Study On Erosion Potential Of Yangtze Subaqueous Delta

In recent decades, many deltas in the world have suffered erosion because of the decreasing of sedimen flux to the sea that caused by human activities such as the operation of dams. The change of delta evolution affcts a lot to the ecological environment and social economy and becames a core content of many international research programs. Under this context,the erosion rate and the limiting erosion (the limit condition of erosion) are important scientific topics worth researching.Whether the erosion has occurred or not and how to determine the erosion rate, that depending on the contrast between the erodibility of sediment and the hydrodynamic of the estuary. These factors are various at different deltas or different phases of one delta. It is very important to study the classic case.As the third longest river in the world, the Yangtze Delta is an important economic region in our country and even in the world. In recent years, the suspended sediment supply from Yangtze has drastically decreased as a result of human activities, such as the operation of dams. Under this context, the temporal trend of accretion/erosion in the Yangtze subaqueous delta is an important scientific topic worth researching. Although a lot of researches about the evolution have been done, the research on the erodibility of subaqueous delta sediment in large scale is relatively rare, especially for the study of erosion potential under the context that the suspended sediment supply from Yangtze has drastically decreased as a result of building stairstep reservoirs in Jinsha River, South-to-North Water Transfer Project and Three Gorges Reservoir. The large-scale erosion of Yangtze subaqueous delta will not only endanger the seabed engineering (such as hallow buried cable, pipeline and the pier of Donghai Bridge), but also reduce the area of wetland and threat to the sea dike. Therefore the research on the erosion potential of subaqueous delta is very important to theoretical significance and application value.Surface sediment samples at large sacle and core sediment samples at typical area of Yangtze subaqueous delta have been collected under the patronage of NSFC, and the grain size, water content of the sedment samples have been analysed in the lab, then calculated the critical shear stress for erosion of the sediment by using Van Rijn (1993) and Taki (2001) formul which generally recognized by other scholars and analysed the distribution of the critical shear stress for erosion in the plane and vertical. By calculating the bed shear stress per ten minutes in full tidal hours with hydrographic data and comparison between critical shear stress for erosion and bed shear stress, the erodibility of the bed has been estimated. On the basis of the analysis, the erosion potential has been forecasted. The main results are as following:1) The surface sediment from Yangtze Estuary to the offshore area showed an obvious spatial pattern that the grain in rivermouth area is finer than upper area and offshore area. The median size (d50) of surface sediment in estuary area ranged from4.9to227.9μm,45.7μm on average, and in offshore area ranged from5.1to293μm,111.8μm on average. The composition of surface sediment in upper reach of North Branch, North and South Channel and the hole South Branch was mainly sand (d50>63um). And in lower reach of North Branch, North and South Channel, the whole North and South Passage and the offshore subaqueous delta area, the composition of surface sediment was mainly mud (dso<63μm). The surface sediment in northeast was coarser than in southwest of offshore area. The low-value of area showed at the front of subaqueous delta outside the mouth of North and South Passage which is the main subsiding area of suspended sediment at Yangtze Estuary. The median size (d50) of core sediment ranged from4.1to173.3μm,19.2μm on average. The median size of core sediment fluctuated with small amplitude in vertical direction, and there was no coarsening characteristic for most core sediment.2) The water content (the ratio of water weight and dry weight of sediment) of surface sediment in Yangtze Estuary ranged from22.3to88.9%,45.1%on average.The low-value water content area was the main subsiding area of suspended sediment at the front of subaqueous delta. In high clay concentration area, the water content of surface sediment was higher (the coefficient of correlation was0.635) The water content of surface sediment in offshore area ranged from25.4to106%,45.7%on average.The water content of core sediment ranged from22.7to79.8%, 49.9%on average. The water content of core sediment fluctuated with small amplitude in vertical direction, decreased a little from surface down.3) The critical shear stress for sediment erosion in Yangtze Estuary ranged from0.079to0.174N/m2,0.11N/m2on average. The critical shear stress for sediment erosion in offshore area ranged from0.075to0.195N/m2,0.129N/m2on average.The τcr at the front of subaqueous delta outside the Sorth Branch system (North Channel, North and South Passage) was much lower than inner estuary and relict sandy area. The critical shear stress for sediment erosion of core sedimen ranged from0.082to0.153N/m2,0.101N/m2on average, there was a small amplitude increase trend from surface to bottom for most core sediment.4) The rotating flow at the three typical hydrodynamic measurement stations at the front of subaqueous delta was evident.The vertical average velocity ranged from0.28to1.53m/s and the bed shear stress ranged from0.002to1.398N/m2.In spring tidal period, the average bed shear stress was0.139N/m2at S1,0.309N/m2at S2,0.302N/m2at S3. In neap tidal period, the average bed shear stress was0.074N/m2at S1,0.224N/m2at S2,0.180N/m2at S3. The bed shear stress was mainly larger during spring tide than neap tide, and fluctuanted over several orders of magnitude in a tidal cycling. The comparison between critical shear stress for sediment erosion and bed shear stress at typical positions of subaqueous delta showed that there were both τc>τcr period and τc>τcr period in a tidal cycling. The τc was larger than τcr at most time.5) Combining to the critical shear stress for sediment erosion at Yangtze subaqueous delta, the bed shear stress at normal weather, a large quantity of hydrodynamic measurement by other scholars, it could be inferred that the subaqueous delta sediment (at least within2m in thickness) could be eroded under the condition of high velocity phase of a tidal cycle (erosion increases under the storm conditions). Obviously, suspended sediment could also fall deposition in the low velocity phase of a tidal cycle. In the long time scale (years or decades), whether the bed erosion occurs or not depends on the balance of erosion and sedimentation (which is positively correlated to suspended sediment concentration) in the short scale (tidal cycle). According to (1) the erosion rate is several centimeters per year at the front of Yangtze subaqueous delta,(2) the critical shear stress for sediment at the study area changes a little in vertical (within2m in thickness),(3) the suspended sediment supply from Yangtze will keep decreasing, so the erode will last and even get stronger. Though at the inner estury area, the bed could be eroded under the condition of high velocity period, but the suspended sediment could fall deposition at long slack period, result in a balance between erosion and deposition in long-term scale (The deposition flux is even larger than erosion flux in deep-water channel projectat the North Passage).These conclusions are very important for long-term coastal protection and harbor engineering.