The Methodology And Framework Study Of Urban Debris Flow Risk Assessment

The Richter scale 8.0 earthquake in Wenchuan on 12 May 2008, in Sichuan Provence, southwest of China caused serious casualties and property loss, and caused serious damage to local geologic environment, this not only blocked the reconstruction process in stricken areas, but also give austere challenge to ecologic recovering and the continuable development. There distributed numbers of landslides, collapses, potential unstable slopes in strong seismic zone, these geologic hazards has formed enough moveable materials ,this is benefit to debris flow pregnant. Based on the first step investigation results in earthquake area, there have more than 5 000 geologic hazards distributed in Wenchuan earthquake area. Most of these geologic hazards are landslide, collapse, unstable slope and debris flow. The investigation date of 44 serious damaged countries show, there have more than 872 potential debris flow gullies in this area, among them, 90 debris flow gullies belong to super-huge magnitude, 91 huge magnitude, 300 middle magnitude and 355 little magnitude. The survey result come from Chinese Ministry of Land and Resources show, there are more than 24 country-level cities and 100 towns or villages was endangered by those potential debris flow,On 24, September 2008, only 4 moths after"5.12"Wenchuan earthquake, in the strong damaged areas Wenchuan and Beichuan occurred regional debris flow, caused more 40 people die or missing, thousands acres fertile land and hundreds of houses be buried and damaged, the magnitude and the power of this debris flow was rare, and no debris flow can has equal power to this in other areas. This regional debris flow show that the debris flow in earthquake area has come into a active stage, and the debris flow will be one of the most severe geologic hazards in this area during the future 5 to 10 years. How to carry out the risk assessment and risk control to those potential debris flow is necessary and important, at the same time, this is a prominent problems in reconstruction processing. So to constructing the systematic urban debris flow risk assessment in earth quake area is significant to mitigation plan and reconstruction. Because of the lack of systematic debris flow risk assessment method for earthquake urban, based on the geology theory, geomorphology theory, system science, manage theory and relative theories research progress, based on the date from occurred debris flow investigation and interpretation from high resolution remote sensing image(the resolution is 0.5m), the thesis carried out systematic research of urban debris flow risk assessment method in strong shocked earthquake area. Based on the framework of risk assessment (hazard and vulnerability), added the urban materials bear capability to debris flow factor, to put forward a new debris flow risk assessment framework for urban debris flow. Based on the assessment index construction, use the software of GIS(Arcview), AutoCAD, Matlab to statistics and calculate relative date, carried out risk calculate and assessment, formed a systematic method of earthquake area urban debris flow risk assessment, the processing include: date collect→hazard assessment→hazard subarea→vulnerability assessment→debris flow bear capability assessment→potential loss evaluate→risk assessment. In total, the thesid carried out the relative content study as fellow:(1) Basic theories analysis of risk assessment. Based on numbers of existing relative researchs, we summarized these theories systematicly, promoted and supplemented the risk assessment contents, then, mixed the the urban objict's sustainability to debris flow into risk assessment framework based on existing risk framework (which comprised of hazard and vulnerability).(2) The study of pattern theory of utban debris flow risk assessment. Based on the relative theory analysis and revise of exiting risk assessment content of debris flow risk assessment, we analyzed the urban debris flow risk assessment method and content, constructed then relationship between risk and hazard, vulnerability and the resist capability of potential suffers, at the same time, we studied the sustainability of urban objicts to debris flow, include the assessment index system and method.(3) The technology and method study of urban debris flow in strong seismic area. By combining the remote sense (RS) and GIS technology and the characters interpretation of debris flow gully and urban objicts in strong seismic area, the thesis carried out spatial analysis, statistic and calculate of debris flow hazard and vulnerability under the GIS sofeware. At the same time, combined the investigation of typical debris flow in strong seismic area to inspect and verify the interpreted result from remote sense image, finally, getting the debris flow hazare map, vulnerability map and risk map.(4) The application of debris flow risk assessment theories.We taking Wenchuan city as an example which is one of the serious effected by"5.12"earthquake to apply the risk assessment theories constructed in the thesis, after the investigation of the two debris flow gully which both are threaten the city, and the potential runout distance calculation of the two debris flow, carried out hazard assessment, vulnerability assessment, the sustanbility of the urban object, risk calculation ande risk subarea study, finally, analyzed the possible debris flow risk and it's potential danger area.Base on those study, the thesis come to these conclusions.(1)We summarized the basic characters of debris flow in earthquake area. After the analysis of the mechanism and active characters of occurred debris flow, drew many typical characters of debris flow in strong earthquake area:①The frequency of debris flow in earthquake area has the upturn tendency, based on the precipitation date analysis of Beichuan country after the"5.12"Wenchuan earthquake, the critical value of precipitation of debris flow relative lower than before earthquake, after the earthquake, the proceeding accumulate precipitation value before debris flow occur was 14.8～22.1% before earthquake, and the precipitation value of per hour was 25.4～31.6% before earthquake;②From the aspect of debris flow type, the torrent before earthquake has the tendency of convert to low volume weight debris flow after the earthquake, from the aspect of debris flow pregnant environment, many gully-debris flow has the tendency of convert to gully-grade mixed debris flow;③From the magnitude of debris flow, the magnitude after the earthquake is tens times of before earthquake.(2)Based on the topography statistic of occurred 72 debris flow in Beichuan country, we geting the debris flow basin characters:①Compared to another areas, the debris flow basin areas distributing ranges is more wider in strong seismic area;②Compared to another areas, most of the debris flow longitudinal river slope range is more wider, the value are fall between 130～140% in earthquake area,;③The average slope in debris flow initiation area in strong seismic area is 33°, similar to another areas(25～40°), most of debris flow tap drain length are fall between 1.5～6 km, compare to another area debris flow, the value has a little low.④Most of the debris flow alluvial fan slop are fall between 2～10°.(3)The thesis put forward the debris flow run out distance evaluate model, based on this model, building the debris flow hazard assessment method in earthquake area. Sported by the GIS tool, statistic and 51 debris flow run out distance which has occurred on 24 August, 2008, and analysis the basin topography characters, then get the run out distance assessment model.(4)Put forward the method of debris flow hazard subarea assessment, which is based on the topography date of debris flow potential accumulate area slope analysis which draw from the digital elevate model(DEM), then, proposing the debris flow subarea method which is based on the accumulate area slop analysis and the distance from gully. The classify method is: if the accumulate area slope <10°,this match with the high hazard area, the slope fall in 10～15°, match with middle hazard area, the slope fall in 15～20°, match with the low hazard area. As the same method, if the distance from gully <100m, match with high hazard area, the distance fall in 1～200m, match with the middle hazard area, the distance fall in 200～300m, match with the low hazard area. After the application of this method of 3 debris flow in Beichuan country, the result show that this subarea method is feasible.(5)The thesis constructed the index system and assessment method of resist capability to debris flow for earthquake area urban, based on the analysis of the factors which effect the object's capability of resist to debris flow, constructed the assessment index system, and calculated the weights of each index, then mixed the resist capability to debris flow into risk assessment system, after the classification of capability, give each classified a certain value, and put this value to calculate with hazard and vulnerability assessment result, finally, get the risk assessment results.(6)Put forward the semi-quantitative assessment method based on the index evaluation. Based on the remote sense image interpretation and investigation of study area, classified the objects in urban area (including population, houses, life line facilities, agriculture land), based on the relationship analyze between building type and vulnerability (the different population density and different building type has different vulnerability), evaluate the relative value to different type of objects, then, multiplication these index value to express the vulnerability. Finally, give a semi-quantitative vulnerability assessment result according the criterion of classification.(7)Put forward systematic risk assessment method of strong seismic area urban debris flow, and carried out semi-quantitative risk assessment. The thesis put a integrative study, including the definition of risk, the content of risk assessment, assessment steps and assessment method, carried out systematic risk analyze, risk calculation, and risk assessment. Through the calculation of quantitative evaluated index, put forward the classification criterion of risk, the research has practical significance to complete the risk assessment theories and method.