| Wenchuan earthquake triggered a large number of mountain landslide, collapse, debris flow and other geological disasters in the earthquake area, which provided a rich source of loose material to make Debris flow happen after M8.0 Wenchuan earthquake. And the landslides matter accumulated in the channels, easily forming natural blockage bodies.With the excitation effect of several heavy rainstorms, it begins to form a new type of debris flow, the Break Debris Flow, which further expanded the risk area of debris flow. Since the Break Debris Flow will burst suddenly and the quality of fluffy solids it carried is heavy, the runout Volume is huge.By summarizing and arranging the topographic and geologic data, precipitation data, image data and mudslides field survey data in the study area, this paper will analyze the causes of typical the Break Debris Flow and its blocking features. Consulting predecessors' researches on runout volume of debris flow, this paper will conduct a regression analysis on the common and break type of debris flow in Wunchuan earthquake area from 8 main characteristic parameters in the basin, such as the amount of debris flow, the relative elevation basin, the longitudinal length of the channel, the total amount of source material, the channel damming body, the main groove average longitudinal gradient, the total dynamic reserves and the volume of the channel. There are following conclusions:(1) By analyzing the runout volume characteristic of 3 typical break debris flow in the seismic zone, it found that the break of blocking body within break debris flow channel mainly dues to the scour and erosion of rainfall and upstream flow within the channel. The features blocking and collapse point have a big similarity with each other. The formation of the blocking and collapse point must go through 4 processes: the clogging stage, the runoff convergence and undercurrent stage, the burst and damage stage, and subsequent mudslides stage. There are some basic features. its density has a obvious increase stage by the blockage body outburst, the flow discharge along the channel sections shows the sharp fluctuation phenomenon. The effect of blockage interception increases the substance potential in the channel, thus causes the debris flow became more destructive power.(2) By taking the slumped body, the channel provenance, the volume, area and dynamic reserves of slope provenance within 48 Watershed in Wenchuan earthquake area(including break debris flow and common debris flow) as sample data to conduct an analysis, this paper shows that there exists a significant quadratic, exponential, cubic correlation between static reserves and dynamic reserves of the slumped body, the channel provenance and the slope provenance and accordingly establishes the calculation model of total provenance and dynamic reserves of debris flow. The instantiated verification results of Niujuan Gully show that the relative error between estimate value of debris flow provenance prediction model and field measured value is 8.73% ~ 19.30%. And the estimate value could be larger or smaller than field measured value. By analyzing conversion rate which different types of provenance could convert to the runout volume, it comes to a conclusion that most of the runout volume after the outburst of blocking body in the inner trench of Niujuan Gully came from the its accumulation body(the conversion rate reached 18.71%). Accumulation body made the greatest contribution to the runout scale of debris flow. Followed is slumped body provenance and the contribution rate of slope provenance is the lowest.(3) Through the analysis of the runout volume, drainage area, relative height difference, total provenance, the longitudinal length of the channel from 34 common debris flow gullies in Wenchuan earthquake area, a high correlation could be found between these four influencing factors and the runout volume. Therefore, this thesis will use Matlab software to conduct a multi-factor regression analysis of the 4 influencing factors and the runout volume and establish a multi-factor statistical prediction model for common debris flow. The test results show that the relative error of the model is in the allowable range and prove that the model is effective for the prediction of the research area or other watershed areas which have the similar geological environment background.(4) To analyze the causes of break debris flow is to analyze the formation conditions and block collapse characteristics. According to different instability and failure mechanisms, the instability of natural blocking body can be divided into three basic modes: overtopping damage, piping damage and slip surface damage. The instability of the blocking body in break debris flow channel is overtopping damage based and it burst mode is mainly overflow erosion outburst. Due to abruptness of blocking body outburst, high intensity and a huge quantity of fluffy solids once it carried, it is easier to form a disaster which is more destructive, more harmful and with high possibility of blocking the river.(5) Taking the runout volume, the amount of blocking body, the basin area, the relative height difference, the total provenance, the longitudinal length of the channel of 9 debris flow gullies in Wenchuan earthquake area as example to conduct a correlation analysis, it will find that there has a correlation between 5 influencing factors and the outrush volume of outburst debris flow. So, this thesis will use Matlab software to conduct a multi-factor regression analysis of the five influencing factors and the runout volume of break debris flow and establish a multi-factor statistical prediction model for the runout volume of break debris flow. The test results show that the relative error of the model is in the allowable range and prove that the model is effective for the prediction of the research area or other watershed areas which have the similar geological environment background.(6) The comparative analysis of the outrush volume model of common debris flow and break debris flow, taking Taoguan Gully as an example, shows that the errors of two established models are small and the runout volume from the model of break debris flow is larger than that of common debris flow. It is the result of sample data combined with amplification effect of the burst of break debris flow. In the same basin, the runout volume after outburst is much larger than that after non-outburst.(7) By using FLO-2D to do a numerical simulation under the condition of outburst and non-outburst in Huaxi Gully, it will show that the runout volume under non-outburst condition is 24.48 × 104m3 and the runout volume under which is 12.61×104m3 larger than that of model prediction, 1.77 times larger than that under non-outburst condition, and 1.16 times larger than the runout range. The runout volume, the scope of the accumulation area, and the average stacking depth under outburst condition are twice as much as that under non-outburst condition. The runout volume under outburst condition is 5.58×104m3less than the actual survey value and its relative error is-11.39%. The results of numerical simulation show that the error of model is in the allowable range and the accuracy simulation results is 88.61% which show simulation results have good effects. Therefore, we should pay special attention to break debris flow in Disaster Prevention Design and Risk Assessment.In short, the mathematical model can quickly predict the runout volume of debris flow. Numerical simulation can intuitively get the runout volume, accumulation form of runout material and its runout range. Furthermore, and its corresponding terrain data is relatively more precise. |
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