Research on the sediment recovery saturation coefficient in river channels downstream of reservoirs

The sediment recovery saturation coefficient is a key parameter for predicting riverbed scour and siltation downstream of reservoirs. Due to the unknown nature of calculation formulas for relevant parameters and the complexity of calculations， it has been difficult to apply this parameter widely in practice. Based on the non-equilibrium sediment transport theory and the vertical sediment concentration distribution formula， this study derives and optimizes a calculation formula for the comprehensive sediment recovery saturation coefficient αz. Specifically， through the vertical integration and sediment transport rate balance relationship， a physical derivation method is proposed for α1 （the ratio of near-bed sediment concentration to vertical average sediment concentration during unbalanced sediment transport） and α2 （the ratio of near-bed sediment-carrying capacity to depth-averaged sediment-carrying capacity）. During the calculation of α1， the near-bed sediment concentration is difficult to determine. Based on the unsaturated adjustment coefficient c， the calculation of α1 is transformed into determining the coefficient c. However， there is also great difficulty in determining c. Relying on flume experiments， it is found that under bed material particle sizes of 0.28 and 0.18 mm， the coefficient c exhibits a power-law relationship with S/S* （the ratio of depth-averaged sediment concentration to the sediment-carrying capacity of flow）. The parameters are fitted to verify the calculation accuracy of the formula for the dynamic change of c under uniform sediment conditions. Based on the flow and sediment data （2003-2023） from a typical reach downstream of the Three Gorges Reservoir， an empirical formula for the bottom recovery saturation coefficient α0 is proposed， and the variation process of the comprehensive sediment recovery saturation coefficient αz is further clarified. The proposed calculation method can be applied to numerical simulation for scouring and silting prediction. However， the applicability of c under other particle size conditions and that of α0 under the scenario of extreme coarsening of bed sediment need to be further explored， pointing the way for future research.