Antiseepage and drainage measures in reservoirs

Abstract

In hydraulic construction practice there are frequent cases in which it is necessary to construct reservoirs or railings ponds on highly pervious foundations. In such cases the complex of hydraulic structures has, as integral parts, the antiseepage and drainage elements which are intended not only to reduce substantially the seepage losses but also to lessen as much as possible the harmful effects of the seepage on the adjacent zones. However, the substantiation of the type and size of these elements involves significant methodological difficulties, since the existing analytical, analog, and numerical methods of seepage analysis and investigation have been developed for comparatively simple conditions. For this reason, the use of these methods under real conditions characterized by unsteady and three-dimensional seepage, nonhomogeneity of the porous medium, presence of several feeding regions, relief of the seepage flow. etc. may lower the reliability of the results. Nevertheless, experience with seepage investigations carried out by the Institute of Hydromechanics of the Academy of Sciences of the UkrainianSSR for different railings ponds and reservoirs in the Ukraine [3, 4, 7, and 8] has shown that the existing methods may be used even for comparatively complex hydrogeologic conditions. It is recommended that the analysis be carried out in two stages.In the first stage, the water losses are ealcuiated for the free seepage phase, when the seepage from the railings pond has not yet been merged with the ordinary ground water level. It should be pointed out that the existing methods of analysis of free seepage [1, 5, 6] have been worked out basically for irrigation canals which are filled nearly instantaneousiy and have a very small longitudinal slope. Tailings ponds are characterized by prolonged filling, considerable slope, and variable bottom width (in railings ponds of the gully and plain-gully type), which do not permit using the existing methods of analysis in their pure form. A modification proposed for these methods is described below.The entire railings pond depression is divided, by area, into separate portions within whose limits free seepage occurs simultaneously. Within these portions the bottom is eousidered to be horizontal. The size of these portions depends on the wetting time of the pervious mass, the bottom slope, and the rate of rise of the water at the upstream side. In the ease of uniform permeability of the mass, the wetting time of this mass and the seepage flows for each of the portions are determined by applying formulas presented in the literature [5. 6]. The analysis is made by the method of successive approximations, during which, on the basis of the filling graph for the tailings pond, the acting heads, the areas of the portions, and the wetting time of the pervious mass are linked with each other.In the ease of highly pervious soils covered by far less pervious soils or by an antiseepage facing made from soils of low permeability, initially for each of the analytical portions the wet...