Debris flow is a kind of natural disaster commonly seen in arid and semiarid mountainous regions, where landslides are often induced by runoff water from rainstorm and ice and snow melt water scouring loose rock strata and mountain bodies, silt and stones are lnixed and carried by runoff water， forming viscous flow with a large volume of stones like cement mortar. Acted by gravity and inertial forces, such flow quickly runs along hillslope surface and channel and forms large-scale debris flow.
China is a mountainous country, 69% of which is occupied by mountains and hills. Due to specific natura1 and human geographica1 environments, 23 provinces,  municipalities 、 and autonomous regions in mountain areas are threa.tened and endangered by debris flow hazards, among which Yunnan, Sichuan, Tibet, Gansu, Shanxi， Ningxia, Liaoning, etc. are suffered greatest losses. Based on incomplete statistics, more than 10,000 debris flow gullies distritute in China, making grave damage to nationa1 economy and inhabitants' lives and property; 2 billion RMB Yuan losses is caused annually; Nearly 100 cities and counties are directly imperiled; Traffic in mountain areas is a1so often blocked; on the basis of initial estimates, 20 main or branch railway lines traverse over 1400 debris flow gulliesj In the highway networks, the lines between Sichuan and Tibet, Yunnan, Shanxi, Gansu, etc. are most endangered by debris flow hazards. Just along Sichuan-Tibet line distribute more than 1000 debris flow gullies. Beside above-mentioned important cities, towns and traffic lines, de bris flows also endanger farmlands, water conservancy facilities, rivers, etc. in different extents. For instance， in recent years more than 3600-middle-scale or smallscale hydropower stations and 50 reservoirs were destroyed by debris flow activities; Every year a large amount of silt was brought by debris flows debouched from 107 gullies along Xiaojiang river into the river, raising the riverbed for 10-20cm and causing heavy damage to local industrial and agricultural production and traffic.