火后泥石流成灾的时空演化机制与防控方法研究

The post-fire debris flow is one particular type of debris flows, which is closely associated with forest fire and always occurs in burned area. Because plenty of vegetation has been burned and the soil texture has been damaged by the high temperature of forest fires, the unit weight, pore porosity, permeability and other physical and hydrological properties of the fire-infected soil are changed dramatically, abundant ash layer and loose debris are generated. Therefore, the post-fire debris flow usually has the characteristics of high unit weight and viscosity. Correspondingly, the post-fire debris flow is distinguished from traditional debris flows distinctly in term of the initiation mechanism. So far, the research on the initiation mechanism and kinetic characteristics of post-fire debris flow home and abroad is still progressing slowly. The spatio-temporal evolution mechanism and control methods, as the key scientific issue of the post-fire debris flow, will be analyzed in this project. Taking several post-fire debris flow ravines in Sichuan province for instance, through field investigation, in-situ tests and indoor physical modelling tests, the structural features of soil in the burned area and the types of activated loose source material, as well as the physical properties and permeability of soil at various depth in different time scale and spatial location are studied systematically to reveal 1) the geological structure combination conditions, which controls the initiating mode (eg. runoff-dominated erosion and infiltration-dominated shallow landslide); 2) the spatio-temporal evolution; 3) the dynamic characteristics and block-collapse effect of this kind of debris flow. Furthermore, the control methods of such kind of debris flow will be put forward to provide some scientific suggestions and designing parameters for the mitigation engineering of the post-fire debris flow.

火后泥石流是指林火发生后火烧迹地附近发生的、与林火紧密联系的泥石流。作为泥石流地质灾害的另一特殊类别，因林火燃烧植被及其高温破坏土壤结构,导致烧焙影响层容重、孔隙率、渗透性等物理水理性质发生剧烈改变，而出现大量灰烬层和松散泥沙，形成的泥石流常表现出高容重、大粘度流体特征，其成灾机理与普通泥石流具有显著差异。目前国内外有关火后泥石流启动成灾机理及动力学特性研究仍十分薄弱。本研究以火后泥石流成灾的时空演化机制及防控方法这一关键科学问题，以四川省内数条典型火后泥石流沟为研究对象，从不同时间尺度、不同空间位置火烧迹地土壤层地质结构特征、对应不同影响深度下的物理力学性质和渗透特性入手，通过现场勘探试验和室内物理模拟，揭示以“坡面地表径流”和“浅表层滑坡”为启动模式的地质结构组合条件、时空演化机制、此类泥石流动力学特性及其堵溃效应，进而提出相应的防控方法，为火后泥石流灾害的防治和参数设计提供科学依据。

火后泥石流是发生于火烧迹地与林火密切相关的一类特殊泥石流，以林火燃烧植被及其高温破坏土壤结构、强烈的坡面侵蚀和大粘度流体特征为主要特点，其时空发育特征与林火烈度和火后植被恢复密不可分。本研究以火后泥石流成灾的时空演化机制及防控方法这一关键科学问题，以四川省凉山州、甘孜州近80余条典型火后泥石流沟为研究对象，从不同时间尺度、不同空间位置火烧迹地的孕灾条件、火后泥石流成灾机理、易发性预测模型和综合防治方法进行研究，获得如下成果：.（1）针对火烧迹地土壤取样开展的物理、水理和力学性质，揭示林火对土壤结构扰动的影响深度不超过4cm。火烧迹地土壤具有独特的斥水性，其含水率、干密度及孔隙度改变随火烈度增加而增强，随植物根系腐烂，抗剪强度逐渐降低。室内模拟火烧条件下，揭露了高温条件下火烧迹地土壤表层有机质向土壤内部的迁移机制和与之相对应的斥水性增强规律，火烧以后土壤中O、Si、Al、Fe和K元素均有提高。.（2）结合凉山州、甘孜州火烧区，系统开展火烧迹地土壤火烧后不同时间尺度降雨条件下，现场坡面侵蚀深度监控量测，并结合部分火烧区现场降雨模拟侵蚀实验，通过修正通用土壤流失方程（RUSLE），揭示了火烧迹地产流产沙及坡面侵蚀规律，并建立火后泥石流坡面物源动储量预测模型。.（3）提出了控制火烧区火后泥石流易发性5个影响因子，采用专家经验法和机器学习法（随机森林和逻辑回归），构建了火后泥石流易发性评价模型、降雨阈值和一次冲出规模预测模型。.（4）提出了火后泥石流从坡面侵蚀到沟道侵蚀的三段式物源起动模式，基于时间序列的火后泥石流物源启动从坡面径流侵蚀到大规模滑坡起动补给三阶段成灾机理。建立了火后初期坡面侵蚀和后期滑坡及残枝树干堵塞的泥石流堵溃系数取值方法。.（5）研发了包括坡面植被修复、枯木固坡或清除、沟内编织袋装土堆砌多级简易拦挡等的火后泥石流的应急处置与“沟内拦挡+沟口停淤”永久治理技术，各自适用条件。