The Spatial Patterns And Recent Changes Of The Sediments In The Yangtze River And Its Subaqueous Delta

Sediment is a fundamental element in riverine and coastal environments.It not only contains the source information,but aso reflects the local situation of hydrodynamics,which is the indispensable information to study sedimentary environment.Thus,the study of sediments is a fundamental subject in estuarine and coastal science.Riverine sediments play a very vital role in shaping estuarine and coastal geomorphology.To study the spatial pattern of the Yangtze sediments(including bed and suspended sediments)and their recent changes along the mainstem and in the estuarine region,the coarsening of sediments in the Yangtze Delta front and the corresponding reasons,and the erodibility of sediments in the Yangtze Delta,this study collected sediment samples in the Yangtze watershed and the delta,gathered the data of sediment and water discharge in the main gauging stations,and conducted hydrodynamic observations in the field.To be specific,this study mainly included the spatial distributions of the sediment grain size and the suspended sediment concentration(SSC),the recent changes of the sediments under the influence of human activities,the accumulation/erosion of some key river reaches,the response of the Yangtze Delta,the erodibility of the sediments by applying the Bottom Boundary Layer system,and predicted the erosion and deposition trend of the Yangtze Delta.The main results and conclusions are attached:1)Under almost nature conditions(i.e.the impacts of humans are relatively small),the riverbed and suspended sediments of the Yangtze River both show downstream fining trends.During 2000~2003(before the conduction of Three Gorges Dam,TGD),the riverbed sediments in the upper reach of Three Gorges were gravels,the sediment grain size in the Three Gorges reach was usually coarser than 30 mm,the average riverbed grain size was 0.290 mm in the reach between Yichang and Shashi,0.151mm in the reach between Shashi and Hankou,0.187 mm in the reach between Hankou and Datong,and 0.116 mm in the reach between Datong and Xuliujing.During1956~1968(pre-dam period),the suspended sediment grain size was 0.029 mm at Xiangjiaba Station,0.019 mm at Zhutuo Station,0.017 mm at Yichang Station,0.007mm at Hankou Station and 0.009 mm at Datong Station.However,after the constructions of dams and the water-soil conservation projects etc.in the Yangtze basin,the spatial patterns of sediment grain size were destroyed.During 2008~2012(after the impoundment of Three Gorges Reservoir),the sediment grain size in the Three Gorges reach decreased significantly to 0.011 mm,the average grain size increased to 11.5 mm in the reach between Yichang and Shashi,increased to 0.236mm in the reach between Shashi and Hankou,increased to 0.195 mm in the reach between Hankou and Datong,and increased to 0.171 mm in the reach between Datong and Xuliujing.During 2013~2015(post-dams and soil-conservation period),the suspended sediment grain size decreased to 0.006 mm at Xiangjiaba Station,decreased to 0.012 mm at Zhutuo Station,decreased to 0.009 mm at Yichang Station,increased to 0.015 mm at Hankou Station and increased to 0.011 mm at Datong Station.During 1956~1968(pre-dam period),the sediment discharge increased downatream in the reach between Xiangjiaba and Yichang,but decreased downstream in the reach between Yichang and Datong.The sediment discharge was 260 Mt/yr at Xiangjiaba Station,560 Mt/yr at Yichang Station and 510 Mt/yr at Datong Station.However,during 2013~2015(post-dams and soil-conservation period),the sediment discharge pattern in the reach between Xiangjiaba and Yichang was first an increase and then followed by a decrease.The sediment discharge was only 1.6 Mt/yr at Xiangjiaba Station,then increased to 41 and 69 Mt/yr at Zhutuo and Cuntan Stations,respectively,then decreased to 14 Mt/yr at Yichang Station.At the same time,the spatial pattern of sediment discharge in the reach between Yichang and Datong changed to a downstream increasing trend.The sediment discharge increased from 14Mt/yr at Yichang Station to 27 Mt/yr at Shashi,to 79 Mt/yr at Hankou,and then to118 Mt/yr at Datong.Since 1950s most of the fluvial sediment has been trapped in reservoirs,except for the finest portion.Furthermore,soil-conservation since the 1990s has reduced sediment yield.From 1956-1968(pre-dam period)to 2013-2015(post-dams and soil-conservation),the sediment discharge from the sub-basins decreased by 91%;in the main river,the decreases of sediment flux at all the main gauging stations were more than 75%,especially the decrease by 99%at Xiangjiaba station in the upper reach.Because the water discharge was minimally impacted,the SSC decreased to the same extent as the sediment flux.During pre-dam period,the SSC in the northwest sub-basins(Jinshajiang,Jialinjiang and Hanjiang basins etc.)are much larger than those in the southeast basins(Lakes Dongting and Poyang basins etc.).Now,the difference of SSC between the northwest and southeast basins has been greatly reduced,and the SSC in the Jinshajiang basin is even lower than that in the Lakes Dongting and Poyang basins.At the same time,the reaches in the downstream of the dams experienced severe erosion.During the process of erosion,the finer portion of the riverbed sediments would be easier to be eroded,which let the riverbed and suspended sediments become coarser compared to their original level.The original spatial patterns of sediment grain size and SSC in the Yangtze basin no longer exist anymore.According to the estimate,the Yangtze sediment discharge will decrease to lower than 110 Mt/yr by 2050,and further decrease to lower than 100 Mt/yr by 2090.2)The generalized estuary region of the Yangtze River is defined as a~700 km-long section from Datong Station(tidal limit)to the subaqueous delta at 30-50 m isobaths.The most seaward observation site in this study is about 660 km long from Datong Station.Results show that bed sediments mainly include sand in the uppermost500-km section between Datong and Xuliujing,whereas silt dominates the downstream reach of Xuliujing.In contrast,the median grain sizes of the suspended sediments are found to be greater in the lowest 100-km section(8-13?m)than in the upper sections(5-6?m)due to strong exchanges between suspended and near bed sediments.The suspended sediment concentration(SSC)was found to be low(<0.1g/L)and homogenous in the uppermost 100-km section,downstream of which the SSC increased rapidly to>1 g/L and both surface-bottom and intratidal variabilities occurred.The spatial pattern of sediments reflected the variation of hydrodynamics.The tidal effects were almost undetectable in the uppermost 100-km section,but the mean tidal range gradually increased downstream to nearly 3 m at the river mouth.Flow was generally unidirectional in the uppermost 400-km section,although its velocity changed in response to flood/ebb tidal dynamics;in the lowest 260-km section,bidirectional flow developed and the tides turned to be in control gradually.3)The average median size of the sediments in the Yangtze delta front coarsened from 8.0?m in 1982 to 15.4?m in 2012.This coarsening was accompanied by a decrease of clay components,better sorting and more positive skewness.The annual wind speed and wave height slightly increased by 2%and 3%,respectively,and the tidal range showed no change trend,which suggested little change in ocean dyanmics.At the same time,the riverine sediment discharge decreased by 70%,and the riverine suspended sediment grain size increased from 8.4?m to 10.5?m.Moreover,the delta morphology in the study area changed from an overall accretion of 1.0 cm/yr to an erosion of-0.6 cm/yr.Thus,the sediment coarsening in the Yangtze delta front was thus mainly attributed to the delta's transition from accumulation to erosion.When the rate at which China Coastal Currents(CCC)move sediment offshore of the delta area is greater than the rate of riverine sediment supply,net erosion occurs.Considering that there is no evidence that the capability of the CCC to transport sediment southward has declined in recent years and the sediment flux from the Yangtze probably will further decrease,the coarsening of the sediments in the Yangtze Delta is expected to continue.4)Deltas are widely threatened by sediment starvation.Erosion potential is an important indicator of delta vulnerability.According to hydrodynamic measurements and analyses of seabed sediments,the bed shear stress due to combined current-wave action under normal weather conditions varied from 0.08 to 0.67 N/m~2,and averaged0.29 N/m~2.It even increased to 2.56 N/m~2 during storms.The critical bed shear stress for erosion in the Yangtze estuary varied from 0.08 to 0.23 N/m~2,and averaged 0.13N/m~2.The period when bed shear stress due to combined current-wave action under normal weather conditions exceeds the critical bed shear stress for erosion accounts for 63%of the total observed period on average and can reach 100%during storms.This explains why net erosion has occurred in some areas of the subaqueous delta.The upper sediment cores suggest that the properties of the sediments in the top~2 m are similar to the surficial sediments,i.e.erodibility changes little vertically.Based on the subbottom seismic profiles and the corresponding deeper sediment cores,the modern sediment grain sizes in the uppermost 10-20 m are still similar to the surficial sediments in the subaqueous delta.Although the compation may increase and the water content may decrease downward,we believe the increased critical shear stress of the modern sediments is still smaller than the bed shear stress due to combined currents and waves.As a whole,the ocean dynamics are strong and the sediments are easy to be eroded in the Yangtze Estuary.So,the erosion will continue over the next several decades and even a few centuries under the background of sediment flux decrease.