Simulation of Water Quality under Different Reservoir Regulation Scenarios in the Tidal River

Reservoir regulation is one of the effective ways to improve water quality. However, when water quality requirement is considered, reservoir operation is complicated due to tidal regimes in the tidal rivers. The study was carried out in the tidal river reach between Dongjiang Hydro-Project and Shilong in Dongjiang River. To evaluate the effect of reservoir regulation on water quality, a two-dimensional hydrodynamic model combined with water quality has been developed. Under the scenarios of differrent flood discharge from the Shima River and different release discharge of Dongjiang Hydro-Project, water quality is simulated using the two-dimensional model. NH3-N and COD concentrations are taken as the major indicators for water quality. Results indicate that (1) water quality is related to the tidal fluctuation, NH3-N and COD concentrations increase at flood tide and decrease at ebb tide; (2) There is no obvious over-limit of COD concentration, NH3-N concentration is found as the main pollution impacts water quality at water intake; (3) flood discharge from Shima River flows back to the upper reach of water intake during higher high tidal period, NH3-N concentration non-compliance occurs, flood tide is the main factor which impacts water quality; (4) reservoir regulation of Dongjiang Hydro-Project causes a significant improvement on water quality, release discharge of Dongjiang Hydro-Project is determined to insure the water supply safety, which keeps tidal-induced backwater always flowing to lower reach of the water intake. The study provides a hydrodynamic perspective on water quality improvement due to reservoir regulation in the tidal river.