The Propagation Mechanisms Of The Glacier-unrelated Alpine Long-runout Debris Flow In Dongyuege In Nu River

The Nu River watershed is one of the most affected areas by debris flow hazards in Yunnan Province and even in China.Frequent debris flows have become a major factor in restricting the local economic development and the successful completion of the “poverty alleviation campaign”.Nu River debris flows are typical of alpine long-runout viscous debris flows occurring in nonglaciated areas.However,the previous relative studies are mainly focused on the aspects of glacial and typical clay-mineral slurry debris flows,the results of which cannot explain the long-runout mechanism and peculiarity of the flow behavior of the glacier-unrelated alpine long-runout debris flows whose deposits do not contain typical clay minerals,such as kaolinite,montmorillonite and illite etc.Therefore,it is more pertinent and urgent to carry out systematic research on such disasters.Taking the catastrophic debris flow occurring in Dongyuege(DYG)Creek on the left bank of Nu River in Nujiang,Yunnan in August 18,2010 as the case study,the long-runout mechanisms of the glacier-unrelated alpine long-runout debris flows are investigated using field investigation,remote sensing image and aerial photo analysis,laboratory observation and experiments,which could help to deepen people's understanding of this type of debris flow disasters,and provide scientific basis for disaster prevention and reduction and risk assessment.The main results are summarized as follows:1.The DYG debris flow originates as gully wall collapses in the front of snowline in the tributary of main gully,which can be classified as glacier-unrelated alpine debris flow.The debris flow has large volume and high density,classified as large scale viscous debris flow,but it does not contain typical clay minerals and has obvious characteristics of sandy silty matrix.The DYG debris flow has a total travel distance of 11.3 km and propagates 3.3 km down an unusually low gradient of <5° upstream of the depositional fan,the phenomenon of long runout is prominent.2.The pore water escape experiments and excess pore pressure monitoring experiments show that small woody debris(SWD)can help the slurry without typical clay minerals maintain pore water and excess pore pressure(i.e.,the persistence of debris flows),facilitating the DYG debris flow to travel an extensive distance over the low-gradient channel.3.SWD favors the persistence of debris flows because of its large specific surface area and low density,which makes it able to absorb fine particles and able to be suspended in slurry flows over long timescales.In well-vegetated mountainous areas,SWD should be taken into account in the assessment ofdebris-flow hazards.4.A series of small-scale flume comparative experiments show that biofilms can significantly reduce flume resistance and greatly assist the propagation of experimental debris flows.By filling depressions and covering surfaces,slippery biofilms consisting mainly of diatoms and their extracellular mucus reduce the skin friction of the flume and greatly promote the propagation of overlying fluids.5.Acting as lubricating layers,well-developed biofilms were found on the underwater channel surfaces and they likely played a key role in the DYG debris-flow runout.Most of the debris transported during the DYG event was deposited on overbanks,and the sediment that caused the disaster was transported to the populated fan region through the stream-bed clad in the thick biofilms.Owing to their impacts on the development and width of the temporary debris dam breach,the stream-bed covered with biofilms became a direct contributor to the debris-flow hazard.The development of perennial streamflows in gullies can be viewed as an indicator of susceptibility to debris flows threatening creek fans.The underwater parts of channel cross-sections should be regarded as slip or low-friction boundaries,and the parts above stream levels should be considered as no-slip boundaries.6.The rheological tests using small volume rheometer and self-made large volume rheometer show that DYG debris flow slurry is shear thinning,and the shear thinning behavior is more obvious when the shear rate is relatively low.By rotating the slurry in a single-cylinder agitating apparatus,it is found that the grain agitation play a positive role in elevating the excess pore pressure.Interparticle collisions can favor grains to fluctuate and float on the pore fluid,and then elevate the excess pore pressure consequently.With the increase of velocity,more grains with greater diameter float on the pore fluid,resulting in the excess pore pressure rises further.7.The elevated excess pore pressure produces a positive feedback that reduces the viscosity and make slurry becomes thin,facilitating the flow mobility.The improved flow mobility,in turn,elevates the excess pore pressure and promotes the continuous thinning of slurry,working in a positive feedback loop(mobility-rheological behavior).The upper limit of this positive feedback loop is all the debris liquefied.This positive feedback loop is favor of improving the velocity and momentum of the debris flow after the short-lived dam breaks with narrow breach,which makes the mixture just like the “jet flow” that spouts out of the Creek mouth and buries the fan suddenly.It is the “jet flow” that produces the gentle-slope fan with an average of 2° and large diffusion angle of 170°,causing a devastating to people and infrastructure on the fan.