Recent Channel Morphology And Micro-topography Characteristics Of The Typical River-lake Confluence In Yangtze River

Dongting Lake and Poyang Lake are southern to the Yangtze River and join to the middle reaches of the Yangtze River,forming a typical river and lake confluent area.In the confluent area,the flow mutual jacking and the remarkable change of sediment erosion and deposition,easily affect navigation conditions and threaten shipping safety.Besides,for the Yangtze River,Dongting Lake and Poyang Lake are the important flood regulating places where channel morphology processes are intimitely related to flood control and disaster mitigation.Therefore,the confluence of river and lake are the key and difficult point of the Yangtze River basin management,development and protection.The interference of human activities on rivers and lakes has increased in recent years.Under the influence of large-scale water conservancy projects based on Three Gorges Dam,river sand mining and conservation of water and soil,sediment discharge in the middle and lower reaches of Yangtze River has been decreasing significantly.River cutoff and waterway regulation project partly changed the boundary conditions of the confluence river,frequent sand mining activities in lake also affect the channel morphology between lake and river.Those result in the change of channel morphology and the constantly adjustment-adaptation of micro-topography features.Based on multi-beam bathymetric,ADCP and bed surface sediment samples data in 2016 and 2017 as well as the bathymetric data in 1998,2006,2013,and 2015,the morphological change and micro-topography features within the confluence of Yangtze River and Dongting Lake and the confluence of Yangtze River and Poyang Lake are analyzed to explore the response of the morphological change and micro-topography features to the human activities.The main achievements are as follows.(1)From 2006 to 2015,the shoreline and center sandbars change is not obvious.0 m shoal eclipsed mostly and the area of 3 m,5 m increased.The majority of cross-sections were eroded,mainly including the deep troughs remarkably swung in the main channel and the deep scour of the entire channel.It is characterized by left erosion and right deposition,groove erosion and shoal deposition.The study area scoured obviously and net scour amount reached 14096.5×10~4 m~3,the average erosion depth is about 1.47 m and the rate of annual erosion is 1409.7×10~4 m~3 From the upper to the lower reach,the channel erosion gradually weakened,and near shore scour is remarkable.(2)There are five types of micro-topographys in the confluence of Yangtze River and Dongting Lake,including smooth surface,dune,hollow,erosional channels erosional bank slope and so on.Dune most widely distributed in the study area,accounting for 38.9%,and large dunes were predominant.These dunes varied from0.11~8.20 m in height with a wide range of wavelength from 6.46 to 281.70 m.Erosive micro-topographys,such as sour pits,erosional channels,erosional bank slopes,accounted for 12.1%of the study area,mainly distributed in the flow rushing inshore side and channel erosion area.(3)From 1998 to 2013,The shoreline of the confluence of Yangtze River and Poyang Lake is basically stable.The center sandbars have moved obviously,mainly in the form of erosion of the head and moving downstream of the whole sandbars,and new sandbar emergence.0 m shoal mostly eclipsed and the area of 5m,10 m increased.Most of the cross sections are scouring.The main change forms are single and double trough transformation,broadening and deepened of the channel.The study area scoured obviously and net scour amount reached 5820.7×10~4 m~3,the average erosion depth is about 0.42 m and the rate of annual erosion is 388.0×10~4 m~3.The channel concave bank erosion and convex bank deposition on the whole,and the inshore scour is significant.(4)There are five types of micro-topographys in the confluence of Yangtze River and Poyang Lake,including smooth surface,dune,sour mark,hollow,sand mining pit,erosional bank slope and so on.As the main micro-topography of the study area,dune accounted for 46.5%,at the wavelength of 3.11~252.95 m,0.10~5.48 m in height,with large dunes and medium dunes mainly.The erosional channel is located in the middle of the confluence,broadening and deepening from upstream to downstream.The scour marks and sand mining pits are mainly distributed in the Poyang Lake channel.(5)The reduction of sediment discharge and river regulation project are closely related with channel scour.With the construction of the upstream water conservancy project,the erosive environment will continue,and affect the channel evolution of the confluence continuous.(6)Compared with the two confluence shows that the annual erosion in the confluence of Yangtze river and Dongting Lake(16.9cm/a)is more obvious than the confluence of Yangtze river and Poyang Lake(6.5cm/a).The dune size showed significant differences between two area.The former dunes average wavelength of24.62 m,the average wave height is 1.46 m,and the latter average wavelength of 9.44m,the average wave height is 0.49 m.