Reformation Machenism And Features Of Dynamic Sediment In Flood Channels Of The Changjiang Estuary

Study of a flood channel or flood-dominated channel of estuary is not only an important content in estuarine dynamic geomorphology and dynamic sediment, but also is a practical problem nearly related to the selection of harbor sites, regulation of sea-entering waterway, bank revetment and land reclamation, and even the comprehensive exploitation of estuaries. Supported by the State Natural Science Foundation Project "Formation mechanism and evolution processes of flood channels in the Changjiang estuary", this paper study systematically the formation and evolution, erosion and deposition, and features of dynamic sediment of flood channels in the Changjiang Estuary, and put forward a development model of the different flood channels.Based on the data collecting from previous study results and measuring actually in the spring and neap tide during the flood season in 2001 and dry season in 2003, this paper proposes that the concept of the flood channel should include a more comprehensive implication. All negative landforms, which are shaped by flood-dominated current, such as souring hole, scouring channel and flood channel, are belong to the study area of flood channels. The character of flood channel can be determined by dominate current, dominate tidal volume, dominate sediment concentration, channel geometry configuration and direction of bed sediment transport and so on.Influenced by flood dominated current in the flood channels, the characteristics of surface sediments, such as grain size, light and heavy minerals, micro-paleontology and magnetism in the flood channel are very different from those in the ebb channels, this kind differences is the response for complex hydrodynamic in estuary.Changes of sediments characteristics of core A and C reflect the sediment environment varied during channel formation and development of flood channel. The open and close environment before and after the channel formation makes different sediments deposit in the flood channels. At the same time the various sediments give the important information about the surroundings difference. Although application of activity of 210Pb in determining age of sediments of estuarine and coastal environment is very difficult, it provides important information about sediment environment and hydrodynamic.Supported by GIS, quantitative changes of erosion and deposition of the Xinqiao Channel are calculated. These results combing sediment characteristics of vertical profile illustrate the formation mechanism and evolution process. Make a comprehensive view of all study results about the flood channel, a model of formation and evolution of the flood channels was proposed. According to this model, not all flood channels formed by ebb channels divided by sand spit develop toward death, some of them may be keeping the current situation or transform to the reserve channel with the change of runoff and sediment concentration from upper reach. Although lower part of the Xinqiao Channel deposits in recent years, and upper part of the Nanxiaohong Channel is developing towards the ebb channel, they will keep developing with the flood channel features according to the model.