| Wenchuan earthquake triggered a large number of mountain collapse andlandslides, which provided a rich source of loose material to make Debris flowhappen after the earthquake. And the landslides matter accumulated in the channels,easily forming natural blockage bodies. Under the scouring of rainfall runoff and theerosion of upstream flow, they may be prone to instability and outburst, which causesudden high intensity flood outburst in the channel, and then form the Break DebrisFlow. The occurrence and development of such debris flow has distinct characteristics,showing rapid disaster, large scale, strong destruction, disaster chain of river damming,and so on, which suggest that it is much more serious than the common rainstormdebris flow. Before the earthquake, the Break debris flows mostly occur in Xinjiangand Tibet, in western China, and mainly belong to the glacial lake outburst debris flow;and after that, owing to the large natural blockage body in the channel, it is easy tostimulate the formation of huge and high-risk break debris flow under the action ofthe rainstorm in the earthquake zone, such as the Hongchun gully debris flow onAugust13,2010, the Gaojia gully debris flow on July3,2011, and the Qipan gullydebris flow on July11,2013.Before the earthquake, people have insufficient awareness and understanding ofthe break debris flow, due to the low frequency of occurrence and the complex triggermechanism of the glacial lake outburst debris flow; and after the earthquake, there aremany break debris flows occurred in the earthquake zone, which provides somevaluable natural samples to us to get further awareness and understanding of suchdebris flows. To be able to make more effective risk assessment, early warning andforecasting, and engineering control methods, this paper has the dynamic processcharacteristics of the break debris flow analyzed and discussed as follows:First of all, through analyzing the variation of the key activity indicators alongthe flow area of three typical debris flows selected in the seismic zone, it concludesthat the break debris flow instability is mainly caused by the upstream water erosion.And the basic features of such debris flow include: its density has a obvious increase stage by the blockage body outburst, and the bulk density will fluctuate within a smallrange when the source distribution is uniform along the flow area and the blockagebodies alternate appear; the flow discharge along the channel sections shows the sharpfluctuation phenomenon, and the sediment concentration will increase by theblockage body outburst, resulting in the flow discharge amplification, and then theflow discharge reaches a maximum; the effect of blockage interception increases thesubstance potential in the channel, thereby increasing the conversion of the flowkinetic energy, which make the flow have a great impact, and thus perform a strongdestructive power.And then, according to the formation mechanism characteristics of the naturalblockage body in the debris flow gully, the forming process of the blockage body isdivided into two stages, that is, the initial formation stage and the strength formingstage; meanwhile, according to the characteristics of its composition, the blockagebody is divided into two categories, that is, the rockfill body and the accumulationbody; furthermore, considering the different effects of the blockage bodies on theformation process of the Break debris flow, the best stability blockage body in thedebris flow gully is defined as the primary one. Through the analysis on thecharacteristics of the blockage body unstable modes and the field investigation, thepaper concludes that the blockage body failure in the break debris flow gully mainlybelongs to the overtopping mode in the seismic area, and the glacial lake outburstdebris flow is mainly in the mode of seepage flow. By contrast with the dynamiccharacteristics of the typical break debris flow and the common rainstorm debris flow,the process models of the break debris flow formed by the natural blockage bodyoutburst in the meizoseismal areas are divided into three typical break chain modes.By the analysis of the rockfill body and the accumulation body using the mechanicalbalance principle, we establish and verify the simplified instability criterion of theboth blockage bodies.In addition, by the two ways of qualitative description and quantitativecalculation, this paper respectively analyzes the variation characteristics of the flowdischarge of the primary blockage body and the downstream flow evolution. Itgeneralizes the change process of the burst flow discharge into three stages, that is,initial phase, failure stage, and the falling phase; and divides the change process of thedownstream flow discharge into two categories, namely, type S1and type S2. Basedon the assumption of a one-dimensional model that the primary blockage body isbreached in the form of instant all burst, there are some formulas put forward to calculate the peak discharge and its flow velocity of the sections of the blockage bodyand the channel downstream with the Saint-Venant equations, at the same time, thispaper establishes the peak discharge calculation methods by introducing the sedimentcoefficient and the flow evolution coefficient. Validation results show that thecalculation method is better fit to the case of the debris flow gully whose channel onlyhas one main control blockage body that is breached in the form of instant all burst.Finally, the paper analyzes the dynamic processes of the Break debris flow by thephysical simulation results, and the fundamental mode of the natural blockage bodyfailure into break debris flow is proposed as follows: With the upstream flood ordiluted debris flow scouring effect, the channel blockage body may be broken on thefailure mode of overtopping, seepage flow, or slope stability, and then, it will lead tohigh intensity gully bed erosion and sediment transport, eventually, it may be evolvedinto debris flow. After that, taking the Qipan gully debris flow for example, asystematic integrated numerical approach is synthesized as follows: firstly, the CFDsoftware FLUENT is applied to generate the blockage body break hydrograph, andsecondly, the flow hydrograph is used as input to drive the software FLO-2D which isused to simulate the evolution progress of the downstream flow. |
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