Research On Hydrologic Effect Of Large-Scale Shallow Lake On The Eastern Line Of The South-to-North Water Transfer Project

Nansi Lake is the largest freshwater lake in northern China,the area of which is 31700 km2.The elevation of the lakeshore surrounding Nansi Lake is below 36.79m and the low-land results the waterlogging difficult drain automatically.Due to the low elevation of river bed and small gradient of rivernet,the high water level of lake causes heavy backwater influence to the water level of riverway.Therefore,drainage of the lakeshore shows a strong response to water level of Nansi Lake.Nansi Lake is a typical shallow water lake,the average water depth under normal water level of which is 1.5m and the water level of lake is a critical factor of the management of water resource and flood control in Nansi Lake.South-to-North Water Transfer East Route Project connects Gaoyou Lake,Hongze Lake,Luoma Lake,Nansi Lake,Dongping Lake these 5 nature lakes and aims at draw water from the lower reaches of Yangtze River through 13 pump stations step by step to supplement water supply to eastern Huanghai Plain and Shandong Peninsula.Nansi Lake is a key regulation and storage node of the Project as a transport and reception district.According to the overall regulation of the Project,the diversion period is non-flood season(October to May).Supply and demand of water resource in Shandong Peninsula has been strained increasingly owing to the heavy drought since the Project operated in 2013 and diversion of water happened during the flood seasons to meet emergencies.Considering the water demand for socio-economic development along the Project areas and the extreme events caused by environment change,diversion of water during flood seasons to relieve conflict of water resource in Shandong Peninsula tends to be more frequent.Large-scale diversion of water participates in hydrology cycle directly that will cause changes of hydrological effects such as rise of water level of lake,change the spatial and temporal distribution of the lake surface and velocity.Nansi Lake Basin is in the transition zone between northern and southern climate,the sharp turn between drought and flood.The shallow lake and the sensitive response of lakeshore waterlogging to the water level of lake make Nansi Lake Basin affected by waterloggingevents frequently.Therefore,simulating water level,temporal distribution of the lake surface and velocity of Nansi Lake,and properties of waterlogging in lakeshore area under the conditions of "normal regulation" and"diversion in emergency" to demonstrate the hydrological effects of large-scale freshwater lake to the water diversion is essential to scientific arrangement and regulation of reception districts and becomes a new research focus.To answer the question above,this paper chooses Nansi Lake and lakeshore area as the research object.An one-two dimensional coupled hydraulic model of Nansi Lake-into-lake rivernet-lakeshore is proposed to simulate the properties of lakeshore influenced by waterloggingevents under different water level conditions of Nansi Lake.The response pattern of waterloggingarea to the change of water level of lakes is demonstrated through quantitative analysis.The proposed model is also applied to simulate the properties of lakeshore in Nansi Lake influenced by floods under"normal regulation" and "diversion in emergency" two conditions of the South-to-North Water Diversion Project as well as the influence caused by the diversion of the Project during the first month to the water level of Nansi Lake,the spatial and temporal distribution of the lake surface and velocity.The main conclusions and results as follows:(1)A hydrodynamic model coupled 1D and 2D of Nansi Lake?into-lake rivemet?lakeshore is proposed.This model reflects the processes of drainage of lakeshores into rivernet by pump stations then into Nansi Lake through into-lake rivernet and the processes of the regulation of second level dams and drainage pumping stations well.The data of 2007.07 and 2008.07 were applied to the calibration and validation of the proposed model.Comparing the measured and calculated process of water levels of Nanyang,Makou,Second Level Sluice of Lake(lower),Huishan these four stations and the result demonstrates that the model has high accuracy of simulation and good ability of generalization.(2)Simulate the relationship between lake-level fluctuations and floods in lakeshores around Nansi Lake under rainfall of 5-,10-,20-years return periods,and the result shows that:When the initial water level of uper lake is 33.5m and water level of lower lake is 31.8m,the area of inundation districts where the water depth is over 10cm is 777.87km2,875.54 km2,965 km2 respectively,the area of inundation districts where the water depth is over 50cm is 44.41 km2,126.2 km2,200.08km2 respectively.Comparing with the second scenario that the initial water level of uper lake is 33m and the lower lake is 31.3m,corresponding increment is 1.4%-16.63% and 6.94%-36.12%.As for the scenario that the initial water level of uper lake is 34m,the lower lake is 32.3m,the increment is 2.37%-3.44%and 13.12%-51.02%respectively comparing with the second scenario It can be seen that even the change of water level is same,the higher the initial water level is,the lager the inundation area is.And the lager the submerged depth of lakeshores is,the more obvious the inundation area change is.(3)Choosing year 2003 as the typical year,simulate how the diversion in emergency during flood seasons of the South-to-North Water Diversion Project influences floods of Nansi Lake.The result demonstrates that the area of inundation districts where waterloggingdepth is over 10 cm and 50cm increases 3.04%and 13.32%respectively.In emergency scenario,the second level dams releases water 4 days ahead of time and the volume of discharge increase 0.249 billion m3 comparing to the no-diversion scenario.Enhancing the accuracy of rain forecast and the forecast period is meaningful to improve water security.(4)The mean annual water level of diversion stage(October to June of the following year)is applied as the initial water level of Nansi Lake without considering of diversion,and the operation water level according to the regulation is applied as the initial water level with considering of diversion to simulate the waterlogging process during different rainfall duration in the research area.And the result shows that under 3 scenarios with different recurrence interval of 5 years,10 years and 20years,the increment of inundation area where the water depth exceeds 10cm owing to the diversion is 32.53km2,33.01km2,13.26km2 respectively,within 1.4%-1.63%.And the increment of inundation area where the water depth exceeds 50cm owing to the diversion is 36.12km2,11.13km2,6.94km2 respectively,within 19.11%-36.12%.Affected by diversion,the waterlogging area reappears in the research area two hours in advance.(5)Choosing the year 2001 as the typical year to simulate the influence caused by the diversion of the Project during the first month to the water level of Nansi Lake,the spatial and temporal distribution of the lake surface and velocity.It can be seen that:the diversion of the Project changeover the daily decreasing trend of water surface of Nansi Lake in October 2011.After the one-month diversion,the water level of superior lake increases 0.46m,the duration time of the water level exceeding the mean annual water level is 28 days,the water level of junior lake increases 0.7m and the duration time of the water level under the lowest ecological water level decreases 16 days,comparing to the water level without diversion.South-to-North Water Transfer East Route Project increases the velocity of channels nearby Nanyang Lake and Zhaoyang Lake and the impact of water surface area and velocity caused by the Project to the junctions between Nanyang Lake and Dushan Lake,Zhaoyang Lake and Huishan Lake is most considerable.