| Artificial river refers to the hydraulic engineering dug by human beings to connect river systems and facilitate transportation,which plays an important role in promoting the development of social economy.The lower reaches of the Yangtze River has the densest river network in China including natural rivers and artificial rivers Since the last century,with the constructions of a series of hydraulic and waterway regulation projects,there have been abundant studies on their influence on the geomorphology evolution of the lower reaches of the Yangtze River.However,there are few reports on it of the confluence of artificial river and main stream,the geomorphology evolution of which has a significant impact on surrounding shipping and nearshore social and economic development which is of great importance.This dissertation chooses the north and south confluence of the Grand Canal and main stream which are runoff-dominated,as well as the cconfluence of five south-north oriented artificial diversion channels from Baozhen to Xijiagang at the south coast of Chongming Island and the North Channel,which are tide-dominated in the Yangtze Estuary.Based on the bathymetric data from 1998 to 2019,we utilized an over-and under-water integrated mobile mapping system which integrates the multibeam bathymetric system,acoustic doppler current profiler(ADCP)and 3D laser scanner,as well as the sub-bottom profile carried on another vehicle,to finish a comprehensive survey of the typical confluence,including over-and under-water geomorphology,current speed,shallow strata structure,water sample and surface sediments in July 2018,September 2019 and January 2020.Then,this thesis focuses on the geomorphological evolution,micro-topography and shallow strata features of the confluence of artificial rivers and main stream dominated respectively by stream and tide,and discusses the affecting factors and evolution tendency of them.The main achievements are as follows.(1)There were two phases of the erosion and deposition in the Zhenyang Reach:the river channel deposit obviously from 1998 to 2013 with the net deposition amount was 5.3×10~7 m~3 and scoured obviously from 2013 to 2019,the net scour amount reached 22.0×10~7 m~3.The north shore of the confluence sharply deposit and the south shore scoured from 1998 to 2013,while from 2013 to 2019,the north shore scoured obviously and a part of the south shore deposit.The change of the erosion and deposition of the river channel resulted in the swing of the-5 m and-10 m isobathic lines.Isobathic lines of the north shore of the north confluence moved mostly to river channel and the one of the south shore moved to shore from 1998 to 2013.However,from 2013 to 2019,the isobathic lines all moved to shore except several areas.(2)The east and west bank of the north river mouth and the west bank of the south river mouth are steep,the maximum gradient of which reached 60°-70°.Megadunes were discovered near the north river mouth with the average wave length and wave height were 133.8 m and 3.1 m,and the maximum wave length and wave height were 230 m and 5 m.The direction of the wave slope was downstream,which indicates the powerful current.There are erosional bank slope and nearshore scour marks at the west bank of the south river mouth,as well as erotional scarp,scour mark and scour pits at the eastern bank of the mouth.(3)The comprehensive influence of varying water and sediment discharge,moving upstream of the tidal and tidal current limit,phase II regulation project and12.5 m deep-water project contributed to the erosion and deposit change of confluence of the Grand Canal and the main stream.With continuous decline of sediment discharge and increase of regulation projects,the confluence of the Grand Canal and the main stream will be scoured in the future.Furthermore,according to the ADCP survey results,circulation flow was discovered near the bottom at the north and south mouths to erode the slope toe.It is necessary to pay attention to the protection of the subaqueous bank slope near the river mouths.(4)The evolution process of the upper and middle North Channel was deposition-erosion-erosion slowed down from 2000 to 2018.The deep channel scoured continuously and the river channel section narrowed.Obvious scour of the river channel resulted in nothward migration of the thalweg and sharp erosion of the Liuyao Sand near the diversion channel mouths from Baozhen to Xijiagang at the south coast of Chongming Island after 2008.The Liuyao Flood Channel was deposited and shallower,whose length is much shorter.The bank slope near the diversion channel mouths is eroded to steeper,with the maximum gradient reached40°which is risky.There are lots of types of micro-topographys in the study area such as smooth surface,dune,scour mark,accumulational bank slope and so on.(5)The Qingcaosha reservoir and the North-South channel bifurcation control project contributed to the increasing ebb tide current duration(TCD)of the upper and middle North Channel.With the comprehensive influence of the Three Gorges Dam and Shanghai Yangtze River Bridge,the upper and middle North Channel turned from sharp deposition during 2000 and 2008 to obvious scour.With the continuous influence of the large scale engineering,the ebb TCD of the upper and middle North Channel will be enhanced.The Liuyao Flood Channel may become shorter and it will receive more ebb tide current.The bank slope near the diversion channel mouths may be eroded to steeper as well.Furthermore,according to the calculation results,it can be inferred that the scour of groyne near the diversion channel mouths will be enhanced.(6)Both reaches have experienced the transition from overall deposition to overall scouring.The scour of the deep channel was significant,and the main stream moved close to the artificial river mouths,resulting in obvious scour of the nearshore channel and steeper bank slope.The change of water and sediment discharge has a relatively greater influence on the runoff-dominated reach and a less influence on the tide-dominated reach.The construction of projects in both reaches has played a key role in erosion and deposition of river channels.To a certain extent the construction of these projects has changed the hydrodynamic field,leading to the migration of thalweg,which has a great impact on the development of nearshore geomorphology. |
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