Abstract:

After impounding, water environment of reservoirs has been significantly changed due to dramatically increased water level, lower flow velocity, poorer diffusion ability and longer retention time in the tributaries. This change can lead to eutrophication which is more likely to occur in tributary than in main stream. Longchuan River, one of the branches of Wudongde Reservoir in Jinsha River, is the major water source to supply drinking and irrigating for Chuxiong City. The change of water dynamic condition in Longchuan River could result in eutrophication after Wudongde Reservoir impounding. A twodimensional, laterally averaged hydrodynamic and waterquality model, CEQUALW2 was taken to perform simulations and predictions of eutrophication in Longchuan Rivers backwater area. The simulation parameters included water temperature, total nitrogen (TN), total phosphorus (TP), flow velocity and chlorophylla (Chla). The simulation results were as follows: firstly, when the operation water levels were high in April, August and September, the Chla concentrations were high and reached 64mg/m3 over 60% distance along the river, so higher risk of eutrophication existed. The lowest risk happened from May to July because of higher flow velocity, and December to February because of lower water temperature. The second, main factors that limited the algae growth were flow velocity and water temperature when the nutrient saline levels were high. For instance, there were obvious stratification and slow velocity in the surface of section near estuary in April and August, so Chla concentrations in the rivers mouth were 70 times and 140 times higher than the sub reservoir tail. The third, there were significant intrusions from Wudongde Reservoir to Longchuan Bay with three forms of intrusion to the surface, middle and bottom respectively.The unique flow characteristic provided a hydrodynamic background of nutrient distributions of Longchuan Bay. Moreover, the densitystratified flow in different direction along depth could decrease the flow velocity of surface water and enhance the capacity of heat storage at certain time, which provided favorable hydrodynamics and temperature for the algae growth in tributary. In addition, the model was also used to evaluate the effects of change of reservoir operation and reduction of nutrient load on algae biomass of branch reach. The modeling results of the four conditions (decreasing water level to 950m in April, cutting TP and TN concentrations of branch into two different nutrient levels, cutting nutrient of branch and main stream synchronously) indicated that decreasing water level and cutting nutrient could improve the water quality of branch and restrict the eutrophication of branch effectively. However, only cutting nutrient concentration of branch was not enough, cutting nutrient of branch and main stream synchronously was more effective. Therefore decreasing water level and “double cutting nutrient” can be taken to restrict the eutrophication in reservoir when great risk for eutrophication existed, and can be used to control algae bloom in the branch of similar ChannelType reservoirs