The Debris Flow Hazard Assessment of Wenchuan Earthquake-stricken Area Based on Watershed Unit Dr Xiong Junnan1, Sun Ming1, Liu Shan2, Peng Chao1 School of Civil Engineering and Architecture, SWPU，Chengdu Sichuan,610500 Dean’s Office of SouthWest Petroleum University, Chengdu Sichuan,610500 ABSTRACT Since the "5.12" Wenchuan earthquake in 2008, Massive debris flow led by the heavy rainfalls has broken out each summer, which causes serious damage to people's lives and property. Given the destructiveness of debris flow, it’s necessary to carry out hazard assessment and prevention work of debris flow in the Earthquake-stricken areas immediately. This article take where earthquake intensity above 7 degrees in Wenchuan as study area, after analyzing the characteristic of debris flow in Whenchuan, we take small watershed unit which debris flow formation as assessment unit, extracts watershed from GDEM data, and established hazard assessment index system with the energy condition, physical condition, as well as rainfall triggering conditions of the debris flow formation. On this basis, we establish debris flow hazard assessment model use extension matter- element theory, carrying out spatial analysis and valuation of each index factor by GIS, and completes debris flow hazard assessment of each watershed in the study area and marking different zones of the debris flow hazard. The experiment shows that the results in good agreement with actual situation of occurred debris flow, the result can offering scientific basis for government to formulate disaster prevention and mitigation decisions. KEYWORDS: debris flow, Wenchuan earthquake-stricken areas, hazard assessment, small watershed unit, GIS INTRODUCTION Debris flow is a sudden disaster which occurring in a mountainous area, and often occurs in small watershed unit in mountain area, which is the combined effects result of geology, geomorphology, hydrology, meteorology, vegetation and other natural factors. After the Wenchuan earthquake in May 12, 2008, the earthquake triggered a large number of secondary mountain disasters in meizoseismal - 6025 - Vol. 20 [2015], Bund. 14 6026 area, such as collapse, landslide, etc., and there are still a lot of mountains standing but cracking. Under the influence of strong aftershocks and heavy rainfall, the debris flow may easily occur in this area. Ever since May 12, 2008, Wenchuan earthquake-stricken area has experienced large-scale debris flow every year. For instance, On the evening of May 12, 2008, debris flow took place around Niu Quan gully of Ying Xiu town, Wenchuan County, Sichuan Province; On the evening of July 31, debris flow occurred in the township of Caoke, Shimian County [4]; On July 17,2009, heavy rainfall triggered large-scale debris flow in the township of Hongkou, Dujiangyan city[5]. Because of extremely weather on August 12 and August 19 of 2010, debris flow and torrential flood took place in many places, among which there are large-scale and outbreak debris flow in LongChi, Dujiangyan city[6]. On July 5, 2011, Massive debris flow took place in Gaojia gully, Wenchuan County. On July 12, massive torrential flood and debris flow took place in Yajiang River, Sichuan province. In the middle of July, 2013, due to heavy rains, debris flow took place in Ya'an, Dujiangyan, Pingwu and many other places of Sichuan. In 2008, debris flow caused more than 450 casualties (including citizens not being found), the property loss and casualty are disastrous in earthquake-stricken areas [4]. Professor Xie Hong analysed the debris flow which occur in 2008 earthquake-stricken area and revealed debris flow are mostly active in small basin where watershed area within 5km2, especially in those less than 3km2[4], there are less debris flow gully area over 5 km2[3]. Professor Huang Runqiu, along with many other experts predicted that a few years or even decades after the earthquake, earthquake-stricken area will becoming an active phase of debris flow. Therefore, a long period of time from now on, Wenchuan earthquake-stricken area will still suffer the threat of debris flow. So, carrying out the debris flow hazard assessment is imperative. During the regional debris flow hazard assessment, the appropriate unit should be selected at first, as model computing element, and the maximum internal similarities and dissimilarities between the units should be considered. Professor Guzzeti divided the assessment units into five types: grid unit, topography unit, uniform conditions unit, Watershed unit and topography unit. Grid unit is widely adopted in regional hazard assessment of debris flow in China, for grid unit are rapid divided by GIS platform and raster data in matrix form can be easily operated by computer. However, the grid unit is lack contact with geomorphology or other environmental information, and this unit is hardly to reflect the actual situation of debris flow occurring. The occurrence of debris flow in Wenchan earthquake- stricken area is closely related to the basic geomorphic units in the mountain, and geomorphic characteristics of debris flow gully, abundant material supply and rainfall are requirements of the debris flow. In order to reflect the integrity of geological environment information where debris flow formation and improve the reliability of the evaluation factor in the process of regional hazard assessment, this paper adopt watershed unit and researched the hazard of debris flow in each small watershed in Wenchuan earthquake -stricken area. The result is important to the prevention and mitigation of debris flow in future in Wenchuan earthquake-stricken areas. Vol. 20 [2015], Bund. 14 6027 THE GENERAL SITUATION OF STUDY AREA 5.12 Wenchuan Earthquake has led a great impact on Chengdu, Mianyang, Deyang , Ya'an, Guangyuan, Leshan, Dazhou, Aba, Ganzi and any other 10 cities and state of Sichuan province, as well as part of Gansu and Shanxi province, among which Severely afflicted 44 county (city) covers 1.58×105km2.This paper take the area where intensity of Wenchuan earthquake between VII to XI as the study area. As it’s shown in figure 1, the research area has covered the 10 counties (cities) which severely afflicted— Wenchuan, Beichuan , Mianzhu, Shifang, Qingchuan, Maoxian, Anxian, Dujiangyan, Pingwu, and Pengzhou; and Li County, Jiangyou City, Lizhou District, Chaotian District, Wangcang County, Zitong County, Youxian District, Jingyang District, Xiaojin County, Fucheng District, Luojiang County, Heishui County, Chongzhou City, Jiange County, Santai County, Langzhong City, Yanting County, Songpan County, Cangxi County, Zhongjiang County, Lushan County, Guangba District, Dayi County, Baoxing County, Nanjiang County, Guanghan City, Shimian County, Jiuzhaigou County of Sichuan province. In addition, it also covers Wen County, Wudu District, Zhouqu County, Kang County, Cheng County, Hui County, Xihe County and Liangdang County of Gansu province, as well as Ningqiang County, Lueyang County and Mian County,in Shanxi province. Figure 1: The Location of Research area Vol. 20 [2015], Bund. 14 6028 The study area located in the northwest of Sichuan Province, and located in geomorphology transition zone of the first ladder to the second ladder in China, there are many high mountain and montane ravines, and the terrain is very steep and develops various gravitational landforms . Besides the flat area in front of longmen mountains, there are few flat places sporadically distributing between river valleys. By stratigraphy the area is the junction of Maerkang zone , longmen Mountain and Sichuan basin. The altitude steps down from west to east, and the crust thickness decreases accordingly but relatively slowly. Affected by the uplift of the qinghai-tibet plateau, the tectonism effect is strong, and the magma activity occurs frequently, metamorphism effect is common. Combined effect by the monsoon climate and terrain conditions, this area has a widely varied regional distribution of rainfall within a year, all of this will provides favorable conditions for the debris flow gestation, formation and development. METHOD TO EXTRACT WATERSHED UNIT DEM(Digital Elevation Models) contains a wealth of topography, hydrology information, they can reflect the terrain characteristics of various resolution, so a large number of surface morphological information can be extracted from DEM. (1) The data source The digital elevation model (DEM) used in this study is the ASTER GDEM data, freely accessed from science data service of Chinese International Academy (http://datamirror.csdb.cn/).The spatial resolution is 30m × 30m, vertical accuracy is 20m, and level of accuracy is 30m, the data format is Geo TIFF, and the reference geoid is WGS84 / EGM96. (2) Extract Watershed Unit Watershed unit division and extraction based on DEM mainly based on hydrological analysis tool to extract the direction of flow, the cumulative amount of cumulant, water flow length, river networks (including classification river network) and division the watershed unit of study area. The process of water flow can be reproduced on the DEM by extracting these basic hydrological factors and basic hydrology analysis, and complete the analysis of hydrological process at last (Tang Guoan, 2006). Many software have provided hydrological analysis functions at home and abroad, such as WMS, ARCGIS, etc. Adopting the hydrological analysis toolbox of ARCGIS 9.3, the process of hydrology experiments are shown in Fig.2. Vol. 20 [2015], Bund. 14 6029 The original DEM The direction of flow Calculation