| Debris flow has great destructive，an important way to reduce its hazard is toestablish monitoring and early warning on debris flow.In the flood season after the5.12WenChuan earthquake, especially the outburst of debris flow in three areaSichuan on mid-August2010, which are characterized by group properties, sudden occurring features, destructiveness and hazard chain effect.It is confirmed bydebris flowinduced bythe occurrence of magnitude7earthquake, on april20,2013in Ya’an, Sichuan Province. Monitoring is to obtain the ideal way to study debris flow data, Conducive to accurate understanding of the start-up and development process of thedebris flow,Early warning can extremely reduce the harmfulness caused by debris flow. Therefore, the critical value of monitoring and warning on debris flow in intense seismic area is established by researching the characteristic of debris flow in three area on WenChuan earthquake for accurate warning, which has important theoretical significance and social meaning.In this paper, the investigating is conductedfor seven debris flow gully in typical of the three area in Wenchuan earthquake zone, the statistical results combinedwith survey data show that the debris deposits source account for more than50%of total content source in Hongchun gully, Shaofang gully, Niujuan gully and Zoumaling gully. In the six debris flow gully as the major object of study, the debris deposits is the main supply source for formation of debris flow, which account for more than40%. Other types of debris flow material sourcearepotential debris depositssource, channel deposits type source and slope shallow accumulation of deposits.By researching the rainfall time and spatial distribution pattern in three area, itis summed up that the rainfall capacitythat account for more than60%of total rainfall throughout the whole year occurs mainly in flood season (from june to september), the rainfall capacity has obvious interannual variation in flood season, it should keep concern on the rainfall monitoring in flood season. Besides, the spatial distribution pattern of a rainfall is uneven, during a rainfall process, it has different rainfall peak in different position in gully, the rainfall capacity and its peak value arealso different between each position and elevation in gully. During the study of relationship between rainfall capacity and elevation, it finds out that the coupling fact ors of rainfall are more than just elevation, there are other factors, for example, general atmospheric circulation, moisture content, mountain chain, air temperature, solar radiation, terrain steep slow and other factors. It also has discovered that it has not obvious linear relation between rainfall and elevation, the rainfallwould not increase with the increase of elevation. Therefore, the choosing of reference rain gauge in the process of monitoring and warning of debris flow in three area, should mainly consider the position in the formative region of debris flow and major source besides the the factor of elevation.In this paper, with reference to debris flow rainfall warning method establishedby Chyan-Deng Jan and Li Wenxi, An instant rainfall triggering index(RTI) and rainfall warning critical values in three area are studied. The rainfall event (direct trigging rainfall) and antecedent rainfall (indirect trigging rainfall) occurred in debris flow are calculated by discussing the partition standard of rainfall process, the definition of rainfall parameters and rainfall warning critical values. The instant rainfall triggeringindex (RTI)for evaluating debris-flow occurrence probability is defined astheproduct of hourly rainfall intensity (I) and the effective accumulatedrainfall (Rt). On the other sides, The Logistic distribution is used to describe debris-flow occurrence probability and determine the critical values of RTI for rainfall-baseddebris-flowwarning in three area where have many debris-flow events. Besides, in order to show the possibility of continuous rainfall process trigging debris flow clearly, the value ofRTI20under that the debris-flow occurrence probability is less than20%is defined as the lower critical value for debris-flow warning, while the value of RTI90above that the debris-flow occurrence probability is higher than90%is defined as theupper critical value for debris-flow warning, the value of RTI60is defined as the middle critical value. The rainfall-based debris-flow warning regime is divided into four regions: the blue region (RTI≤RTI20); the yellow attention region (RTI20<RTI≤RTI60); the orange caution region (RTI60<RTI≤RTI90); the red warning region (RTI≥RTI90), where the blue region is safe area, the yellow attention region is low occurrence area of debris flow, the orange caution region is middle occurrence area andthe red warning region is high occurrence area. In a word, the possibility of occurrence of debris flow is higher when the value of RTI gets closer to the upper critical value.By analyzing the instance of debris flow event occurred in Wenjia gully and Zoumaling gully on July8,2013, Niujuan gully on July10,2013, it gets the rainfal l parameters such as hour raininess during the rainfall and antecedent effective accumulated rainfall. The results are drawn in the rainfall warning figure to verify thepresented debris flow warning model.After the occurrence of debris flow, the deposits are turned into new gully source, the accurate measurement of volume of debris flow deposits is advantageous tothe correction and research of debris flow warning model. Therefore, in order tomeasure the gully landform and volume of deposits debris flow accurately, the three-dimensional laser scanning method is adopted. First, using the three-dimensional laser scanning for debris flow gully and reservoir area of control engineering worksbefore the debris flow, to obtain the first phase three-dimensional point cloud data,after the outbreak of debris flow, it can get the second phase point cloud data. Thetwo-period data are superimposed for comparison, it can get the scope and depthof gully eroded by debris flow, and the volume of deposits debris can also be calculated accurately. |
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