So far most of the research consider that debris flow is a special kind of flash flood slope, where the density of debris in water is more than 60%. But the danger of debris flows compared with flash floods multiplied times. It is because of high energy in debris flow. Particularly, danger is the large rocks moving with great kinetic energy (with large boulders the size of 3–5 m).
Following debris flows occurring in recent years (since debris flow in Muong Lay in 1996 we recorded), the experts said that debris flows associated with landslides. But the real (Nam Luc Commune, in 2012, Ban cavity, 2013) showed that the formation and development of debris flows concern to many special causes. In [11] the authors identified debris flows developed mainly in the neck Proluvium and Colluvium layers or the tectonic debris layer. However, the authors did not analyze deeply the mechanism of formation and development of debris flows in mountains locating in northern Vietnam. In this paper, the authors analyze in depth natural conditions and development of some typical debris floods that happened in Tan Nam, Xin Man 2002, in Pac Nam, Bac Kan, 2008, Nam Luc, Lao Cai, 2012 and in Ban Khoang, Sa Pa in 2013, from which to identify causes and mechanism of formation of debris flows in the mountains of northern Vietnam.
Debris flows occuring in Tan Nam, Xin Man district, Ha Giang at Night of 08/07/2002 killed 13 people, dozens of homes were destroyed and hundreds of hectares of rice and vegetables were buried by sand and gravel. Debris flows occurred in both Na Vai and Lung Chun. However the low zone of Lung Chun also happened the mixture sweeping flood should still greater damage and the death increases.
According to topography, both Na Vai and Lung Chun are located in 100–300 m elevation. Slope of the terrain is sbout 10–150. There are many Colluvium-Proluvium necks, it proved that debris flows occurred in the past.
Based on geology, we see Thac Ba formation consists of quartz sericite schist, quartz-biotite schist-garnet, quartzite (Figure 1). This collection have weathered heterogeneously with many large boulders. Magmatic rocks of the Chay River Formation consists of biotite granites gneise form with larger particles, easy weathering make up the great stone. The ancient stone of the An Phu formation including marble contain graphite, marble dolomite, biotite-muscovite schist alternating, schist with feldspar-calcite strongly weathered. With this block can develops the Colluvi-Proluvium layers which causes large landslides and debris flow generation.
