| Estuaries are sensitive areas of land-sea interaction,located in the area where the confluence of runoff and tidal current interacting with each other.The initiation,flocculation,and sedimentation of fine sediments,as well as the formation and evolution of the estuarine turbidity maximum(ETM),which determines the temporal and spatial distribution characteristics of sediments,and profoundly affects the geomorphological evolution,the security of land use ports and the stability of waterways in estuaries and coastal areas.The dynamic factors and genetic mechanism of the development and evolution of the ETM are very complex,which mainly include the vertical circulation,tidal deformation-induced sediment transport,resuspension,stratification,flocculation and settling process,etc.Most of the studies on the water stratification effect only consider salinity-induced stratification but suspended sediment concentration-induced stratification(SSC-induced stratification)is equally significant in the ETM.However,the influence mechanism of SSC-induced stratification on the ETM is still unclear.Therefore,the salinity and SSC-induced stratification effect and the temporal-spatial evolution of the ETM are analyzed and studied to deepen the understanding of the mechanism of the estuarine sedimentary dynamic process and predict the evolution trend of the ETM of the Changjiang River in the future.In this paper,the response process of the ETM to the salinity-and SSC-induced stratification is analyzed in the South Passage of the Changjiang Estuary,and the physical mechanism is discussed by combining field observation and numerical simulation.We made field observations in July 2018 and December 2018 respectively and collected the observation data in February 2013 and September 2013 at the ETM.The estuarine sedimentary dynamic process was observed by the method of bolted-ship profile combined with a bottom tripod.According to the stratification effect index Richardson number,the stratification characteristics of salinity and/or suspended sediment in wet and dry seasons were analyzed.The empirical orthogonal function(EOF)was used to study quantitatively the main controlling factors of suspended sediment concentration(SSC).The mechanism between salinity-and/or SSC-inducedstratification and sediment distribution is revealed.Ideal experiments are also established through FVCOM numerical simulation to discuss the significant stratification effects,including the differences and relations between salinity-and SSCinduced stratification,and their contributions to the range and strength of the ETM.The main conclusions are as follows.(1)According to the analysis of the measured data,the ebb tide duration is longer than that in flood with tidal asymmetry in the ETM of the South Passage of the Changjiang Estuary.Due to the effect of runoff,the current speed of the spring tide inside the mouth is higher than its downstream position.Due to the seasonal runoff differences and the effects of strong winds and waves in the dry season,salinity stratification existed in the 2018 wet season,while it was strongly mixed in the dry season.The average SSC of the spring tide is higher than that of the neap tide in the dry season,but the maximum SSC of the neap tide is higher than that of the spring tide in the wet season due to the stratification effect.The maximum SSC in 2013 and 2018 was more than 5 g/L in the wet season,which was more than 3 times that in the dry season.In terms of interannual variation,the maximum SSC in the 2018 wet season was located at the mouth shoal,while in 2013 was downstream.(2)Based on the analysis of spatio-temporal distribution characteristics of stratification,it is found that the salinity-and/or SSC-induced stratification in the 2018 wet season is more significant than in the dry season.In the South Passage of the Changjiang Estuary,the salinity-induced stratification is mainly distributed in the upper and middle layers,while the SSC-induced stratification is mainly in the near-bottom layer.In addition to the contribution of salinity and SSC density stratification to the stratification,low-velocity shear promotes stable stratification on the one hand and a large number of near-bottom sediments captured by stable stratification on the other hand increase velocity shear.The final stratification state is the result of the balance between velocity shear and density gradient.(3)Based on the empirical orthogonal function(EOF)analysis method,it is concluded that the main reason for the high SSC(>6 g/L)during the spring tide at the mouth gate in the 2013 wet season is the sediment resuspension caused by the tidal current and the sediment from the underwater delta outside the mouth carried by the high tide.However,during the 2018 wet season,the SSC at the barrier sand was affected by the sediment resuspension,salinity-and SSC-induced stratification during the spring tide,while during the neap tide,it was related to the flocculation settling process and salinity-and SSC-induced stratification,mainly affected by stratification effect.(4)The stratification effect contains salinity-and SSC-induced stratification.Salinity-induced stratification can inhibit the generation of turbulent flow and prevent the sediment near the bottom from diffusing to the upper layer,and the suspended sediment aggregates at the bottom to form the high SSC(>5 g/L).Meanwhile,the suspended sediment density gradient is formed between the high SSC at the bottom and the lower SSC at the upper layer,which leads to the SSCinduced stratification.Salinity-and SSC-induced stratification form a positive feedback mechanism,in which salinity-induced stratification is the prerequisite for SSC-induced stratification,and the ideal experiment results also show that SSC is higher when only considering salinity-induced stratification than when only considering SSC-induced stratification.(5)Based on validated ideal experiments using the FVCOM model,the range and intensity of the ETM are smaller in the dry season than in the wet season,and the spring tide is smaller than the neap tide.The extent and intensity of ETM under the influence of salinity-or SSC-induced stratification in the wet season are much larger than that in the dry season,and the spring/neap tide contrast is not as significant as wet/ dry season contrast.For the SSC-induced stratification,although the range and intensity change during seasons and tidal cycles,the center position of the ETM under its influence remain unchanged,which is located 15 km below the upstream position of Jiuduansha.For salinity-induced stratification,the center of the ETM changes during seasons and tides.The center moves about 5 km upstream during the neap tide compared with the spring in the dry season,and about 10 km upstream in the dry season compared with the wet season during the neap tide.Under the influence of global changing and human activities,frequent extreme events and estuarine engineering constructions make it difficult to update the water depth of the Changjiang Estuary in time,which brings challenges to the refined numerical simulation of suspended sediment.It is necessary to further strengthen the research on the influence of engineering construction,extreme events of short time scale on stratification effect,and the response process of the ETM. |
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