Abstract: The influences of residual cofferdam at the inlet and outlet of diversion tunnel on the river closure were investigated through model test with Huangdeng hydropower station as a case study. The influences were analyzed in terms of upstream water-level fluctuation, hydraulic parameters variation during the river closure, and loss of dumped ripraps. The result indicates that the residual cofferdam of diversion tunnel increases the difficulty of river closure. The residual cofferdam at the inlet of the diversion tunnel has bigger influence on discharge capacity than that at the outlet. And the influence on the discharge capacity decreases when the discharge increases. It's concluded that the cofferdam at the inlet of diversion tunnel should be demolished to the design elevation as much as possible.      

Key words: model test, diversion tunnel, river closure, residual cofferdam of diversion tunnel