Preliminary Study On Quantization Of Channel Form Adjustment And Its Influence On Resistance

As the boundary of river water and sediment movement, the characters of Riverbed are movable and complexity of its 3D space structure. The variety of river bed form and the problems such as flow structure, channel resistance and sediment transport capacity adjustment which is caused by its shape changes are the contents of fundamental research in the river dynamics field. For the channels downstream reservoir, the changes of water and sediment conditions will lead to erosion adjustment of their bed form, which will impact the flood control, irrigation, navigation and many other implications of the river. Therefore, it has important theoretical and engineering value to study erosion adjustment of channel form and some related effects.In this paper, GIS technology and theory of non-linear-fractal are introduced into the river dynamics, the law and measurement method of river bed form erosion adjustment is studied by the approach of observed data analysis and generalized model experiment, and on this basis, the river-type classification, channel resistance estimation, as well as the river bed erosion adjustment and related effects on resistance of the river downstream Three Gorges Project (TGP) were also discussed, the specific contents and main results are as follows:(1) The bed form features of different typical braided river and the law of its erosion adjustment are analyzed by the observed data analysis. In the case of strong constraint boundary, the commonness of erosion adjustment among different types of braided river is erosion of the inlet, and each inlet has a unbalanced turn of erosion, in addition, the central bar and sand bar nearby convex bank often will be a degree of erosion; the difference is that the relative erosion rate of the two branches is different. Although the larger one is still after the local flood dynamic axis, the results are also different, the diversion pattern of the alternate braided river should be changed, the one of non-alternant braided river will be stability. (2) The riverbed surface fractal dimension (BSD) is put forword to measure river bed. Based on improvements in some aspects of Surface area-Scale Method, such as, estimation of surface area, boundary treatment and judgment of non-scale range, and GIS technology was combined to bring up a method on the bed surface fractal dimension calculation that the data sources is DWG format underwater topographic map. The new method is effective and accurate, and has good applicabiliton on the bed surface fractal dimension calculation. The fractal characteristics of river bed surface morphology is discussed by combination with river-pattern, local river regime and river process of typical reach.(3) Based on the combination of general model about different river patterns and observed data, the relationship between bed surface fractal dimension and the adjustment of bed form is studied. There is whole and part relationship between riverbed surface fractal dimension and flat shape, cross-sectional shape as well as longitudinal profile form of the river, the two-dimensional profiles reflect changes in bed form only from a different side or in part, therefore, the bed surface fractal dimension can describe overall adjustment intensity of river bed surface morphology, and reflect characteristic of the river process sensitivity. As different river has different bed form, the river bed surface fractal dimension can reflect the difference among river-patterns, even subclass to some extent. The bed surface fractal dimension should be a distinguished criterion of river-pattern after gather more actual information and unified fractal algorithm. Meanwhile, the fractal characteristics of the Yangtze River middle reaches is studied, the thelweg longitudinal profile profile fractal dimension (TD) can quantitatively reflect thecomplexity of river longitudinal profile sharp.(4) The law and mechanism of resistance changes in the adjustment process under the variational factors is studied. The increase of intensity of flow conditions or the decrease of base-level can make the range of river bed changes greater, and exacerbate the adjustment of channel resistance; the resistance adjustment process of different river pattern are basically similar, there is a first increases and then decreases in the basic process, compared with straight river pattern, as the complexity of the flow structure and rapidity of bed deformation in braided river and bending river, their resistance adjustment process is faster and the variation range is bigger. There is a positive relationship between river bed surface fractal dimension and the bed roughness. The quantitative relationship between them is obtained based on the experiment, the quantitative relationship can be applied to estimate changes of roughness in the same river.(5) The mechanism of low water level changes in Yichang was analyzed and discussed from distribution of erosion and river morphology adjustment on Yichang to Yangjianao channel after the impoundment of Three Gorges Project(TGP). There is a closely relationship between distribution of erosion along the river and the channel morphology on this channel, in comparison, the erosion intensity of bending reach and the branching reach is greater than the overall average; the longitudinal nodes of the channel are mostly braided reach, and the node of alternating-type braided reach is more stable than the one of non-alternating type braided reach; due to the increase of thalweg longitudinal profile fractal dimension in the long channel, the general increase of riverbed surface fractal dimension in node reaches,and coarsening of bed material, the bed resistance downstream TGP has increased, and the increace played inhibition to this low water level decline in Yichang; in the case of in downstream water level can not decline furtherly, Yichang low water level will remain stable with some engineering measures be carried on.