A threshold law for reservoir sediment flushing through an initially covered bottom tunnel

Reservoir sedimentation is of worldwide concern, threatening the sustainable exploitation and management of water resources. Although sediment flushing through bottom tunnels has been widely applied to alleviate reservoir sedimentation, it may not be feasible if the tunnel intake is covered by sediment deposits. To date, however, an effective method has remained missing to determine whether flushing can be accomplished. Here, we develop a threshold law that governs reservoir sediment flushing through an initially covered bottom tunnel. It is derived from the fundamental conservation equations of water-sediment mixture flows. The proposed law is benchmarked against a series of laboratory flume experiments, covering a spectrum of reservoir water levels and cover layer thicknesses. A case study of the Wanjiazhai Reservoir on the Yellow River, China demonstrates that a modified reservoir operation scheme in accord with the threshold law can significantly enhance power generation while sediment flushing viability is ensured. The present findings have implications for the planning, design and operation of reservoirs on sediment-rich rivers, particularly in the period when global climate change may lead to an increased risk of reservoir sedimentation.