Simulation Of Hydrodynamics Of Dead Zone Of Longjing Lake Based On EFDC

Deterioration of lake and reservoir water is a serious problem on the society. In lake and reservoir, the water flow rate is slow and the mobility is poor, which leads to more serious deterioration of water quality. Hydrodynamic mathematical model is an important method of hydrodynamic research in the field of environmental engineering, and it has been widely used in the research of lake and reservoir. The Longjing Lake is very irregular shape which is slender. the Lake is distributed a large number of different sizes lake bay. The lake bay has a poor flow, and the flow velocity is almost zero. Referring to lots of documents and data, the flow velocity is less than 0.1m/s of the area in the lake as dead zone. By flow monitoring in the lake, a lake bay near the Lingyun Bridge is chosen as the research object. Submersible water impeller is chosen to improve the water cycle of lake bay for the improvement of the water quality.The landform of bottom of the Lake Bay is complex, and the shore with irregular boundary. the flow field of Submersible water impeller which is installed in different position of is influenced by the different landform. In order to exp lore the water hydrodynamic impact by the submersible water impeller in different locations, In this paper, using the EFDC model construction of the dead zone of Longjing Lake hydrodynamic numerical model for the study. The main conclusions are as follows:(1) In order to obtain more accurate simulation results, it is necessary to test and verify the EFDC model. The relative error of the velocity of the seven selected monitoring points is 5.27% 、 12.24% 、 8.76% 、 11.62% 、 10.30% 、 20.91% 、24.53%,respectively, and the re lative error of water level of seven monitoring points are less the 1%.(2) It is necessary to consider the following three aspects for the submersible water impeller:1)the characteristic of axis thrust and radial direction diffusion;2)the distribution of phytoplankton in vertical direction;3)the distribution effect of the submersible water impeller on the bottom of the water body. Based on the above characteristics, we have developed three sets of program of station. The location of program one is in the middle point between side of convex bank. The location of program two is in the middle points north five meters. The location of program three is in the middle point between the location of program one and the west side of convex bank.(3) Three sets of the location of program are simulated by EFDC, and the flow velocity distribution chart which the submersible water impeller is installed in different positions is obtained. The flow velocity chart similarity of simulation time interval of second day, fifth day and eighth day reached more than 95%. It shows that the flow field is basically unchanged after stable operation of submersible water impeller. The simulation flow velocity decay rate decreased gradually. The radial direction flow velocity also gradually decreased, and the decay rate is greater than that of the axis direction velocity. After calculation, the simulated flow velocity distribution map of program one is about 45.2 meters long, 10.7 meters width, with an area of about 362 square meters. The simu lated flow velocity distribution map of program two is about 40.3 meters long, 9.7 meters width, with an area of 311 square meters. The simulated flow velocity distribution map of program three is about 39.3 meters long, 9.8 meters width, with an area of about 307 square meters. The maximum simulated flow velocity of the three programs are 0.724m/s, 0.6129m/s, 0.4962m/. In order to improve the dead zone hydrodynamic conditions to the greatest extent, analysis three sets of program simulation flow chart, the program one is chosen as simulate the optimal program.(4) The measured flow velocity distribution map of the three sets of program is drawn by the measured flow velocity data. As the flow velocity is monitored by the push of the impeller, the wind field and the wind field on the water surface. The measured velocity is less than the simulated velocity. The interference has a greater impact on the low flow water,resulting in a significant “jagged” shape of the flow velocity contour. The measured maximum flow velocity of the three sets of program are 0.61m/s 、 0.55m/s and 0.48m/s. After calculation, the measured flow velocity distribution map of program one is about 40.5 meters long, 9.4 meters width, with an area of about 321 square meters. The measured flow velocity distribution map of program two is about 37 meters long, 8.5 meters width, with an area of about 287 square meters. The measured flow velocity distribution map of program three is about 35.5 meters long, 9.1 meters width, with an area of about 276 square meters. The optimal layout scheme obtained is program one.(5) In order to consider the influence of different hydrodynamic conditions on the water quality, the water quality monitoring of the three programs was carried out. The flow velocity and range of program one is largest widest, and the interference de gree of program one is also largest. The water oxygen enrichment rate reached 15% in the lake bay. The oxygen enrichment rate of program two and three is 3% and 5% respectively. Submersible water impeller disturbed water shearing action will inhibit phytop lankton growth and production, resulting the TN concentration of lake bay less than the control point. The average removal rate of chl-a of program one is 32%, while the program two and three are only 14% and 6%. As the oxygen enrichment effect of the submersible, the oxygen concentration is increased, resulting the concentration of TN 、TP、CODMn in lake bay is less than that of control point. The flow velocity of program and the oxygen enrichment effect are best, so removal rate of AN is high, which is 33%,the removal rate of TN、TP、CODMn is low, respectively 18.9%、15.3%s and 22%. The flow velocity of program two and three is low. The removal rate of TN、TP、CODMn is low than that of one. By the conclusion, the best simulated location of submersible water impeller is consistent with the best measured location.