| An estuary is an area where a freshwater river or stream meets the ocean,and fluvial sediment is transported and deposited in the estuarine area.In recent decades,changes in natural conditions and human activities have significantly affected the balance of source-sink systems in estuaries and changed the evolution process of estuaries.Human drivers play an important role in reducing the amount of sediment into the estuaries,such as damming of rivers,waterway construction,and reclamation projects.Engineering projects such as the construction of waterways changed local topographies of estuaries and greatly affected hydrodynamics and sediment dynamics,which further brought feedbacks in sedimentary processes.The estuarine turbidity maximum(ETM)is the zone of highest turbidity in an estuary.It is generally believed that ETM in the Yangtze Estuary(YE)mainly resulted from the combined action of various mechanisms such as tidal action and estuarine circulation.The water and sediment dynamics in ETM directly affect the accumulation of fine-grained sediment,the evolution of estuaries,and the source-sink conversion.Previous studies mainly focused on the variation in distribution of estuarine suspended sediment concentration(SSC)under human modifications.Riverine sediment supply to YE decreased by about75% in the mid-1980 s.As a result,SSC has decreased significantly in YE and subaquatic delta,particularly for South Branch and South Channel.However,SSC in ETM didn’t decreased significantly,which was different from other area.A better understanding of the ETM dynamics in the YE is still needed.There is a lack of quantitative research on the long-sequence and large-scale water and sediment dynamics in the ETM,and studies on the sediment transport capacity and source-sink transition characteristics of the ETM under the comprehensive influence of human activities are also needed.In this study,based on the large-scale in-site measured data of the ETM and subaquatic delta in 1982 and 2013,we aim to explore the variation in characteristics of flow and sediment,suspended sediment flux and spatial distribution pattern in the ETM of the YE.This study also aim to figure out the mechanism of these changes between 1982 and 2013,which definitely will provide support for future estuarine management.In 1982,few reservoirs and dams were built in the Yangtze River Basin,and riverine sediment entering the YE was not trapped and reduced.In 2013,there was a decrease of about 70% riverine sediment input due to more and more reservoir completions.Moreover,the construction of waterways in the estuary and the implementation of reclamation projects have changed the topographic conditions.The year 1982 and 2013 can represent typical years: one is the condition before the intense human activity,and the other represents the condition under human modifications.Based on field measurements conducted in 1982 and 2013,this study analyzed the hydrodynamics(velocity,salinity,stratification)and sediment dynamics(sedimentation)in the North Channel,North Passage,South Passage and offshore areas of the Yangtze Estuary under human modifications.The main findings of this study are shown as follows:(1)The hydrodynamics in the ETM of the Yangtze Estuary changed significantly in the past 30 years.The current velocity increased in the wet season and decreased in the dry season,especially during the spring tide in the flood season,the average velocity increased by 12%: the velocity in the South Passage and North Passage increased by15% and 33%,respectively.The residual current increased and its direction was towards the ETM.The ebb-tide dominancy also increased in 2013.By contrast,the average velocity decreased by about 22% in the dry season,the residual flow decreased slightly,and the ebb-tide dominancy also increased.Considering that upstream runoff didn’t change much,the change of hydrodynamics in ETM is mainly due to morphological change of the YE caused by a large number of estuarine engineering projects.During the flood season in 2013,the average velocity increased by 6% compared with 1982,and decreased by 22% during the dry season.The North Channel is less affected by the estuarine projects.Though the water depth increased but the channel narrowing was not obvious.The average flow velocity in the flood and dry seasons decreased,the residual current decreased,the tidal range decreased slightly,and ebbtide dominancy increased.Affected by the deep-water channel project,the North Passage was significantly narrowed and deepened,amplifying the runoff effect.Under the condition of large runoff during the flood season,the hydrodynamic force enhanced,and the flow velocity in the flood season increased.The residual current also increased,and the ebb-tide dominancy increases significantly.The South Passage was also affected by the construction of the Deep Waterway Project,the development of the sandbar in Jiuduan Shoal,and the implementation of the Nanhui Shoal reclamation project.The ebb-tide dominancy increased.The Nanhui area was affected by the reclamation of the Nanhui Shoal,and the coastal line accreted seaward.Affected by the narrowing and deepening of the South Channel,the hydrodynamics also changed.Overall,the estuarine engineering project had different impacts on hydrodynamics of each channel.The North Channel was slightly affected by the projects.The North Passage,South Passage and Nanhui area were strongly affected by the estuarine projects.The flow velocity all increased during the flood season,the residual current increased,and the dominancy of ebb tide increased.Considering that little change occurred in the watershed runoff and the tidal dynamics outside the mouth in the past three decade,it could be concluded that the hydrodynamics of the ETM was significantly affected by the local engineering projects.(2)In terms of sediment dynamics,the average SSC of the ETM decreased by 19%,and the decrease in the sediment concentration of the neap tides in flood season and the small tides in dry seasons was over 40%.The reduction in fluvial sediment supply played a dominant role in estuarine sediment dynamics.However,further analysis indicated that SSC of the spring tide in the flood season showed a different increase of39%.The vertical gradient of sediment concentration generally increased,but the vertical gradient decreased in the Nanhui mudbank area.At the same time,the relationship between SSC and flow velocity was studied.The results showed that the reduction of riverine sediment supply was not the only factor affecting sediment dynamics in the ETM.The sediment flux from subaquatic delta and local sediment resuspension caused by strong dynamics also had an important impact.In 2013,the average SSC decreased by 19% compared with 1982.The average SSC increased by 39% during the spring tide in the flood season,decreased by 47%during neap tide,and decreased by 40% during the dry season.Spatially,the SSC of North Channel decreased during all tidal cycles,while the SSC of North Passage,South Passage and Nanhui decreased during most of tidal cycles but increased during the spring tides in the flood season.By analyzing the effects of sediment reduction from the river basin,hydrodynamics change caused by local engineering in the estuary,and sediment fluxes from the sea on SSC in the ETM,this study found that the reduction of sediment in the river basin would reduce the SSC in the ETM.However,the local engineering could increase the flow velocity of the ETM.At the same time,the residual current into the ETM increased and more sediment was delivered into the ETM from subaquatic delta,resulting in an increase in SSC in the ETM.Under the combination of sediment reduction in the river basin and estuarine engineering projects,the SSC generally decreased.Only during the spring tide in the flood season,when riverine sediment supply was relatively rich and sediment flux from subaquatic delta was strongest in the year,SSC in the North Passage,South Passage and Nanhui increased.Based on the analysis of the relationship between SSC and velocity during flood and ebb tide,the riverine factor(Datong Station)was studied,and a well-validated empirical formula for SSC and flow velocity was established.The relationship during flood tide remained stable,indicating that under the condition of sediment reduction in the watershed,the relative increase of sediment supply from sea and local resuspended sediment during flood tide were new source of sediment.The relationship between SSC and ebb velocity was different.The source of sediment during ebb tide was from the river basin.With the same velocity,SSC of the ebb tide in 2013 was lower than that in1982,and it was clear that the reduction of sediment in the river basin was the main reason for the decrease in the SSC in the ETM.(3)Focusing on sediment flux in the ETM was important to understand the variation in sediment dynamics.The study found that net input of suspended sediment kept being delivered into the ETM in the past 30 years,but the net input in 2013 was nearly twice than that in 1982.In addition,the suspended sediment flux in the ETM increased by 55% in the flood season and decreased by 58% in the dry season,indicating that sediment transport in the flood season enhanced,and the sediment transport intensity in the dry season was weakened.The sediment transport intensity in the area of the mudbank was higher than that in other areas.For example,in the South Passage and Nanhui,the suspended sediment flux increased significantly during the flooding period.The reduction of sediment in the basin,the narrowing and deepening of the channels caused by human activities led to different changes in hydrodynamics and sediment dynamics during the flood and dry seasons.The average suspended sediment flux in the flood season generally increased,while it decreased in the dry season.During the spring tide in the flood season,the suspended sediment flux in the tidal cycle increased by 184%.During the neap tide in the flood season,the flux increased was 2%;and during the spring and neap tide in the dry season,the suspended sediment flux in the tidal cycle decreased by an average of 58%.In 1982,the sediment during the flood and dry seasons was mainly delivered to the southeast,and during the neap tide the sediment was transported to the northwest.While in 2013,the direction of the suspended sediment flux was generally to the southeast.In 1982,sediment was transported from the South Passage to the outside of the mouth.In 2013,the sediment was partly transported landward to Nanhui coastal area,and was also transported to the northern part of Jiuduan Shoal,becoming part of the sediment transport cycle of the North and South Passage.In 1982,the net sediment input of the ETM was 19.05 t/s.In 2013,the net input into the ETM was 33.79 t/s.In 1982,the net flux of the flood season was close to zero,and the net sediment input mainly occurred in the dry season.In 2013,sediment was imported into the ETM during flood and dry seasons,particularly for the wet season.The results showed that even though fluvial sediment supply decreased,more sediment were transported into the ETM.From a perspective of elevation,erosion occurred from1982 to 2013 and more sediment was eroded from seabed and resuspended.Therefore,it could be speculated that a mobile mud layer was formed near the bottom layer,which was difficult to observe by in-site measurements.(4)Under the combined influence of the reduction in riverine sediment supply and estuarine engineering projects,the ‘mudbank’ in the ETM of the Yangtze Estuary has undergone a ‘source-sink shift’.The ‘mudbank’ is changing from sediment deposition to sediment supply in the past 30 years.In 1982,the South Passage and Nanhui areas were sediment deposition areas.In 2013,these areas were transformed into erosion areas.The paths of sediment transport in these areas also changed: during the ebb tide in 1982,a large amount of riverine sediment was transported to the ‘mudbank’ and deposited herein.In 2013,the sediment was transported landward to the coasts.Part of sediment was deposited on the coastal tidal flats and Hangzhou Bay,and the other part,was resuspended from the South Passage,crossed the embankment and entered the Deep Waterway Channel of the North Passage.The sediment-carrying capacity of the ‘mudbank’ changed,sediment-carrying capacity in the flood season increased,and sediment-carrying capacity in the dry season decreased from 1982 to 2013.During the flood season,the amount of suspended sediment transport in 2013 was larger than that in 1982,and the increase in the amount of sediment transport at flood tide was greater than that at ebb tide.More sediment entered the estuary.During the dry season,the amount of suspended sediment transport in 2013 decreased compared with 1982,and the decrease in sediment transport at flood tide was close to that at ebb.The ‘mudbank’ transferred from deposition to erosion.The results showed that a new pattern of sediment dynamics was formed in the‘mudbank’ area.During the ebb tide period in 1982,a large amount of river sediment was transported to the ‘mudbank’ and deposited.Due to stronger hydrodynamics in2013,it was difficult for sediment to settle.But during the flood season in 2013,more suspended sediment was able to be transported at flood tides and less suspended sediment transport at ebb tides made more net transport of sediment be delivered to the coastal zone.Part of the sediment deposited on coastal tidal flats and the Hangzhou Bay.The other sediment entered the Deep Waterway Channel,providing a new source of sediment for the ETM of the YE to offset the influence of reduction in riverine sediment supply. |
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