| There are various types of debris flow, such as storm debris flow, glacial debris flow, earthquake debris flow, manmade debris flow and the like. Earthquake debris flow had been paid a lot of attention to in our country in 1970s when China entered an active phase of earthquake. Debris flow was also very active at that period of time. The 8 magnitude earthquake breaking out in Wen Chuan County, Si Chuan province on May 12th, 2008 had caused many debris flows, especially the one on September 24th, 2008 near Wei jia gou, Bei Chun County, the ensuing one on July 17th, 2009 and the super-large one on August 13th, 2010. They all had serious influence on the production and life of the disaster areas. The formation conditions of debris flow, from loose solid substance source, micro topography to water source, have undertaken huge change due to earthquakes.On-site investigation indicates that rocks in research areas of Wen Chuan meizoseismal area are mostly hard rocks and a relatively large amount of collapse and landslides exist. There are mainly two types of debris flow: one is the block form when substances caused by landslides and collapse due to earthquakes block water channels and formed barrier lakes; the other is channel start form when lots of solid substances accumulate in channels but don't form barrier lakes, but under the influence of a certain amount of rainfall cause a large scale of debris flow disaster, the fire hose effect. The latter discussed in this paper is channel start form.Based on detailed outdoor investigation and data acquisition of debris flow areas at Hong Kou water channel, Du Jiang Dam, Si Chuan province, this paper studies the substance source at accumulation area of water channels affected by debris flow subsequent to an earthquake and the characteristics of water channels. Through indoor experiment, the start mechanism about how accumulations in flood start channels form a debris flow in meizoseismal area after an earthquake is discussed. At last, the following conclusions are reached:A. Characteristics of channels in meizoseismal areas are as follows: topographic conditions have changed greatly; conditions, especially source condition, gully bed condition and water source condition, in channels bursting into debris flows have also changed. Earthquakes have caused lots of landslide collapses in watersheds, largely increasing loose substance reserve there. Besides, there is, in the watershed, plenty of underlying landslides mass which is likely to be activated under certain influence and conditions some day later and enters into channels to block them. A large number of loose substances also accumulate in the gully bed. Some landslides collapses caused channel block directly, leading to great changes of gully bed condition. And because of changes of source and gully bed conditions, a certain amount of rainfall before an earthquake is more likely to bring a large scale of debris flow after the quake. Such debris flow gully debris flow subsequent to an earthquake is not easily detected, thus will make greater influence on production and people's life.B. According to outdoor investigations, there is a relatively large amount of loose substances accumulated in channels affected by debris flows subsequent to an earthquake. Grains in the rough levels are relatively even with uneven factor Cu less than 3 and are angular with poor rounding degree. It's easy to make mechanics analysis of such kind of grain.C. Experiments indicate that under the same slope average flow speed and unit discharge required when forming a debris flow increase as median particle diameter increases; under the same grading, i.e. when d50 is same, the former decreases as the latter increases. Outdoor ditto.D. During the verification of the existing formula( He Wenshe Formula) experiment data tendency agrees with He Wenshe Formula tendency, proving the experiment is correct and feasible. Differences between the results reveal the limitation of He Wenshe Formula in the utility of critical condition forming a debris flow. There are several reasons: a) The grains'diameters(signified by median grain diametre) in the experiment differ greatly from those applied in the deduction of He Wenshe formula; b) Uneven factor is another limit. There are even grains and uneven grains. Compared with those of He Wenshe formula respectively, the latter are all natural grains. Analysis result indicates that even factor of natural grains is below 3 while uneven factor of natural grains is above 6. The influence of uneven factor towards the experiment can't be ignored, thus d50 and Cu should be introduced to the formula.E. The formula below is the critical flow rate formula forming a debris flow, with the influence of uneven factor and median particle diameter introduced. F. Through outdoor regression verification, the critical unit discharge q starting the debris flow on July 17th, 2009 in Bai Guo Channel and Death Channel. The q value of Bai Guo Channel is 0.52m~3/s·m, equivalent to the debris flow started by the flood bursting out 10 years a time. The a value of Death Channel is 0.58m~3/s·m, equivalent to the debris flow started by the flood bursting out 10 years a time. |
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