Numerical modelling shows an old check-dam still attenuates flooding and sediment transport

Check dams are an effective and widespread measure for soil and water conservation around the world (e.g., in the Chinese Loess Plateau). However, the quantitative impact of a heavy silted check dam with small remaining storage capacity and large deposition area on river flow and sediment transport during floods remained unclear. In this study, to investigate this impact, a physics-based distributed hydrological model (the Integrated Hydrology Model, InHM) was employed to simulate runoff and sediment processes during floods in a small catchment with a check dam in the Loess Plateau, considering different heavy rainfalls and check-dam remaining storages. The model was calibrated and validated against the observed hydrographs and sedigraphs in two flood events and obtained satisfactory modelling performance (the Nash-Sutcliffe efficiencies > 0.70). The scenario modelling results showed that a heavy silted check dam still remarkably reduced and delayed the peaks of river flow and sediment rate, due to the attenuated surface flow movement and sediment transport on the deposition area. The largest flow peak occurred around the 85% silting stage of the check dam rather than the fully silting stage (100%). Even in the fully silted stage, sediment deposition continually occurred on the deposition area with a maximum deposition of 11,700 t sediment in a heavy-rainfall scenario and more sediment deposited on the tail-end regions of the deposition area than other regions after a flood. Moreover, in heavy silted stage backwater on the tail-end areas and gullies largely attenuated local erosion and promoted sediment deposition, indirectly reducing the total sediment yield of the catchment during flood. These results indicated that numerous ageing check dams built in the Loess Plateau and worldwide are still useful to attenuate flood risk and soil erosion, and sound maintenance and management of the check-dam will be beneficial to flood and soil erosion control.