Study On The Mechanism Of Saltwater Intrusion In The Pearl River Estuary

With the global climate change,the melting of polar glaciers has intensified,and the sea level has been rising in recent decades leading to a threat to coastal areas.Sea level rise will cause serious saltwater intrusion,coastal erosion,wetland degradation and other hazards,which would have a serious impact on freshwater resources.Saltwater intrusion directly affects the production and life of residents in the estuary.In order to ensure the stable economic development of the Guangdong-Hong Kong-Macao Greater Bay Area,the impact of sea-level rise on the intrusion of saltwater in the Pearl River Estuary is of great significance.In this paper,the three-dimensional hydrodynamic model EFDC(Environmental Fluid Dynamics Code)was used to numerically simulate the hydrodynamic process of the Pearl River Estuary.Based on thoroughly verifying the model,the study on the dynamic mechanism of saltwater intrusion in the upper reaches of the Pearl River Estuary is carried out from three aspects: runoff,tide and wind field.By analyzing the temporal and spatial distribution of bottom salinity in the dry season,it is found that the saltwater intrusion is closely related to the tide,and the strong saltwater intrusion always occurs 1-3 days before the spring tide.In addition,it is also found that the sustained northerly wind will lead to a strong saltwater intrusion in the upper reaches of the Pearl River estuary for a long time,and the impact will be greater during the neap tide.In order to further analyze the causes of the strong saltwater intrusion in the upper reaches of the Pearl River Estuary in February 2008,the salt flux was decomposed in the cross section of Humen waterway,and the contribution of each factor was evaluated.The results showed that the stable shear term caused by the estuary circulation is the main reason to control the saltwater intrusion;the contribution of advection term caused by runoff is second;the tidal oscillation term caused by the asymmetry of rising tide and falling tide has the least contribution.When the strong circulation in neap tide period overlaps with the continuous north wind force,a strong saltwater intrusion will be formed in the upper reaches of the Pearl River Estuary.Therefore,the periodicity of spring tide and neap tide and the persistence of wind direction can be used to warn the occurrence of super saltwater intrusion in Humen waterway.According to the IPCC forecast,the global average sea level rise will reach 110 cm by 2100.In this paper,numerical simulation is used to study the response of saltwater intrusion in the upper reaches of the Pearl River Estuary under the condition of 50 cm and 100 cm sea-level rise in the future.The results showed that when the sea level rises,the salinity changes most significantly in the upper and middle reaches of the estuary.The salinity at the water intake point of the upstream waterworks will increase significantly,and the days of water intake will decrease significantly.The salinity change in the upper and middle reaches of the estuary is the most significant.By adjusting the runoff to simulate the measure of " Flow modulation ",it is found that " Flow modulation " can still effectively inhibit the saltwater intrusion in the dry season when the sea level rises by 100 cm.There is a good exponential regression relationship between the salinity change and the runoff threshold,and the specific brackish water discharge threshold can be obtained by using the regression curve.It can provide a reliable reference for saltwater intrusion control under the condition of sea level rise in the future.However,during the period of super strong saltwater intrusion,the cost of the freshwater resources is greater than the benefit.It is suggested to adopt other measures such as tide gate project,water curtain project,water storage in advance and other traditional measures to deal with situation that there is no fresh water available during the period of strong saltwater intrusion.