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Characteristics of Sediment-Related Disasters Triggered by the Wenchuan Earthquake

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International Journal of Erosion Control Engineering, Vol.3, No.1, 2010 Technical Note Characteristics of Sediment-Related Disasters Triggered by the Wenchuan Earthquake Guo-qiang OU1, 2, Hua-li PAN1, 2, Jin-feng LIU1, 2, Jian-rong FAN1, and Yong YOU1, 2 1Key Laboratory of Sediment disasters and Surface Process, Chinese Academy of Science (Chengdu 610041, China) 2Institute of Sediment disasters and Environment, Chinese Academy of Sciences (Chengdu 610041, China) The “5.12” Wenchuan earthquake not only had catastrophic primary effects, but also triggered many major secondary effects in mountainous regions including collapse (rock fail, slide, and so on), landslides, debris flows, and the formation of barrier lakes. These secondary disasters had a major influence on the areas affected by the earthquake, as they resulted in significant blocks to aid and seriously slowed down the rescue process. Furthermore, huge amounts of uncompacted debris created by collapse and landslides continue to pose a substantial long-term risk to the safety of the people and to their property as it can form powerful debris flows with strong rains. In this study, the distribution characteristics and physical status were investigated through field surveys and image interpretation. The features and distribution of future sediment disasters were estimated, and suggestions for corresponding mitigation measures were proposed. These will play an important role in protecting the safety of the people and in facilitating the reconstruction of disaster areas. 1. BRIEF INTRODUCTION TO disastrous sedimentary effects, such as slope SEDIMENT DISASTERS TRIGGERED collapse, landslides, and barrier lake formation. BY THE WENCHUAN EARTHQUAKE These major secondary sedimentary effects caused great damage to mountain towns, villages, roads, At 14:28 Beijing time on 12 May 2008, a hydroelectric engineering, and communication devastating earthquake of magnitude 8.0 on the facilities, which not only aggravated the disaster, but Richter scale hit Wenchuan County in the Sichuan also presented major obstacles to disaster relief, Province of southwestern China. By 10:00 am on 25 seriously delaying its progress. The main shock of May 2009, 69,227 people had died, 374,643 people the Wenchuan earthquake was very strong, with an had been injured and 17,923 people were declared epicentral intensity of up to XI on the Mercalli scale, missing as a direct consequence of the earthquake. causing great damage to the mountain surface The area affected by the disaster covers over (Fig.1). Before 12:00 am on 1 August 2009, 303 2 100,000 km , and the breadth of the sweeping region aftershocks over Ms = 4.0 occurred in the area. and the scale of the damage are apparently without Among these aftershocks, 258 were between Ms = precedent in the recorded history of this area. The 4.0 and 4.9, 37 were between Ms = 5.0 and 5.9, and earthquake exerted great damage to Wenchuan, eight were over Ms = 6.0 (Fig. 2, data from China Beichuan, Dujiangyan, Pengzhou, Mianzhu, Shifang, Earthquake Networks Center). Frequent and strong Anxian, Qingchuan, Pingwu, Lixian, Maoxian and aftershocks caused reciprocating damage to Wenxian, all of which lie along the Longmen Fault. mountain surfaces. In summary, many Since the most-seriously affected areas lie in the sediment-related disasters, such as collapse and western mountain regions of Sichuan Province, landslides, were triggered, and a large amount of which contain high mountains, deep valleys, loose solid material was created. According to field complicated geological structures, and fault development, the earthquake induced many 59 International Journal of Erosion Control Engineering, Vol.3, No.1, 2010 Fig. 1 Distribution map of the intensity of the Wenchuan earthquake (from Ministry of Land and Resources P.R.C) investigations and interpretations of remote sensing data, the total amount of soil and water loss in the area most seriously affected by the Wenchuan Fig. 2 Distribution map of the main shock and aftershocks of the earthquake was about 56 billion tons [Chen et al., Wenchuan earthquake (from China Earthquake Networks 2009]. Center, through 1 August 2009) The sedimentary disasters caused by the Wenchuan earthquake have already caused great Therefore, this area always has a high incidence of damage, and it is estimated that about one-third of sediment disasters. Before the earthquake, sediment the recorded deaths, missing persons, and property disaster investigations identified 5184 hidden losses can be attributed to these secondary hazardous mountain locations in the 44 counties sedimentary effects. For example, the intense within the serious disaster area. Among these, 3300 rainfall on 24 September 2008 initiated widespread landslides, 492 collapses, 604 debris flows, and 751 debris flows in the epicenter of the Wenchuan unstable slopes were recorded before the earthquake. earthquake, Beichuan. These debris flows greatly After 20 July 2008, 9671 additional hidden trouble impacted the community of Beichuan County and spots were added in the 44 disaster counties (cities). caused 42 fatalities. Between June and July 2009, Among the 8627 statistically identified hazardous the Sichuan Basin area experienced frequent spots with a certain scale, 3627 landslides, 2383 rainstorms, and in the major disaster areas such as collapses, 837 debris flows, and 1694 unstable Doujiangyan, Pingwu, Pengzhou, and Qingchuan, slopes were recorded [Huang, 2008]. The change in serious landslides and debris flows occurred, sediment disaster occurrence caused by the jeopardizing the safety of local people and the Wenchuan earthquake can be illustrated by the post-disaster reconstruction efforts. statistical data of collapse, landslide, and debris flow The “5.12” Wenchuan earthquake occurred in before and after the earthquake in the serious the structural belt of the Longmen Mountains on the disaster area (Table 1). eastern edge of Tibetan Plateau. Intensively The data in Table 1 show that the Wenchuan squeezed by the Tibetan Plateau and the Sichuan earthquake induced a large number of collapses, Basin, the Longmen structural belt was in a landslides, and debris flows. Before the earthquake, continuously active state before the earthquake. This 1013 sediment disaster spots were identified in 10 of region includes one of the steepest mountain slope the counties (cities), whereas after the earthquake, areas in China. A 5500-m change in elevation occurs sediment disaster spots increased by a factor of 8.82 over a distance of 100 km. This region also to a total of 8933 occurrences. Among the secondary encompasses the headwaters of many rivers in the sedimentary effects, the increase in the occurrence upper Yangtze River catchment [Yin, 2009]. of collapses was perhaps the most remarkable; 1855 60 International Journal of Erosion Control Engineering, Vol.3, No.1, 2010 collapses were recorded after the earthquake, which 142 disasters, the county with the highest number of is 12.28 times the number before the earthquake. disasters (Qingchuan) had 989 disasters, and The second most dramatic difference was seen in Wenchuan had 474 disasters. Along the 213 national landslide occurrence, with 6785 landslides occurring highway from Zipingpu Power Station to Wenchuan after the earthquake, 9.57 times the number before County, 351 sediment disasters developed, with a the earthquake. Two hundred ninety-three debris density of 3.28/km [Yan et al., 2009]. flows were recorded after the earthquake, 1.92 times (3) Concentrated distribution along the two sides of the occurrence before the earthquake. Therefore, the the Longmenshan Fault dramatic increase in the occurrence of collapses The distribution of sediment disasters triggered represents the biggest effect, and it shows that the by the Wenchuan earthquake was mainly determined destructive effect of the earthquake on steep slopes by the location of fault movement that triggered the is especially great. According to the relative seismic activity, and the sedimentary effects are proportions of the occurrence of different sediment distributed along those faults like a ribbon. Because disasters, before the earthquake, collapse accounted the faults that triggered the earthquake were reverse for 14.9%, landslides accounted for 70.0%, and faults, the distribution of the sediment disasters debris flows accounted for 15.1%. After the showed a clear “Upper plate/lower plate effect.” earthquake, collapse accounted for 20.8%, That is, the upper plate had a higher distribution landslides accounted for 76.0%, and debris flows density of sediment disasters than did the lower accounted for 3.2%. This shows that landslides plate, and these events were also characterized by a represented the most common effect of the wider scope and larger scale [Huang et al., 2009]. earthquake, followed by collapse, and debris flow. Through analysis of the relationships between the locations of the mountain hazards after the 2. SEDIMENTARY DISASTER earthquake and the fault that caused the earthquake, CHARACTERISTICS ANALYSIS OF it was shown that the farther an area was from the THE WENCHUAN EARTHQUAKE fault, the smaller was the distribution density of mountain hazards. The region of highest intensity of 2.1 Distribution characteristics and damage mountain hazards was between 0 and 7 km away 2.1.1 Distribution characteristics from the fault in the upper plate; the region of (1) Wide distribution area moderate hazard development was between 7 and 11 According to the investigation of sediment km from the fault in the upper plate and between 0 disasters associated with the Wenchuan earthquake and 5 km from the fault in the lower plate. The vast carried out by the Ministry of Land and Resources majority of larger-scale landslides were about 5 km P.R.C, preliminary results show that the sediment away from faults [Huang, 2008]. disasters induced by the Wenchuan earthquake have 2.1.2 Damage Characteristics a very wide distribution. They cover three provinces Unlike other strong earthquakes in China, the and 84 counties (cities), with a total area of 48 × 104 Wenchuan earthquake induced a large number of km2 [Wu et al., 2008].
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