5　Results and Discussion

5.1　Calculation cells

All the lake area of 134km2 was divided into 9745 triangle cells,5743 nodes with 13750m2 per cell.

5.2　Hydrodynamic modelling and analysis of Chang Lake without water diversion project

The hydrodynamic model was employed for modelling the hydrodynamic characteristics of the Chang Lake.Considering that water level of the lake had little changes in a year,the water level of Chang Lake for calculation was considered as 29.5m according to filed hydrological data,and the corresponding flow field of the lake in water level 29.5m was shown in Fig.2.

The results shown that the flow velocity in the whole lake is very small 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.However,considering the effect of wind and sewage drainage,the water level of the lake has little change less than 0.005m.

5.3　Hydrodynamic characteristics analysis of Chang Lake with water diversion project from the Yangtze River to Chang Lake

Three different water discharge were designed for water diversion from Yangtze River to Chang Lake,such as 50m3/s,100m3/s,150m3/s.Simulated water flow fields of the lake in the three cases with different water diversion flow were shown in Fig.3 to Fig.5,respectively.

The results shown that flow velocity near the area of Miao Lake,and Haizi Lake and the west of Chang Lake,where water flow into Chang Lake from Yangtze River,was distinctive and the flow velocity became slow quickly after water flowing into the main area of Chang Lake.Flow velocity near Gate Liuling and Xijiakou,the outlet of Chang Lake,were relatively fast but weak in the northern bay of Chang Lake.In all,water exchange in Miao Lake,and Haizi Lake and west Chang Lake were distinct,while capacity of water exchanges in the northern bay and main area of Chang Lake were weak.

Flow velocity in Miao Lake,and Haizi Lake and the west area of Chang Lake increased obviously along with the increase of water diversion from the Yangtze River,however,flow velocity in the main area of Chang Lake,and the northern bay has little change with the increase of water diversion.

5.4　Effect analysis of water quality improvement in different water diversion project

5.4.1　Water quality modelling of the lake without water diversion project

According to the surface shape and hydrodynamic characteristics of the lake,some typical points were chosen to analyze changes of water quality along with the water diversion process in different design condition.Changes of the concentrations of COD and TP at these typical points along with water diversion process were analyzed.The location of these typical points were shown in Fig.6.

In current scheme without water diversion project,modelled concentration changes of COD in the typical points were shown in Fig.6.The results shown that the concentrations of COD in the lake will increase along with the sewage drainage and come to dynamic balance and tended to be stable,as shown in Fig.7.

5.4.2　Changes of COD concentration in the typical points in different water diversion process

Changes of water quality were modelled in different water diversion project.The results shown that the concentration of different water index had similar changes in one water diversion project.Therefore,modelled results of COD changes were list as follows while results of other water quality indexes were not list in this paper as space was limited.Changes of COD concentration at the typical points in different water diversion project were shown in Fig.8 to Fig.10.

The results shown that concentration of COD increased firstly and then decreased to balance concentration during the water diversion process.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 small current from the south bay caused water diversion,which resulted in larger polluted area and more replace time of water in the lake.And the balance concentration of COD decreased as the increase of water diversion flux.

In different water diversion process(50m3/s,100m3/s,150m3/s)from the Yangtze River,the balanced concentration field of COD of the whole lake were shown in Fig.11 to Fig.13.

As shown in Fig.11 to Fig.13,when the diversion water flow 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.

When the diversion water flow was 50m3/s,the polluted zone were shaped with the length of 25km nearby the south bay,where the COD concentration exceeded 40mg/L which was the limited concentration of the fifth level in the national surface standard(GB 3838—2002).And the area where the COD concentration exceeded the fourth level was 30km long,expanded to the southeast of the lake.

when the diversion water flow was 100m3/s,the polluted zone were shaped with the length of 17km nearby the southern bay,where the COD concentration exceeded 40mg/L which was the limited concentration of the fifth level in the national surface standard(GB 3838—2002).And the area where the COD concentration exceeded the fourth level was 28km long shorter than that in the scheme of 50m3 water diversion.

When the diversion water flow was 150m3/s,the polluted zone were shaped with the length of 15km nearby the southern bay,where the COD concentration exceeded 40mg/L which was the limited concentration of the fifth level in the national surface standard(GB 3838—2002).And the area where the COD concentration exceeded the fourth level was 18km long much shorter than that in the scheme of 50m3/s water diversion.

According to the modelled results,water areas with different quality and the corresponding percent over the whole lake were summarized in different water diversion scheme,shown in Table 1.

As shown in Table 1,in different water diversion scheme,the water area where COD concentration were lower than the third level of the national surface standard,were 24%,38%and 43%of the lake area,while the water area where COD concentration exceeded the fifth level of the national surface standard,were 38%,28%and 15%.