Study On The Different Computation Patterns On Riverbed Erosion And Deposition Amount

A large amount of research on mathematical models, physical models, sediment experiments, river training, and channel regulation is based on the data of channel scouring and siltation. Consequently, the amount and distribution of deposits are important basic data in scientific experiments, project scheduling and river evolution and regulation. The section topographical method, grid topographical method, and sediment discharge balance method are widely adopted in the calculation of channel erosion-deposition amount. In theory, the calculated results of erosion-deposition amounts should be equal or much the same with different methods within the same river reach and period of time, nonetheless, researchers have found it may not be the case in production practice as well as scientific researches.In this paper, the channel erosion-deposition amount of Yichang-Jianli river reach (which is located in the downstream of the Three Gorges Dam) after the impoundment of the Three Gorges Reservoir have been calculated based on a large number of measured water and sediment data as well as topography data, by using the section topographical method, grid topographical method, and sediment discharge balance method. Some factors influencing the calculation accuracy have been comprehensively researched. The nonlinear fractal theory has been used to study the relation between bed fluctuation degree and section representative. Reasonable proposals for arrangement of observational sections in hydro logical tests and channel measurement are put forward. Specific research contents and main conclusions are as follows:(1) If we want to achieve the calculation accuracy, sections should not be too few in contractive and expansive reaches, as well as bend and braided reaches. When using section topographical method, the distance between sections should be less than2times of river width. The difference between cone and trapezoid method in the calculation of sectional area is small, and both can meet requirements.(2)As grid dimension increases, the true terrain of river channel flats, subtle changes of local topography are weakened, storage capacity under the same water level reduces, and river channel erosion-deposition amount decreases. The grid dimension should not be larger than the distance between measuring cross-sections in calculation when topographic map is used. For topographic map with scale of1:10000, the grid dimension should be less than60m; and for topographic map with scale of 1:10000, the grid dimension should be less than30m; DEM Kriging interpolation is better than the others, and can simulate the original topography relatively exactly.(3) According to the observation data of close-to-bed suspended load from the hydrology bureau of the Changjiang Water Resources Commission, the problem of suspended sediment miss-measurement in the downstream of the Three Gorges Dam is discussed. Miss-measurement exists in the conventional suspension load test in the downstream of the Three Gorges Dam. In addition, accurate amounts of sand mining and sediment diversion are required in sand mining river reaches and distributary channels respectively. After the measured results are modified, the accurate results can be got by the sediment discharge equilibrium method, otherwise, the erosion-deposition amount will be obviously less.(4) The distance between fixed cross-sections in Jingjiang reach is large, and the fixed cross-sections in bend and braided reaches cannot control the terrain variation, thus the results of the two calculational methods have great difference. The representative of fixed cross-sections in local reaches of Jingjiang is not strong, and it’s need to increase fixed cross-sections and adjust the direction of some cross-sections. The difference percentage of erosion-deposition amount has a certain relationship with fractal dimension of bed surface. The difference of results between the grid method and the fixed cross-section method is bigger, as the fractal dimension of the bed surface increases.(5)The precision of three calculational methods are comprehensively evaluated. The main factors influncing the calculation accuracy of sediment discharge balance method include low sediment concentration, miss-measurement of close-to-bed sediment load in the scouring reach, the error of sand mining and the system error of the conventional sediment discharge. The main factors influncing the calculation accuracy of grid topographical mothod mainly include the topography measurement accuracy, especially the measurement accuracy of water depth and some local areas which are difficult to measure, as well as the grid interpolation method and the resolution ratio. The main factors influncing the calculation accuracy of section method mainly include the representative of the cross-section and the measurement error of section distance, apart from the cross-section measurement accuracy and water depth measurement error.