| Yangtze estuary is the combine belt of river and sea and is the Golden coast and Golden waterway. It is the essence zone of Yangtze River basin and has great development potential. Exploitation of Yangtze estuary promotes the development of the local economy. Changes of hydrodynamics, suspended sediment concentration, characteristics of suspended sediment and bed material and geomorphic system are all related to local environment quality, eco-system safety and even the local economy development in the estuary and adjacent sea. Hydrodynamic is the driving force of sediment, ion and nutritive salt and is the source power of dynamic system. Large amounts of nutrients run into the sea with suspended sediment. In the results, the geomorphic system changes under the combined action of hydrodynamics and sediment movement. Under the influence of Three Gorges and the cascade reservoirs, the flow fluxes distribution changes and the runoff and tidal dynamics process also adjusts. So study of hydrodynamics is the base of study of estuarine sediment and physiognomy changes. Establishment of dams keeps a large amount of sediment in the reservoirs, decreases the suspended sediment in the downstream and then changes the suspended sediment in the estuary, especially in the turbidity maximum zone. At the same time, Suspended sediment and deposited sediment in the estuary adjust as the result of sediment grain size and constitution changes in the Yangtze River. Study of coarse particle and fine particle in the suspended sediment is the key point of study of how geomorphic system response to the change of water and sediment fluxes. Flow, sediment and geomorphic system influenced each other. Firstly, seawards water and sediment variations directly influenced adjustment of geomorphic system. The ratio of deposition of front tidal bar and submerged delta decreased and some of them even eroded. The annual seawards water fluxed changed little but the distribution of flow discharge changed. Present study always paid attention to the effect of great flood process. With the operation of Three Gorges and the cascade reservoirs, influence of flow distribution process will be more and more outstanding. Regulation projects in the estuary will also change the local hydrodynamics and sediment transport and have much more influence than runoff variation. Based on those, study of the combine effects of hydrodynamics, sediment transport and geomorphic system is an important issue and has great significant in estuarine study and coastal management.Based on the data of seawards water, sediment and tide, change of water fluxes and tide characteristics in Yangtze estuary is analyzed under the influence of Three Gorges and cascade reservoirs. How tidal limit and tidal current limit responses to the storage of reservoirs and how deep water regulation project affects the stagnation point is analyzed. Suspended sediment concentration, especially in the turbidity maximum zone is analyzed and also how it in North Passage response to regulation projects. Grain size variation characteristics and trend and the influence of Three Gorges and cascade reservoirs is studied. Deposition area of different sediment is affirmed. Influence of hydrodynamic and sediment transport on the boundary of sediment and soil and area of soil is analyzed. Combined with the storage of reservoirs, the evolution trend of estuarine geomorphic is forecasted. Involved hydro engineering projects contains Three Gorges, cascade reservoirs and deep waterway regulation projects in estuary. The main conclusions are as follows.1) Under the influence of Three Gorges and the cascade reservoirs, storage period of seawards water flux declines, and water replenishing period increases. Storage period of high and low tide is declines, while water replenishing period increases. And the effect of Storage period is bigger than that of water replenishing period. Tide range of storage period increases relatively, and decreases for water replenishing period. Tide limit and tide current limit curves including ratio of runoff and tide are presented, which can reflect the process of hydrodynamics changing with runoff tide. Interface position of storage period moves upstream and that of water replenishing period moves downstream. The change is not so big, but the distant of moving downstream is slightly higher than that of moving downstream. Under the influence of regulation project, the position of stagnation point in south passage has little change in years, and the position in north passage under the flow discharge moves downstream. It moves downstream from1997to2004, moves upstream form2004to2007which is influence of both spilt ratio and river channel volume.2) Sediment volume, sediment discharge, flood season and dry season at Yangtze estuary show a decreasing trend. After water storage of Three Gorges and the cascade reservoirs, decreasing trend become faster. Amount of average sediment volume into sea is no more than150million t/y; Suspended sediment concentration decreases with sediment volume into sea decreasing at Yangtze estuary south branch, south channel and north channel. North branch also show a decreasing trend under the same ratio of runoff and tide. Influence by both spilt ratio and decrease of local sediment, the sediment concentration doesn't change much. Area of turbidity maximum zone and sediment volume into sea decrease. Suspended sediment concentration declines by about21.42%during1980-2002compared with that during2003-2012. The drop range is smaller than that at Datong station, which is adjusted by re-suspended sediment and estuary erosion. Under the influence of Three Gorges and the cascade reservoirs, area of turbidity maximum zone is no more than1500km2. The center position of turbidity zone moves upstream1/6longitude. Suspended sediment concentration decrease33.25%in the upstream and downstream in North Passage because of spilt ratio and coarsening of bed material. And it increase in the middle of North Passage because of the over dam sediment and water.3) Sediment with d<0.063mm came from Yichang or upstream of it from1960to2002, and from Yichang and Yichang to Datong or branch river from2003to2012and it decrease along the river. Sediment with d>0.063mm deposited between Datong station and Xuliujing station and sediment with d<0.063deposited in front tidal bars and submerged delta. Result from the sharp decrease of sediment with d<0.063mm, the suspended sediment grain size in Yangtze estuary decreased. Deposited sediment coarsening in the downstream of Yangtze River, estuary and nearby the dams. The phenomenon moves downstream. The boundary of sediment and soil moved upstream during1991to2010because of the influence of runoff and tide changes. And it also reflects the retrogressive erosion process. Decrease of sediment concentration with d<0.063mm result in the decrease of soil area. Variation of spilt ratio changed the location of soil area.4) Bars in South Branch eroded in the front and deposited in the end from1978to2010. The thalweg deposited, the river width increased and the bars eroded. The whole branch eroded. From1007to2007, area of river channel in South Channel and North Channel increased. The great flood process transport the sediment from South Branch, North and South Channel to the sea. Front tidal bars above-2m deposited because of human activity and those above-5m eroded. The front tidal bars stop change when the seawards sediment volume is281million per year and sediment volume with d<0.063is235million per year. With the operation of reservoirs, the front tidal bars will erode. 5) Submerged delta eroded and deposited from1958to1997, deposited from1997to2000and eroded from2000to2009. The empirical curve between deposition ratio of submerged delta and seaward sediment and water is established. Increase of seawards water and sediment lead to the deposition. Sediment transported to the delta and deposited during the great flood period. Changes of flow distribution due to Three Gorges have little influence on the evolution of submerged delta. Based on the study of deposition mechanism, the submerged delta in Yangtze estuary won't erode to the degree of1958in100years and with the coarsening of bed material, the time maybe longer. |
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