Research On The Initiation And Motion Of Gully Debris Flows In Tibetan Plateau

Most of debris flows distributed along deep incised valleys worldwide.The Tibetan plateau has been uplifting since Holocene and suffering intensive invasion by the rivers on the plateau.The dense distribution and large scales of the debris flows on the plateau are attributed to the deep incised V-shaped valleys,which supply sufficient energy for gravitational erosion and mass movement.The world's large scale debris flows with volumes larger than one million cubic meters majorly distributed on the Tibetal plateau.The debris flows on the Tibet are divided into two types:(1)the debris flows along the deep incised rock valleys,e.g.Jinsha River,Nu River and Palongcangbu River valley,and(2)the debris flows along the incised sedimentary basin valleys,e.g.the Tongde basin.Different geological conditions lead to different characteristics of debris flow formation.Debris flows along the incised rock valleys have been affected by tectonic motions and seismic activities,therefore the debris flows mainly distribute along the faults and the seismic zones.The Tongde basin on the northeast edge of the plateau was a sedimentary basin with large amount of sediment deposition long time ago.As the basin has being uplifting and meanwhile incised by the Yellow River,the basin changes from deposition zone to erosion zone.The loose and flow-sorted sediment materials were beneficial to debris flow formation under the conditions of short rainfall in summer.Nevertheless,the occurrence of the debris flows in the basin were more frequent than that of the debris flows in the incised rock valleys.Field surveys found that many collapses and landslides distributed on the both sides of the debris flow gullies.The variation of watershed topography under the condition of debris flows showed that the collapses and landslides were vulnerable to erosion and flushed into the debris flow motion.The erosion of the loose materials on the banks played a significant role on the formation and motion of the both types of debris flows.Both types of the debris flow gullies require certain minimal gully gradients to provide potential energy and certain minimal catchment areas to provide water energy.It is hard for debris flow formation under the condition of small drainage area even though the gulley gradient is high;similarly,it is hard for debris flow formation with gentle gully gradient even though the drainage area is large.Experiments were designed to reveal the mechanisms of debris flow formation and motion.It was showed that the average velocity of debris flow head was proportional to the flood discharge and the gully gradient and inversely proportional to the volume of the debris flow head.The flood discharge and the gully gradient represented the supply of energy,while the volume of the debris flow head represented energy consumption by the particles.The experiments also indicated that the maximum effective basal normal stress was at the debris flow head.The average velocity of the water flow was faster than that of the particles flow,and both were faster than the velocity of the debris flow head;therefore the water and particles flew behind the debris flow head passed their kinetic energy to the flow head.Bank erosion enhanced basal resistance and energy consuming,but the higher efficiency of energy supply from the flow body to the flow head made the head velocity of the debris flows for bank erosion-dominated conditions was higher than that for bed erosion-only conditions.Bank erosion reduced the condition of debris flow formation and increased the scale of debris flows.