Distribution Characteristics And Influencing Mechanism Of Suspended Sediment Concentration In Branched Channels Of Yangtze Estuary

Due to the seasonal variation of runoff,the periodic variation of tidal intensity and the estuarine geomorphology in the estuary area,the hydrodynamic processes between flood and ebb,distribution of suspended sediment concentration(SSC)and salinity exist regional and seasonal differences.In addition,because of the interaction of runoff and tide current,the mixing process of salt and fresh water,the fine particle silt of estuary is easy to occur flocculation and resuspension.The SSC in the estuary also has significant asymmetric characteristics,which varies with spring and neap tide as well as the distribution of surface and bottom layers,and produces a periodic net discharge of the sandiment.Therefore,it is important to study the spatial and temporal distribution,asymmetric characteristics between flood and ebb and mechanism of SSC in large tidal estuaries,which is of great significance to the evolution of estuary deltas and the regulation of estuary channel.The Yangtze Estuary is a branching channel with medium tidal intensity.It's gradually widened from Xuliujing to the 10m depth contour outside the estuary,divided into North-South Branch,North-South Channel and North-South Passage by Chongming Island,Changxing Island and Jiuduansha.Due to the differences of morphology,hydrodynamics and salinity in branching estuary,the SSC has significant seasonal and regional characteristics.Based on hydrological survey in July 2013 and January 2014 at 12 stations in the Yangtze Estuary,include eight typical channels in the Yangtze estuary,upstream Xuliujing node and downstream 10 m depth contour,the planar and vertical distribution of SSC,asymmetry between flood and ebb,suspended sediment transport flux in the Yangtze Estuary is analyzed by hydrological statistical analysis,Kriging spatial interpolation and transport mechanism.The main conclusions are as follows:1.Planar variation characteristics and influencing mechanisms of SSC in the branched estuary of the Yangtze River(1)The average SSC of branched channels in the Yangtze Estuary is 0.60 kg/m~3,which is significantly higher than 0.14 kg/m~3 at upstream Xuliujing,and 0.35 kg/m~3 at the downstream 10 m depth contour outside the estuary.From different channels,the SSC in the North Branch Estuary is higher than that in the South Branch Estuary.High SSC occurs in the turbidity maximum region of lower North Channel,North Passage and South Passage,and decreases from north to south.The low area of SSC is appear in the South Branch,which close to Xuliujing.Next is appear in the upper North Channel and South Channel,the SSC is close to 10 m depth contour outside the estuary.(2)The SSC of dry season is generally higher than that of flood season in the typical channels of the Yangtze Estuary.The performance of the South and North Branch Estuaries is different.In the North Branch Estuary,the SSC in flood season is higher than that in dry season,while the South Branch Estuary is contrary.From the variation rate along the river course,the variation rate of SSC from the South Channel to the South and North Passage shows a scissors difference distribution,reflecting that the runoff effect in flood season has affected the distribution of SSC in the South Channel and upper North Channel,while in dry season the SSC is still controlled by tide.(3)The SSC of spring tide is generally higher than that of neap tide in the typical channels of the Yangtze Estuary.Distinguishing among different region,the SSC of spring and neap tide in the North Branch Estuary is significantly higher than that in the South Branch Estuary,the change between spring and neap tide is more significant in the South Branch Estuary.The SSC in different channels of South Branch Estuary is significantly affected by tidal effect,while that in the turbidity maximum region is more obviously affected by the changes of spring and neap tide.From the variation along the river course,the SSC changes between spring and neap tide are mainly concentrated in the South Channel and the following channels.The contribution rate of spring tide to SSC variation is more significant.(4)There is a good correlation between SSC and velocity current in plane.The peak values both occur in the turbidity region of the South Branch Estuary and decrease to both sides.Further study shows that there is a power function relationship between SSC and velocity in bottom layer.Besides,the position where salinity changes dramatically corresponds to the position where SSC increases rapidly.In the flood season,it is mainly in the turbid region of the South Branch Estuary,while in the dry season it moves up to the position of upper North Channel and South Channel.The flocculation settling process of fine sediment in estuary has an important influence on the distribution of suspended sediment concentration.2.Vertical variation characteristics and influencing mechanisms of SSC in the branched estuary of the Yangtze River(5)The SSC in the bottom layer is significantly higher than that in the surface layer in general,and this phenomena is more obvious in the maximum turbidity region of estuary.The periodic variation of SSC in bottom water with fluctuation tide is more obvious than that in surface water.The peak value of SSC in bottom layer mostly appears around 1-2 hours after the maximum velocity occurs,and the low value mostly appears 1-2 hours after the turn of tide.The peak value of SSC in surface layer occur lags behind that in bottom layer,and the trend is relatively low.(6)From the difference between surface and bottom layers of flood season and dry season,the difference in flood season between surface and bottom layers is mainly concentrated in the river mouth,while in dry season,the biggest difference between surface and bottom layer upward to the South and North Channel.In the flood season,the SSC at the flood tide is higher than that at the ebb tide of the bottom layer of river mouth,but the difference between the surface and the bottom layer at the ebb tide is greater than that of the flood tide.In dry season,the significant difference of SSC between the surface and bottom layers is mainly distributed in the river mouth and the South and North Channel,the trend is consistent with that in flood season.However,the regional distribution of bottom high SSC is wider,the peak value decreases and the difference reduces,reflecting the weakening of runoff and the enhancement of tides.(7)From the difference between surface and bottom layers of spring tide and neap tide,the SSC of spring tide is significantly higher than that of neap tide,and the bottom layer is more obvious.In spring tide the maximum SSC of surface and bottom layers are both distributed in the river mouth.The difference of SSC between surface and bottom layers in the ebb tide is greater than that in the flood tide,reflecting the strong tidal effect during the spring tide and the obvious resuspension of bottom sediment.In the neap tide,the peak value of SSC is not significant,and the difference between surface and bottom is significantly reduced.This is due to the sediment deposition caused by the weakening of tidal effect,resulting in the reduction of SSC.Sediment settlement at the bottom is faster,the differences of SSC between flood and ebb tide as well as surface and bottom layer are significantly reduced.3.Asymmetry characteristics between flood and ebb and influencing mechanisms of SSC in the branched estuary of the Yangtze River(8)The evolution of the estuary controlls the overall SSC difference between the North and South Branches.In recent years,the SSC of the Yangtze River runoff has declined significantly.During the observation period,the SSC in upstream Xuliujing is significantly lower than the 10 m depth contour outside the estuary.The evolution of river regime affects the interaction intensity and range of runoff and tidal currents,resulting a pattern of SSC in the North Branch of the Yangtze Estuary is larger than that in the South Branch,as well as in flood tide is larger than that in ebb tide.(9)The flood and dry seasons affect the redistribution process of SSC in the estuary.The changes of flood and dry season in the Yangtze Estuary is mainly reflected in the variation of runoff.Because the runoff of dry season is much smaller than the flood season,the distribution of SSC in the South Branch Estuary shows that the dry season is larger than the flood season.Form the variation along the river course,in flood season the area which SSC is less than 0.20 kg/m~3 can reach to the upper North Channel and South Channel,while in the dry season,it is only distributed from Xuliujing to South Branch.(10)Spring tides has a stronger impact on SSC asymmetry than that of neap tides.In spring tide,the sediment resuspension in flood and ebb tides of turbid region is more significant.The SSC in the upstream estuary from Xuliujing to the South Branch is low,reflecting the weak influence of tidal dynamics on the south branch.In the maximum turbidity region,the SSC in flood and ebb tides of spring tide is significantly different and higher than that in both sides.The results show that the processes of flood and ebb tide has resuspension effect on sediment,and the resuspension effect of flood tide is greater than that of ebb tide.(11)The seasonal wave condition affect the SSC asymmetric distribution in the turbidity maximum zone of the estuary along the north-south direction.In the flood season,the maximum SSC occurs in the process of flood tide in the South Passage,and decreases in turn to the North Passage and the lower North Channel,reflecting that the SSC in the flood season is strongly affected by the southeast-south wind wave in summer,while in the neap tide,the SSC mostly below 0.20 kg/m~3,and the characteristics of the maximum turbidity zone in the estuary are not obvious.In the dry season,the distribution of SSC is contrary to that in flood season.The maximum SSC appears in the lower North Channel,and decreases in turn to the North Passage and the South Passage,reflecting that the SSC in dry season is strongly affected by the northward wind wave in winter,while in the neap tide,the variation trend of the SSC is consistent with that in spring tide,but higher than neap tide in flood season,mainly due to the weakening of runoff in dry season.(12)The high sediment concentration near the bottom in the spring tide contributes significantly to the SSC asymmetry between flooding and ebbing stages at the river mouth.In the spring tide of flood season,the high SSC at the bottom layer is mainly concentrated in the river mouth,because of the strong tidal effect,the sediment resuspension at the bottom is obvious,and the asymmetry between flood and ebb is obvious.The trend of the dry season is consistent with that of the flood season,because of the weakening of runoff and the enhancement of tidal effect,the peak SSC difference between surface and bottom moves up to the vicinity of the North and South Channel,and the distribution of the high SSC at the bottom is wider,but the value decreases.High SSC at the bottom of spring tide is significantly higher than that at neap tide,which contributes significantly to the asymmetry between flood and ebb at the river mouth.4.Suspended sediment transport flux characteristics and mechanisms in the branched estuary of the Yangtze River(13)During the investigation period,the average tidal net sediment transport per unit width of the North Branch Estuary was 28.0 t,and the direction was land-oriented.The South Branch Estuary was 24.4 t,which was between 29.3 t in Xuliujing and 3.2 t in the 10 m depth contour outside the estuary,and was sea-oriented.From the different channels,the peak value of per sediment transport of flood and ebb tide appears in the area of maximum turbidity zone of North Channel,North Passage and South Passage,but the net sediment transport is higher in the lower North Channel and North Passage,the South Passage has turned into land-oriented and the value is smaller.(14)In general,the per unit width sediment discharge of the typical channels in the Yangtze Estuary is greater in the dry season than in the flood season,but the net sediment discharge is greater in the flood season than in the dry season.In flood season,the net sediment transport of the North Branch Estuary is land-oriented,while that of the South Branch Estuary is sea-oriented.In dry season,the net sediment transport of the South and North Branch Estuary is sea-oriented.Both flood and ebb tides,the sediment transport of the spring tide is significantly larger than that of the neap tides,but the net sediment transport of the neap tide is larger than that of the spring tide.(15)From the stratified sediment transport,the high sediment transport layers inside the estuary mostly occur between 0.2h and 0.8h,and the vertical net sediment transport is characterized by low surface and bottom layer and high middle layer.In the10 m depth contour outside the mouth,the net sediment transport in the upper layer is large outside the North Channel mouth,while that in the bottom layer is significantly higher than middle and upper layers outside the South Passage mouth.(16)Advection transport is the main contribution of sediment transport in the branching channels of the Yangtze Estuary,followed by tidal pump effect and net vertical circulation transport.In the North Branch Estuary,the contribution rate of tidal pump effect and net vertical circulation transport are increasing from west to east,and have become an important contribution to sediment transport in the area of maximum turbidity zone.