Study On Morphological Evolution And Siltation In Deep Waterway Due To Channel Re-construction In The North Passage, Yangtze Estuary

By using bathymetric chart,discharged flow data,hydrological data and back-siltation data,recent change of the riverbed and silting pattern of the deep waterway in the North Passage of the Yangtze Estuary has been studied in this paper. The main contents could be reviewed as follows:The regulation engineering of deep waterway,including two wide spacing jetties and nineteen long groins,had led to an apparent morphological evolution in the North Passage.The main channel of the up,middle and down side in the North Passage had successively eroded and its groin field had significantly deposited.At the same time,sediment has been deposited on the entrance region.Erosion and deposition had responded rapidly to the construction of the regulation engineering.There was about one year of lagging between the time of the erosion happened in the deep channel and the time of the construction of the regulation engineering finished in different stage.The siltation lag of time in the groin field varied with the initial depth, but the average deposited thickness was about 0.Sm per year.Volumetric analysis demonstrates there is an increasing trend of the siltation in the North Passage after 2002,because of the difference in duration and quantity between erosion in the deep channel and deposition in the groin field.The water volume of the North Passage had reduced by≈9%(280 million m³)between 2002 and 2006.Sediment budget reveals that the main sediment deposited in North Passage sourced from the river and the ocean.The decreasing water volume was attributable to shoaling in the groin field. Its triggering factors for the increased sedimentation are the navigational improvements(jetties and groins)after 1998,which altered the passage boundary and destroyed the equilibrium state on the average ebb and flood sediment fluxes.The establishment of a stable estuary is attributed to a reduction rate in depth of the groin field.The forecast on the deposited sediment quantity and continuous infilling time in the groin system is about 325×10⁶m³ and 6-7 years respectively.The South Passage and North Passage are the third branched reach of Yangtze Estuary.After implementing Yangtze Estuary Deepwater Navigational Improvements, there are a decreasing trend in the ratio of ebb flow diversion through the North Passage and an increasing trend through the South Passage.Sediment has deposited on the entrance region in the North Passage,with an length of about 12km long,and eroded on the upper region in the South Passage,with an length of about 18km.But the total cross-section area of the entrance reach in the North Passage and South Passage had decreased from 1998 to 2006.There is a good correlation between the total water volume and the yearly averaged ebb flow ratio in the North Passage,and between the cross-section averaged depth of the entrance reach in the North Passage and South Passage and their yearly averaged ebb flow ratio respectively.According to the relationship formula,the change of the ebb flow ratio and the cross-section averaged depth of the entrance reach in the North Passage and South Passage are evaluated.Due to change of boundary conditions,Back-siltation in the deep channel had undergone an obvious change after the implementing Yangtze Estuary Deepwater Navigational Improvements,which had a decrease in the back-silting intensity of the whole navigational channel and increase in local reach.The yearly back-silting thickness of the whole navigational channel decreases from 2.93m to 1.46m of the 1^ststage and l.00m of the 2^ststage,respectively.The location of the high back-siltation moves from the downstream channel during 1^ststage to the upstream channel during the 2^ststage.The spatial distribution of back-silting intensity along the navigational channel was closely related to its elevation difference between dredged trough and riverbed.The time distribution of back-silting intensity was closely related to the flow runoff.The back-silting amount increased and moved downward in the flood season and decreased and moved upward in the dry season.The main back-silting location centralized in the upstream channel after the 2^ststage.The main factors resulting in back-silting include riverbed topography,tide currents,sediment load and salinity.The mechanism of back-silting in upstream channel involves:â‘ an increase in the elevation difference between the dredged trough and the riverbed.â‘¡the worse dynamic conditions.â‘¢the rich source of sediment.â‘£mixing of the salt water and the fresh water.The dredging maintenance became difficult due to the centralization of back-silting in the upstream channel.It is necessary to take further measures to reduce the back-silting intensity.According to the theory and practical experiences of the navigational improvements,and the analysis of back-silting mechanism,six steps were proposed,including:â‘ Heightening south jetty.â‘¡Stabilizing the upstream riverbed.â‘¢Adjusting channel axis.â‘£Revising the width of regulation line.⑤Adjusting the radius of curvature.â‘¥Protecting the entrance riverbed of South Passage from further scouring.