火后泥石流致灾机理及动力学特性研究

Post-fire debris flow is described as a type of debris flow occurs in the burned area which linked closely to the forest fire. As another particular type of the debris flow phenomenon, post-fire debris flow has fluid characters with high unit weight and high viscosity, because the debris flow mass contains plenty of ash caused by the combustion of organic matters and abundant loose sediment resulting from the dramatic changes of physical properties (eg. unit weight, porosity etc) of soil, which are attributed to the destruction of soil structure by high temperature in forest fire. Thus, the post-fire debris flow can be distinguished distinctively from common debris flow through its formation mechanism. So far, the research of the initiation and disaster-causing mechanism of post-fire debris flow in and abroad is still weak. This research aims at discussing the initiation and disaster-causing mechanism and kinetic characteristics of post-fire debris flow taking several post-fire debris flows in Sichuan province for research objects. Based on the features of soil structure in burned areas, types of the loose source material, physical properties and permeability of soil in different depths, the research focus on revealing the combination features of geologic structure in the condition of runoff-dominated erosion and infiltration-dominated shallow landslide failure combining with in-situ exploration and tests, and further discusses the disaster-causing mechanism and block-collapse effect of the kinetic characteristics of post-fire debris flow. The research results can provide scientific basis to the design parameters and the mitigation of this kind of natural hazard. Broad application prospects is expected greatly in enriching and improving the research of debris flow hazards, as well as the theoretical level of study on 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，因火烧形成的灰烬层及林火燃烧产生的高温破坏土壤结构，导致其容重、孔隙率等物理性质发生剧烈改变而出现大量松散泥沙，形成的泥石流常表现出高容重、大粘度流体特征，其形成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致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启动致灾机理及其动力学特性这一关键科学问题，以四川省内数条典型火后泥石流沟为研究对象，从火烧迹地土壤层地质结构特点、松散物源类型、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，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，此类泥石流致灾机理及动力学特性中的堵溃效应，对丰富和完善泥石流灾害研究，深化火后泥石流研究理论水平，并为此类灾害的防治提供科学依据和设计参数具有广阔的应用前景。

火后泥石流是一种与林火紧密相关的特殊泥石流，其形成机理与动力学特性与普通泥石流有显著差异。本研究以四川西南地区数条典型火后泥石流流域为研究对象，通过现场调查、遥感解译、室内外土工实验、降雨模拟实验等手段，对火烧迹地植被烧毁特征、火烧迹地坡表土壤性质变化、火后泥石流物源启动模式及致灾机理、火后泥石流动力学特性及关键参数取值等关键问题进行了详尽的调查研究。结果表明：1）林火烧毁了植被和枯落物，使植被覆盖率大幅下降，在坡表形成了灰烬层；同时林火燃烧产生的高温烧焙土壤，使烧焙影响层内的土壤天然密度和含水率降低、干密度和孔隙度增加，内聚力降低，斥水性增强，渗透性下降，从而在坡表形成了松散土壤扰动层；灰烬层和松散土壤扰动层随坡面径流起动，作为一类特殊物源参与火后泥石流；2）火后泥石流物源启动可分为3个阶段:坡面侵蚀阶段、高含沙水流沟道侵蚀阶段和泥石流沟道刨蚀阶段；火后当年，由于坡面存在大量灰烬层和松散土壤扰动层，火后泥石流启动以坡面侵蚀为主；火后2-5年内，由于火烧区浅表层土壤中根系减少，火烧区根—土复合体抗剪强度显著降低，加上沟道下切，坡脚临空面高度增加，导致火后泥石流起动物源主要来自于浅表层滑坡；3）早期火后泥石流浆体以细颗粒为主，为粘性泥石流，其流速可按照粘性泥石流计算公式得出；由于残枝树干和滑坡联合堵沟溃决造成火后泥石流峰值流量被逐级放大，结合形态调查法和雨洪法反算得出宽缓沟道型火后泥石流的堵塞系数范围为2.74~7.64。另外，综合考虑流域形态、火烧严重程度、降雨条件、物源动储量及分布、植被特征等火后泥石流起动影响因素，采用混合机器学习方法和多元线性拟合，建立了火后泥石流发生概率、小时临界降雨阈值及一次冲出规模的预测模型；并且基于RUSLE模型提出了有效的火后泥石流流域坡面侵蚀物源动储量估算方法。本研究成果进一步深化了火后泥石流致灾机理及其动力学特性研究理论水平，可以为此类灾害的防治提供科学依据和设计参数。