| Changjiang River is the largest river system in China with the secondary sediment flux just after the Yellow River. The Changjiang River plume exists in the offshore region in the whole year. The seasonal variations of Changjiang plume is very obvious, the plume in summer time is evident, under the impact of tide, wind and shelf currents, the plume can reach to the Jeju Island. The discharge in winter time is smallest, the scope and route of Changjiang plume are very different with these in summer time. Meanwhile, the suspended sediment exits in the Changjiang freshwater, the sediment discharge also varies seasonanlly, which would influence the topography, channel dredging and land inning. It is very important for the water and material transports and the biochemistry process in the East China Sea to study the seasonal variations of Changjiang plume.In this paper, the field surveys in the East China Sea were analyzed about the horizontal and vertical distributions in the summer, winter and spring. Meanwhile, the impacts of typhoon and low/high river discharge on the salinity distribution were discussed. Under the normal conditions, the plume was mainly transported towards Northeast, east and south in summer, spring and winter, respectively. The typhoon can influence obviously on the horizontal and vertical distribution in the plume region. Under the low river discharge, the transport direction of Changjiang plume did not vary a lot, but the range was much smaller than that in normal years.To study the seasonal variations of freshwater and material based on physical dynamics and quantify the response of Changjiang plume under violent weather (typhoon) and abrupt change of river discharge, a three-dimensional hydrodynamic and sediment numerical model was established. The model domain covers Datong station in Changjiang channel, Bo Sea, Yellow Sea and East China Sea. River discharge, wind stess, tide and shelf current were considered in the model. HSIMT-TVD scheme was used to calculate the advection term in transport equation, which can guarantee high accuracy; CUDA technology was employed in this model, which can increase the calculation efficiency and will not influence the accuracy. Since the topography is complicated in Changjiang Estuary, the wet-dry method was used to simulate the moving intertide zone. The offshore region of Changjiang Estuary is open seas, therefore the impact of wind stress is significant. In this paper, the WRF model was employed to calcuted the wind field in the study area, which had a high resolution. The water depth in the river mouth is relatively shallow comparing to the open sea, the impact of wave is significant on the sediment resuspended and transport process, the SWAN model which covered the Bo sea, Yellow sea and East China Sea was established to provided wave parameter to the hydrodynamic and sediment model.Since the typhoon is frequent in summer, three series of differnet routines of typhoon were designed based on the 8th Typhoon Neoguri in 2014, which were meant to analyze the impacts of typhoon on the Changjiang plume. Before the center of typhoon pass through the adjacent sea of Changjiang Estuary, the residual currents has been changed and leads to the rotation of Changjiang plume. When the center of typhoon pass through the plume region, the stratification of plume is altered due to the strong mixing of the wind. After the typhoon, the Changjiang plume begins to recover to the normal pattern in summer, which will take about one week.The numerical model was used to study the response time of the Changjiang (Yangtze) River plume to river discharge by artificially increasing the runoff over a short period and investigating the variation of salinity in the plume region. The time lagged between the change of river discharge and the change of salinity reaches the 10% of the adjusted value is considered as the response time in this study. The response time in the plume region differed slightly when the river discharge during the spring tide and the neap tide was increased. Specifically, the response times near the river mouth and in the plume edge～1 days and more than 15 days, respectively. The brackish water volume were also calculated to determine the variations in the plume extensions over time. A tracer was released to study the transport time from the Datong station to the sea using the concept of mean water age. The tracer transport time ranged from 10 days near the river mouth to more than 50 days at the edge of the plume, which is much longer than the response time of the surface salinity. The seasonal variations of plume response process and response time were also discussed.As for the transport time scales of dissolved material, the water age method was employed to analyze the time need for transporting material from Datong Station to offshore region. The numerical experiments indicated that the seasonal vatiations of water age scope and magnitude were obvious. In the river mouth, the water age is about 40 days in winter and 10 days in summer. Comparing the control experiment to the experiments without wind/tide, the results indicated that the impact of wind/tide is significant on the distribution of water age. Specially, when wind condition is removed, the patterns of water age are similar but different magnitude; when tide is removed, the water age is distributed more along the shore.As for the spatial and temporal variations of suspended sediment in the Changjiang Estuary and adjacent sea, the distributions of suspended sediment concentration (SSC) on different seasons were shown based on the survey data in the past decades. The yearly-averaged remote sense data from satellite from 1998 to 2014 were used to analyze interannual variations of SSC. The seasonal-averaged remote sense data from 2003 to 2014 were used to analyze the seasonal variations of SSC, the EOF mothod was employed to analyze the characteristics of SSC in each season. The numerical model was used to simulate the seasonal variations of SSC under climatological condition, and the impacts of wave and shelf current on the transport of SSC were investigated by conduting single factor experiments, in which the effect of wave is more significant in winter and the effect of shelf current is more significant in summer. |
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