3D Numerical modeling of Sediment Deposition in a Hydropower Reservoir
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The purpose of this assignment was to test and evaluate SSIIM 2 on its ability in numerical modelling of sediment transport in the hydropower reservoir Banjë in Albania, which is a 14 km long, and narrow hydropower reservoir. As far as the preconditions allowed, a purpose was also to obtain indications of the sediment transport in the reservoir. Numerical modelling of sediment transport in hydropower reservoirs is used in an increasing extant in research and in commercial companies to foresee and mitigate for the sediment transport and depositions and thus volume loss of reservoirs, blocking of intakes and hydraulic structures in hydropower plants, and turbine erosion from sediment particles by planning operational strategies for the hydropower plant operation regime and sediment removal. Reduction of reservoir volume globally is important in the struggle for increased share of renewables in the energy sector, where hydropower is an important part, and it is believed that numerical modelling of sediment transport facilitates the management of sediments. Three dimensional modelling allows for more detailed simulations of sediment transport in the reservoirs than lower dimension simulations and physical modelling. SSIIM 2 has been used to set up a three dimensional numerical model of Banjë hydropower reservoir in Albania, a reservoir with a volume of about 400 million m³. The emphasis on the testing of the software was on evaluation of different grid that were suitable for modelling in such a large reservoir. The hindrance most encountered was the difficulty of having the computations converge for the water flow computations. The factor which was varied between different grids that were tried out was the cell dimensions, where the grid with the largest cell dimensions proved the only grid to reach convergence for all simulation scenarios set up. The distortion ratio of the grid is 10,7, and the cell surfaces in the horizontal plane is about 800m². The grid is structured and orthogonal. Data for calibration of the model is yet to exist for the hydropower project, and the sediment input data is of relatively low quality. However, the order of magnitude of the sediment data from the hydropower project is provided, which allows for simulations to be carried out that can indicate the sediment concentration distribution in the reservoir and the depositions. The scenarios modelled were an estimated flood event and an annual maximum average weekly inflow value. Both showed that sediment concentrations critical for turbine erosion will not approach the dam. Banjë reservoir is large reservoir where sediments will be likely to cause mainly for reduction in reservoir volume. However, the sediment concentration from the tributaries near the dam should be measured in order to obtain more accurate simulations. The conclusion is that SSIIM 2 appears suitable for modelling sediment transport in a reservoir as large as Banjë, but with further development of the grid, and of the use of different algorithms, relaxation coefficients in the iteration processes, and right handling of cells near the geometry borders.