QUANTITATIVE EVALUATION OF ENGINEERING TREATMENTS FOR PREVENTION OF DEBRIS FLOW AT WENJIA GULLY

After Wenchuan Earthquake in China, debris flows as a kind of geohazard occur frequently in Wenjia Gully. The debris flows have already cost millions of property and casualty losses to the local community. For the purpose of preventing debris flows, geological engineering treatments have been implemented in multiple locations of Wenjia Gully. The adopted preventive measures include water and sediment separation upstream, reinforce bottom of channel reinforcement and slope protection midstream, damming against and silting downstream. To evaluate the effectiveness of these treatments, this paper presents a data-driven method to conduct this research by collecting data during five seasons of heavy rainfalls. Survival analysis, Bootstrap method and Extreme Learning Machine(ELM) are selected to build data-driven models. By implementing survival analysis models, the survival probabilities of locations without treatment decrease to 0%.The survival probability of the locations with treatment stay at 55.6%after the five rainfall seasons. Meanwhile, the maximum hourly rainfall in the post-treatment period is 2.571 times as the one in pre-treatment period. Total rainfall volume of post-treatment period is 1.232 times as the one in pre-treatment period. Total time period of rainfall of post-treatment is 5.435 times as the one of pre-treatment. Comparing the survival probabilities, the effect of treatments are significant in the prevention of debris flows. After resampling by bootstrap method, the predictive results from Extreme Learning Machine indicate that, without geological treatment, the probability of having debris flow under 10 times of heavy rainfall is 100%.It is much higher than the observed 30% between the year of 2011 and 2015. The predicted debris flows magnitudes are also significantly higher than the observed ones with treatment. Hence, geological engineering treatment is crucial in reducing and preventing geohazards.