occurs in the form of a groundwater wedge and during falling of the level the part of the water stored in the soil is released into the reservoir.The water-management effect of the operation of the subsurface reservoir can be estimated by means of the water-balance equation used in calculating streamflow regulation in the following form:where Qsti~ is the storage discharge, i.e., the difference between the inflowing Qinij and regulated Wregij discharges; Quseii, Qseepij, Qwdrij, respectively, the discharge being used, s e . . . . eepag discharge, and withdrawals of water from the reservoir; At, duration of the calculated time interval; i, number of the section; j, number of the time interval.With consideration of the interaction of subsurface and surface waters Eq. (I) will take the following form:where Qsubsij is the effect of interaction of the subsurface and surface runoffs.The effect of the interaction of the subsurface and surface runoffs depends on the hydrogeological parameters of the water-bearing rocks (permeability coefficient, hydraulic diffusivity, storage coefficient, etc.).Furthermore, the selection of the scheme of the hydrogeological conditions and method of calculation is important.The dynamics of groundwaters in such a formulation corresponds to a free-surface, unsteady, nonuniform seepage flow.It is necessary to verify that in an examination of the given problem, seepage into the bank strip, which does not enter into the zone of influence of seepage under the retaining structures, is taken into account.In the general case the problem reduces to the solution of a nonlinear differential equation of three-dimensional seepage flow.For the case of flow in a homogeneous bed the equation looks so:Translated from Gidrotekhnicheskoe Stroitei's<vo, No. 2, pp. i4-i6, February, 1983.