Morphological Change Of Estuarine Channel In Response To Human Activities

The morphological changes and bedforms of the channel beds have been the foci of several scientific enquiries.With the increase of the population,demands on water and land resources is becoming intense increasingly,hence it is critical to manage large rivers and estuaries in an effective and sustainable way,and the prediction of flow,sediment transport and both channel bed and bank stability.The study focus on the morphological changes and bedfroms of the channel beds has important significance for exploring the flow and sediment conditions.Human activities have pronounced effects on the hydrodynamics and morphology of the large-scale alluvial rivers and estuaries in recent years.It is well known that the sediment supply at Datong station showed a declining trend in the recent years because of the construction of the Three Gorge Dam,and many river engineering practices in the Yangtze River estuary,e.g.channel deepening,and levee and port construction,have greatly changed the morphology and topography of the river main and distributary channels.These results in the morphological changes and bedforms of the channel beds constantly adjusted and adapted.During the past century,more than 50,000 dams have been constructed in the Mississippi River Basin,which led to sediment discharge of the Mississippi River has decreased by over 50%,especially the Old River Control Structure(ORCS)(RK 500),whcih significantly influnence the hydrodynamics of the Mississippi River estuary.Multi-beam bathymetric measurements and subbottom profilers made in 2012,2014 and 2015 were utilized to investigate microtophographic bedforms of the Yangtze River estuary,while the multi-beam bathymetric measurements during 2012 over four 1.6-8 km long reaches in the Mississippi River estuary were used to analyze the riverbed micromorphologic features.Bottom sediment samples were collected for grain size analysis to assess the physical properties of the bedforms.The digitized bathymetric data were used to assessed the morphological changes of the estuarine channels.The study draw a systemic comparative analysis of current development and morphological change of bedforms in the Mississippi River estuary and the Yangtze River estuary and explore the response of the morphological changes and bedforms of the channel beds in the large-scale estuaries to the human activities.The main conclusions are as follows.1)The estuary channel and submerged delta experienced erosion because of intense human activities.As the scouring trend continues,the upper reach of the mouth bar area also experienced erosion.Declining of the sediment supply will induce moving upstream of the submerged delta.Therefore,the length of the mouth bar will be shortened.2)Under the influence of the Three Gorge Dam(TGD)which began to impound water in June 2003,the sediment supply in the past decade showed a declining trend,which may have contributed to bed erosion of the South Branch,the South Channel and the middle and upper reaches of the North Channel.After execution of the Qingcaosha Reservoir project project started,a large part of the upper North Channel was confined by levees,causing narrowing of the channel from 7.1 km to 4,3 km.This has caused large channel incising and bed scouring.As the scouring trend continues,the upper reach of the mouth bar area also experienced erosion.During 2002-2007,net volume loss and average annual net volume loss reached 2.3 ×108 m3 and 0.46 × 108 m3 yr-1 in the South Branch,respectively.For the South Channel,during the period 2002-2013,the bed net volume loss and average annual net volume loss reached 1.36 × 108m3 and 0.12 × 108 m3 yr-1,respectively.During 2002-2013,net volume loss and average annual net volume loss reached 6.54 × 108 m3 and 0.65 × 108m3 yr-1 in the North Channel,respectively.From 2002 to 2013,net volume loss and average annual net volume loss reached 0.86 × 108m3 and 0.08 × 108m3 yr-1 in the Hengsha Passage,respectively.A large amount of the eroded sediment from the south entrance of the Hengsha Passage was trapped,resulting in substantial accretion in the Yuanyuansha Waterway and the upper reach of the North Passage because the construction of the Qingcaosha Reservoir project and the Deepening Waterway Project may have indirectly affected the morphology of the Hengsha Passage.During 2002-2013,intense channel erosion occurred in the middle and upper reaches of the South Passage,while gradual accretion occurred in the lower reach because of construction of the Deepening Waterway Project,and the net volume gain and average annual net volume gain reached 0.68 × 108m3 and 0.062 × 108 m3 yr-1,respectively.The increasing saltwater intrusion found in the Yangtze River estuary may be a result of deepening of channels due to either dredging or erosion.3)The sea-bottom sediments distributed in.Yangtze River estuary is mainly composed of clayey silt,silt,sandy silt,silty sand and fine sand.Clayey silt is the dominant sediment type(30%of all 169 sea-bottom sediment samples)and is primarily deposited in the lower reaches of the Yangtze River estuary(Turbidity Maximum Zone),including the North Passage(14%),middle and lower reaches of the South Passage(5%)and the mouth bar area of the North Channel(8%).Fine sand(20%)is present in the middle and upper reaches of the Yangtze River estuary,Hengsha Passage and the primary channels of the Turbidity Maximum Zone,with the medium size in the range of 130-178 ?m and 105-237 ?m in the middle and upper reaches of the South and North Channels,130-200 ?m in the Hengsha Passage,136-204 ?m in the upper reaches of the South Passage,120-175 pm in the mouth bars area of the North Channel,and 144-165 pm in the local of the North Passage,respectively.Due to the sediments discharge from the Yangtze drainage basin rapidly decreased and the implementation of the large-scale estuarine engineering within the North Channel,especially the construction of the Qingcaosha reservoir project in June of 2007,there was significant erosion in the middle and upper reaches of the North Channel,and local erosion occurred in the mouth bar are'a of North Channel,resulting in a significant coarsening of the bed material in the area.Due to the construction of two training walls and ten groins within the North Passage,the ebb flow diversion ratio of the North Passage and the South Passage changed significantly and the ebb flow diversion ratio of the South Passage increased significantly.Consequently,the upper reach of the South Passage experienced significant erosion,resulting in a significant coarsening of the bed material in the area.And the hydrodynamic of the local area of the North Passage changed obviously because of the dredging project,which may be associated with the coarsening of the bed material in the local area of the middle and lower reaches of the North Passage.4)In addition to four kinds of common bedforms:smooth surface,gully,scour mark and dune,there are also two kinds of bedfroms under the human intervention:dredging mark and hollow.The gully mainly developed in the North Channel and the scour mark mainly occured in the Hengsha Passage.There is a large range of dredging marks and hollows distributed in the North Passage.5)There was a large area of dunes developed in the middle and upper reaches of the North and South Channles and in the Hengsha Passage.Much larger dunes were detected in the upper and middle reaches of North Channel,much small dunes occurred in the upper and middle reaches of the South Channel,while even smaller dunes were measured in the Hengsha Passage.There is little difference in the geometry of these dunes.Dunes in the Yangtze River estuary can be classified in four major classes:very large dunes,large dunes,medium dunes and small dunes.Large dunes were predominant,amounting to 51.5%.These dunes varied from 0.12 to 3.12 m in height with a wide range of wavelength from 2.83 to 127.89 m,yielding a range in height to wavelength of 0.003-0.136.There was a relatively close relationship between dune height and dune wavelength.6)Dunes in the Mississippi River estuary can be classified into four size groups:very large dunes,large dunes,medium dunes and small dunes.Large dunes were predominant,amounting to 70.3%.Dune size and bed roughness differed between the straight reaches and meandering bends,as well as between the upper and the lower river reaches.When compared with the dunes in the straight reaches,the dunes in the bends were much larger and had higher bed roughness(0.91 vs 0.68),indicating the impact of turbulent flow on dune formation.Dune size increased with increasing water depth at the cross section of a straight reach,while it decreased with increasing water depth at the cross section of a bend.When compared with the dunes in the lower river reach,the dunes in the upper river reach were significantly larger and showed much higher bed roughness(1.12 vs 0.68),suggesting the impact of slope and sand source on riverbed deposition.A clear difference in the geometry of dunes was found between the lower and upper river reaches,but no difference was found in the dune geometry between the straight reaches and the meandering bends.Across these river reaches,there was a relatively close relationship(r2=0.76)between dune height and dune wavelength.7)The paired Student's t-test showed statistically significant differences(p<0.0001)between dune size,the geometry of dunes(height/wavelength),and symmetrical characteristic(stoss slope/lee slope)of the Mississippi River estuary and Yangtze River estuary.Compared with dune size in the Mississippi River estuary(i.e.mean length and height of 17.62 m and 0.87 m),much larger dunes(i.e.mean length and height of 20.08 m and 0.91m)developed in the Yangtze River estuary.There is little difference occurred in the dunes roughness.It is likely that the dune size of the Mississippi River estuary will increase in the future because of channel-bed erosion caused by backwater effects in the large lowland river,while the dune size also will growth in the future in the North and South Channels of the Yangtze River estuary because of intense human interventions.