Simulation of non-stationary two-phase mass transfer in a large reservoir

Abstract

The Kuibyshev Reservoir was formed on the Volga River and is the largest in Eurasia. The length of the reservoir along the Volga is 510 km with its maximum depth of 41 m and average depth of 8 m; the area of water mirror is 6,450 km2. Conditions of runoff formation in catchment area, as well as seasonal and daily regulation are the reasons for unsteady regime of the reservoir. Specificity of flow control in the Kuibyshev Reservoir predetermines water masses motion, flow speed pattern, as well as degree of water saturation with suspended sediments and, therefore, reservoir bed siltation intensity. The lowest flow velocity in the reservoir is observed in summer-autumn period of year; at this time, the reservoir bottom is reshaped least actively. During spring filling, flow velocity in the reservoir is at maximum, hence, favourable conditions to transfer small and large fractions of suspended sediments are created. Most of them enter the reservoir during its spring time filling, although sometimes, sediments maximum was observed earlier, before reaching the peak of flood wave in the reservoir. Flow pattern unsteadiness leads to increase or decrease in flow transfer capacity and, hence, changes in bottom levels. Complex configuration of banks, large depth gradients and basin morphology’s particulars pre-determine instability of sedimentation processes in water area. 3D hydrodynamic model of the reservoir (A. Rakhuba) forms the basis for calculating bottom level motions of the Kuibyshev reservoir. This model is supplemented with analytical formula for sediment consumption (M. Shmakova) and hydraulic ratios to calculate erosion and sediments accumulation (M. Shmakova). Four ice-free months runoff simulation of the Kuibyshev reservoir includes the following main water content phases: low water period and periods of snow flood and rain floods. The study of spatial and temporal patterns of bottom reshaping has shown that greatest intensity of such reshaping occurs during the period of large level gradients, i.e. high water. Flood swelling is featured by intensive bottom erosion. When flood recesses, transfer capacity decreases that leads to sedimentation. The greatest motions in bottom reshaping occurs in the places of channel contraction, which is explained by sharp changes in channel capacity.