| The mostly of southwestern regions in china are in the complex geological conditions and geographical environment, the crustal faulting is widely distributed, and the climate conditions are different in the space-time. In recent years, with the accelerated process of urbanization in the region, and the relative concentration of economic and population, especially the frequent occurred of the earthquake and the strong rainfall, the geohazards occurred frequently, and had caused the serious personnel casualties and the huge economic losses. People are more and more aware of the importance of preventing and reducing the geohazards for the urban economic development and sustainable development in the region, it is urgent to carry out the regional risk assessment for geohazards so that we can control and manage the geohazards effectively. Therefore, how to carry out the regional risk assessment for geohazards in this area is a major issue that must be faced in the urban development and construction and the planning of preventing and reducing the geohazards.Due to Wenchuan County is the epicenter of the 5·12 Wenchuan earthquake, a great number of landslide occurred in the Wenchuan area, a large number of loose deposit accumulated in the valley, the regional geological environment suffered devastating damage. It has been eight years since the Wenchuan earthquake broke out, but the surface and the ecological environment of the Wenchuan County still have not been the level before the Wenchuan earthquake, especially under the inducement of the strong rainfall in recent years, the old landslides were reactivation and expanded, even triggered the form of debris flow as the deposit source, and further aggravated its destructive. In a word, the geohazards will be in frequent and multiple periods for a quite long time. Based on the above, this paper takes Wenchuan County as an example, relying on the geohazards survey project-“the major geohazards investigation, comprehensive assessment and early warning research in the southwest area” from the china geological survey and the international cooperation project-“the study on the regional geohazards risk assessment and mapping method in the china typical area” from the state key laboratory of geo-hazard prevention and geo-environment protection. Based on the full collection of other worker's data, combined with the landslide and debris flow disasters that were interpreted precisely by using the high resolution remote sensing image, and were further validated through the field investigation, the Wenchuan County regional geohazards risk assessment was conducted by the combination of the GIS and mathematical statistical analysis models. The purposes of this paper is to provide a reference to the local government for urban planning and construction and disaster prevention and mitigation planning. The main results obtained are as follows:1, the statistical analysis of the distribution characteristics of the geohazards and the disaster bearing bodiesThe three dimensional real scene of Wenchuan County was constructed by using the terrain data and high resolution remote sensing image photographed after the Wenchuan earthquake, the landslides and debris flow were interpreted precisely and identified by using the three dimensional real scene, and further validation through the field investigation, and the geohazards and the disaster bearing bodies spatial information database is established. Based on this, the distribution characteristics of the geohazards and disaster bearing bodies were analyzed in statistics. These data provided basic support for subsequent risk assessment.2, the geohazard regional hazard assessmentAccording to the spatial distribution and development characteristics of landslide disasters in the Wenchuan County, six influence factors such as the seismic intensity, faults, drainages, elevation, slope, and lithology were chosen to be the assessment indices for the landslide regional susceptibility assessment in the Wenchuan area, the weight of each factor and quantitative value of each factor internal classification were determined by using the AHP and Information method, based on the above, the landslide regional susceptibility assessment system was constructed, the landslide regional hazard assessment model was established, combined with the rain data and the landslide regional susceptibility assessment system, for the Wenchuan County landslide regional hazard assessment.According to the development characteristics of debris flow disasters in the Wenchuan County, six influence factors such as the slope, aspect, lithology, peak ground accelerations(PGA), drainages, and faults were chosen to be the assessment indices for the debris flow regional susceptibility assessment in the Wenchuan area, the comprehensive discrimination value of the landslide probability distributed in each factor internal classification was determined by using the mathematical probability method at first, and the scoring criteria was established, based on the above, the scores of each debris flow gully distributing in six influence factors were obtained as the scoring criteria, then, the weight of each factor were determined by using the AHP method, the debris flow regional susceptibility assessment system was constructed, the debris flow regional hazard assessment model was established, combined with the rain data and the debris flow regional susceptibility assessment system, for the Wenchuan County debris flow regional hazard assessment.3, the geohazard regional vulnerability assessmentAccording to the background conditions of geography and geological environment in Wenchuan County, three influence factors such as people, building and road(highways, provincial road and county road) were chosen to be assessment indices for the landslide regional vulnerability assessment in Wenchuan area, and people data was mapped to the building point data, the density value of each factor were dimensionless through the normalization, obtained by using the kernel density algorithm, then, the weight of each factor were determined by using the AHP method, the landslide regional vulnerability assessment model was constructed for the Wenchuan County landslide regional vulnerability assessment.In view of the damage characteristics of the debris flow disasters, firstly, according the sediment volume of each debris flow, the surface of each debris flow and the relief of each debris flow obtained by statistical, the maximum threat zone of each debris flow gully are obtained by using the Tangchuan formula, combined with the catchment of debris flow, based on this, the total vulnerability of the maximum threat zone of each debris flow gully, obtained by using the landslide regional vulnerability and maximum threat zone of each debris flow gully, are regarded as the regional vulnerability value of the debris flow for the Wenchuan County debris flow regional vulnerability assessment.4, the geohazard regional risk assessment and zonation mapAccording to the comprehensive analysis of each elements for the geohazard risk assessment, the result of the geohazard regional risk assessment was obtained by using the spatial overlay operation and the normalization for result of the geohazard regional hazard assessment and vulnerability assessment, the higher the value, the greater the risk, the risk zoning was made by using the natural break point method, at last, the zoning map of the landslide and debris flow regional risk assessment were obtained.The results of the landslide regional risk assessment shows: the area of the high risk zone is 220.5 km~2, 5.4% of the total area in Wenchuan area, distributing mainly in the Yingxiu town, Xuankou town and along the Minjiang River; the area of the medium risk zone is 996.2 km~2; the area of the low risk zone is 2866.3 km~2, 70.2% of the total area in Wenchuan County, distributing mainly in the western part of Wenchuan County and the northern part of Wenchuan County.The results of the debris flow regional risk assessment shows: the number of debris flow gully belonging in the high risk area is 12, 8.2% of the total debris flow gully in Wenchuan County, distributing mainly in the Yingxiu town along the Minjiang River; the number of debris flow gully belonging in the medium risk area is 58, distributing mainly in the Wenchuan city and Sanjiang town; the debris flow gully of the low risk distribute mainly along the Yuzixi River and in the Yingxiu town along the Minjiang River. |
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