The Fluvial Bedform And Hydrodynamic Controls Along The Middle And Lower Yangtze River(from Wuhan To Estuary)

As the linkage between land and sea, river is the branch in land of the global water cycle and also the most important part of Land Surface Process which involved in the Global Change. Rivers play a very important role in ecosystem and economics, especially in the blooming China now. Proportion of sediment and water is the key to learn about the environment change in the fluvial basin. Fluvial sediment is a representation of portion of the sediment and water. Fluvial bedform is the movement of bed-load. Research on the distribution and controls of the bedforms in different flux is a way to understand the transfer and aggradations of fluvial sediments.The Yangtze river is located in the east coast of the Euro-Asian continent which is sensitive in reflecting the global change. In the Yangtze drainage basin, the climate is influenced by the interaction of seasonal monsoon, meanwhile the rainfall is influenced by the summer monsoon. The intensive human activity as the Three Gorges Dam will affect the environment of the middle and lower Yangtze area. It is interesting thing to study the circumstance change between the pre-Dam and the post-Dam periods. In this thesis, a database was built including the digital pictures of the bedforms and the topography of the dunes.According to the database, four types of bedforms (Type I, II, III and IV) were recognized. Height of Type I dunes were over 4m.Type II was over 2m and below 4m. Type III was below 2m but could be visible recognized from the sonar image. The plane bed was treated as one part of Type IV. Considering the vertical precision (7.5cm) of the sub-bottom system, we classified these ripples that can not be recognized from the image into Type IV with plane bed.100 fluvial transects with 10-km interval along the river channel were sampled. Grain size was tested in the laboratory. Relevant hydro-morphological data including river width and water depth (bankfull and mean discharge), and water slope were also calculated.Data in given graph clustered well. Five distinctive clustering zones (A-B-C-D-E, from upstream to downstream) were observed in representative graphs. Zone A have two type bedform: the bigger lunate type which is over 5m height and lengths 20-70m; the smaller sinuous transverse (crest-parallel) type which was under 2m and shorter than 10m. Dunes (Type I and II) in Zone B were in the length of several to 60-70m but smaller than Zone A. Large-scale compound dunes occured in the Zone C.The underlay big dunes hold the height of 5-7m and length 154-267m. The upper small dunes was 2-2.5m high and 10-20m in length (057/056 sample transect). Generally speaking, the lower one was more symmetrical than the upper ones. The scale of bedforms as a whole reduced in Zone D which is 5.6-7.7m in height and 56-102m in length. Lunate and sinuous transverse dunes appeared contemporarily in Zone D and were more symmetrical than the upper Zones. Zone E diminish in bedform scale which was symmetrical and 1.6-2.4m in height and 140-155m in length.The different bedform characteristic in Zone A-E rooted in the variable hydrodynamic environment. According to the ADP velocity, some classic fluvial hydrodynamic parameters were simulated in study area. Froude Number almost did not change in Zone A-C (about 0.1). It dropped dramatically below 0.08 and to 0.02 in Zone E. Boundary Shear Stress got the maximum in Zone B. The simulated velocity in the flux of 60,000 m³s^-1 reduced from Wuhan to the estuary. It was 2.2-3.5ms^-1 in Zone A and deduced to 1.0 ms^-1 in esturary. The Unit Stream Power ω had the same trend with T but get in top earlier in Zone B. Renault Number increased continuously in Zone A-C and reduced in the last D-E. Renault Number got the peak in the Zone C. The experiential index T which can show the bed-load transfer got the average of 10.6. T reach the peak in Zone C (14.7).In the light of the field data from ADP and Geochrip and the simulated parameters, it is possible to search for the reason of the regional difference of the bedforms. Zone A is the transition between the mid-Yangtze meandering channel and the branching river channel in the lower-Yangtze. The channel narrowed by the lower Quaternary terraces especially the bedrock and node in the Zone B (Huangshi-Wuxue). Blocked floods by the narrow channel caused the lower water slope and the mass deposition of sediments which offered the large-scale dunes (I and II). High R.S.I (Ripples Symmetrical Index=3-4.1) indicated the strong movement of bedload. Dunes developed in high-energy areas in Zone B but it was smaller than these in Zone A. Zone C was the typical branching river channel with bigger cross section area, higher water depth , river width but obvious lower water slope. The highest curvature of the river channel in Zone C (average 1.32) accompanied with abundance meanders and sand bars in the channel. Despite of the high T and Renault Number, Zone C was generally calmer than B in hydrodynamic environment. Intense bedload transport can be seen in the R.S.I (3-3.5). The compound dunes indicated obvious sedimentmovement in flood and lower water period. The 1.03 curvature of the river channel in Zone D show the straight river channel pattern. Parameters fell down in this Zone with smaller but more symmetrical dunes. When Yangtze went into estuary in Zone E, fluvial and tidal stream interacted. Bidirectional stream breeds the symmetrical dunes. Type IV bedforms bloomed in here as the biggest cross section area and river width.It is important to say that large-scale bedforms (type I and II) is inclined to locate in the areas where the slope is 0.2-2.0 × 10^-5 and cross section area 10,000-35,000 m². Two grain-size regions (0.02-0.05 mm and 0.15-0.35 mm) are more easy to yield large-scale bedform, especially the latter one. As the import role of floods to form dunes, velocity in higher flux was simulated to find the potential linkage. Under the flux of 60,000 m³s^-1 velocity of 2.3-3.5 ms^-1 is needed in Zone A-B and 1.2-2.2 ms^-1 in Zone C-D.The Renault Number and T but not other parameters is more dependable to indicate the large-scale dunes in the mid- and lower Yangtze River. ADP velocity almost did not change dramatically in 1000 km study area.The diversity of local geology and geomorphology lead to the difference between diverse river reaches. This research calculated the size, charactered the plane geometry of bedform . It is concluded that the distribution of the bedform is a good way to learn about the portion of the transfer and aggradation of sediment in the River channel.