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 ( 610041, ) 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 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 in the 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, , , , 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

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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

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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 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]. Over the larger area, landslides, collapses, debris flows, and barrier lakes. sediment disasters occur to different degrees. No With the added occurrence of rainstorms, many other external force can result in such a large sediment disasters occurred, causing major injuries number of simultaneous sediment disasters and over and fatalities among local populations as well as a such a wide area. high degree of property loss and much damage to (2) High density of sediment disasters traffic arteries in and around the affected villages, Another characteristic of the sediment disasters towns, and cities. triggered by this earthquake is the very high number of resultant mountain hazards in the unit area (Fig.

3). The 5.12 Wenchuan earthquake induced 18,997 collapses and landslides. The average county had 61

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Table 1 Comparison of collapse, landslide, and debris flow occurrence before and after the Wenchuan earthquake in the most serious disaster area. Region Collapse Landslide Debris flow Total Before 7 43 71 121 Wenchuan earthquake After earthquake 1379 1181 125 2685 Before 21 211 23 255 Beichuan earthquake After earthquake 42 1124 38 1204 Before 35 47 17 99 Mianzhu earthquake After earthquake 11 993 24 1028 Before 21 57 13 91 Shifang earthquake After earthquake 70 733 6 809 Before 18 76 5 99 Qingchuan earthquake After earthquake 5 432 6 443 Before 8 91 16 115 Maoxian earthquake After earthquake 72 647 34 753 Before 2 25 2 29 Anxian earthquake After earthquake 25 686 10 721 Before 19 29 1 49 Dujiangyan earthquake After earthquake 38 93 17 148 Before 12 60 5 77 Pingwu earthquake After earthquake 43 528 19 590 Before 8 70 0 78 Pengzhou earthquake After earthquake 170 368 14 552 Before / / / / Lixian earthquake After earthquake 73 340 39 452 Before / / / / earthquake After earthquake 26 106 0 132 * Data from WU S.R, 2008 and HUANG R.Q, 2008.

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Fig. 3 Distribution map of mountain hazard spots before and Fig. 4 Prefabricated houses destroyed by debris flow in after the earthquake (Data from Huang R.Q., 2008) Maliuwan

(1) Threat to villages, towns, and cities, and resultant injury and death It is estimated that the sediment disasters induced by the earthquake were the direct cause of 20,000 deaths. About one third of the fatalities can be attributed to the sediment disasters in the major disaster area. Thirty-one disastrous landslides caused 4996 fatalities in Sichuan Province. The landslide with the most disastrous results was the Wangjiayan landslide in Beichuan County, which caused 1600 deaths. Debris flows also caused great damage in the major affected area. For example, the Fig. 5 Debris flows in Xiangshui Gully in County earthquake area experienced debris flows during 22–24 September 2008, leaving 14 people missing and many people injured in the Maliuwan village of Leigu Town. A debris flow also buried many prefabricated houses, and a set of infrastructures were damaged (Fig. 4). On 23 July 2009 a debris flow suddenly broke out in the Xiangshui gully of Kangding County in Sichuan Province causing 15 fatalities and leaving 39 people missing (Fig. 5). (2) Threat to main traffic arteries As a direct result of the earthquake, many collapses, landslides, debris flows, and barrier lakes developed, which blocked the roads by which rescue services might reach the disaster area and significantly Fig. 6 Road buried by debris flow in Yingchanggou Gully delayed rescue progress (Fig.6). These sediment The 213 National Highway was interrupted for 7 disasters threatened the safety of the important days as a result (Fig. 7). On 25 August 2009, many traffic artery during the time of earthquake relief. debris flows developed along the section of the 213 For example, the pier of the Chediguan Bridge on National Highway from to Wenchuan, again the 213 National Highway was smashed by large causing interruption of traffic on the 213 National rocks, causing the death of six people and injury to Highway. 12. 63

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Fig. 7 Chediguan Bridge smashed by large rocks Fig. 9 Large amount of sand washed into the main river

Fig. 8 Cultivated lands destroyed and buried by debris flow

(3) Threat to people’s livelihoods and living quarters The secondary sediment disasters washed away and submerged many houses and made many Fig. 10 Rainfall records at Tangjiashan rainfall station on 23 and villagers homeless again. In addition, these 24 September 2008 (Data from Tang C.) secondary processes destroyed much cultivated land along the river banks, rendering them unsuitable for induces the occurrence of sediment disasters. After future cultivation, or at least making it very difficult an earthquake, the critical rainfall required to trigger to re-convert them to land that could be easily sediment disasters decreases. For example, the cultivated (Fig. 8). People’s livelihoods and living hourly rainfall intensity and the critical accumulated quarters were thus seriously threatened. In addition, precipitation that induced debris flow after the the secondary sediment disasters induced by the Chichi earthquake in Taiwan decreased by one-third earthquake washed much sand into the main river compared to that before the earthquake [Lin et al., (Fig. 9) as well as causing violent uplift of the main 2003]. Taking the “9.24” debris flow in Beichuan river, causing serious damage to the ecological County as an example, Tang and Liang [2008] environment along the river banks. analyzed the changes in occurrence conditions of 2.2 Change in conditions of occurrence of debris flows after the earthquake. The antecedent sediment disasters precipitation (accumulated during the prophase of Precipitation is a very important condition that the debris flows) was 320–350 mm, and the critical

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International Journal of Erosion Control Engineering, Vol.3, No.1, 2010 rainfall intensity required for the occurrence of 1999]. Debris flows occurred in belts and groups debris flow was 55–60 mm/h in Beichuan County. after the Luhuo M7.9 earthquake in 1973 [Tian, Based on the rainfall records of the Tangjiashan 1986], and they have continued to occur frequently rainfall station (Fig. 10), the antecedent precipitation to the present day. Therefore, the development of accumulated during the prophase was 272.7 mm, sediment disasters in the Wenchuan area is worrying, and the critical rainfall intensity inducing debris and to protect people’s lives and property, as well to flows was 41 mm/h. Therefore, the prophase allow restoration and reconstruction work to be precipitation and critical precipitation for the carried out smoothly, the need for reliable occurrence of debris flow change considerably from sediment-disaster forecasting is urgent. before to after an earthquake. The prophase 3.1 Prediction Method precipitation and the critical precipitation required 3.1.1 Monitoring of temporal and spatial for the occurrence of debris flows after the variation in mountain surfaces earthquake were smaller than those before Fast and effective means of monitoring the earthquake. Taking Beichuan county as an example, dynamic changes underlying sediment disasters in the prophase antecedent precipitation necessary to affected area are available using GIS and remote initiate debris flow decreased by about 14.8–22.1%, sensing technology. Topographic changes in and the hourly rainfall intensity decreased by about mountain surfaces and spatial distribution 25.4–31.6%. characteristics and changes in sediment disasters after earthquakes can be determined macroscopically. 3. TENDENCY PREDICTION AND Such data combined with the information on the COUNTERMEASURES distribution of aftershocks allows for effective This large-scale earthquake resulted in dramatic prediction of the development tendencies of changes to the landscape in the affected area, with sediment disasters. On the basis of this, the entire major increases in the reserves of loose solid earthquake disaster area can be divided into zones material and a much-enhanced threat of sediment according to the hazard levels, providing crucial disasters. Major earthquakes in history have caused information for choosing the location of large debris flows, landslides, and other sediment reconstruction sites after the disaster and for disasters for decades or even centuries after the main designing major infrastructures. earthquake occurred. For example, in the case of the 3.1.2 The prototype observation of typical earthquakes that occurred in the Xiaojiang fault zone, disaster spots Yunnan Province, in 1733 and 1833, sediment On the basis of temporal and spatial variations disasters such as collapse, landslides, and debris in the mountain surface, as determined by flows have continued during the 200–300-year monitoring, along with additional knowledge about period following the earthquakes and have occurred the distribution and occurrence of disasters, the frequently up to the present time. Among the major prototype observation of a typical collapse and effects triggered by Xiaojiang Earthquake, 15 debris landslide can be carried out. By choosing typical flows have been reported in the Jiangjia gully, with disaster spots, the deposition of solid matter, the about 3,000,000 tons of sand transported per year change of physical and mechanical properties, water into Xiaojiang River, often blocking Xiaojiang and content, displacement, and groundwater level can be causing considerable damage. The Zayu M8.5 monitored. The mechanism by which these sediment earthquake that occurred in Tibet on 15 August 1950 disasters occurred following the Wenchuan was followed by an outbreak of nearly 1000 debris earthquake, given the conditions of a new flows in the 40 years after 1953. More than 600 underlying surface and the characteristics and debris flows occurred in the 20-year interval from development tendency of the disasters, can be 1953 to 1973, and an estimated total of up to 1.5 × determined, providing a basis for future disaster 108m3 of solid material was transported [Zhu et al., prevention and reduction.

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3.1.3 Experimental and theoretical research disasters during the emergency phase after an On the basis of surface monitoring and earthquake. The model results can provide the prototype observation, experiments (field necessary basis for the enacting particular experiments, indoor simulation experiments) and emergency measures while considering the potential theoretical research examining the mechanical for sediment disasters. problems of sediment disasters after a major earthquake should be carried out. Using with remote sensing and prototype observations, models can be Before earthquake developed to estimate the likelihood of occurrence After earthquake O u and the dynamics of sediment disasters in the tb re ak Wenchuan earthquake disaster area. A model for A T r r ea evaluating the danger of sediment disasters in the an Outbreak and cause sit io damage rainfall line Wenchuan earthquake disaster area can also be n N Z o on n- e developed. ou A tb Stimulate rainfall(mm) Stimulate re re Research into how to predict the tendency for a ak sediment disasters in the Wenchuan earthquake disaster area, as well as adoption of the methods of Critical rainfall line of debris flow formation surface monitoring, placement observation, and pertinent research, should also include analysis of Antecedent precipitation(mm) and comparisons with the research achievements in Fig. 11 Diagram of debris flow forecast other areas with long observation histories and rich research data (such as the Xiaojiang watershed in The major earthquake destroyed the weak Yunnan Province, Tibet, and Yakedake in Japan). mountain surface in the Wenchuan earthquake Using this information combined with the specific disaster area. Because of the earthquake load, the details of the geological environment in the integrity of the mountain surface was destroyed, and Longmen Mountains region, a model for the the shear strength decreased so that the slope prediction of sediment disasters following the became unstable. Rain causes a decrease in slope Wenchuan earthquake can be developed to provide a stability once it infiltrates the surface. Therefore, the scientific basis for disaster prevention and reduction secondary sediment disasters, such as collapse, and to guarantee successful reconstruction in the landslides, and debris flow are likely to be very disaster area, thereby preserving people’s lives and active for a long time in the earthquake area. property. The occurrence of debris flows after the 3.2 Predicting the tendency of sediment disasters earthquake shows a strong hysteretic quality. The in the emergency phase movement of the abundant solid materials loosened The establishment of a reliable model that can by the earthquake was determined mainly by rainfall, predict sediment disasters following an earthquake which led to the debris flows observed in this region. represents a long-term and continuous research After an earthquake, debris flows will increase in challenge. occurrence as the rainfall threshold needed to trigger Focusing on the need for immediate a debris flow decreases (Fig. 11). For example, in post-disaster reconstruction and victim resettlement, the region of Hongkou in Dujiangyan and in attention should be paid to existing information from Yinchang Gully in Penzhou, no occurrences of related areas, combined with data on the debris flows had been recorded in a very long time geographical characteristics of the Longmen before the earthquake. However, following the Mountains derived from remote sensing data. These earthquake, many debris flows have occurred. sources of information can be combined to yield a preliminary prediction of the tendency for sediment

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3.3 Countermeasures 3.3.2 Long-term measures 3.3.1 Emergency measures Due to the hysteretic nature of sediment To ensure the smooth progress of post-disaster disasters following an earthquake, we should reconstruction and resettlement work, it is essential undertake not only emergency measures, but also to take appropriate countermeasures to prevent more long-term measures to protect infrastructures and damage from being caused by sediment disasters important sites during post-disaster reconstruction after the Wenchuan earthquake. projects. (1) Disaster investigation and risk assessment (1) Regional risk zonation Through systematic field investigation and By using 3S technology, RS, GIS, GPS, discovery of previously hidden areas at risk for combining field investigation and prototype sediment disasters and by assessing the risk of future observation, and after analyzing the temporal and sediment disasters for a particular area, we can spatial distribution characteristics and development select a relatively safe area for the site of potential of sediment disasters, we can establish post-disaster reconstruction. zoning based on regional risks. The results of such (2) Emergency engineering measures zonation can provide a basis for the general layout In certain regions where reconstruction is for post-earthquake reconstruction. needed but where sediment disasters cannot be (2) Developing comprehensive disaster prevention avoided, it is essential to identify the characteristics, and mitigation programs scale, and potential damage from the sediment Based on the monitoring of typical disaster disasters through detailed field investigation. In points, prototype observation, and experimental conducting such investigations, we can take study, we can develop comprehensive disaster appropriate emergency engineering measures to prevention and mitigation programs that combine protect reconstruction projects. engineering and ecological measures, monitoring (3) Monitoring and warning and warning systems, and emergency evacuation As sediment disasters threaten important measures. We should protect the environment from protection objects, villages, cities, highways, et al., it damage induced by engineering activities and carry is essential to establish a comprehensive disaster out ecological environment-restoration programs monitoring and early-warning system to ensure the during post-earthquake reconstruction to achieve the protection of lives and property. restoration of the mountain environment, as well as (4) Treatment of barrier lakes to prevent or mitigate sediment disasters in the With respect to the barrier lakes triggered by the long-term. earthquakes, we should initially carry out a risk assessment of dam breakage. In terms of ACKNOWLEDGEMENT: The work was high-danger barrier lakes, it is essential to use supported by The National Natural Science artificial methods to mitigate any immediate danger Foundation of China (Key Program) (Grant No. from dam breakage, or at least reduce risks and 40830742). losses as much as possible. (5) Mass monitoring and prevention REFERENCES To avoid further loss of life and property, it is Chen X.Q., Li Z.G., Cui P. (2009): Estimation of soil erosion essential to establish a mass monitoring and caused by the 5·12 Wenchuan earthquake, Journal of Mountain Research, Vol. 1, pp. 122-127. prevention system for timely detection and early Yin Y.P. (2009): Great Wenchuan earthquake: seismogeology warning of potential sediment disasters. In this way, and landslide hazards. Beijing: Geology Press. when sediment disasters do occur, we can evacuate Huang R.Q. (2008): Distribution and mechanism of landslides people as quickly as possible. induced by the Wenchuan earthquake. China–Japan Symposium on Seismic Disaster Prevention and Mitigation, Chengdu. Wu S.R, Shi J.S, Yao X. (2008): Analysis and evaluation of

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