sign in sign up
<<
Home , Cangxi County, Dayi County, Dazhou, Dujiangyan City, Fucheng District, Heishui County

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， ,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, , Sichuan Province; On the evening of July 31, debris flow occurred in the township of Caoke, [4]; On July 17,2009, heavy rainfall triggered large-scale debris flow in the township of Hongkou, 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, [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 , 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, , , Ya'an, , , , 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 , , , Qingchuan, Maoxian, Anxian, Dujiangyan, Pingwu, and ; and Li County, City, Lizhou , , , , , , , , Luojiang County, , City, , , City, , , , , Lushan County, Guangba District, , , , City, Shimian County, 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 , there are few flat places sporadically distributing between river valleys. By stratigraphy the area is the junction of Maerkang zone , longmen Mountain and . 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 and filling extraction depressions

Calculation the con- Generat e catchment area Extraction of river network fluence of cumulant

Watershed division

Figure 2: Flow chart of watershed extraction

Watershed area is the main parameter in determining the hydrodynamic conditions of the valley. Through the analysis of debris flow data in Wenchuan area after the earthquake, debris flow activities are found centering in the catchment area coverage is less than 5 km2. While adopting GDEM extracting small watershed, the author determines to regard the watershed extracted by a cumulative number of 500 as a computing unit for risk assessment.

HAZARD ASSESSMENT MODEL OF DEBRIS FLOW (1) Evaluation index system Because of the Wenchuan earthquake, the stability of mountain was destroyed and the valley erosion got more severely, which left a large number of loose debris, and thus caused the diversity, variability, uncertainty and incompleteness of the topography, geology and precipitation conditions of the development as well as formation of debris flow after the earthquake. How to apply a specific technology to process these formation conditions and then carry out the hazard assessment of debris flow is an important part in disaster mitigation after the earthquake. The analysis of debris flow hazard index is mainly analysis the forming conditions affected by historical geology and quake geological process. Debris flow is the combining result of internal and external cause. The internal cause include geology, geomorphology and basin of material supply conditions, the external factor mainly include rainfall, which is also the triggering condition of debris flow. The regional hazard assessment of debris flow are determine the possibility of occur debris flow through analyse and evaluate the environment in the gully. Therefore, this paper takes fully consider all kinds of factors associated with debris flow while established the evaluation index system. According to the data collection and results of correlation analysis , we find out eight indicators most closely related to debris flow in Wenchuan earthquake-stricken areas, respectively are slope of basin, basin gradient of gully, average precipitation in many years, the variation coefficient of annual rainfall, fault density, seismic intensity, strata lithology, and land use. Vol. 20 [2015], Bund. 14 6030

The degree of the basin slope not only affects concentration time, but also decides the possibility of eluvia material’s participation in debris flow activity. According to the analysis, the basin slope of 30°～40° in Wenchuan earthquake-stricken area are most prone to occur debris flow, followed by 20°to 30°, the most stable slope is 80° to 90°, 0°to 20°.

Gradient of gully (average gradient of gully bed) is the value of steep，usually reflecting the steep of hillside. Gullys which get a relatively big gradient usually match with large hillside slope, strong gravitational erosion in the basin, abundant loose solids, enormous potential energy, and provide good condition for rapidly changing static energy into kinetic energy. According to the analysis, the gradient between 50 ‰ -500 ‰ is the watershed most vulnerable to debris flow.

The rainfall of river basin is reflect by coefficient of variation between average annual precipitation for many years and years of average annual precipitation. Debris flow is more likely to break out when the average annual precipitation amounts to 500-1200mm or when the rainfall for 24 hours gets 50mm-150mm.

The formation lithology of a basin, mainly referring the weathering resistance and anti-erosion capability of rock, largely determines the basin’s reserves of loose debris. Generally speaking, lithological layers made by weak rocks, poor in cementation diagenesis or layered with soft and hard rock are more vulnerable and produce more loose materials than those which are uniform lithology and hard rocks.

Earthquakes often damage the stability of slope, and thus cause the collapse and landslide which provide loose solid substances for debris flow. Meanwhile, the seismic activity increased erosion in gully, which is conducive to the formation and development of debris flow. The seismic intensity, indicates the degree of the earthquake effects on the earth as well as the constructions, is a factor showing the seismic contribution in producing solid material,

Tectonic movements make the stratum curve or diastrophism, and bring damages to rock and stratum of basin. As it’s shown in the analysis, the farther distance the basin gets away from the fault, the weaker effect the basin suffers to produce debris flow. Therefore, to some extent, the density of the fault affects the formation of debris flow.

Land use is a direct reflection of human activity. Irrational human activities to some extent exacerbated the formation of debris flow, such as steep slopes farming, destruction of forests and vegetation, waste rock of construction dumping anywhere. （2) Evaluation model Accoding to the above analysis of evaluation index, we can determine 5 levels of interval values showing the factors of debris flow occurrence. Accordingly, Extension element theoretical model used for assessing debris flow hazard in Wenchuan earthquake-stricken areas is built as follows: Vol. 20 [2015], Bund. 14 6031

Hazard degree p , groove gradient , groove gradient value p,G ,X1 average slope , average slope value  S,X stratigraphic and lithology , stratigraphic and lithology value mean 2 R,X 3 P fault density , fault density value  F,X 4 earthquake intensity , earthquake intensity value  Rp = = E,X 5 the comprehensive land use index  the comprehensive land use index , L,X 6 annual average precipitation value annual average precipitation , P,X annual precipitation variation mean 7  precipitation variation coefficient index , C,V8X coefficient index  precipitation variation coefficient index ,   is the level of debris flow hazard assessment, while G, Smean, R, F, E, L, Pmean, and Cv are respectively the 8 indexes, Xi is the value of each index.

When the quantity value which expresses correlation function by matter-element selects one point on number axis, matter-element meets the requirements of the extent of scope. Each evaluation object characteristic of the correlation density in each grade can be calculated:

 ρ(x , x ) i 0i ∈  − x i x 0i  x 0i K j (x i ) =  (2) ρ(x i , x 0i )  ∉ ρ − ρ x i x 0i  (x i , x pi ) (x i , x 0i )

a oi + b oi boi − a oi ρ(x i , x 0i ) = x i − − In this formula， 2 2

a + b b - a ρ(x, x ) = x - pi pi - pi pi p i 2 2

， ， ， ， ， The interval xo = [a b] xp= [A B] and x o ⊂ x p |xo|=|b-a|

Use the formula(3) to calculate degree of membership Kj（x）, and then decide debris flow hazard level with the Kj（x）,

n K j (x) = ∑λiKi (xi ) (3) i =1

This model divided debris flow hazard assessment into five grades: higher hazard, high hazard, medium hazard, low hazard, lower hazard. Vol. 20 [2015], Bund. 14 6032

HAZARD ASSESSMENT AND RESULTS ANALYSIS According to the above model, the hazard of debris flow in each watershed unit in Wenchuan earthquake-stricken area were determined ,and we can divide the degree of debris flow hazard into five levels — very severe zone , severe zone , moderate severe zone, low severe zone and very low severe zone. The debris flow hazard zoning result in Wenchuan earthquake-stricken areas are shown in Fig.2.

Figure 2: The results of debris flow hazard assessment in Wenchuan

earthquake--stricken area

CONCLUSIONS AND DISCUSSION Using watershed as the evaluation unit for debris flow hazard assessment, they can reflect the geomorphology characteristics of debris flow occurs and spatial characteristics of gully, which make assessment process can reflect the physical mechanism of debris flow occurs, and improved the reliability of evaluation result. Furthermore, the final results of the evaluation practically and intuitively display the hazard values for each watershed. In this paper, decision-making department Vol. 20 [2015], Bund. 14 6033 may take corresponding measures directly when taking measures in dealing with basins vulnerable to debris flow and reducing the damages.

ACKNOWLEDGEMENT This Study was supported by Project of Sichuan province department of education(13ZB0202), Geographic National Condition Monitoring Engineering Research Center of Sichuan Province (GC201409), Open Research Fund by Sichuan Engineering Research Center for Emenrgency Mapping &Disaster Reduction (K2014B009), Supported by scientific research starting project of SWPU(2014QHZ034).

Corresponding author: Xiong junnan，Ph.D, Associate professor, majoring in 3S technology and mountain hazards risk assessment, School of Civil Engineering and Architecture, SWPU, Chengdu, Sichuan, China. 610500, Tel: +86 13541223403, e-mail:[email protected].

REFERENCES [1] YU Bin,XIE Hong,WANG Shi-ge,etc. Activity tendency of debris flow gully in Wenchuan County after "5.12"Wenchuan earthquake[J], JOURNAL OF NATURAL DISASTERS, 2011,20 (6):68-73.

[2] XIE Hong,ZHONG Dunlun,JIAO Zhe,etc. Debris Flow in Wenchuan Quake-hitArea in 2008[J], JOURNAL OF MOUNTAIN SCIENCE. 2009,27(4)：501-509.

[3] JIA Changeheng. Analysis on the Charaeteristics of Xiaogou Drainage Area 7.17 DebrisFlow oceurred at the Hongkou Township of Dujiangyan City[J], ANALYSIS RESEARCH. 2013,8(1):38-42

[4] Liu Yang,Tang Chuan,Li Wei,etc.Sensitivity Analysisi on Mass of Debris Flows on the Longchi Region Dujiangyan,Sichuan Province[J], JOURNAL OF CATASTROPHOLOGY, 2013,28(2):107-113.

[5] YANG XING. Wenchuan earthquake meizoseismal area of Potential Sudden debris flow’s Hazard zonation and Evaluation Based on 3S Technology[D], SiChuan,ChengDu: Chengdu university of technology, 2012.

[6] Tom Parsons, Chen Ji, Eric Kirby. Stress changes from the 2008 Wenchuan earthquake andincreased hazard in the Sichuan basin[J].Nature,2008，454（7203)：509–510.

[7] William B. Ouimet. Landslides associated with the May 12, 2008 Wenchuan earthquake:Implications for the erosion and tectonic evolution of the Longmen Shan[J]. Tectonophysics.2010,491(1-4):244-252. Vol. 20 [2015], Bund. 14 6034

[8] HUANG Runqiu，LI Weile. RESEARCH ON DEVELOPMENT AND DISTRIBUTION RULES OF GEOHAZARDS INDUCED BY WENCHUAN EARTHQUAKE ON 12TH MAY,2008,Chinese Journal of Rock Mechanics and Engineering, 2008,27(12):2585- 2592.

[9] FanstoGuzzentti, AlbertoCarrara, MauroCardinali, Paola Reichenbach. Landslide hazard evaluation a review of current techniques and their application in a multi scale study Central Italy[J].Geomorphology. 1999,31:181～216.

[10] WANG Xiaopeng, PAN Mao, XU Yueren. Debris Flow Hazard Zonation Based on Cathment unit [J]，JOURNAL OF MOUNTAIN SCIENCE，2006,24(2):177-180.

© 2015 ejge