Numerical study on the cumulative effect of supersaturated TDG through the spillway

Abstract

The cascade development of water resource will lead to cumulative effect of supersaturated total dissolved gas (TDG) on the ecological environment, which will cause lethal disease to fish. The TDG dissipates slowly in deep reservoir areas and if supersaturated TDG can't recover through the spillway then it will lead to continuous accumulation in the stilling basin. In order to investigate the cumulative effect of supersaturated TDG of hydropower cascades, a validated TDG numerical model with the VOF method and DES model was used to simulate a spillway discharge of Tongjiezi dam with supersaturated TDG and 100% TDG inflow. In this model, bubble size change is calculated by bubble number density equation and TDG is simulated by transport equation considering the gas-liquid mass transfer. Comparing with the TDG 100% inflow case, it indicated forebay TDG level caused by the upstream dams should be considered and involved in the downstream TDG evaluation. This paper also proposed an engineering measure with baffle blocks in the spillway which can reduce entrained bubble mass and TDG saturation level at the entry point than the case without blocks. It shows baffle blocks in the spillway can effectively reduce the cumulative effect of supersaturated TDG. It is a critical step for the mitigation of accumulation of supersaturated TDG through the spillway and the results provide reference about mitigation measure of supersaturated TDG for hydropower cascades.