Sedimentary Dynamic Processes Of The North Branch In The Changjiang Estuary

As a bridge connecting the river and sea,estuarine channel is a key zone and main carrier of the land and ocean interaction.As the most developed economy region in Shanghai china,the channels located at the mouth of the Yangtze River have played the important water transport value.However,due to drastic reduction of fluvial sediment and intensive human activities in the estuary,channel morphology of the Changjiang estuary have undergone serious changes,especially for the North Branch(NB),the first order bifurcation of the Changjiang Estuary.Frequent human intervention in the NB result in a qualitative change from the river-dominated to tide-dominated estuary in recent several decades.At present,the characteristics of net landward sediment transport triggers severe siltation in the NB,which significantly blocks the shipping industry.In addition,frequent reclamation continues to decrease both the tidal flat area and water area of the NB,which not only alters the ecological environment,but also aggravate flow path and the divergence of flood-ebb tides,furtherly worsening the sedimentary environment and intensifying shrinkage of the NB.Therefore,researches on the response of the NB morpho-sedimentary changes to human activities is particularly important for improving the navigation environment.Based on bathymetric data from 1958 to 2020,field measurements on sediment and flood-ebb tides from 1982 to 2020,water and sediment discharge records at Datong Station since 1955,as well as diversion ratios of discharge and sediment of the North-South Branch bifurcation,this paper expounds the sedimentary environment change characteristics and sediment transport trends in the NB,explores the morphological change processes and associated influence mechanism of the NB,and thus brings forward a reasonable prediction of the NB's future development pattern as well as its regulation measures through geo-science analysis methods and techniques,including grain-size experiment,ArcGIS software,McLaren sediment transport model and empirical orthogonal function(EOF).The main conclusions include:(1)Since 1982,sediment in the entire NB shows a fining tendency.The flood event has an obvious driving effect on sediment coarsening in the NB.The average median diameter of sediment in the NB ranges from 4.3?in 1982 to4.8?in 2011 and then 5.8?in June 2020,indicating a fining trend.In 1982,the main sediment type in the NB is sand,with the sand component accounting for 68.8%.In2011,the main sediment types in the NB are sandy silt and silty sand.Compared with1982,the content of silt has increased 24.3%.In June 2020,sediment in the NB is dominated by mud,silty sand,and sandy silt.with a sharp 37.4%decrease of sand compared with 1982.In August 2020,after extreme flood peak,the average median diameter of the sediment in the NB is 4.8?,which has reduced by one size compared with the sediment in June of the same year.Besides,flood generates better sorting negative skewness and narrowing of particle size frequency distribution curve in the NB sediment.(2)There is always a net landward sediment transport in the NB.The flood does not change the direction of net sediment transport in the NB,but significantly blocks the landward transport of sediment along the NB.The results of McLaren one-dimensional sediment transport model show that the NB exists the northwestward(landward)sediment transport in 1982,2011 and June2020.But the flood weakens the net landward sediment transport trend along the NB.EOF analysis of the NB sediment shows that runoff in August 2020 was significantly stronger than June in the same year.In addition,the coarse sediment carried by the flood has a significant impact on the sediment distribution in the NB.(3)From 1958 to 2020,the entire NB becomes shallower,with the channel volume below-5 m isobaths decreasing sharply.Siltation in the NB slows down since 2005,after weathering the large scale of accretion in the last century.Based on the digital elevation model(DEM)from 1958 to 2020,corresponding discharge and sediment data,it is found that the average water depth of the NB decreases by around 44%from 5.9 m in 1958 to 3.3 m in 2020.The volume below-5m isobath reduces from 370 million m~3 in 1958 to 56 million m~3 in 2020,indicating a shrinkage of nearly 6.6 times.The historical channel profiles show that the NB is narrowing every year,with the mouth bar developing northward gradually and crushing the flood channel.From 1958 to 2005,the NB is dominated by net siltation,with a siltation volume of 0.76 billion m~3.Since 2005,the NB experiences both erosion and accretion,and the amount of erosion and deposition has decreased significantly.River bend angle increase,river width shrikage,tidal flat area reduction and flood-ebb tidal asymmetry aggravation induced by land reclamation are the main factors and influence mechanisms that lead to the deposition in the NB.A sharp decrease of fluvial sediment caused by the operation of the Three Gorges Dam since 2003 and the narrowing project in the mouth are the principal factors driving the weakening of the NB siltation in recent years.(4)The NB maintains the dynamic-sediment-geomorphology evolution pattern in the future.It is recommended to cutoff the curve channel and restore the intertidal tidal flat to extend the life of the NB.According to the sediment fining characteristic and net landward sediment transport tendency in the NB,as well as the interaction between flood and flood tide in2020,the tide-dominated nature of the NB will remain unchanged in the future,and flood tide will continue control the branch.Due to mouth narrowing,the tidal prism decreases,and flood-ebb tidal asymmetry weakens,while discharge and ebb current enhance accordingly.Subject to riverine sediment reduction and fine sediment supply from the sea,NB sediment may further fining in the future.At the same time,the NB will keep the net landward sediment transport tendency,and thus it will still be dominated by siltation in the future.It is suggested that the regulation of NB should follow the concept of‘manual intervention first,later ecological restoration later'.On the basis of the narrowing project,a series of measures,including upper NB dredging,curve cutoff middle-lower NB intertidal flat restoration,and flow path smoothing of flood-ebb tides are suggested to enhance the ebb tide,thus to maintain the channel volume of the North Branch.