Reconstruction of reservoir water level-storage relationship based on 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 (WLS) relationship by significantly reducing the storage capacity, further threatening the flood control safety of reservoirs in long-term scheduling and operation. However, reconstructing the WLS relationship has long been challenging, especially on a large scale, due to the difficulties of traditional field bathymetric measurement. To fill this knowledge gap, a method to estimate the reservoir WLS curve based on the capacity loss induced by sediment accumulation is proposed in this study. To assess the potential negative impact caused by inaccurate WLS curve, flood regulation calculations for reservoirs are performed individually using six design flood hydrographs with return intervals ranging from 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 exceeding the design flood level (γ). Based on hydrological and sediment data and operational information over ten years, a cascade of nine reservoirs in the Wujiang River in China is selected to conduct the established method. The results show that sediment accumulation is more severe in reservoirs located in the middle and upper reaches of the Wujiang River, leading to the most significant reduction in capacity loss volume for Hongjiadu (180.3 million m³) and the largest loss rate for Suofengying (25.02 %) reservoirs. Using the current design WLS curve for flood regulation calculations, Z* is underestimated by 7.11 m and 1.84 m, and γ by 2 % and 3 % for Suofengying and Dongfeng reservoirs, respectively, compared with the reconstructed one. This underestimation increases with the length of the return interval. This indicates that when storage capacity is considerably reduced, continued use of the existing design WLS curve may significantly underestimate flood regulation risks, thus posing potential safety hazards to the reservoir itself and downstream flood protection objects.