中文摘要：

2008年9月24日汶川震区的北川县暴雨导致区域性泥石流发生，这次9．24暴雨泥石流灾害导致了42人死亡，对公路和其他基础设施造成严重损毁。本研究采用地面调查和遥感解译方法分析地震与暴雨共同作用下的泥石流特征，获取的气象数据用于分析泥石流起动的临界雨量条件。本文探讨了研究区泥石流起动和输移过程，并根据野外调查，分析了泥石流形成的降雨、岩石和断层作用，特别是强降雨过程与物源区对泥石流发生的作用。根据应急调查发现北川县境内暴雨诱发的泥石流72处，其分布受岩石类型、发震断层和河流等因素控制。根据对研究区震前和震后泥石流发生的临界雨量和雨强的初步分析，汶川地震后，该区域泥石流起动的前期累积雨量降低了14．8％-22．1％，小时雨强降低25．4％～31．6％。震区泥石流起动方式主要有二种，一是由于暴雨过程形成的斜坡表层径流导致悬挂于斜坡上的滑坡体表面和前缘松散物质向下输移，进入沟道后转为泥石流过程；二是“消防水管效应”使沟道水流快速集中，并强烈冲刷沟床中松散固体物质，导致沟床物质起动并形成泥石流过程。调查和分析发现沟内堆积的滑坡坝对泥石流的阻塞明显，溃决后可导致瞬时洪峰流量特别大。研究结果表明了汶川震区已进入一个新的活跃期。因此，应该开展对汶川地震区的泥石流风险评估和监测、早期预警，采取有效的工程措施控制泥石流的发生和危害。

英文摘要：

The intense rainfall on 24th September 2008 initiated widespread debris flows in Beichuan County, the epicenter of the Wenchuan Earthquake. These debris flows greatly impacted the community of the Beichuan County and caused 42 fatalities. The debris flows also damaged many roads and other infrastructures in the mountainous areas. Field reeonnaisance and interpretations of aerial photographs were used to identify debris flow occurrences. Precipitation data were then utilized to analyze critical rainfall conditions leading to the debris flow initiation. This study examines the process of debris - flow initiation and transport in the study areas and presents field observations on the roles of rainfall, lithology, and faults. Particular emphasis is given to the roles of intense precipitation and source areas. The character and spatial distribution of lithology, faults and topography appear to control the initiation processes of debris flows in the mountainous area. The study resuhs show that after the Wenchuan Earthquake, critical accumulated precipitation and hourly rainfall intensity necessary to initiate debris flow reduced to as low as 14.8% -22.1% and 25.4% -31.6% respectively comparing with those before the earthquake. Through emergency field investigation, 72 debris -flows triggered by the rainfall were identified. Almost all debris flow gullies in the drainage basins have contained many seismic landslides, which supply debris flow source areas with huge loose material. Two types of failure mechanisms initiated the debris flows： the first initiation involved the mobilization of shallow landslides and the second process, which was explained with term of the ＂fire - hose effect＂ initiated debris flows. The overland flow of water resulting from intense rainfall was concentrated in steep channels and scoured debris from gully -bed loose deposits. The study also indicated that the region affected by the Wenchuan Earthquake become more vulnerable to debris flows in this period. Therefore it is important to a