6　Conclusion and Discussion

A 2-D hydrodynamic and water quality model was developed and employed to analyze the effect of water diversion from the Yangtze River to Chang Lake.Hydrodynamic and water quality before and after the water diversion projects were modelled using the developed model.

The results shown that the flow velocity in the whole lake is very slow in current case without water diversion project.Main lake area of Miao Lake and Haizi Lake has weak circumfluence by the drainage of sewage outfalls.Water diversion had obvious effect on the flow velocity near the waterinlet of the water diversion project,and had little effect on the northern bay and the main water area of the lake.

Results of modelled water quality shown that concentration of COD increased firstly and then decreased during the water diversion process,especially in the south of the lake.The reason was that pollutants in the west of the lake were transferred to the east of the lake,and entered the main lake area by slow current from the southern bay caused water diversion,which resulted in larger polluted area and more replace time of water in the lake.When the diversion water flux increased from 50m3/s to 150m3/s,water quality were improved gradually with shorter polluted zone,lower concentration of COD,and the increase of water area with good quality.