Study On Debris-flow Activity Patterns And Hazard Driving Forces Of The Alpine Vallery Area In Nujiang River Basin

The study on debris flow hazard mechanisms not only provides a theoretical basis for scientific prediction and prevention of disasters, but also is the foundation of debris flow disaster prevention and mitigation. Mountainous valley area in Nujiang basin is one of the most serious debris flow hazard regions in China. There is a unique disaster-forming environment and disaster mechanism.However, debris flow disaster mechanismshave not been studied enough. It is urgent to carry out in-depth systematic study on it.According to remote sensing image interpretation and field discrimination of disaster, the spatial distribution of debris flow and history in Nujiang alpine-gorge area have been basically identified. It has acquiredthe disaster-forming environmental information of research area and constructed spatial information database of debris flow. By multiscale temporal analysis, temporal and spatial patterns and activity characteristics of debris flow have been clearly studied. The systematic analysis on debris flow disaster focuses on three key driving forces:hydrodynamic (precipitation conditions), the driving force of geological (tectonic, seismic, lithology) and gravity driving force (micro-topography, loose material). From the regional scale to explore the mechanism of debris flow disaster; it combines the recent typical major debris flow disaster analysis of study area, focusing on the multi-scale precipitation characteristics, loose material source, micro topography and other aspects. The study use mathematical statistics, remote sensing interpretation methods and techniques in order to analyze the mechanisms of typical debris flowsfrom mudslides basin scale.Developing research of debris flow hazard mechanisms in Nujiang alpine-gorge region helps to make it clear on debris flow hazard process and to propose effective hazard prevention strategies and programs for disaster prevention. Moreover,it can provide scientific reference for planning and construction of major projects in mountain and mountain towns in terms of disaster prevention and mitigation. The main conclusions in the study are as follows:(1) Based on the triggering factor, debris flow in Nujiang alpine gorge area can be divided into three categories:storm debris flow, the landslides slump triggered ones (boulders and driftwood outburst debris flow), and snowmelt-triggered debris flow, among which storm debris flows are the main ones while gully debris flow thatis widespread in China is few found in study area.(2) Debris flow hazardin Nujiang alpine canyon area have characteristics including increasing frequency, double rainy season (Fugong above), nocturnal, certain cyclical and unstable; debris flows development along the Nu River on both sides of the zonal, and widely distributed with semi-hard rock group in formation lithology; debris flows in research area with high concealment, high density disasters, low frequency, large-scale, rapid outbreak, sabotage and other strong features. Among them, strong concealment is the most essential feature of debris flow activity in the study area.(3) spatial and temporal distribution of precipitation in the mountains and canyons of Nujiang directly impacts the debris flow activity. Nujiang fracture determines the debris flow space development. Under the mix influence of precipitation, stratigraphic lithology and different folding belt, the right bank of Nujiang mainly breaks out granite boulder dilute debris flow, and viscous debris flows on left bank. The study area is a typical alpine valley landscape, with dominant high heave middle altitude mountain, development with pinniform shaped water system, and featuring a steep debris flow micro-topography, full of sufficient potential energy. According to the analysis of the key drivers of disasters, in Nujiang mountain valleys, there are mudslides activities with very strong hydrodynamic driving force of gravity, and medium strength of the driving geologicalforce. It is a loose material source controlled debris flow because loose mobile material is not very widely distributed.(4) The main factor to induce Nujiang alpine valley area debris flows is the strong local rainfall. Gongshan "8.18" debris flow was a outburst viscous type debris. It was a massive once-in-a-century debris flow. By comparison, Bingzhongluo "6.24" and Latudi "5.10" debris flows were slump triggered mudslides dilute debris flows, mixed with a lot of boulders, driftwood, causing huge impact force and serious damages. Shawa River "7.09" debris flow was a small-scale viscous debris, however, it caused heavy casualties, because the disaster broke out in the middle of the night. In spite of that, the debris flow gully was highly covert and workers had weak awareness of disaster preventionbuilding the work shed on debris flow fans.