As is known, proper drainage of spillway aprons considerably or even completely relieves the apron from uplift under the slabs as a resuk of the large pressure reduction in the surface flow in the region of the hydraulic jump [1,2]. Moreover, surcharging of the apron by the surface flow can be attained by the appropriate arrangement of drain holes and by blocking off free access of water under the apron on the downstream side. This allows a considerable reduction of the thickness and volume of spillway aprons.The effect of relieving the apron from uplift is used widely in design and, in particular, in the design of the apron of the Kanevsk hydroeleotric station.The installation of drain holes in the apron in the region of the hydraulic jump creates the possibility of occurrence under the apron of large fluctuating pressure pulses, causing the filter under a drained apron to operate in a fluctuating-flow regime.Problems of the calculation of a fluctuating flow in coarse-grained material and its effect on graded filters with consideration of inertia forces and other circumstances were investigated in detail by N. N. Belyashevskii and N. G. Bugai [3][4][5]. Their studies developed formulas for finding the scouring gradient in the case of contact scour by longitudinal steady and fluctuating flows for materials of different grain size for interlayer coefficients K = (Ds0/ds0) < 40 and scouring gradients ~s -< 1.0. These design relationships were obtained on the basis of experiments with quite uniform compositions of the rabble for ~ = (Ds0/D10) __< 5. Such rubble formed a rigid undeformable filter skeleton.The maximum pressure gradient in the fluctuating flow ~m depends on the distance between drain wells and the parameters of the filter itself and also on the magnitude of the fluctuating press~e pulses in the surface flow. G.A. Yuditskii's generalizing study is devoted to pressure fluctuations in the surface flow [6]. When calcuiating fm it is also necessary to take into account that the probability of the maximum pulses of opposite sign occurring simultaneously over adjacent drain wells is extremely small [7].Thus, exhaustive data were obtained for a well-founded calculation of the fraotional composition of a graded filter under a drained apron operating in a fluctuating-flow regime. However, these investigations pertained to rather uniform rubble materials with no intermixing of the contacting layers, wNch imposed limitations on the selection of the fractional composition of the multilayered graded filter and especially its first layer contacting the sand foundation.