Experimental Study at the Reservoir Head of Run-of-River Hydropower Plants in Gravel Bed Rivers. Part I: Delta Formation at Operation Level

Abstract

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 104 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 at 1:1 scale. Experiments were carried out under mobile-bed conditions at flow rates which correspond to 50%–80% of a 1-year flood HQ1. Even at the head of the reservoir, which is least influenced by the backwater effect of the RoR, sediment transport practically ceases for sediment fractions >14 mm for a flow rate of 0.7 × HQ1. The whole sediment load coming from the undisturbed upstream section accumulates at the head of the reservoir. This delta formation is accompanied by a substantial rise in water levels. A spatio-temporal scheme of the delta formation was derived from the experiments. The study proved that the delta formation increases the flood risk at the head of the reservoir. Conversely, reservoir drawdowns at flood events of high probability may be a promising strategy to enhance sediment connectivity under the specified boundary conditions.

RAISE – Research on Alpine sediment processes, morphodynamics and ecosystem behaviour considering extreme events

“…This section summarizes the findings of delta formation experiments at the head of run-of-river hydropower plants as reported in Sindelar et al (2020). Human activities and climate changerelated effects have disturbed natural sediment dynamics in riverine systems on a global scale.…”

“…The impounded section is termed "reservoir". The operation where the reservoir water level is lowered by opening the flood gate(s), such that a free surface flow develops is termed "flushing" (Sindelar et al, 2020).…”

Sediment Management: Hydropower Improvement and Habitat Evaluation

“…The results of the flushing experiment by Reiterer et al (2020) showed the possibility of remobilizing delta depositions by (partial) drawdown flushing within a reasonable period (about 9 h in a 1:1 scale). The erosion of existing headwater delta deposition (compare to [2]) was found to be retrogressive and twice as fast as the preceding delta formation process. A spatiotemporal erosion scheme points out these findings.…”

“…For the first category-to derive an advanced process understanding-the papers of The flood aspect, in terms of sediment management, was the central focus of Sindelar et al (2020). On the one hand, the study proved that the delta formation increases the flood risk at the head of the reservoir; on the other hand, the results showed that reservoir drawdowns at flood events of high probability were a promising strategy for enhancing sediment connectivity under the specified boundary conditions.…”

“…For strategic management, the findings of the SI allow a differentiated discussion concerning if and how hydrological thresholds can be implemented into management practice. On the one hand, a clear move forward concerning an optimization in flushing operations, due to an adjustment of a clearly defined hydrological threshold for flushing at run-of-the-river hydropower stations, was presented by [2]. On the other hand, in [6], it was presented how thresholds can be misleading in terms of the ecological evaluation of, for example, habitat specific turbidity.…”