| The saltwater intrusion in the Changjiang estuary is closely related to the drinking water security of Shanghai.Quantitative simulation,analysis,and prediction of saltwater intrusion using numerical models are important methods to ensure the water supply security of Shanghai.With climate change and toporaphy evolution,higher requirements for horizontal resolution of the model grid have been raised.For example,during the severe saltwater intrusion event in February 2014,to accurately simulate the strong water level rise generated by the strong north wind and ensure the accuracy of material transport in the estuary,local grid refinement is required.The Hengsha Channel,as the only channel connecting the North Channel and North Passage,has a significant impact on saltwater intrusion in the Changjiang estuary.To describe this channel in the model,local grid refinement is also necessary.Based on the Un FECOM(Unstructured quadrilateral grid,Finite-Differencing,Estuarine and Coastal Ocean Model)developed by our research group in the past two years,a numerical model for the saltwater intrusion in the Changjiang Estuary,which domain covers the Bohai Sea,Yellow Sea,and parts of the East China Sea,was established in this dissertation.(1)A large domain multi-level nested numerical model for the saltwater intrusion in Changjiang Estuary was established,which covers Bohai Sea,Yellow Sea,and part of the East China Sea.The model accurately simulated the strong water level rise and severe saltwater intrusion caused by the persistent strong northerly wind in February2014,confirming that the Ekman transport induced by the north wind is the main driving force.The nested grid model successfully reproduced the saltwater intrusion process,and improved the simulation accuracy and computational efficiency.(2)Quantitative analysis of the impact of increases of various river discharge on saltwater intrusion in the Changjiang Estuary under a persistent strong northerly wind.Numerical experiments were designed to investigate the effects of increases of river discharge on the inhibition of saltwater intrusion and extension of reservoir water intake time during the severe saltwater intrusion event in February 2014.The net water diversion ratio,net water flux,and net salt flux in the upper reaches of the North Channel increase accordingly with the increases of river discharge.At the water intake of the Qingcaosha Reservoir,by increasing the discharge from 1,000 to 8,000 m~3/s(in increments of 1,000 m~3/s),the longest continuous unsuitable water intake time was reduced by 0.5,2.5,3.5,4.0,10.5,13.0,15.0,and 18.5 days compared to the actual longest continuous unsuitable water intake time of 23 days,resperctively.When the river discharge increase is less than 4,000 m~3/s,there was no significant reduction in the longest continuous unsuitable water intake time,but when the river discharge increase value reaches 5,000 m~3/s,the longest unsuitable continuous water intake time begin to decrease significantly.(3)The dynamic mechanism of prolonged north wind and tidal patterns on saltwater intrusion during the strong northerly wind conditions was revealed.During the strong northerly wind period in February 2014,there were two episodes of intermittent southeasterly winds lasting for two days each.In numerical experiments,these two episodes of southeasterly winds were set by north wind speeds of 6 m/s and12 m/s,respectively.The results showed that the prolonged strong north wind period of four days intensifies the saltwater intrusion in the Changjiang Estuary.Furthermore,as the wind speed increases from 6 m/s to 12 m/s,the landward Ekman transport is strengthened,causing the saltwater front to advance further upstream,resulting in larger increases in salinity at the hydrological stations and the water intake of the Qingcaosha Reservoir.The saltwater intrusion during the strong northerly wind period in the neap tide is more severe than that in the spring tide.(4)The water and salt flux exchanges in the Hengsha Channel under the influence of the winter monsoon and their impacts on the spatial and temporal distribution of saltwater intrusion in the Changjiang Estuary was investigated.A nested grid was employed to refine the small-scale Hengsha Channel region,and numerical experiments were conducted by different wind speeds and directions during the dry season.The simulated current and salinity fields in the Hengsha Channel and adjacent domain,as well as the water and salt fluxes through the channel,were analyzed.The results showed that,when the northwest,north,northeast wind speed is 6 m/s,during the neap tide,the net water flux is from the North Channel to the North Passage through the Hengsha Channel,with the highest value of 1,306 m~3/s under the north wind.During the spring tide,the net water flux is from the North Passage to the North Channel through the Hengsha Channel with the highest value of 593 m~3/s.When the northwest,north,northeast wind speed was 12 m/s,the landward Ekman transport is enhanced.During both neap and spring tide,water was transported from the North Channel to the North Passage through the Hengsha Channel.The net water flux under north wind is the highest,reaching 5,763 m~3/s during the neap tide and 2,249 m~3/s during the spring tide.The simulation results also provided detailed information on the distribution of water diversion ratio and their variations between the North Channel and South Channel,and between North Passage and South Passage during the dry season for different wind speeds and directions.Wind speed and direction had a significant impact on the water flux in the Hengsha Channel and the water diversion ratio between the North Channel and South Channel,and between North Passage and South Passage. |
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