Maintenance Mechanism For Large High-speed Long-runout Debris Flow In Dongyuege Creek,Nujiang River On Low Gradient Gully Bed

The debris flow of Dongyuege Valley broke out on the morning of August 18,2010,which caused a huge loss of personnel and property.The debris flow occurred in the Nujiang basin which is covered by vegetation.In recent years,the debris flow occurred now and then,but those that can really develop into a devastating debris flow is very rare.The total length of the channel is about 13.5 km,and the channel width changes alternately.The average width is about 40 m,and the average gradient of the ditch bed is 150‰.The main trench is narrow and in the main ditch has a low gradient(3 °?5 °)ditch bed of 3 km.Under this condition,maintaining its huge amount of flux and maintain its mobility will be the key to whether the debris flow can reach the mountain pass.In order to study the maintenance mechanism of the debris flow in the process of migration,this paper studied the particle size composition and density of debris flow and the effect of small woddy debris on the remote maintenance of debris flow.The following results were obtained:(1)Compared with the debris flow in the area that lacks vegetation,the average flow,the maximum flow rate,the average density and other related characteristics of the debris flow in the debris flow of Dongyuege Valley are larger,and the scale and destructive power of the debris flow are also larger.(2)The fractal dimension of the debris flow accumulation in different sections of the debris flow of Dongyuege Valley has a certain variation with the range between 2.8403 and 2.9435,and the average fractal dimension is 2.8975.Debris flow particles showed a typical exponential distribution and the index value is small.The high fractal dimension(2.8975)and the small index of the debris flow in Dongyuege Valley show that the debris flow accumulation body has the characteristics of particle size composition of highly viscous debris flow.(3)The debris flow in Dongyuege Valley has been developing continuously along a path of up to 13.5 km.The structure of the particles is constantly changing and the structural strength is increasing.The particle size enveloped by of the debris flow slurry is gradually increased and particle sorting is getting worse.The changes of these debris flow provides the conditions for maintaining high volume and high carrying capacity for debris flow.At the same time,it also provides the necessary conditions for liquefaction maintenance in the process of long-distance migration of debris flow and determines the fluidity of debris flow and thus creates the conditions for the development and evolution of debris flow.(4)Adding the small woody debris to the soil-water mixture without woody debris can increase the yield stress and viscosity of the mixture.The addition of small woody debris increases the structural strength of the slurry,making the structure less susceptible to damage,and more energy is consumed in the process of fluid migration,Therefore,the fluid is more likely to be suspended and agglomerate a large amount of loose materials,hence provides the condition for the expansion of the debris-flow scale.(5)In the area which is well vegetated,there are a lot of different kinds of woody debris in the channel,which can obviously affect the properties of the mixture after mixing with the ordinary soil-water mixture.Under certain conditions,it is easier to promote the transformation of the two-phase fluid into one phase.The results show that the addition of small woody debris(less than 1 m in length and less than 0.1 m in diameter)will affect the fluidity of debris flow and reduce fluid loss and particle sorting in the process of migration.Thus,in the mountainous areas abundant vegetation,the presence of small woody debris provides a good material condition for the transition from immature debris flow to mature debris flow.(6)Dongyuege debris flow affected by grain composition,density and small wood debris.so in the migration process,they ensure the basic conditions of debris flow development and maintenance and create favorable conditions for the debris flow in the low gradient gully bed maintenance.