Study On Mechanism Of Bank Failure In The Alluvial River

Based on fluid mechanics, sediment transport mechanics, fluvial process, soil mechanics, geomorphology etc., this dissertation gives the detailed theoretical and experimental researches on characteristics, process and mechanism of the bank failure in the alluvial river by means of the field investigation, the data analysis and the model experiment. The main parts of the dissertation are illustrated as follows.(1) Based on the field data on the bank failures, and on several types of the bank failures and their processes in the physical model, a new classification and analytic models of the bank failures were given in this dissertation. Considering soil properties, slope shape, osmotic pressure and lateral erosion of the bank etc, a general formula of the bank stability coefficient was derived on the mechanic analysis of the bank failure.(2) Influence of bed degradation and aggradation on the bank failure firstly includes that bank stability coefficient will decrease after bed degradation and bank will fail when the further degradation. Secondly, bank stability will increase after the bed aggradation. And thirdly, lengthwise cracks at the top of the cohesive bank will occur and accelerate the bank failure, and the critical height formula of bank failure was given in this dissertation for bank slopes with broken lines in the initial state and the failed state.(3) Erosion mechanism in the concave bank was studied according to the water and sediment movement and the distribution of the shearing stress in the curve reach, and the stable cross-section equation was derived for the cohesive concave bank. Bank will seriously fail at the inner of the upstream, apex and the outer of the downstream in curved reach, of which there is a greater erosion rate at the apex. The lateral erosion of the bank, especially erosion at the bottom of the bank, results in increases of motive force and its moment and decreases of resistance and its moment, which leads to decrease of stability coefficient and the bank failure. Stability of the falling failure (shear and toppling) was emphatically studied with the critical caving width, which was derived in this dissertation. A form of the retreat rate equation was given according to that bank retreat includes erosion and failure of bank.(4) According to the caving mechanism at the bottom of the bank, the stability was analyzed for the bank with the dual composition or the bank with a sand layer. There are several forms of sliding failure, sinking failure and falling failure (shear and toppling) forthe dual composition bank with some differences, and the sliding failure and the falling failure for the bank with a sand layer. The critical caving width of the falling failure was given when the shear failure or the toppling failure of the bank occurs.(5) This dissertation has studied the process and characteristics of bank failure during the flooding period and in the changing process of the water level. The internal friction angle and the cohesion of the bank soil will greatly decrease after long soil soaking during the flood period, which results in a decrease of the stability coefficient. For the cohesive bank, water pressure in the rising process of water level is effective to raise the bank stability, the water pressure in the rising process of the water level is effective to raise the bank stability, and the possibility of the bank failure will increase in the falling process. And for the non-cohesive reach, the bank stability doesn't change very much when the water level slowly rises, and the bank stability coefficient will reduce when the water level slowly falls.(6) The bank failure takes an important role in accelerating seepage and seepage failure (such as piping and soil flow) of the embankment foundation. The critical width of the beach area was given when the soil flow occurred behind the embankment or at the bottom behind the embankment.(7) Analyzed the field data on ? caving, the concrete conditions were given in this dissertation such as boundary conditions and flow conditions a...