Delta formation in reservoirs of run-of-river hydropower plants in gravel bed rivers – experimental studies with nonuniform sediments

This study concerns scaled physical model tests of the delta formation process at the head of a run-of-river hydropower plant (RoR). It forms part of a larger research project to provide a scientific base for RoR sediment management strategies in medium-sized gravel bed rivers. The physical model consisted of an idealized river having a width of 20 m, a mean slope of 0.005, a mean flow rate of 22 m³/s and a 1-year flood flow of 105 m³/s. The model scale was 1:20. For the experiments five different grain sizes were used covering a range of 14 to 120 mm. Sediment feeding rates for each grain size were calculated using the surface-based approach of Wilcock & Crowe (2003) which was calibrated in preliminary tests in a straight channel. Experiments were carried out under live-bed conditions at a flow rate which corresponds to 70% of the 1-year flood. This discharge is able to mobilize all sediment fractions and is yet low enough to not evoke reservoir drawdown. It is therefore well suited to study delta formation. Two sets of test runs were performed: (i) normal flow conditions to provide a reference case which is undisturbed of any backwater effects and (ii) delta formation at the head of the reservoir at turbine operation (i.e. at operation level). The experiments revealed that even at the head of the reservoir which is least influenced by the backwater effect of the RoR plant sediment transport practically ceases for sediment fractions > 14 mm at 1:1 scale. The whole sediment load coming from the undisturbed upstream section accumulates at the head of the reservoir accompanied by a substantial rise in water levels.