| After "5.12" earthquake event,a large amount of seismic secondary disasters which including landslides,collapses,debris-flows were induced in the meizoseismal area.Especially,the clusters of large-scale debris-flows triggered by heavy rainfall process of "9.26" in 2008,"8.13" in 2010,"7.10" in 2013,"7.03" in 2011 and "7.05"in 2016 in the meizoseismal area,which brought serious injuries and huge economic losses to local residents and huge destruction to the reconstruction engineering of wenchuan earthquake area.The active period of geological disaster in wenchuan earthquake area will last 20 years,and high-incidence season for debris flow disasters will over the next 10 to 15 years.Due to the steep topography and widely distributed co-seismic disasters,resulting in co-seismic debris flows in earthquake area also has the characteristics of gully-type debris flows and slope-type debris flows,the outbreak conditions and law of co-seismic debris flows is different from the debris flow in aseismic erea,also is different from gully-type debris flow and slope-type debris flow.Therefore,the necessity and significance to research on the types and movement condition of debris flows for the future warning and prevention.After the wenchuan earthquake,a large deposit of potential landslides,collapses,and landslides distributed in the basin,which provided large amounts of provenance for co-seismic debris flows.And the physical properties of provenance in wenchuan earthquake area with a low clay particles content,large void ratio,low degree of consolidation,good gradation,low grinding roundness and other,which greatly reduce the stability of the debris flow provenance,namely the steep slope terrain,relatively smaller watershed area of co-seismic debris flows,lead to the threshold value of rainfall is greatly reduced.Because of the special erosion landform condition of low-middle mountains in wenchuan earthquake area,nanely the slope of mainly channel of some debris flows are≥400 ‰,some gullies even can reach 600‰.Therefore,under the condition of early continuous heavy rainfall,the special small watershed and large longitudinal slope topography which lead to the fast catchment speed of drainage basin,fast moving speed of mountain torrent,strong impact strength,all which leading this type debris flow has features as the longer rushed out distance,the shorter outbreak time,the more difficult prevention and the higher possibility of damming river.On this basis,compared with the previous study on debris flow types,the urgent steep-channel debris flow defined as the channel type debris flow with the channel longitudinal gradient of debris flow is≥400‰,according to the statistical analysis and theoretical analysis of watershed topographic feature of debris flow gullies and stability of provenance in earthquake area.The most of deposit of collapses and landslides deposited on the inner channel which according to special landform of steep slope of drainage area(Abbreviation:landslide accumulation body in gullies slope),which will serious intensified the co-seismic debris flows’ channel congestion,which can even forming barrier dam in the gullies.Under the condition of heavy rainfall and catchment,the mountain torrents moved fast as "fire hose" effect and scoured provenance.According to the source of provenance of the urgent steep-channel debris flow in the Wenchuan earthquake area,the provenance of the urgent steep-channel debris flow classified into the channel landslide-accumulation and the channel-deposits,and according to the distribution location of provenance divided into the forming-region provenance and circulating-region provenance.Starting mode of urgent steep-channel debris flow can be divided into three modes;1),Channel landslide-accumulation starting;2),Forming-region channel-deposits starting;3),Circulating-region channel-deposits starting.And the starting meachine of provenances of urgent steep-channel debris flows can be classified as saturated seepage failure starting and mountain torrents washed starting,according to the soil mechanics and hydrodynamics.The rainfall condition is the mainly exogenous reason which induced urgent steep-channel debris flow in Wenchuan earthquake zone.The water content of deposit located in urgent steep-channel debris flow gullises suddenlly increased under the rainfall condition,and the cohesion and internal friction angle of deposit decline dramatically,when the shear strength of deposit is smaller than the shear stress,progressive failure occurs along the shear sliding surface to aggravate the channel damed,and under the scouring effect of mountain torrents,induced the channel landslide-accumulation "Secondary Starting".Since the steep vertical slope of channel and the accumulation of debris flow gullies,the starting model of channel provenance of urgent steep-channel debris flow is similar to slope-type debris flow,the formation of pore pressure and osmotic pressure in deposits increased under rainfall conditions,so that the shear strength of channel deposits decreased,the deposits shear failure occured along latent sliding surface;And the mountain torrents formed in upstream of mountain torrents with the high hydrodynamic pressure similar the "fire hose" effect to scoured the deposits in circulating-region.Urgent steep-channel debris flow "fire hose effect,controlling factors as follows:l),Because of the confluence speed,high conversion efficiency of potential energy to kinetic energy,the flow rate increased rapidly moving along the channel,and the initial runoff at the front end of the flow depth is approximately zero,so the runoff formation high dynamic water pressure difference,and the initial flow depth is approximately zero,all which leading to the high dynamic pressure formation.2),because the "funnel beam,due to width of forming-region channel in upstream Wider,and the circulation area channel width in downstream narrower,which leading to the increases of the flow depth and kinetic energy under mass conservation condition,and then the formation of a high dynamic pressure.After the earthquake event,\a great deal of geological disaster prevention project constructed in earthquake area,due to actively prevented earthquake disasters behavior of the state and the local,and the prevention and control engineering of debris flow mainly by obstruct,block and row,and silt dam,resulting in runout scale and rushed out of the distance of the debris flow greatly reduced by the attenuation of dynamic characteristics such as flow rate,flow velocity,bulk density.Based on the theory of fluid mechanics and elastic-plastic mechanics,the dynamic model of debris flow and dynamic parameters of debris flow were established.In summary,through to the terrain slope,drainage area,relative elevation,Shape factor and other factors,the urgent steep-channel type debris flow initial established in the Wenchuan earthquake area.On this basis,three deposits starting modes established through the provenance distributing and provenance source.And base on the different damage accumulation of different failure angle,the potential failure angle mode of landslide accumulated in channel was established;further the accumulated geometric model,seepage failure model,runoff starting model was established.On this basis,according to a debris flow "fire hose effect",NS continuous equation,the "Bernoulli effect" and the classical Newtonian mechanics theorys,kinetic parameters of urgent steep-channel debris flow,erosion model based on flow depth and density was established.Based on the attenuation characteristics of the velocity and the flow rate of debris flow moved under the dam condition,the motion characteristics and attenuation coefficient of the debris flow under the dam condition was established.Provide new insights and a certain reference for understanding and prevention of debris flow in Wenchuan earthquake area. |
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