Adaptive Behavior Of Channel Morphodynamics And Micro-geomophology Evolution Under Human Activities From Hankou To Wusongkou Reach Of The Yangtze River

Alluvial rivers can automatically adjust channel morphodynamics to adapt the change of water and sediment discharge and channel boundary.This process is known as river channel adaptive behavior.This adaptive behavior indicates the trend of the channel morphodynamics.Research on river channel adaptive behavior associated with flood control and drought,wading engineering safety,the development and utilization of shoreline resources.It is well known that the Yangtze River is a“golden waterway”in China.Since1950s,the channel boundary and water/sediment discharge had changed due to huge projects,e.g.water and soil conservation project,river regulation project,the Three Gorges Dam,the Qingcaosha reservoir,and the deep waterway regulation project in the Yangtze River Basin and Estuary.Thus,the law of river morphodynamics has undergone change.How the river channel morphodynamics adapt to these changes needs to answer.Therefore,it is of great significance to study the self-adjustment mechanism of river morphodynamics from the Hankou to Wusongkou reach.The subaqueous bathymetry data in 1998 and 2013,multi-beam data,flow velocity data and riverbed sediment samples were used to study the river evolution and deposition process and riverbed micro-geomorphology.The erosion and deposition patterns from Hankou to Wusongkou reach between1998 and 2013 indicate that the river channel strengthening erosion and some parts a slight deposition.In which,erosion volume of river bed approximately 2.96×10~8 m~3and the deposition volume of 0.68×10~8 m~3 from the Hankou to Hukou reach with a net erosion volume of 2.29×10~8 m~3.In the Hukou to Datong reach,the erosion volume of river bed approximately 2.91×10~8 m~3 and the deposition volume of 0.29×10~8 m~3 with a net erosion volume of 2.62×10~8 m~3.At the same time,the erosion volume of river bed approximately 21.6×10~8 m~3 and the deposition volume of 3.1×10~8 m~3 from the Hankou to Hukou reach with a net erosion volume of 18.5×10~8 m~3.The main reason led to this change was large projects built in the Yangtze River Basin and Estuary.Although the erosion phenomenon was observed along the Hankou to Wusongkou reach,the self-adjustment behaviors were different in Hankou to Hukou reach,Hukou to Datong reach,and Datong to Wusongkou reach.Adaptive behaviors of the river channel for projects in the Yangtze River Basin and Estuary are:the river channel below 0 m experienced erosion from Hankou to Hukou.The area of 5~10 m is the principal part response to human activities.The river channel below 0 m also experienced erosion from Hukou to Datong with the area of 0~5 m is the principal part response to human activities.The hydrodynamic conditions are complicated in the reaches of Datong to Wusongkou.It is influenced by runoff and the tidal cycle.However,the sediment discharge reduction is the main reason for erosion due to the Three Gorges Dam and soil and water conservation project.Overall,the area below10 m is the principal part response to human activities.The contribution rate of river channel erosion in the mainly adaptive area are 60.2%?44.9%and 49.1%,respectively.Several erosion and deposition phenomenon in local reach are related to the channel boundary change by the large projects.Measurements of riverbed micro-geomorphology at the segment of bifurcation between the South and the North Channel were conducted in the YRE(Yangtze River Estuary).A new type of subaqueous dune was discovered with a large amount of elliptical pits were discovered in the trough of dunes.This structure,newly identified as a catenary-bead dune,consists of a catenary dune and its associated elliptical pit bed form.Based on this finding,the nomenclature of“morphology of dune associated with accompanying bed form”was first proposed.Analysis points that this new type of dune is an adaptive behavior of riverbed micro-geomorphology owing to the sediment load reduction,riverbed sediment coarser,and the flow changed by large projects.The subaqueous dunes account for 80.3%,62.1%,64.3%and 27.5%in the Jiujiang to Hukou reach with 21 km long,Hukou to Daton reach with 210 km,Datong to Xuliujing reach with 490 km and Xuliujing to Wusongkou reach with 70 km,respectively.Compared with the dune size before 2003,the dune size,observed in2014,2015 and 2016 from the Jiujiang to Wusongkou reach,showed an increase trend.In a word,the river channel from Hankou to Wusongkou reach is experiencing an adaptive behaviors of river channel erosion,riverbed sediment coarser,erosion micro-geomorphology development,and river channel form change.These changes are related to human activities in the Yangtze River Basin and Estuary.