| description abstract | Reservoir sedimentation is of worldwide concern, involving not only reservoir benefits, but environmental and ecological issues. Sediment flushing is one of the most common and effective methods for alleviating sedimentation and its negative impacts. To date, however, it has remained unknown whether sediment flushing can be accomplished through a bottom tunnel once its intake has been covered by sediment deposits. Here, a total of 65 runs of flume experiments were conducted to investigate the threshold conditions for occurrence of sediment flushing. The results demonstrate that higher water level, smaller cover layer thickness, shorter tunnel length, and steeper tunnel slope favor the occurrence of sediment flushing. Physically, as the gate located at the tunnel intake is opened, the sediment deposits immediately upstream of the intake collapse and enter the tunnel, entraining water. A mass of hyperconcentrated water-sediment mixture is generated, in which the amount of sediment depends on the cover layer thickness. If the water level is sufficiently high, the cover layer thickness is low, the tunnel slope is steep, or the tunnel is short, the pressure difference between the inlet and outlet of the tunnel suffices to drive the water-sediment mixture to move down to the outlet, overcoming the resistance due to the tunnel boundary. This way, sediment flushing is accomplished. Based on dimensional analysis of the observed data, a threshold condition is proposed for the occurrence of sediment flushing. It is also shown that an equilibrium scour hole is formed immediately upstream of the tunnel intake, and an empirical model for the scour hole geometry is proposed. The present findings can be exploited to improve sediment-rich reservoir planning, design, and operation for benefit maximization and eco-environmental wellbeing. | |
