| Recently building artificial islands in lakes has occurred in many areas of China,which not only effectively solves the problem of silt accumulation caused by river dredging,but also promotes the ornamental value of the lakes to a certain extent.However,the artificial islands in the lakes will make the hydrodynamic conditions of the lakes get worse and lead to the water body renewal time becoming longer as well as increase the risk of lakes eutrophication.Therefore,rationally arranging the number and shape of artificial islands is of great significance to improve the hydrodynamic conditions of lakes and promote the temporal and spatial improvement of the water quality of lakes.Based on the EFDC(Environmental Fluid Dynamics Code)numerical simulation technology,this thesis takes the artificial Lake Majiadang in Yancheng,Jiangsu Province as the research object to build a two-dimensional hydrodynamic-water quality model.On the basis of considering the factors such as the location,size,and offshore distance of the artificial island,this thesis draws on the philosophy of orthogonal test construction,designs a variety of artificial island layout schemes for the artificial island morphology,and conducts numerical simulation research on the hydrodynamic conditions of the lake,the water body renewal ability and the lake surface floating capacity under the artificial island layout schemes,so as to obtain the optimal artificial island layout scheme.It provides theoretical support for scientific decision-making of lake artificial island layout.The main work of this thesis is as follows:firstly,this thesis analyzes the basic situation of natural conditions,regional meteorological conditions and hydrology in Majiadang area,and also obtains the inflow and outflow of artificial lakes in typical years by using long series of the rainfall data.On the basis of summarizing the layout of artificial islands in lakes at home and abroad,combined with the actual situation of Majiadang artificial island layout,the layout scheme of Majiadang artificial island which based on different artificial island forms is designed by referring to the philosophy of orthogonal test construction.In this study,based on the EFDC model,the two-dimensional hydrodynamic model of Majiadang artificial lake was established by using the orthogonal curvilinear grid.The analysis shows that the model well fits the complex boundary of Majiadang artificial lake,and the calculation results of the hydrodynamic model can reflect the hydrodynamic characteristics of Majiadang artificial lake.The established hydrodynamic model was used to simulate the hydrodynamic conditions of the artificial lake and also the overall velocity of the lake as well as the velocity near the artificial island was used as the main reference,the results show that after continuous water diversion for 6 days,the flow field in the lake area is stable.From the perspective of the flow velocity of the lake area,the overall flow velocity of the whole lake area has little change under the nine artificial island layout schemes,and the flow velocity of the eastern lake area is higher than that of the western lake area.The flow velocity analysis near the artificial island shows that the scheme three is the best:the island 1 is circular,and the flow velocity is 3.3 cm/s;the flow velocity near island 2 is most affected by morphology,and the maximum crescent flow velocity is 2.83 cm/s;island 3 is the crescent-shaped flow velocity of 2.43 cm/s.In addition,there is a correlation between the hydrodynamic conditions of island 1 and island 2,the hydrodynamic conditions of island 3 are relatively independent,and the shape of island 2 directly affects the circulation intensity on the southwest side of island 2.On the basis of this hydrodynamic model,the convective diffusion model of Majiadang artificial lake was further constructed,the conserved pollutants in the artificial lake of nine artificial island layout schemes were simulated,the water body update time was used as an index to characterize the water renewal capacity of the lake area,and the observation point was set in the low flow rate area simulated by the hydrodynamic model.The results show that the tracer concentration in the east lake area is significantly lower than that in the west lake area with 30 days as the replenishment period,and the tracer concentration around the artificial island is reduced to a small level after continuous water diversion for 18 days.In terms of water body renewal time,the scheme three is the best.When the island 1 is round,the water body renewal time on the west side of the island is the shortest,about 4-8 days;when island 2 is crescent shaped,the water renewing time is about 1.5 days;when island 3 is crescent shaped,the water renewing time is about 1.5 days.Further,taking the ranking of the arrangement of artificial islands under the hydrodynamic conditions of artificial lakes and the renewal capacity of water bodies as statistical variables,based on the reciprocal of the arithmetic mean of the reciprocal of the two sets of statistical variables,the comprehensive ranking of the hydrodynamic conditions and water regeneration capacity of artificial lakes under the artificial island layout scheme was determined.Finally,the established hydrodynamic model of Majiadang artificial lake,based on the Lagrangian particle tracing method,released particles in the southwest lake area and the southwest side of the island 2 that may have circulation in the hydrodynamic analysis,and the insolubleness of the lake area was simulated,and the survival ratio of the particles was used as the main judgment basis to characterize the ability of floating matter out of the lake.Through the numerical experiment comparison of the nine artificial island layout schemes,it was found that the retention ratio of the nine schemes showed a downward trend as a whole,but when the retention ratio of the particles moved around 17 days,the longer the period of time,the larger the retention ratio,and after 30 days transport,the retention ratio of the scheme 6 had a minimum retention ratio of approximately 25%.Further,based on the ranking of the artificial island arrangement under the hydrodynamic conditions of the artificial lake,the regeneration capacity of the water body,and the floating body out of the lake,and the reciprocal of the arithmetic average of the reciprocal of the three sets of statistical variables,the hydrodynamic conditions of the artificial lake,the water body renewal capacity,and the floating body out of the lake under the artificial island layout scheme were comprehensively sorted.The layout of artificial islands supports the construction of artificial lake landscape belts,improves the surrounding landscape and environment.The hydrodynamic conditions,water body renewal ability and floating material output capacity of lakes under different artificial island arrangements are systematically simulated,and it is comprehensively analyzed to determine the optimal artificial island layout scheme,for letting the lake planning become more reasonable and make the artificial island development more scientific,and also providing ideas and references for the layout of artificial islands in depression lakes with similar characteristics. |
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