| With approximately 44% of world’s population living within 150 km coastline, coastal retreat influenced directly by the reduction of river-supplied sediment has been a wild concern in the world since 1900s. Thus, the core project of International Geosphere Biosphere Programme(IGBP) and Land Ocean Interaction in the Coastal Zone (LOICZ) has been to survey diverse factors on sediment flux reduction. Considering the inner aggregation and buffering effects of large basins, the study on morphodynamic process and self-adjustment mechanism of coastal and estuarine areas is currently assuming increasing importance at the global scale. Over 100 billion metric tons of sediment has been sequestered in reservoirs constructed largely within the past 50 years, and this situation is more obvious in African and Asian rivers. Ranking first and third in China and the world respectively regarding to its river discharge, the Yangtze River Estuary experienced great changes in river discharge and sediment load under the influence of Three Gorge Dam and other marine projects, thus the water, sediment and topography would adjust themselves to adapt to these changes. The study on morphodynamic process and self-adjustment mechanism of coastal and estuarine areas is undoubtedly of considerable realistic significance.Yangtze River Estuary and channels’linear and nonlinear self-adjustment processes under the influence of artificial regulation engineering and basin reservoirs have been analyzed based on the maritime charts since 1997 and recent field works. Firstly, whole estuarine erosion and deposition patterns, the detailed mophodynamic evolution process of typical area were analyzed. Then based on fractal geometry, whole estuary topography flow paths’fractal dimension values were calculated, and stability analysis for the active shaped bars influenced by local engineering was conducted. The conclusions are as follows,The erosion and deposition patterns of whole Yangtze Estuary from 1997 to 2013 indicate that the morphology features slightly erosion in upper estuary, lasting deposition and seaward move for the mouth bar crest and strengthening erosion at the nearshore seabed, the dominant cause is changed from nature factors to intensive human activities. Firstly, the riverbed in South Branch (SB), South Channel (SC) and the upper and middle reaches of North Channel (NC) are eroded overall, the sediment above bed is very active, surface sediment is coarsened, bed surface sand dune is becoming more distinct, and the nearshore seabed outside river mouth is being eroded slightly, which are attributed by river runoff and sediment load directly. Secondly, there is a deposition in the reaches of North Branch (NB), the mouth of NC and the mouth bars of North Passage (NP) and South Passage (SP), the reasons for which are not only the dynamic structure in estuarine mouth bars, but also the supply of sediment resuspension in local area and offshore area. By the way, the severe erosion and deposition in some reaches have relationship with estuarine engineering construction. It indicates that Yangtze River estuary is in slow adjustment and adaptation with changing natural factors and intensive human activities recently.The evolution process of the South Branch shows stable water flow regime and continuing riverbed’s erosion since the completion of the Three Gorge Dam project with decreased sediment discharge and almost unchanged river discharge, while the erosion intensity has become smaller since 2006 and the riverbed attains a new quasi-equilibrium state. The erosion volume below 5 m contour accounts about 90% of total erosion amount, and it shows a feature of "erosion in channels while deposition on shoals" with deposition on shoals since 2007. Changes of sand dunes exert significant influence on the channel stability in the South Branch. In order to prevent frequent changes of erosion and deposition in the South and North Passage bifurcation, the Xinliuhe shoal protection and Nanshatou channel submerged dike projects were constructed between 2007 and 2009. Although the project has made great progress in fixing shoals and limiting flow, it still cannot prevent the trend of riverbed’s intense erosion in the front areas of Xinliuhe shoal and Nanshatou channel. And the flow state around the Baimao shoal will be continuous and strengthened, growing stronger in the south waterway and weaker in the north waterway. The split ratio will keep below 30% in the north waterway of the Baimao shoal, while the mainstream tends to be shifted northward when passing the Qi-Ya-Kou. As the end portion of Biandan shoal has shifted downstream and extended to the Xinqiao waterway, which is from the south part of Biandan shoal in the middle reach of the South Branch. At present, the end portion of Biandan shoal becomes very active and experiences several erosion and deposition process.The mouth bar apex moves seaside coutinously and the 5m bathymetric contour of shoaly land shows same general seaside movement but in a slower speed rencently. Meantime, certain areas along the 5m bathymetric contour have been eroded retrogressed, which is related to estuarine projects.For the whole offshore area, the erosion situation mostly occurred after 1997. Obvious erosion occurred in the seabed out of the NB mouth bar area and thus leaded to deeper slope than before. Generally, the seabed area out of the NC mouth bar area changed from deposition to erosion trend between the 10 m and 20 m bathymetric countours, while the area deeper than 20 m bathymetric shows continuouse deposition. Being in the same situation, the seabed area out of the NP mouth bar area also changed from deposition to erosion with a slight deeper slope. For the the seabed area out of the SP mouth bar area, it shows slight change with little erosion after 1997. In a word, the seabed experienced erosion with a deeper slope recently.Based on the digital nautical charts data and the box dimension of fractal geometry, the stability for active shaped bars in the South and North Passage bifurcation area under the influence of recent near-shore projects (Xinliuhe shoal protection and Nanshatou channel submerged dike projects, Qingcaosha reservoir and the Central sandbank reclamation projects) is analyzed. The results indicate that the area’s fractal dimension value of the active shaped bars at -5 m contour within study area ranges from 1.7538 to 1.7823 during 1997 to 2012. The value increases obviously during the projects (2007-2009a), which is much higher than that before and after the projects. Variation of the value D before the projects is influenced by the 1998 flood and retention of the Three Gorge Dam, but it is overall coordinated with the river and sediment discharge. After the projects, the value D decreases and tends to stabilization, but it is higher than that before the projects, which means the shaped bars are still very active after the projects and in the self-adjustment process. Particularly, the larger amount of the river discharge from upstream results in the lower stability and the higher value D of the active shaped bars, moreover, the value D could also be enlarged by the drastic changes of riverbed and hydrodynamic during the projects. Conclusively, the higher energy results in higher fractal dimension value. Compared with the traditional methods, including the deposition and erosion amount calculation, the depth contour changes, and the cross-sectional studies, fractal dimension value D could show the whole changing process of the active shaped bars within the study area to some extent. Therefore, as a new stability analysis method, it could provide certain reference significance to the stability analysis for the active shaped bars under the influence of near-shore projects.We also conducted some studies to pick out the relationships of fractal dimension values, boxing sizes, confluence cumulative threshold of the whole Yangtze Estuary flow paths. The no scale interval of boxing sizes is 1.6-4.2 km, and the optimum confluence cumulative threshold is 7000. The fractal dimension values of the whole Yangtze River Estuary flow paths increased obviously from 1.1312 to 1.2222 and 1.3805 in the year of 1900,1997 and 2013 respectively, which indicated the flow paths became more complex than before. It may be directly related to the unstable river discharge and massive marine projects.In summary, the Yangtze River Estuary is currently in the process of adjusting its channel geometry, sediment carrying capacity, riverbed sedimentation composition, land-ocean mutual functions and riverbed slopes, thus to meet the influences of river discharge and sediment load changes and estuarine marine projects recently. |
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