长江上游泥石流灾害的特征

长江上游地区地质条件复杂、新构造运动强烈、地震活跃、地形起伏巨大、降水丰沛而集中，对泥石流发育极为有利；而长期不合理的人类经济活动(如过度采伐森林、毁林开荒、陡坡耕作、建设和采矿不合理弃渣等)则对生态环境造成强烈破坏，加剧了泥石流灾害的发生，使长江上游成为我国泥石流活动最强烈及灾害最严重的地区。据统计，1753~2002年的200多年中，区内仅一场泥石流致死百人以上的特大灾害点就达17个，累计死亡近6 000人；致死百人以下的灾害点更多，致死十人以下的灾害点不计其数，死亡人数总和远远大于上列数字；有60余个市县政府驻地城镇受泥石流危害或威胁，乡镇则多达上百个。由泥石流造成的人员伤亡每年数十到数百人，经济损失10~15(亿元／a)。其灾害的严重性由此可见一斑。泥石流灾害已成为制约长江上游地区经济发展的重要因素之一。研究泥石流灾害，掌握其特征，是长江上游防灾减灾所必需，对促进长江上游地区社会经济发展和西部大开发具有重要意义。长江上游己查明的泥石流沟6 800余条。其特殊的地理位置和巨大的地形高差及岩性差异，使区内发育的泥石流不仅数量众多，而且类型石相当齐全，除无火山泥石流外，几乎 包含了世界上目前己发现的其他各种类型的泥石流。在青藏高原边缘等岭谷高差变化大的地貌过渡带，泥石流极为发育；沿深大断裂带和强烈地震带泥石流集中分布；泥石流主要出现在流域面积小于10 km\+2的小流域内，分布呈非地带性特征。泥石流活动频率高，每年都有成百上千处发生，导致了泥石流灾害频繁。如云南东川的蒋家沟，从1965~2001年的37年中，共发生泥石流459次，平均12．4次／a；甘肃武都柳弯沟平均每年暴发泥石流11次。此外，泥石流搬运泥沙石块的能力惊人，搬运物以粗粒物质为主，据干流和支流的69个泥石流堆积物样品分析，堆积物中粒径大于20 mm 的石块占43.%~64.8%，平均50．4％；由此，常常造成其汇入的主河被堵塞而形成淹没和溃决后水毁等次生灾害。泥石流冲刷与淤积能力极强，运动中可将基岩下切10余m，金沙江、雅砻江、大渡河、岷江及其支流被泥石流堵塞成湖，形成淹没灾害，溃决后对下游造成严重的水毁灾害，均屡见不鲜。

FEATURES OF DEBRIS FLOWS IN THE UPPER REACHES OF THE CHANGJIANG RIVER

The upper reaches of Changjiang River are the most serious regions of debris flow hazards. Nearly 60 counties and cities as well as hundreds of towns are under menaces of 6 800 gullies of debris flow in various kinds and scales claiming annual deaths of tens to hundred and causing annual economic loss amounting to 120~180 million US dollars. Debris flow gullies are scattering in geomorphologic transitional strips with great relief and concentrating in fault zones. Debris flow is a principal cause lagging and limiting the economic development in mountainous areas there. Further studies on theory and control measurements are in imminence. The upper Changjiang is flowing eastwards across two geomorphologic transition zones of the largest relief in China: 1.transition zone from the first terrace, i.e., the Qinghai-Xizang Plateau, down to the second terrace, i.e., the Yunnan-Guizhou Plateau, where rainy and glacial debris flows are developed;2.transition zone from the second terrace descending down to the third terrace, where the dominating agents are rainy debris flows.The great difference of altitude in the transition is in element responsible for the distribution. In particular, the concentration of debris flow in the first zone is mainly dependent upon the more favorable conditions compared to the second: the higher altitude, greater relief, and more concentrated precipitation, etc.