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In order to improve the water quality and the ability to prevent the risk of water environment pollution in the tidal reach of Xiaoqing River, numerical simulation of water quality was carried out in tidal reach of Xiaoqing River. The MIKE21 was used to build a hydrodynamic- water quality coupling model. The main exceeding pollutants COD, TN and TP were selected, and the model was calibrated and verified by the measured water quality data. The water quality changes of the tidal reach under three scenarios of different upstream water quality, differ- ent pollution reduction conditions and sudden water pollution in different water periods were simulated. The results show that the calibrated and validated hydrodynamic-water quality coupling model is well suitable for the tidal reach of Xiaoqing River. When the upstream water is class IV, COD and TP are basically achieved, and TN exceeds the standard. When the upstream water is class V, none of the indicators are up to standard. The upstream water quality should be controlled within the range of class IV as much as possible, and the TN pollution control of the basin water body should be strengthened. Reducing 30% of point and non-point source pollution can effectively reduce COD and TP pollution, but the improvement effect on TN is not good. The influence of polluted water on water body lastes longer and the concentration is higher in the migration process of normal periods. In the wet period, the migration speed will be faster and the emergency treatment time will be less. [ABSTRACT FROM AUTHOR]

The diffusion of offshore salt in inland rivers along with tidal waves has an important effect on the aquatic ecological environment of the estuary area.This paper,taking the Wanquan River Estuary as an example,used the Mike21 model to simulate the salinity distribution along the channel under different runoff and tidal levels and analyzed the change rules and leading factors.The results show that the river system and riverbed topography at the Wanquan River Estuary reduce the upward diffusion of salt propelled by the tide and that the salt is mainly distributed in the waters near the estuary.The salinity distribution along the upper reaches of the river decreases rapidly,and it basically remains unchanged when the tidal level increases by 0.1 m and 0.3 m,respectively.A sudden increase in salinity occurs where a curved or narrow channel changes the local water flow and salt accumulates.The upward diffusion range of salt is mainly affected by runoff. When the runoff reaches 106 m3/s and 148 m3/s,the tidal level needs to increase by 0.1 m and 0.3 m respectively before it has an impact.Different degrees of change in runoff and tidal level have different effects on salinity distribution at the estuary,and channel conditions play a key role.