Study On Engineered Geomorphology Characteristics And Risk Assessment In The Yangtze Estuary

The geomorphological evolution of the Yangtze River Estuary has been focused by the geographical researchers,engineers,urban planners and decision makers for its important impacts in the riverine production and security of flood defense,engineering infrastructure,as well as the shipping and economic development of cities along the watershed.Since the mid-20^th century,a large number of engineering projects have been built along the river channels changing the nature of geomorphic patterns and evolution.And then,definitely,they in turn limited the land use along the riverine channels,even resulting to outbreak of bank collapse hazard.Therefore,it is quite urgent and necessary to make an over-and under-water integrated observation and relative targeted researches.However,subject to poor observation and complex dynamic conditions in the studied region,a high-precision and high-resolution topography investigation via traditional means of observation and measurement is hardly achieved.In addition,the differences between onshore and subaqua principles of topographical surveying increased the difficulty of achieving seamless fusion of terrain data.With development of advanced measuring techniques,more efficient instruments have been put into service in topography measurement,and small unmanned surface vehicle?USV?are increasingly being applied to observe complex,sensitive and dangerous shallow waters in the engineering project,which makes it possible to study the engineering geomorphology characteristics and its risk assessment quantitatively.For this purpose,we build a over-and under-water integrated mobile mapping system which integrates the multibeam bathymetric system,3D laser scanner,acoustic doppler current profiler,position and orientation system,as well as intelligent unmanned surface vehicle?USV?.Then,a comprehensive survey of topography,hydrodynamics,and surface sediments was carried out on the three regions,including Luochengzhou sandbar and Manyusha sandbar in the Kou'anzhi reach,and Hengsha survey area of the North Channel,which characterizing intensive engineering projects recently.In the end,this paper furtherly discusses the geomorphological characteristics and its formation mechanism of typical river projects and evaluates the potential risks of engineering geomorphology.Key findings in this paper are as follows:?1?We solve the key problem of time synchronization and coordinate registration during the process of building the over-and under-water integrated mobile mapping system.The time synchronization of 3D laser scanner and multi-beam sounding system can be completed by using GPS output$GPGGA data coordinate with 1PPS signal.Then,establish the spatial position relationship between the coordinate systems of each sensor.After calculating the measurement results via means of the multi-rotation matrix model,the spatial registration is completed in the geodetic coordinate system of WGS-84,so as to achieve the integrated measurement of land and water topography.This rapid intelligent mobile measurement system greatly improves the work efficiency and precision of the topographic survey of the engineering reach,and provides more accurate data support for the Geomorphic evolution mechanism of the wading engineering.?2?Luochengzhou sandbar belongs to the curved river controlled by tidal.This reach has been continuously scour from 1998 to 2019 with a net scouring amount of1.27×10⁸ m³,the left inlet of Luochengzhou is characterized by alternating scouring and silting;while the right is scouring.The local scour geomorphology formed within the scope of project,we found two scour pits,No.1 is located between the spur dike LR1 and LR2 with a deepest point of approximately 32 m,No.2 is located behind the spur dike LR2 with a deepest point of 36 m.And the No.2 scour pit is only 100 m away from the river bed with the slope is up to 40°.Under the influence of Luochengzhou Project,local flow field was changed,moreover,the riverbed sediments of this area were silty sand with poor scouring resistance,local scour pits are formed.?3?Manyusha sandbar belongs to the straight river controlled by tidal.This reach has been alternating scouring and silting from 1998 to 2019 with a net scouring amount of 1.06×10⁸ m³,under the influence of Manyusha Project,Manyusha shore silting,while both beds scouring,forming a channel form of“W”.The local deposite geomorphology formed within the scope of project,and we found medium and small scale sand waves with the wave length of 5.22-42.46 m,the wave height of 0.25-2.44m,and the sand wave index?wave length/wave height?of 6.96-47.18.The ridge line of this topography was curved,while dip angles on both sides were asymmetric.The steep slope of the sand waves faced the downstream,and the lee-side was significantly steeper than the upstream side.Under the action of Manyusha Project,although the whole section of this straight reach is in the erosion trend in recent years,there is still some deposition in the shallows in the middle of the river,which is helpful to maintain the stability of hydrodynamic axis and guarantee the navigable conditions of deep channel on both sides.?4?North Channel belongs to the artificial curved river controlled by tidal.This reach has been continuously scour from 2002 to 2018 with a net scouring amount of2.8×10⁵ m³,especially in the south bank,the nearshore was scoured down and formed-20 m isobath in 2011.We found a scour pit with a length and width of approximately1500 m and 200 m,the deepest point is approximately 27 m,and is only 200 m away from the north bank of Hengsha.A series of projects in North Channel changed the boundary of Yangtze Estuary,causing the downstream of the main stream south gradually close to the bank,formed the scour pit?5?The risk of instability of the deposit geomorphology of the sand waves at Manyusha and the No.1 scour pit at Luochengzhou is relatively low.But the No.2 scour pit at Luochengzhou has a high instability,especially in the east nearshore area.Further development in the future will threaten the bank safety.The scouring range had extended to the project structures,where were at risk of instability.The risk of instability of the scour pit at north of Hengsha is relatively low,however the mainstream is still act on the bank,which makes the development trend of this scour pit uncertain.This is a potential threat to the stability of bank safety.