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沟床物质起动形成泥石流的临界条件
Alternative TitleInitiation condition for debris flows generated by gully bed materials
Language中文
王洋
Thesis Advisor崔鹏
2016
Degree Grantor中国科学院大学
Place of Conferral北京
Degree Name博士
Degree Discipline岩土工程
Keyword沟床松散物质 泥石流起动 宽级配 临界条件 起动模式
Other Abstract

沟床松散物质起动形成泥石流,易发生在小流域、陡峻、狭窄的沟道,由于发生频率低,物源积聚时间长,突然暴发,起动区域很难确定,因此,野外实测资料难以获取,相对于滑坡导致泥石流没有得到很好的研究。该类泥石流不仅数量多,而且危害大,因此沟床松散物质在水动力作用下是否形成泥石流,以及如何形成泥石流成为亟待解决的问题。该类泥石流的起动机理主要有两种,一种是水流的入渗引起沟床土体失稳形成泥石流;另一种是水流的冲击作用导致沟床土体起动形成泥石流。这两种机制是否能够解释各种类型的沟床松散物质起动形成泥石流的过程,仍旧缺乏依据。 沟床松散物质颗粒组成、坡度、流量是影响泥石流起动的关键因素。本文通过开展水槽实验,分析以上三种因素对泥石流起动过程的影响。并结合土力学与泥沙运动力学的理论分析方法,探索沟床松散物质起动形成泥石流的机理。主要内容如下:(1)采用均匀沙、两种粒径混合沙、宽级配沙、宽级配砾石土,开展不同水动力条件下的泥石流起动实验,探讨泥石流起动过程中泥石流龙头的高度、速度及分选性、沟床内部孔隙水压力的变化特征,分析沟床松散物质颗粒组成与泥石流起动过程之间的关系。改变了平均粒径越小、细颗粒含量越多越易形成泥石流的观点。泥石流起动过程包括泥沙颗粒起动和泥石流龙头形成两部分,水动力条件与泥沙颗粒起动临界条件差距过小,易形成含沙水流;差距过大,不利于稳定龙头的形成,易形成山洪。(2)引入综合考虑床沙粗细质量比及粒径大小的暴露度和坡度影响的泥沙颗粒滚动起动模型。通过理论推导,得到随着坡度(当暴露系数大于0.5时)或暴露系数的增大,泥沙颗粒滚动起动的临界条件呈先减小后增大的趋势。并采用两种尺寸的水槽开展不同粗细质量比的床沙起动形成泥石流的实验,验证了理论分析的结论。(3)通过分析泥石流龙头高度、速度、分选性、传感器监测的孔隙水压力,将泥石流的起动模式划分为以下四类:水流冲刷作用起动、层移质起动、溯源侵蚀起动、水流冲刷与渗流共同起动。并根据实验现象和泥石流的密度,对不同水动力及颗粒组成条件下,泥石流的起动模式进行分区。(4)沟床松散物质的物理力学性质对泥石流起动具有重要影响。以2mm为粗细颗粒粒径分界,随着细颗粒含量的增加,沟床物质起动形成泥石流的临界坡度和临界流量呈先减小后增大趋势。拐点位置宽级配沙的渗透率、休止角较小,粗细颗粒的中值粒径比值较大导致非均匀沙中粗颗粒较易起动,更易形成泥石流。综合考虑水流的冲刷作用以及沟床内部剪切力作用,建立泥石流起动的临界坡度模型;综合考虑土体的不均匀性、连续性、粗、细颗粒的影响建立泥石流起动的无量纲临界流量模型,并采用野外监测数据进行检验。

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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.

Document Type学位论文
Identifierhttp://ir.imde.ac.cn/handle/131551/18920
Collection山地灾害与地表过程重点实验室
Affiliation中国科学院成都山地灾害与环境研究所
Recommended Citation
GB/T 7714
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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