Operational regulation of a hydropower cascade based on the mitigation of the total dissolved gas supersaturation

Spillway discharge leads to total dissolved gas (TDG) supersaturation in the plunge pool, and these gases cannot dissipate to restore equilibrium after traveling tens or even hundreds of kilometers downstream of the spillway. Supersaturated TDG is detrimental to fish and may cause gas bubble disease. The negative impact is enhanced when cascade reservoirs are in operation. Field observations of the TDG concentrations were performed in the lower reaches of the Dadu River in the summers of 2009 and 2012. The relationships between the TDG concentration and various impact factors were analyzed. Increasing the unit discharge rate and downstream water depth can accelerate the production of TDG supersaturation, a ski-jump was a better pattern for dissipating energy than an underflow, and the power generation system did not contribute to the TDG production. The TDG dissipation showed that the dissipation coefficient increased with an increasing flow rate. An investigation was conducted to explore the cumulative effect of a hydropower cascade. A controlled cascade reduced the TDG production by reducing the occurrence of a discharge at the downstream cascade is beneficial. Based on the field observations, which were focused on operational regulation, mitigation measures for TDG supersaturation for the watershed management of a hydropower cascade were suggested and discussed.