|Alternative Title||Initiation condition for debris flows generated by gully bed materials|
|Place of Conferral||北京|
|Keyword||沟床松散物质 泥石流起动 宽级配 临界条件 起动模式|
Debris flows initiated by loose gully bed materials are likely occurred in the small watershed, steep and narrow gully. For the low frequency of occurrence, long time of materials accumulation, sudden eruption and concealed areas of initiation, it is difficult to obtain the field data of debris flow initiation in gully. Therefore, this kind of debris flows has not been well studied as debris flow induced by landslide. For the obvious characteristics of great quantity and serious effects, whether the gully bed material can be initiated by hydrodynamic force and how the debris flows can be initiated are the urgent problems to solve. There are two main kinds of initiation mechanism, one is debris flow initiated for the instability of gully bed material induced by permeability. The other is debris flow initiated by the erosion of gully bed material induced by the scouring effect of flow. Whether these mechanisms can explain the processes of debris flows initiated by various types of gully bed material are lack of basis. The composition of loose gully bed material, gradient and flow volume is the key factor for debris flow initiation. Based on the flume tests, the effects of three factors on the processes of debris flow initiation are analyzed. The main conclusions are as follows: (1) Uniform sand, mixture sand of two different grain diameters, wide grading of sand, wide grading gravel soil are used to carry out the flume tests of debris flow initiation under different hydrodynamic condition. The height, velocity and sediment sorting property of debris flow head, and the change of pore-water pressure are used to discuss the relationship between the composition of gully bed material and the initiation of debris flow. The traditional viewpoints that the smaller the grain or the more the fine grain, the easier the debris flow initiation are changed. The process of debris flow initiation includes two parts: the initiation of sand grain and the formation of debris flow head. When the difference between hydro-dynamic condition and the critical condition of sand initiation is too small, sediment laden flow is formed. When the difference is too great, debris flow head is difficult to form and flooding is formed. (2)The initiation mode of sand grain rolling is built on the basis of exposure degree and gradient. The exposure degree analyzes comprehensively the effect of the diameters and mass contents of different grains. The critical condition of sand grain rolling is achieved by theoretical analysis. The critical flow velocity of sand grain rolling decreases firstly, and increases than as the increase of exposure degree or gradient (When the exposure degree is greater than 0.5). Two different scales of flume tests are used to carry out flume tests to study the debris flow initiated by different mass ratio between coarse and fine grain. The flumes tests verified the results of theoretical analysis. (3)Based on the analysis of the height, velocity, the sediment sorting of debris flow head, and the change of pore-water, the initiation mode of debris flow is classified into four groups: debris flow initiated by flow scouring action, debris flow initiated by laminated load, debris flow initiated by retrogressive erosion, debris flow initiated by flow scouring and permeability. According to the experiment phenomena and the density of debris flow, the initiation mode is divided under different hydrodynamic condition and grain composition. (4) The physical and mechanical property of loose gully bed material has great effect on the initiation of debris flow. As the mass content of particles (<2mm) increases, both the critical gradient and critical flow volume decrease at first and then increase. The wide grading gravel sand of inflection point has small repose and permeability, and the coarse grain is easily to be initiated for the great ratio between coarse and fine grain, so debris flow is easily to be formed. The theoretical model for predicting the critical gradient of debris flow initiation is built based on the combination of scouring action and shear stress action in gully bed. The critical discharge model considering the effect of the size of coarse and fine grains, non-homogeneity and continuity of soil is built and verified by field data.
|王洋. 沟床物质起动形成泥石流的临界条件[D]. 北京. 中国科学院大学,2016.|
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