Reconstruction of the reservoir water level–storage volume relationship based on the capacity loss induced by sediment accumulation and its impact on flood control operation

Abstract. Sediment accumulation in reservoirs can change the predefined water level–storage volume (WLSV) relationship by significantly reducing the storage capacity, which threatens the flood control safety of reservoirs in long-term scheduling and operation. However, reconstructing the WLSV relationship has long been challenging, particularly on a large scale, because traditional field bathymetric measurement is difficult. To fill this knowledge gap, this study proposes a method to estimate the reservoir WLSV curve based on the capacity loss induced by sediment accumulation. To assess the potential negative impact caused by an inaccurate WLSV curve, reservoir flood regulations are performed individually using six design flood hydrographs with return periods of 200–10 000 years as reservoir inflow. The flood regulation risk is quantified using the maximum flood regulation water level (Z*) and the ratio of periods when the design flood level is exceeded (γ). Based on over 10 years of hydrological data, sediment data, and operational information, a cascade of nine reservoirs in the Wujiang River Basin in China was selected to apply the established method. The results showed that sediment accumulation was more severe in reservoirs in the middle and upper reaches of the Wujiang River, which caused the most significant decrease in capacity loss volume for the Wujiangdu Reservoir (172.8 million m3) and the largest loss rate for the Suofengying Reservoir (25.02 %). The current design WLSV curve for flood regulation calculation underestimated Z* by 7.11 and 1.84 m and γ by 2 % and 3 % for the Suofengying and Dongfeng reservoirs, respectively, compared with the reconstructed one. This underestimation increased with the length of the return period, which indicates that when storage capacity considerably decreases, continued use of the existing design WLSV curve may significantly underestimate the flood regulation risks and consequently pose potential safety hazards to the reservoir and downstream flood protection objects.