Numerical Simulation Of Residual Water Level Along The Coast Of The Eastern China Seas And In The Changjiang Estuary

Estuarine residual water level is an important factor affecting water depth and water level,which is affected by river,ocean,meteorology and many other elements together.Up to now,huge amounts of research have shown that it mainly depends on river discharge,tide and wind stress,and it changes significantly with time and space.The Changjiang River,as the largest river in China,the seasonal runoff of which plays an important role in the residual water level of the Changjiang Estuary.Meanwhile,the contribution of oceanic factors like shelf circulation can not be ignored.It is of great scientific significance and application value for estuarine water level prediction,water resources utilization,seawall design,flood protection and navigation to study the temporal and spatial variation in residual water level and its causes.The Changjiang River flows through 11 provincial districts of China,and eventually empties into the Eastern China Seas.In a broad sense,the Eastern China Seas generally refer to the vast expanse including the Bohai Sea,the Yellow Sea and the East China Sea.In this paper,the research focused on the the coastal part of China in the south Yellow Sea and the East China Sea from the Shandong Peninsula to the Changjiang Estuary.The measured water level data and the three-dimensional numerical model for estuary,coast and ocean,ECOM-si,were used to analyze and simulate the temporal and spatial variation as well as dynamic mechanism of the residual water level along the coast of the Eastern China Seas and in the Changjiang Estuary.The main research conclusions are summarized as follows:1.By analyzing the measured data in the Changjiang Estuary in 2016 and 2017,it was found that owing to the effect of the runoff,the upstream residual water level was higher in the upstream than in the downstream in the Changjiang Estuary,and the one in the North Branch is lower than the one in the South Branch,because a relatively smaller amount of river water flows into the North Branch.The residual water level in the South Channel is higher than the one in the North Channel,and it is higher on the south side than the one on the north side in the same channel due to the Coriolis force that makes the water turn to right.The temporal variation shows that the residual water level was higher in summer and autumn than the one in winter and spring on account of the seasonal variation of runoff and wind.The maximum residual water level in 2016 appeared in July,while it appeared in October in 2017.By numerical simulation under the climatological condition,the results show that the annual minimum residual water level appears in January while the maximum one occurs in September in the Changjiang Estuary.The interaction of the higher river discharge and the northly wind in September is the reason why the residual water level is highest in this month.2.The results of the sensitive numerical experiments under different wind conditions show that when northly wind prevails in the Eastern China Seas,the residual water level rises with wind speed increasing,and the residual water level is lower in the north and higher in the south along the coast of the Yellow Sea and the East China Sea.The residual water level inside the Changjiang Estuary is higher in the upstream than in the downstream.However,when the wind speed keeps growing,the residual water level in the downstream will exceed the one in the upstream.When the northly wind speed reaches 15 m/s,the residual water level at Baozhen hydrological station is higher than the one at Nanmen hydrological station and when it reaches 20 m/s,the residual water level at Baozhen hydrological station is higher than the one at Chongxi or Xuliujing hydrological station.When the wind speed is fixed at 10 m/s,the residual water level in the Eastern China Seas is highest under northeast wind,second highest under northly wind,and lowest under northwest wind,because the northwest wind is an offshore wind blocked by the Shandong Peninsula while northeast wind is an onshore wind in the study area.3.The residual water level along the coast of Jiangsu and in the Changjiang Estuary rose significantly under the persistent and strong northly wind in February 2014.The surface waters accumulated at the south coast forcing by local wind,which resulted in the residual water level higher on the whole south shore of the South Branch during the period of persistent and strong northly wind.Since the width of the upstream South Branch is relatively narrow,there’s not much difference in the residual water level between the north coast and south coast.Under the persistent and strong northly wind,the landward Ekman transport of the water outside the Changjiang Estuary formed an estuarine circlulation that flowed into the North Channel and out of the South Channel,which intensified the rise of residual water level in the downstream.The results of EMD decomposition proved the phenomenon that the coastal trapped wave propagated along the coast of the Eastern China Seas to the Changjiang Estuary and resulted in the elevation of residual water level.It is also found that the contributions of the second and third modes are larger than the first mode,barotropic Kelvin wave mode,during the period of persistent and strong northly wind.If shifting the acutual wind force in February 2014 backward to the period of spring tide,the total water level at the intakes of three reservoirs will rise up,with the value at the water intakes of Dongfengxisha,Chenhang and Qingcaosha reservoir reaching 1.98,2.13 and 2.21 m,respectively.