On The Mechanism Behind The Nature Behaviour Of Sediment Dynamics Within The Turbidity Maximum Zone In Reponse To Reclamations In The North Channel Of Yangtze Estuary

The turbidity maximum zone（TMZ）is the region where the suspended sediment concentration（SSC）is significantly higher than that in adjacent water in estuary.The formation of TMZ is due to the accumulation of sediment under the dynamic interaction of runoff and tide,and it regularly migrates within a certain spatially range.The dynamics of suspended sediment in the TMZ plays an important role in the evolution of the channel,shoal and sand bar,as well as the distribution of nutrients and pollutants.Reclamation projects conducted in estuaries modify the channel geometry,intercept flow and sediment exchange between the shoal and channel,and result in variations of dynamics of runoff,tide and sediment.As the area with the strongest interaction between runoff and tide in estuary,the nature behaviour of sediment dynamics within the TMZ in response to reclamations needs to be further studied.The North Channel（NC）of the YE is the main channel of the Yangtze River into the East China Sea where minor artificial dredging takes place.The constructions of Qingcaosha Reservoir（2007-2009）and the third to eighth phase Hengsha East Shoal Promoting Siltation（2006-2018）have significantly modified the channel geometry of NC.Therefore,NC is a suitable area to study the mechanism behind the nature behaviour of sediment dynamics within the TMZ in reponse to reclamations.The systematic study is carried out based on the data collections,in-situ observations.The main contents and conclusions are as follows:1.After constructions of Qingcaosha Reservoir and Hengsha East Shoal Promoting Siltation,the TMZ of NC in the dry season was narrowed,its landward boundary moved seaward.The ebb dominance was enhanced,the tidally averaged SSC at the landward boundary of TMZ decreasedEight Landsat satellite remote sensing images in the dry season from 1995 to 2019were collected and processed,which were similar in terms of riverd discharge,tide type and time.In-situ measured data of flow velocity and SSC at three stations were also collected,which were measured synchronously at three stations along the main channel of NC during spring and neap tides in the dry season of 2003 before constructions of reclamations.In the dry season of 2018,in-situ observations of flow velocity and SSC were taken at three stations during spring and neap tide;particle sizes of seven surface sediment samples collected along the main channel were measured in laboratory.The relative surface suspended sediment concentration（RSSSC）retrieved from remote sensing image was proposed to quantify spatial variations of TMZ in NC.The flow dominance coefficients and vertical profiles of tidally averaged SSC at observation stations were calculated.From 2006 to 2019,the area-averaged SSSC within NC decreased by 0.21 kg m^-3,landward boundary of the TMZ moved seaward by 6 km,the location of the seaward boundary wandered within a 3 km range from 122°12’E to 122°14’E during the spring tide in dry season and the length of TMZ decreased by 6 km.During the neap tide in dry season,the averaged SSSC decreased by 0.05 kg m^-3,landward boundary of the TMZ moved seaward by 17 km,the seaward boundary of the TMZ wandered within a5 km range from 122°21’E to 122°24’and the length of TMZ decreased by 18 km.Compared with measured data in 2003 before the constructions of reclamations,in the dry season of 2018 after reclamations,the ebb dominance of NC was enhanced and the stagnation point moved seaward.At the landward boundary of TMZ,the tidally averaged SSC over spring and neap tides decreased,the tidally averaged SSC on bottom decreases by 0.43 kg m^-3 over spring tide and 0.71 kg m^-3 over neap tide.The median grain size of surface sediments in the main channel increased in the upper reach of sand bar,which had little change in the main channel on sand bar.2.At the landward boundary of TMZ in NC,during the spring tide in dry season after reclamations,there was regular tidal mixing asymmetry in the channel with the landward net sediment transport and reversed tidal mixing asymmetry on the shoal with the seaward net sediment transport.During the spring tide in dry season of 2018,a transect survey was carried out at the landward boundary of TMZ where the flow velocities,salinities and SSC were measured synchronously at five station.Parameters such as stratification ratio,eddy viscosity,eddy diffusivity and vertical mixing coefficient were calculated to describe temporal variations of stratification,the intensities of vertical mixing and eddy diffusion and the degree of tidal mixing asymmetry.The sediment transport rates induced by advection and tidal pumping and net sediment transport were also calculated at five stations.From the maximum ebb to ebb slack,the stratification ratios S_r were reduced from 0.00%to 0.09%at three stations in the channel and from 0.00%to 0.03%at two stations on the shoal;the stratification in channel was higher than the shoal.The depth-mean eddy viscositie（?） and depth-mean eddy diffusivitie（?）ranged between1.46×10^-3～1.39×10^-2 m²s^-1and 2.13×10^-3～1.14×10^-2m²s^-1 respectively at three stations in the channel,which were higher than that at two stations on the shoal with values ranged between 8.34×10^-3 m²s^-1～1.70×10^-2 m²s^-1and 6.25×10^-3～1.50×10^-2 m²s^-1.The vertical mixing coefficientsΩwere 0.12,0.16 and 0.06 at three stations in the channel showing regular tidal mixing asymmetry,which were-0.02 and-0.27 at two stations on the shoal showing reversed tidal mixing asymmetry.Therefore,after the maximum ebb,the stronger stratification in the channel restrained the vertical mixing;the vertical mixing over ebb tide was lower than that over flood tide,showing regular tidal mixing asymmetry.The stratification decreased rapidly on the shoal;the vertical shear induced to stronger vertical mixing;the vertical mixing over ebb tide was higher than that over flood tide,showing reversed tidal mixing asymmetry.Within a whole spring tidal cycle,sediment transport rates induced by tidal pumping were landward in the channel and seaward on the shoal,whose values were higher than seaward sediment transport rates induced by advection,resulting in the landward net sediment transport in the channel and seaward on the shoal.3.The reclamations narrowed the channel and enhanced the scouring significantly in the channel in the lower segment of NC.Data were collected and processed of bathymetry of NC in 1995,2002,2007,2012and 2018,bathymetry of Southern margin of Hengsha East Shoal in 2004,2010 and2016.Based on the characteristics of geomorphology evolution and channel volume within NC in recent 30 years,the effects of reclamations in estuary on geomorphology evolution of NC were analyzed.Meanwhile,the high-resolution subaqueous geomorphology at the north of north levee of Deep Waterway Project and the northeast of Qingcaosha Reservoir were obtained by multibeam sonar system Sea Bat 7125 in2016 and Sea Bat T50P equipped on unmanned surface vessel in 2020 respectively.Characteristics of local scour induced by Qingcaosha Reservoir,Hengsha East Shoal Promoting Siltation and Deep Waterway Project were analysed.The Qingcaosha Reservoir project narrowed the upper segment of NC by 3.5 km;the Hengsha East Shoal Promoting Siltation project fixed the 23.8 km coastline on the south bank in lower segment of NC.From 2007 to 2018,the constructions of recalmations resulted in the extension of the deep channel below the-10 m isobaths by6.7 km in lower segment;the channel volume below-5m isobaths increased 8.65×10⁷m³with the annual averaged erosion rate of 7.86×10⁶m³yr^-1,which was the 7 times of that pre-reclamations with the rate of 1.07×10⁶m³yr^-1（1995～2006）.From 2012 to2018,in lower segment of NC,the erosion rate within the channel below-5 m isobaths was 1.49×10⁷m³ yr^-1and deposition rate on the north shoal was 3.02×10⁷m³yr^-1.Meanwhile,from 2007 to 2018,there were 9.16 km scour along the embankment in the northwest and north of Qingcaosha Reservoir and 8.88 km scour along the north levee of Deep Waterway Project,with the maximum scour depths of 13.1 m and 5.5 m respectively.The Hengsha East Shoal Promoting Siltation and Deep Waterway Project intercepted the southward flow over Hengsha East Shoal.From 2005 to 2016,there was a 19 km scour along the north levee.Due to the coaction of flow along the levee and diffusion flow at the levee head,the maximum scour depth around the head of North levee was 7.2 m.The mechanisms behind the nature behaviour of spatial variation of TMZ and the lateral variation of sediment transport on the landward boundary of TMZ were discussed as follows:1.In the lower segment of NC where TMZ located,ebb dominance has been enhanced due to deepening and narrowing of the channel resulted from reclamations;the stagnation point and landward boundary of TMZ moved seaward.In addition,bed sediment coarsened and its resuspension was weakened.Consequently,the bottom tidally averaged SSC and depth-mean of the tidally averaged SSC decreased.2.Resulting from the constructions of reclamations,the differences of water depth between the shoal and channel increased on the transection at the landward boundary of TMZ,which strengthened the stratification difference between the shoal and channel in a tidal cycle.Under the impact of regular tidal mixing asymmetry in the channel,the flood tidally averaged SSC was higher than ebb tidally;the sediment transport rate induced by tidal pumping and the net sediment transport were landward.Under the impact of reversed tidal mixing asymmetry on the shoal,the sediment transport rate induced by tidal pumping and net sediment transport were seaward.