工程地质学报  2018, Vol. 26 Issue (6): 1562-1573   (3158 KB)    
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  • 收稿日期:2018-02-05
  • 收到修改稿日期:2018-06-15
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    胡卸文
    王严
    杨瀛

    引用本文  

    胡卸文, 王严, 杨瀛. 2018. 火后泥石流成灾特点及研究现状[J]. 工程地质学报, 26(6): 1562-1573. doi: 10.13544/j.cnki.jeg.2018-073.
    HU Xiewen, WANG Yan, YANG Ying. 2018. Research actuality and evolution mechanism of post-fire debris flow[J]. Journal of Engineering Geology, 26(6): 1562-1573. doi: 10.13544/j.cnki.jeg.2018-073.

    火后泥石流成灾特点及研究现状
    胡卸文①②, 王严, 杨瀛    
    ① 西南交通大学, 地球科学与环境工程学院 成都 610031;
    ② 西南交通大学, 高速铁路运营安全空间信息技术国家地方联合工程实验室 成都 610031
    摘要: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,因林火高温燃烧植被进而影响表部土壤结构,导致土壤影响层内密度、孔隙比、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、大黏度流体特征,其成灾机理与普通泥石流具有显著差异。通过对国内外有关火后泥石流启动成灾机理及动力学特性研究的相关文献查阅,并从火后泥石流的启动类型及运动学特征、影响因素以及预测预报、防控措施等进行了综述,显示火后泥石流国外从1936年开始研究,且以美国、澳大利亚和西班牙的研究成果较为全面和系统,而国内除了林业系统有所涉及、且也主要从水土流失、植被恢复等方面考虑外,火后泥石流的研究还没引起我国学者的广泛关注,作为泥石流地质灾害的另一特殊类别基本上尚未进行系统深入的研究,也很少查到国内对火后泥石流方面的研究成果,可以说我国的火后泥石流研究目前尚属空白。针对国内外研究现状,结合我国山区频发森林大火形成泥石流造成的危害性,提出了未来应加强对火后泥石流的成灾机理、时空演化机制及有效防控方法的系统研究。
    关键词火烧迹地    火后泥石流    成灾机理    时空演化    防控方法    
    RESEARCH ACTUALITY AND EVOLUTION MECHANISM OF POST-FIRE DEBRIS FLOW
    HU Xiewen①②, WANG Yan, YANG Ying    
    ① Faculty of Geosciences and Environment Engineering, Southwest Jiaotong University, Chengdu 610031;
    ② State-province Joint Engineering Laboratory of Spatial Information Technology for High-Speed Railway Safety, Chengdu 610031
    Abstract: The post-fire debris flow is another special type of debris flows that closely related to forest fire and generate in burned area. Obvious difference in term of the initiation mechanism has been find between post-fire debris flows and general debris flows. Fire combusts the surface vegetation and furtherly destroys the texture of underlying soil for the high temperature of fire. The unit weight, pore porosity, permeability and other physical and hydrological properties of the fire-infected soil are changed dramatically in fire, and abundant residual ash layer and loose debris accumulated on the slope after fire. Therefore, post-fire debris flows usually have the characteristics of high unit weight and viscosity. A review of research on the generation mode, kinetic mechanism, influence factors, debris flow forecasting and mitigation measures of this kind of debris flows is put forward, referring to relevant literature at home and abroad. Post-fire debris flows are first studied in 1936, detailed study on which is mostly conducted in America, Australian and Spain. But few can been find in China, except the researches in agroforestry, in which the study are mainly about water and soil loss, vegetation combust and recovery. As another special type of debris flow hazards, systematic study of post-fire debris flows has not attracted enough attention in China, which reflects the poor study of this kind of debris flows. Based on the key problem mentioned above, some suggestions are came up with to enhance the research on the generation and spatial-temporal evolution mechanism and effective protection of post-fire debris flows.
    Key words: Burned area    Post-fire debris flow    Generation mechanism    Spatial-temporal evolution    Mitigation measures    

    0 引言

    森林火灾是林区常见灾害, 它除了受自然因素(如雷击)诱发外, 据不完全统计, 约90%以上的森林火灾与人类活动有关, 最为常见的如除夕燃放烟花爆竹、清明节墓地祭祀用火不慎以及林区随意乱扔烟头都极易诱发森林火灾。据联合国粮农组织的统计数据显示, 世界每年发生森林火灾将近22万次, 主要是加拿大、美国、俄罗斯和中国等国家, 年均过火面积均超过5×105 hm2, 而根据中国林业网的统计数据, 近5年来全国发生森林火灾532起, 其中129起发生在四川省。

    林火灾害的发生不仅对生态环境质量和人类的健康与安全构成重要影响, 其另一特点是往往火后不久一遇降雨, 就会形成泥石流灾害, 这种因火烧迹地附近形成的过火型泥石流在国际上习惯称之为“火后泥石流”(post-fire debris flow)或“火相关泥石流”(fire-related debris flow), 是最危险的林火次生灾害之一, 它不仅形成大范围水土流失, 而且对位于火烧迹地下游乃至沟口的当地居民及交通设施构成严重威胁和危害。据美国学者对火烧迹地已经发生的泥石流数量统计, 大约40%的火烧迹地发生了泥石流灾害, 而我国对此尚无系统的统计资料, 但从四川省近10 a发生过森林火灾地在随后的泥石流发生数量统计, 比例约在50%。可见, 一次森林火灾在其后可能诱发泥石流的可能性较大。

    火后泥石流尽管从形成条件与一般常规泥石流并无区别, 但由于在泥石流物源提供、启动致灾机理以及运动学模型上的特殊性和差异, 非常有必要查明并掌握其发生条件、致灾机理及其动力学特性、时空演化过程以及有效的防控技术, 这不仅是森林覆盖区火后灾害风险评估的重要理论基础, 也是为火后泥石流预测预报和防灾减灾提供重要依据。

    1 火后泥石流基本特点

    2014年6月1日18时40分, 四川省甘孜藏族自治州乡城县正斗乡仁额拥沟流域中上游的元根山因高压输变电线路电塔施工人员用火不慎, 发生森林火灾, 火灾发生地海拔3580 m, 大火整整燃烧持续4 d。该次火灾过火面积大, 达8000多亩(图 1), 约占整个流域的四分之一, 火灾后地表裸露, 坡体上形成了大量松散固体物质(图 2)。过火后的2014年6月8日, 受局地暴雨影响, 火烧迹地附近松散物源随即启动, 导致仁额拥沟发生了大规模泥石流, 泥石流造成乡(城)-得(荣)县道中断, 主河河水雍高, 房屋、农田被淤埋, 大量道路、桥梁被冲毁(图 3), 造成直接经济损失达两千万元, 给当地居民生产及生活造成严重影响。同时在继“6·8”泥石流后, 此沟还分别于2014年6月30日、7月10日、2015年8月24日、2016年7月23日再次发生不等规模泥石流, 由于当地政府监测预警措施应对得当, 对位于沟口乡政府所有居民及学校师生果断采取紧急避让, 数次泥石流幸未对当地居民造成人员伤亡。

    图 1 四川乡城县仁额拥沟域森林大火后的火烧迹地及坡面植被 Fig. 1 Burned area and slope vegetation in Ren'e Yong Basin in Xiangcheng, Sichuan province a.摄于2014-6-4; b.摄于2014-7-10

    图 2 乡城县仁额拥沟火后坡表土层结构松散及沟道侧蚀滑坡 Fig. 2 The loose soil after fire and channel incision and landslide a.摄于2014-7-10; b.摄于2016-8-6

    图 3 乡城县仁额拥沟火后暴发泥石流掩埋乡城-得荣县道(a)及冲淤沟口正斗乡政府并堵塞主河(b)(摄于2015-9-12) Fig. 3 Post-fire debris flow in Ren'e Yong valley bury roads from Xiangcheng to Derong and Zhengdou village and further block the river(shoot at September 12th, 2015)

    2016年3月16日17时, 四川甘孜州九龙县三岩龙乡柏林村因大风吹断枯木将输电线路打断引发森林火灾, 火场海拔4000多米, 以高山松、青杠林和杂灌为主, 过火面积约60 hm2。同样是火灾过后, 所涉及的4条支沟因暴雨分别在2016年5月6日即暴发首次小规模泥石流, 随后在2016年8月6日则全范围暴发且形成较大规模泥石流。另外2016年8月9日、8月23日又相继暴发小规模泥石流, 冲毁沟口4栋居民房屋, 阻断乡道交通(图 4)。

    图 4 甘孜州九龙县三岩龙乡柏林村火后暴发泥石流掩埋沟口居民建筑及乡村公路(摄于2016-12-20) Fig. 4 The post-fire debris flow buried residence and village road in Sanyan Long Village of Jiulong Country in Sichuan Province(shoot at December 20th, 2016)

    除上述典型案例外, 最为典型的还有发生在2005年5月中旬至6月初在四川省凉山州木里藏族自治县连续发生的3次森林大火, 地处青藏高原的木里是个林业资源县, 该县相继发生东孜乡“5·17”、水洛乡“5·23”、唐央乡“6·1”重大森林火灾, 3处场地过火面积分别为1412 hm2、996 hm2和2214 hm2, 上述3处火灾均系人为引发, 东孜乡火灾是当地居民在野外生火做饭余火未扑灭, 水洛乡火灾则是一名流浪到当地突然烧火引起, 而唐央乡火灾系当地一工程在放炮施工时不慎引起, 上述火灾同样在当年雨季即暴发泥石流灾害。

    上述现象说明, 发生在山区的森林大火过后, 只要遇到合适的降雨条件, 很快在火烧迹地附近物源启动而构成泥石流灾害, 它与常规泥石流相比, 不论在松散物质来源及类型、激发雨强、启动机理以及沿途动力学特性包括防控方法和技术等诸方面都有较大差异, 突出表现出以下特点:

    (1) 火后泥石流的发生与林火密切相关。火后泥石流作为火烧迹地发生的次生地质灾害, 其形成过程、易发性和规模都与林火特征(如火烈度分布、流域过火面积、过火区域空间连续性等)密切相关。

    (2) 火后泥石流流体细颗粒含量明显增多, 呈现黏性特征。由于植被燃烧造成大量的灰烬残留, 以及林下土壤的结构和植物根系受到严重破坏, 极大地改变坡面的径流条件和流域汇流条件, 因此在松散物源类型上突出表现是火后燃烧的灰烬富集, 且因燃火对坡表部覆盖层的烧结松弛效应极易造成浅表层滑坡, 综合作用会产生大量泥沙, 与常规泥石流比较, 火后泥石流堆积扇中细小沉积物的含量显著增大, 且均以黏性泥石流为代表。

    (3) 火后泥石流暴发所需降雨阈值显著降低。由于林火造成植被对降雨的截排能力丧失或降低, 原有植被林冠层和林下枯枝落叶层对坡面土壤的保护被严重削弱, 同时受到土壤斥水层及土壤结构变化的影响, 表层灰烬及烧结扰动使得物源更易启动, 具体表现在火烧迹地泥石流启动的降雨阈值会明显降低。

    (4) 火后泥石流发生还与土壤影响层内“斥水层”分布密切相关。大量的研究已经表明, 森林火灾会造成一定深度内的林下土壤斥水性新增或显著增强。其作用机理主要是通过不溶于水的物质包裹在土颗粒表面而阻挡水分入渗, 增加坡面径流量。火烧迹地土壤斥水性的强度会随着火灾后时间的推移逐渐降低, 在土壤斥水层被穿透以前, 坡面侵蚀率极高, 而在其被穿透以后, 地表径流系数和植被逐渐恢复, 坡面侵蚀减弱, 泥石流易发性随之降低。

    (5) 火后泥石流具有特殊的启动机理。一般情况下, 火后泥石流可以由坡面地表径流冲刷或浅层土壤滑坡单独引发, 也可以由两种启动方式共同作用。细粒灰烬层坡表径流和浅层滑坡引起的泥石流可以在高强度、大面积的火烧迹地同时发生。根据国外学者的研究, 两种启动方式在时空上具备一定的差异性, 如坡面径流引发的泥石流多发生在火灾后的前18个月, 而滑坡触发泥石流多是在火后2.3~10 a(De Graff, 2015)。

    (6) 火后泥石流沿途动力学特征的差异性。火后泥石流启动后, 不论是灰烬富集地表径流还是浅层土壤滑坡启动, 伴随着大量燃烧后的枯树枝干介入, 再加之泥石流运动过程中, 对沟道沿途淘刷造成的浅层滑坡补给作用, 两者联合作用使得沿途经常出现堵塞溃决, 进而导致泥石流流量放大效应显著, 危害性增强。

    (7) 火后泥石流暴发周期与植被恢复密切相关。森林火灾会造成大量植被燃烧和岩土体裸露, 同时植被死亡也会造成根系的逐渐腐烂, 降低土壤抗剪强度更易形成浅层乃至较深层滑坡。随着时间的推移, 火烧迹地植被又会逐渐恢复, 地表坡面径流逐渐减小、土壤影响层结构再生, 这些都有可能导致火后泥石流暴发具有一定的周期性。

    2 国内外研究现状及分析

    火后泥石流的研究最早起源于20世纪30年代, Eaton(1936)最早将火烧迹地发生的洪灾定义为泥石流灾害, 但是有关火后泥石流的深入研究则是起步于20世纪70、80年代, Wells(1981, 1987)在对美国加利福尼亚州的火烧迹地调查中, 明确提出火灾与泥石流之间的密切关系。近30 a来, 火后泥石流研究主要集中在美国西部加利福尼亚州、蒙大拿州、科罗拉多州和犹他州等地, 以及澳大利亚的东南部和地中海地区。Gartner et al. (2005)统计了美国606个火烧迹地的地表侵蚀数据, 其中216次侵蚀形式为泥石流。Cannon et al. (2003a)通过对加利福尼亚南部86个火烧迹地调查发现, 35%的地区发生了火后泥石流, 其余的只是产生高含沙水流或者两者都不发生。Cannon(2005)得出约40%(37/95)的火烧迹地发生了泥石流灾害。Nyman et al. (2011)统计了2003~2009年发生在澳大利亚维多利亚州的16个大规模地表侵蚀事件中, 有13件为火后泥石流。据Nyman et al. (2015), 发生于澳大利亚东南部的“黑色星期六”火灾过后, 调查发现了319个火后泥石流堆积扇。

    与普通泥石流的启动物源来源于因集中降雨导致的常规坡面水土流失、浅表层滑坡、早期崩滑堆积物坍滑或矿渣相比(崔鹏等, 2003a, 2003b; 陈晓清等, 2006; 徐友宁等, 2009; 余斌等, 2010a; 洪磊等, 2017), 对特殊因素(如地震、森林火灾)诱发的泥石流灾害, 其物源类型特殊且丰富, 启动模式也不相同, 且易发性急剧增加, 堵塞溃决效应突出, 例如2008年5·12汶川特大地震后形成的北川老县城魏家沟2009-9-24泥石流(唐川等, 2008)、银厂沟区域群发性泥石流(黄勋等, 2013)、绵竹市清平文家沟2010-8-13特大型泥石流(余斌等, 2010b; 游勇等, 2011)等, 普遍以高频率、多点暴发和冲出规模巨大为显著特点(谢洪等, 2009)。崔鹏等(2010)指出其主要原因是地震造成崩滑物源的剧增、沟道堵塞和大量植被的毁坏。而林火造成火烧迹地的泥石流易发性突然增大、堵溃效应同样明显, Prochaska et al. (2008)研究发现同一地区, 在相同的降雨情况下, 火烧区域发生泥石流灾害, 而非火烧区域则没有发生泥石流灾害。Cannon et al.(2008, 2011)和Moody et al. (2001)发现在以前没有预报过的地方在火灾过后普遍发生了泥石流灾害。Riley et al. (2013)通过对美国988条泥石流的统计研究发现, 相较于非火后泥石流, 火后泥石流中小规模事件的发生频率显著提高。

    2.1 火后泥石流启动机理研究

    Cannon et al. (1998)通过航片和现场调查, 解译了1994年发生于科罗拉多州的火后泥石流发生过程, 其物源主要有坡面松散物质、坡脚崩坡积物和沟道松散堆积物。Meyer et al. (2001)研究了美国爱达荷州一发生于1996年的火后泥石流灾害, 发现是由1989年森林火灾火烧迹地崩积物启动补给的。Nyman et al. (2011)对2003~2009年发生在澳大利亚东南部包括火后泥石流在内的火后侵蚀进行了系统统计, 显示其共有特征为渗透率低、分布广泛的片蚀和细沟冲刷以及强烈的沟道侵蚀。邸雪颖等(2013)指出了火后泥石流的4大特征:林火的强度、烈度和频率与火后泥石流的规模有直接的联系; 林火造成大量的坡面细小堆积物, 尤其是灰烬层和泥沙, 它们是构成火后泥石流动储量的直接来源; 同时泥石流的发生周期与植被的恢复密切相关; 火后坡面斥水层的形成直接影响着泥石流物源的启动。

    2.1.1 火后泥石流的启动类型及诱因

    Cannon et al. (2005)指出火后泥石流物源启动的主要类型为地表径流冲刷和降雨入渗触发的浅表层滑坡。其中大多数火后泥石流以地表径流冲刷为主。Wondzell et al. (2003)表明太平洋西北部的火后泥石流物源启动以浅表层滑坡为主, 主要由于土体饱水和植物根须腐烂造成的土体黏聚力下降而触发, 而内陆地区则是地表径流冲刷和浅表层滑坡兼而有之。Santi et al. (2008)通过对美国科罗拉多州、加州和犹他州的46个火烧迹地的泥石流灾害调查得出, 沟道侵蚀物源是火后泥石流的主要物源。Meyer et al.(1997)提出地表径流冲刷引起的火后泥石流有两种形式, 一种是坡面土壤侵蚀而形成泥石流, 另一种是流动中的高含沙水流侵蚀沟道中松散物源而转变为泥石流, 其中后一种形式更为常见(Cannon et al., 2003a, 2003b, 2003c; Santi et al., 2008; Gabet et al., 2008; Kean et al., 2011; Parise et al., 2012)。

    以降雨入渗触发滑坡转化泥石流的例子在火烧迹地并不常见, Parise et al. (2012)统计指出, 只有12%火后泥石流是由浅层滑坡所触发, 滑坡厚度可能为几十厘米~6米。浅层滑坡可能发生在火灾后的第一个雨季(Morton, 1989; Cannon et al., 2005), 也有可能是火灾后1~2 a(Meyer et al., 2001)、10 a甚至30 a(May et al., 2003)。Wondzell et al. (2003)指出火后土体的剪应力和抗剪强度取决于土壤沉积物的类型和厚度, 同时土壤含水率的增加加重了土体的重度和孔隙水压力, 进而造成火后土体的内摩擦角和黏聚力的降低。周德培等(2003)指出植物根须的加筋作用锚固效应和支撑效应会增强坡体的稳定性。而植物根系的固坡作用受到根系类型、根系强度的影响(付江涛等, 2014), 而植物根系对土壤的加筋作用主要体现在黏聚力的增强, 而对内摩擦角影响不大(刘益良等, 2016; 祁兆鑫等, 2017)。Swanson(1981)研究发现植物根须的腐烂会导致土体黏聚力的下降。Megahan(1983)对爱达荷州火烧迹地的水文特征研究发现, 林火过后土壤的孔隙水压和地下水流动速率加快。Wells(1987)在研究土壤斥水层时发现, 斥水层上方的土壤表层的饱水会形成浅表层滑坡。Cannon et al.(2005)认为火后泥石流峰值流量的增加, 加强了沟道的侧蚀作用, 也是火烧迹地滑坡发生的诱因。

    坡面径流冲刷引发的火后泥石流相对较为常见, 如在美国落基山脉(Meyer et al., 1997; Cannon et al., 2001b)、加利福尼亚州(Wells, 1987; Santi et al., 2008)和科罗拉多州(Cannon et al., 2008; Dewolfe et al., 2008; Dennis et al., 2015)等, 但是在太平洋西北部却几乎没有(Wondzell et al., 2003)。研究表明大多数火后泥石流都是由坡面径流冲刷所启动, 并在流通过程中不断裹挟松散物质(Santi et al., 2008; Nyman et al., 2013)。坡面径流冲刷触发的火后泥石流灾害主要有以下两点原因:火后土壤易蚀性和地表径流量的增大(Wondzell et al., 2003; Cawson et al., 2012; Parise et al., 2012)。

    相较于非火烧区, 火烧区域的坡面径流冲刷独具特点, Wells(1987)在1987年对美国加利福利亚州西南部的火烧迹地研究发现, 坡面径流冲刷形式以冲沟网络为主, 并强调其泥石流物源启动过程中的关键作用。一般来说, 火灾之后的1~2 a内坡面径流冲刷的强度最大, Hubbert et al. (2012)对美国加州威廉姆斯火灾区观测得出, 灾后第一年的沉积物冲刷量达到了85%。灾后植被的恢复情况对于土壤的易蚀性有着极大的影响, Cerda(1998)指出火烧迹地的坡面径流量随着时间的增长逐渐减小, 而渗透率逐渐增大, 而这一变化主要取决于植被的恢复情况。

    2.1.2 火后泥石流的运动特征

    Cannon et al.(2005)指出火后泥石流的固体物质含量会在坡面流的汇聚和运移过程中以滚雪球的方式不断增加, 从高含沙水流转变为泥石流。Cannon et al.(2001b, 2001c)提出高含沙水流转化为泥石流的充分条件是其要裹挟足够多固体物质, 以致其流体性质发生改变。Gabet et al. (2008)指出火后物质运移形式从高含沙水流到泥石流的转变需要流域过火面积到达一定的阈值。Parise et al. (2012)也表明, 高含沙水流到泥石流转变的阈值与产沙区面积和坡度有关。但即便是其短暂达到泥石流的形成条件, 其流体形态是在高含沙水流和泥石流之间来回浮动的(Cannon et al., 2005)。Gabet et al. (2008)指出火烧区域泥石流的流动不仅会造成原有沟道的下蚀, 而且会在松散堆积体上形成新的沟道; Santi et al. (2008)调查得出坡面径流汇流以后在其运动过程中不断裹挟松散物质, 其沟道侵蚀能力逐渐增大, 据统计, 泥石流形成后其沟道侵蚀率会激增到7.3倍, 并指出堆积堵塞堤坝在火后泥石流中十分常见。Elliott et al. (2004)采用Flo-2d对火后泥石流的沟道运移过程进行了数值模拟。

    由于林火对原有植被的破坏, 火烧迹地的泥石流碎屑物质中含有大量的树枝和树根等木材, 严重影响着火后泥石流的冲淤特征。2015年, Short et al. (2015)在对美国蒙大拿州西部的某一火后泥石流研究发现, 林火过后木材碎屑物质增加了50%, 并且造成沟道的拥堵和碎屑物质的淤积。Beyers et al. (2005)研究发现, 火烧迹地的大型的碎屑木材分布于沟道之中, 容易发生堵溃效应, 可造成该流域的洪峰流量最大增加3个等级。

    2.1.3 火后泥石流成灾模式研究

    林火造成树冠层和枯枝落叶层的缺失, 流域内对降雨的截留能力丧失或极大降低, 这是造成火后流域径流量激增的主要因素(Stoof et.al, 2012; Cannon et al., 2003a, 2003b; Martin et al., 2001), Hanshaw et al.(2009)的对火烧区和未火烧区的降雨监测发现, 火烧区到达地面的降雨量是未火烧区的1.7倍, 同时林冠层能够截留35% ~78%的降雨量。Chen et al.(2013)指出在森林植被覆盖区, 火烧以前流域产流模式是蓄满产流, 而火灾以后则为超渗产流。即火烧迹地在短期降雨过程中的产流能力激增。

    国际上的大量研究表明, 由于地表覆盖物的燃烧以及土壤斥水性的存在, 火烧迹地土壤抗侵蚀能力急剧下降(Moody et al., 2013)。Beyers et al. (2005)指出, 林火过后土壤的结构崩坏, 密度增加, 孔隙比下降, 易蚀性增大, 火烧迹地的洪峰流量急剧增加。Moody et al. (2001)指出火后当年火烧迹地侵蚀率增加了约200倍。Nyman et al. (2015)指出火烧区域产沙量比非火烧区域大出2~3个量级。Woods et al. (2010)Gabet et al. (2011)通过试验表明草木灰会堵塞粗颗粒土壤表面孔隙, 降低其渗透系数, 进而增大坡面径流。Gabet et al. (2008)指出草木灰增大了地表径流量和泥石流流体密度, 提高了泥石流流体的物质运移和侵蚀能力。Moody et al. (2001)指出火灾过后土壤的抗冲刷能力下降和灰烬层的存在, 面对显著增大的坡面径流, 泥石流启动的降雨阈值大大降低。Lavee et al. (1995)指出火后地表粗糙程度的降低放大了地表径流。在国内, 刘发林等(2015)通过在火烧迹地进行降雨模拟实验得出, 火后林下土壤的产沙量与土壤裸露面积成正比, 但是对于降雨条件下的泥石流形成过程的研究较少。为了揭露火后泥石流的物源启动过程, Staley et al.(2014)通过雷达扫描的方式研究了火烧迹地侵蚀过程主要包括坡面片蚀、坡面汇聚、坡面运移以及沟道的下切侵蚀4个过程。Langhans et al. (2017)则指出火烧迹地的坡面泥石流对粗颗粒无黏性土壤具备较大的侵蚀能力。

    此外, 林火对有机质的燃烧会在土壤表层形成一层斥水层(Shakesby et al., 2006; Hubbert et al., 2012), 斥水层对于坡面径流的产生具有促进作用(Doerr et al., 2000)。Wells(1987)提出土壤斥水层的存在是坡面细沟冲刷形成的原因之一, 其形成机制是由于降雨入渗受到斥水层的阻挡, 上部透水层受到坡面径流冲刷形成湍流, 湍流进一步掏蚀斥水层, 直到下切侵蚀深入到斥水层下部土体之中, 此时土体渗透性加大, 下蚀停止。一般来说, 火后土壤斥水层对地表渗透性的影响不会超过一年。Gabet(2003)研究表明火后半干旱地区地表斥水层的形成减少了降雨入渗, 导致地表径流量和沉积物运移量的快速增加。

    因此就火后泥石流成灾模式而言, 火后首次集中降雨会很快在火烧迹地形成汇流, 因土壤斥水性影响, 明显以坡面地表径流为特点而形成首次以灰烬层为主的泥石流, 伴随泥石流在汇入沟道内的沿程侵蚀, 又会铲刮沟床及两侧坡脚, 导致沟道两侧局部边坡稳定性变差; 在后期集中降雨作用下, 这些潜在不稳定斜坡逐渐转化为沟道浅表层滑坡, 进一步堵塞沟道而溃决, 因此后期泥石流不仅频繁、而且流量放大效应极为显著, 这就是火烧迹地泥石流形成的共性过程机制。

    2.2 火后泥石流启动的影响因素

    火后泥石流与非火烧迹地所发生的泥石流有明显区别, 其发生是多个因素共同作用的结果, 并不是所有的火烧迹地都会发生泥石流灾害。Nyman et al. (2015)认为影响火后泥石流易发性的因素有林火烈度、坡度、土壤干燥性和降雨强度。Cannon et al. (2000a)通过对比新墨西哥州3个不同火烧迹地的坡面特征、沟道特征和火行为特征得出, 火烧迹地的地形地貌和地层岩性是区别预测洪水和泥石流的重要指标, 随后Cannon(2001a)又加入了是否产生斥水层这一指标。

    2.2.1 地形地质条件

    从地形条件上看, 相较于缓坡, 陡坡更利于火后泥石流的形成, Meyer et al. (1997)在研究黄石公园所发生的火后泥石流中发现, 陡峭火烧迹地所发生的泥石流导致严重的侵蚀和下坡沉积物的重新分配。Cannon et al.(2005)分析了大量的火后泥石流数据(Gartner et al., 2004; Gartner et al., 2005), 试图通过讨论基岩岩性和地表松散物质、流域面积和平均坡度、过火面积和火强度、斥水层等因素对于火后泥石流启动的敏感性, 来确定火后泥石流的发生条件; 在以坡面径流侵蚀为主的火后泥石流流域中, 70%以上的基岩岩性为沉积岩和变质岩, 而在以浅表层滑坡为主的火后泥石流流域中, 90%以上的基岩岩性为风化花岗岩; 山坡和沟道的大量松散物源和火烧迹地土壤的易蚀性是火后泥石流启动的必备条件; 火后泥石流沟的流域面积多小于25 km2Cannon et al. (2003c)通过对3个典型火烧迹地泥石流沟对比发现, 沟道物源动储量对于火后泥石流的规模有一定影响, 如果不能够提供足够的裹挟物质, 会降低火后泥石流的危害程度。Gabet et al. (2008)通过研究美国蒙大拿州西南部泥石流发现, 泥石流的规模随着坡面面积的增加呈指数增长。Nyman et al. (2015)指出相较于湿润地区, 干燥的火烧迹地更容易发生火后泥石流灾害, 且具有更大的规模。

    2.2.2 火行为条件

    林火烈度(或林火强度)对于火烧迹地的洪峰或泥石流流量具有密切联系。Mitsopoulos et al. (2006)在研究地中海地区的火后泥石流中发现, 高烈度林火与火后流域内洪峰流量的激增有明显的关系。Ffolliott et al. (2003)对美国亚利桑那州2002年的林火研究得出, 高烈度林火发生后的洪峰流量约为火烧之前的900倍。Hyde et al. (2014)通过对美国蒙大拿州和爱达荷州的97条火后泥石流沟研究发现, 林火烈度越高, 植被残留越少, 泥石流启动所需的坡面面积和坡度阈值就越小。Nyman et al. (2015)指出火行为特征细小变化会显著影响火后泥石流的发生频率。李菁等(2010)指出火灾强度会影响到火后植被的恢复情况, 火灾边缘区相较于中心区域的植被恢复速率要高。

    2.2.3 坡面面积和降雨阈值条件

    大量研究发现, 火后泥石流的发生需要一定的坡面面积和降雨阈值(Cannon et al., 2008, 2011; Staley et al., 2012; Hyde et al., 2014; Dennis et al., 2015)。Cannon et al. (2001c)通过对科罗拉多州某一火烧迹地泥石流启动条件研究发现, 以坡面径流冲刷为主的火后泥石流启动与其过火面积有很大关系, 得出了关于坡面面积和坡度的泥石流启动阈值函数, 并提出这个阈值会随着不同的坡面物质类型而发生变化。Cannon et al.(2003b, 2008)在研究科罗拉多州的火后泥石流时发现, 80%的短时间暴风雨触发泥石流灾害, 并通过对火后泥石流事件中记录的降雨强度-持续时间数据的线性回归分析, 得出了其针对性的降雨阈值, 同时这一阈值也具有时空变化特征, 不同地区其地形和地质条件不同, 造成降雨阈值的差异, 随着火烧迹地植被的逐渐恢复, 其降雨阈值会不断增大。Cannon et al. (2011)在2011年将火后泥石流的降雨阈值分为4个等级, 分别对应不同规模的泥石流灾害, 并以此来确定人员疏散和调度的等级。Staley et al. (2012)通过统计数据发现火后泥石流的激发雨强普遍小于峰值雨强。Kean et al. (2011)研究发现, 火后泥石流的体积与短时间降雨强度密切相关, 但是与总降雨量和坡面土壤的含水率关系不大。

    2.3 火后泥石流的预测预报

    由于林火造成流域内地质环境条件的改变, 极大降低了泥石流发生的降雨阈值, 对于火后泥石流, 常规泥石流预测预报方法的适用性大大降低, 很有可能误报漏报, 造成严重损失。目前国内外对火后泥石流的预测研究主要集中在其发生的可能性、频率以及规模。

    研究表明, 控制火后泥石流规模的关键因素是其峰值流量, Gartner et al. (2004)统计了61条坡面径流冲刷为主的火后泥石流的峰值流量从2~240 m3·s-1不等。根据Wells et al.(1981, 1987)的统计数据, 火后泥石流体积约为600~30 000 m3Cannon et al.(2003a, 2005)分别针对不同的岩性做出了以过火面积为变量的峰值流量预测函数, 同时也通过多元数据统计分析得出了关于过火面积、坡度和平均小时雨强的峰值流量预测函数。Gartner et al. (2008)通过对美国西部三州53条火后泥石流沟的调查, 利用多元回归分析, 整合流域地形、不同火烈度条件下的过火面积、土壤特性、岩性、降雨强度和降雨量等因素, 分别对不同区域和地质条件下的泥石流体积(规模)进行预测。Gartner et al.在2014年又进一步提出了两种火后泥石流规模的预测模型:长时间预测模型和两年内短时间的应急预测模型, 并通过了实际数据的对比验证, 预测结果更为可靠。2015年, Gartner et al. (2015)提出了一个基于沟道坡度、平面曲率和沟道长度的火后泥石流易发性预测函数。Friedel(2011)以数据驱动理论为基础, 采取遗传程序的方法, 针对泥石流规模进行了建模, 最终得出利用流域平均坡度、总过火面积和总降雨量这3个指标建立的非线性方程, 其预测误差最小。Cannon et al. (2010)提出了基于大数据的预测火后泥石流发生可能性模型和预测火后泥石流冲出规模的模型, 前者是关于过火面积、土壤性质、流域形态和降雨量的函数, 后者是关于流域坡度、过火面积和降雨量的函数。但是这些预测模型都是基于已有的数据库, 且只针对数据库所在的区域预测效果良好, 而对其他地区的火后泥石流预测较为乏力。对于没有区域降雨阈值的地方, Staley et.al(2017)提出了一种基于当地的降雨、水文响应特征和地形数据的火后泥石流预测模型。

    2.4 火后泥石流防治

    De Graff(2014)提出火后灾害评估与防治至少考虑到10 a。火后泥石流的物源启动具有分散性, 所以为了减少火后泥石流的发生, 除了常规的拦砂坝、导流堤和植被恢复等措施外, Dewolfe et al. (2008)还采用了原木侵蚀障碍和稻草覆盖的方法, 实践证明, 采用各种方法进行综合治理的效果很好。在火灾之后第一年, 坡面侵蚀率很高, 原木拦截的效果最好, 其次为秸秆覆盖, 反而是种草的防治效果最差, 这是由于地表径流的强烈侵蚀作用造成的。Fox(2011)研究发现原木拦渣坝的治理效果最好, 尤其是对于粗颗粒碎屑物质的拦截特别有效, 可允许细颗粒黏土和粉细砂通过, 如果将原木拦渣坝和原木侵蚀障碍两者结合治理效果更佳。

    3 有待进一步研究的问题

    从已查阅到的正式发表文献还可以看出, 火后泥石流国外从1987年开始研究, 且以美国、澳大利亚和西班牙的研究成果较为全面和系统, 而国内除了林业系统有所涉及、且也主要从水土流失、植被恢复等方面考虑外, 加之火后泥石流多出现在偏僻山区, 不易被发现, 因此火后泥石流的研究还没引起我国学者的广泛关注。作为泥石流地质灾害的另一特殊类别基本上尚未进行系统深入的研究, 可以说我国的火后泥石流研究目前尚属空白。已有研究主要关注林火对土壤性质和植被的影响(张玉红等, 2012)、火后植被恢复(赵凤君等, 2013)等方面。而国外的研究都是从火生态学、水文学、地理学, 并以水土流失的角度开展, 基本涉及了火后泥石流启动方式、影响因素、启动降雨阀值、预测预报以及防治措施等各个方面, 且以表观勘测、统计发育规律的定性分析为主, 而从地质灾害的角度考虑相对偏弱, 具体表现出对火后泥石流启动成灾机理的定量化、时空演变以及有效防控方法研究涉及很少。以下问题有待于深化研究:

    (1) 不同火烈度下的火烧迹地土壤影响层内是否真正存在所谓的“斥水层”?其埋深及厚度是多少?

    (2) 火烧迹地因植被根系烧毁和表土焙烤综合效应, 坡表不同深度下的物理力学性质、渗透特性到底如何变化?其影响深度界限是多少?

    (3) 不同火烈度下的火烧迹地, 其地表坡面径流冲刷深度及其差异?

    (4) 与沟底冲刷牵引无关、发育在火烧迹地部位因降雨形成的浅层滑坡规模(含深度、边界)受控因素?

    (5) 不同火烈度下的火后泥石流, 考虑沟道两侧滑坡堵沟、以及沿途火烧后残枝树干汇入而产生的联合堵溃效应及其堵塞系数如何确定?

    (6) 一次森林火灾过后, 火后泥石流成灾在时间和空间两个维度上如何演变?时间进度上是由“发育期-衰退期”两阶段还是由“发育期-旺盛期-衰退期”3阶段演变?另外在空间上处于不同时期的火烧迹地物源启动又会如何变化?

    (7) 对火后泥石流灾害如何进行有效防控?

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