| Rainfall induced debris flow is very complicated. Based on the rainfall conditions of debris flow,the forecasting, monitoring and early warning on debris flow has become one of the study hot and important means of disaster prevention and mitigation in recent years. However, due to the rainfall process itself is complicated and changeable, the process of rainfall influence on solid materials supplying and debris flow triggering is more complex, which makes the critical rainfall condition of debris flow is very difficult to accurately obtain. As a result, the raining process and its influence on debris flow initiation has been the weak linkto the mechanism and the critical rainfall conditions of debris flow.In this paper, Dadu River Basin (Xiaojin-Ganluo section) , Anninghe-Zemuhe fault zone and the Wenchuan earthquake seriously disturbanced zone were selected as the study area, where debris flows are widely distributed, disaster mechanisms are typical and disaster situation is serious. Based on the comprehensive collection and in-depth analysis on the debris flow events and corresponding rainfall data (including hourly rainfall process the daily rainfall), the concept of rainfall pattern was introducedand the raining process corresponding to debris flow initiation have been characterized and reflected. Through rainfall pattern, the influence and triggering action of rainfall on debris flow from the regional and temporal scale was compared and the critical rainfall conditions of debris flow occurrence under different rainfall patterns was explored. This study has theoretical significance in improving the recognition on debris flow initiation and practice reference for the forecasting,disaster prevention and mitigation on debris flow hazard.Through the comparsion on domestic and foreign research status, data collecting and analyzing, physical simulation experiments of debris flow initiation conducting,mathematical statistics, some achievements and conclusions have been obtained in the following 6 aspects:(1) The introduction of rainfall pattern concept associated with debris flow occurrence and the establishment of rainfall pattern system. In this paper, the rainfall pattern concept which ought to be applied to climatology was introduced and corresponding concept of rainfall pattern associated with debris flow occurrence and the framework of rainfall pattern system was put forward which makes it possible to characterize and reflect the rainfall process on debris flow occurrence. The concept of rainfall patternassociated with debris flow occurrence is based on rainfall intensities and duration curve. The framework of rainfall pattern system of debris flow is formed based on temporal scale, spatial scale, intervals between rainfalls, rainfall peak values and trend of rainfall. The introduction of rainfall pattern of debris flow occurrence and the rainfall pattern system establishment lay the foundation for rainfall pattern respond to debris flow.(2) The exploration and construction of rainfall pattern system associated with debris flow occurrence based on 24h and 1h time scales. Through comprehensively assessment on the contribution between the precipitation before debris flow occurred and the intraday precipitation when debris flow occurred, the rainfall pattern associated with debris flow occurrence based on 24h scale was divided into 4 types,including disaster-controlling type by heavy rainfall with short duration (Pattern A),disaster-dominating type by heavy rainfall with short duration (Pattern B),mutual-triggering type by antecedent effective rainfall and intraday rainfall (Pattern C)and disastr-dominating type by antecedent rainfall (Pattern D). According to the the relationship between temporal distribution of hour rainfall and the time when debris flow occured, rainfall patterns based on 1h scale wereclassified 4 types: no peak type,pre-peak type, middle-peak type and post-peak type.(3) The division and comparativ study on rainfall patterns of 24h scale and debris flow occurrence based on. Based on the analysis of debris flow formation conditions and the disaster characteristics in the Dadu River Basin (Xiaojin-Ganluo section),Anninghe-Zemuhe fault zone and the Wenchuan earthquake seriously disturbanced zone, the antecedent effective rainfall 7 days before debris flows occurred, the intraday rainfall and the total rainfall for 315 debris flow events occurred in recent years have been counted and corresponding rainfall patterns associated with debris flow occurrence were classified in accordance with 24h time scales. From the overall classification results, it can be seen that debris flow occurrence corresponded to the 4 types of 24h time scales are all accounted for more than 20%.Among them, the mutual-triggering type by antecedent effective rainfall and intraday rainfall (Pattern C)was the most one with a 29.5% percentage, indicating that the occurrence of debris flow is usually the consequence of bothantecedent rainfall and the heavy rainfall with short duration. Followed are disaster-controlling type by heavy rainfall with short duration (Pattern A) and disaster-dominating type by heavy rainfall with short duration (Pattern B), accounting for 24.76% and 24.44%, respectively, showing the triggering action of short-duration rainfall on debris flow occurrence. About 21.27%debris flows belong to the disaster-dominating type by antecedent rainfall (Pattern D),indicatingthe contribution of antecedent rainfall to debris flow occurrence. At the same time, there was a spatial heterogeneity on rainfall patterns of 24h scale.Pattern B and Pattern D take a less percentage in the Dadu River Basin (Xiaojin-Ganluo section). Pattern B counts a little more in the Wenchuan earthquake seriously disturbanced zone. In Anninghe-Zemuhe fault zone, Pattern Bis significantly less than other regions. According to the comparison on the rainfall distribution associated with 315 debris flows, the contribution weight of antecedent effective rainfall and intraday rainfall can be estimated as 0.65.(4) The comparative study onrainfall patterns based on 1h scale associated with debris flow occurrence. According to classification method, rainfall patterns with 1h scale were classified according to the relation between the peak hourly rainfall distribution and the occurrence time of debris flow for 31 events in the 3 study regions. The results show that no peak type was rare, while peak type was the vast majority. Up to 54.8% of debris flows, which occurred synchronized with peak hourly rainfall arised,belong to the middle-peak type. About 32.3% debris flows belong to post-peak type and occurred after peak hourly rainfall appeared. Pre-peak type debris flow was fewer, accounted for nearly 10%.(5) Physical simulation experiments of debris flow initiation conducting and the related phenomenon analyzing. In this paper, taking Xiongjia gully in SW China as an example and on the basis of artificial rainfall experiment, initiation of gully-type debris flow has been studied. Experiment results indicate some relation between rainfall intensity and gully erosion, failure mode of soil mass, initiation mechanism and characteristics of debris flow. Based on experiments, conclusions were drawn as follows: 1) Under strong precipitation, infiltration rate and soil water content in different depth are inversely proportional with rainfall intensity. Intense rainfall favors overland flow, gully runoff and erosion, but not conducive to water infiltration. 2)Slope failure modes and debris flow initiation mechanism are various under different rainfall and runoff condition. Under the condition with rainfall intensity of 55mm/h,the slope failure mode presents soil liquefaction and landslide. Accordingly, debris flow initiation mechanism belongs to landslide transformation. However, under the condition of intense rainfall and runoff, gully beds are easily to be eroded and slopes are prone to collapse. Then, debris flows occurred with initiation mechanism of entrainment. 3) In terms of debris flow characteristics, debris flow occurrence process consists of several intermittent flows. In addition,debris flow magnitude and flow viscosity are not consistent with rainfall intensity. On the contrary, under condition of intense rainfall of 65mm/h and 75mm/h, debris flows tend to be watery. But with rainfall condition of 55mm/h, the flow viscosity is higher. Experiment results are well consistent with the natural debris flow occurrence from Xioangjia gully. Experiment results indicated the relation between debris flow occurrence and rainfall process or pattern to some extent.(6) Study on the critical rainfall conditions of debris flow occurrence based on 24h and lh time scales. Selecting the intraday rainfall, antecedent effective rainfall and total rainfall as precipitation parameters, the critical rainfall conditions of debris flow occurrence based on 24h time scales was established by single-line method and the reletion between intraday rainfall and antecedent effective rainfall in the Dadu River Basin (Xiaojin-Ganluo section),Anninghe-Zemuhe fault zone and the Wenchuan earthquake seriously disturbanced zone. As for the critical rainfall condition of debris flow occurrence based on 1h scale, average hourly rainfall and rainfall duration were selected as as precipitation parameters and R-D curves between average hourly rainfall and rainfall duration have been established. Based on the R-D .curves, critical conditions of hourly rainfall in the Dadu River Basin (Xiaojin-Ganluo section) , Anninghe-Zemuhe fault zone and the Wenchuan earthquake seriously disturbanced zone have been established.The study on the critical rainfall condition and the forecasting and warning on rainfall induced debris flow is very complex and usually influenced by multiple factors. In this paper the rainfall patternand corresponding influence on debris flow occurrence wasstudied and the critical rainfall conditions were also explored.However, there are still some problems and limitations, for example the representativeness and comprehensiveness of debris flow disasters(sample problem),the reliability precipitation monitoring data, the influence of earthquake, and so on. Therefore,the study are preliminary and many scientific problems such asthe scientific division and system establishment of rainfall pattern associated with debris flow occurrence,debris flow initiation mechanisms under different rainfall pattern should be further studied. |
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