Study On Saltwater Intrusion Under Continuous Strong Northerly Wind In The Changjiang Estuary

Saltwater intrusion is one of the most important phenomena in estuaries.It not only have influence on estuarine circulation,sediment transport,biological and environmental conditions,but also related to freshwater resource management.Saltwater intrusion in the Changjiang Estuary is controlled by many factors,such as river discharge,tide,bathymetry,mixing,wind,etc.And the related dynamics process is complicated.In February 2014,Changjiang Estuary experienced an extreme saltwater intrusion event.This event interrupted the freshwater intake of Qingcaosha Reservoir,the biggest reservoir for Shanghai City.As a result,the drinking water supply of Shanghai City is threatened.In this work,based on observation and numerical model results,the process and mechanism of this extreme saltwater intrusion event is investigated.In summary,the main works and conclusions are:(1)Based on measurements of salinity,water level,discharge,wind speed and direction in February 2014,it is discovered that the salinity increased abnormally in the Channels of Changjiang Estuary and salinity increase was bigger in the North Channel than South Channel.The salinity increase was accompanied by continuous strong northerly wind and surges.Analysis of observational data suggests that the extreme saltwater intrusion is not caused by low river discharge.Instead,it is triggered by strong northerly wind.(2)Based on a high-resolution 3D hydrodynamic numerical model of the Changjiang Estuary,the saltwater intrusion event is reproduced,and the mechanism of intrusion is investigated.When driven by the February climatological wind field and residual water level on the open boundary,the model shows no extreme saltwater intrusion.When driven by the realistic wind field of February 2014 and the climatological residual water level on the open boundary,the model shows an abnormal salinity increasing in Changjiang Estuary,which suggests that strong northerly wind triggered this intrusion event.However,the magnitude of modeled salinity is still lower than observation.When driven by realistic wind field and wind induced surge on the open boundary,the simulation accuracy of modeled salinity phase and magnitude are greatly improved.This suggests that the extreme saltwater intrusion is caused by both local northerly wind and wind driven surge.The fundamental dynamics behind this event is the landward Ekman transport driven by the continuous strong northerly wind;it drives anticlockwise horizontal circulation which enters from North Channel and exit from South Channel.(3)Under climatic wind condition in February,the dikes of the Deep Water Project in the Changjiang Estuary prevent the southward transport of relatively low-salinity water at the mouth of the North Channel,resulting in the weakening of saltwater intrusion in this channel,which is favorable for the water intake of the reservoir.However,under the persistent strong northerly wind that occurred in February 2014,relatively high-salinity water in Subei is transported southward to the mouth of Changjiang Estuary and blocked by the dikes,causing a large amount of high-salinity water advected into the North Channel.As a result,an anticlockwise horizontal circulation entering from North Channel and exiting from South Channel is formed,and salinity in the North Channel increase,which is unfavorable for water intake of the reservoir.As the horizontal circulation increases,the vertical velocity shear and stratification decrease in the North Channel but increase in the North Passage.(4)Using the same numerical model,the intensity of saltwater intrusion under different scenarios are examined and summarized.Parameters explored are wind duration and speed,tidal phase and river discharge.Results suggest that strong northerly wind of 4 days in February 2014 is able to trigger an extreme saltwater intrusion.With the same northerly wind condition,the intensity of saltwater intrusion is stronger in neap tide than spring tide.When the northerly wind is stronger than 10m/s,the net water transport in North Channel becomes landward;when the northerly wind is stronger than 7 m/s,the net salt transport in North Channel is landward.Under the scenario of February 2014,an extra discharge of 8000 m~3/s is needed to make sure Qingcaosha Reservoir can intake the fresh water in each day of this month.