Safety Assessment and Retrofit Techniques of Quake Lakes

Xingguo Yang, Hongwei Zhou, Hongtao Li, Zhaohui Yang, Lu Qiao, Yuanyuan Lin

July 13,2009 CONTENT

1.1 Introduction 2.2 Main Tasks and Basic Data Collection in Safety Assessment 3.3 Risk Level of Quake Lakes 4.4 Risk Assessment of Landslide Dam 5.5 Improvement and Protection of Quake Lakes 6.6 Conclusions

July 13,2009 1 INTRODUCTION

• The great Wenchuan Earthquake formed more than 100 Quake lakes with a dam taller than 10 m, a water storage capacity larger than 1.0×105 m3 and a catchment area greater than 20 km2. • Statistical data show that these are the largest number of Quake lakes created by a single earthquake in human history.

July 13,2009 1 INTRODUCTION

• These Quake lakes have the following common features: – Extensive and dense distributed; – Formed in rivers where the landslide dams had already formed within 40 years, indicating that the river banks are not stable; – Treated or self-burst, not causing any disaster or casualty, and – 90% of them are in stable, healthy conditions.

July 13,2009 2 Main Tasks and Basic Data Collection in Safety Assessment • The main tasks of safety assessment of Quake lakes include: – Assessment of risk level of Quake lake; – Assessment of stability of landslide dam; – Assessment of impact to the upstream and downstream areas, and – Comprehensive evaluation of risk of quake lake.

July 13,2009 2 Main Tasks and Basic Data Collection in Safety Assessment • Related standard has detailed provision for basic data collection. It is suggested that: 1. The latest data from site survey be collected for the evaluation of its impact on the upstream and downstream areas, since the features of river channels has dramatically changed after earthquake, landslide or mud- rock flow. EXAMPLES

2. Use of hydrology data. EXPLAN

July 13,2009 3 Risk Level of Quake Lakes

• The related standards categorize Quake lakes according to three factors: – Size, – risk of landslide dam and – breach loss. • They are classified as – Extremely High Risk, – High Risk, – Medium Risk and – Low Risk. • More details can be found from references.

July 13,2009 Size Definition of Quake lake

Reservior capacity of Quake lake Quake lake size (x108 m3)

Large =1.0

Medium 0.1~ 1.0

Small I 0.01~ 0.1

Small II < 0.01

July 13,2009 4 Risk Assessment of Landslide Dam • 4.1 Breach modes of landslide dam • 4.2 Shape characteristics and environment of landslide dam • 4.3 Breach form • 4.4 Breach flood routing calculation

July 13,2009 4.1 Breach modes of landslide dam

• Generally, landslide dam breaches are classified as – global breach and local breach according to size of breach, or – instantaneous breach and gradual breach according to breach speed. • It is commonly believed that landslide dam is similar to man-made earth-rock fill dam in terms of structure and breach modes, and its breach mode generally belongs to gradual breach.

July 13,2009 4.1 Breach modes of landslide dam

• But there are obvious differences in configuration and composition between landslide dam and man-made earth-rock fill dam. The breach modes of man-made earth-rock fill dam may not apply to landslide dam. • It is suggested that breach of landslide dam be classified as the following three types: – overtopping breach, – drain breach, and – group breach.

July 13,2009 Breach Modes

• Overtopping Breach – Overtopping global breach – Overtopping deep breach – Overtopping shallow breach • Drain Breach – Drain global breach – Drain deep breach • Group Breach – Natural group breach – Drain group breach

July 13,2009 4.1.1 Overtopping breach

• Overtopping breach is the most dangerous and also hard to control. • The main cause is the poor anti-erosion ability and stability of the landslide dam, coupled with rapid rise of water level and lack of measures to divert or drain water from the reservoir. – Overtopping global breach – Overtopping deep breach – Overtopping shallow breach

July 13,2009 4.1.2 Drain breach

• The disposal experience of Tangjiashan Quake Lake shows that construction of sluice channel is one of the key engineering measures to proactively reduce or eliminate breach risk. However, it is less likely that breach risk can be totally eliminated for landslide dams mainly composed of soils, due to factors including defect in discharge channel design, improper construction method, and limited construction period or intensity. – Drain global breach – Drain deep breach

July 13,2009 4.1.3 Group breach

• Group breach occurs when Quake lakes breach due to flood caused by breach of Quake lakes in upstream. Depending on the causes of Quake lake breaches in upstream, it can be categorized as natural group breach, and drain group breach. – Natural group breach – Drain group breach

July 13,2009 4.2 Shape characteristics and environment of landslide dam • Due to limit of road access, evaluation of the inner structure of landslide dams is not necessarily correct during emergency disposal stage. • Through in-situ observation, it is found that the shape characteristics and environment of landslide dam have great value for short-time safety assessment.

EXAMPLE: Dabachang July 13,2009 4.3 Breach form

• Breach form mainly refers to shape, depth and width of breach. • Current technique standards mostly assume that it has a rectangular or trapezoidal shape, and suggest that breach width be determined by composition of landslide dam, geological condition and hydrology data.

July 13,2009 • Based on the experience from emergency disposal of more than 10 Quake lakes including Tangjiashan Quake Lake, we suggest that the following studies be carried out regarding breach form: 1. Speculate the composition and inner structure of landslide dam according to the geological conditions of origin place of landslide and rock collapse; 2. Predict the development of breach shape according to composition and configuration of landslide dam, and design of sluice channel; 3. Predict breach depth according to the inner structure of landslide dam and the hydrology data; 4. According to breach depth, composition of the dam, back-calculate the bottom width and slope ratio of both sides of the temporarily steady sluice channel, and analyze breach form, and 5. Calculate the maximum discharge flow rate according to breach form, and carry out flood routing calculation. EXAMPLE: Tangjiashan July 13,2009 4.4 Breach Flood Routing Calculation

• Currently, it is common to use one- dimensional unsteady flow model or empirical formula to carry out flood routing calculation. • It is suggested that the following risk factors be considered: – Quake lake group effect – River channel characteristic and parameter selection

July 13,2009 5 Improvement and Protection of Quake Lakes • Based on the improvement and protection of 19 Quake lakes in Mianyuan River and Baisha River, the following suggestions can be made: 1.1 Collect the most recent topography data; 2.2 Mitigate the danger by lowering the reservoir level in multiple times by limited amount; 3.3 Utilizing drain and scouring in reservoir discharge; 4.4 Retrofit goal should meet the short- and medium- term needs. Systemic improvement should be implemented after long-term monitoring;

July 13,2009 5 Improvement and Protection of Quake Lakes 5.5 Open channels with soft liner like gravel and gravel in steel-bar basket for bottom protection or slope protection are recommended as general water discharge facilities; 6.6 Utilizing construction materials from nearby borrow sites; 7.7 Monitoring and data collection should be done before improving the river bank; 8.8 Systemic planning and comprehensive treatment are crucial for river channel improvement; 9.9 Deploying an integrated survey and monitoring system, establishing Quake lake health file and implementing a long-term health status scheme, and 1010.Safety is the prerequisite for development and utilization.

July 13,2009 6 Conclusions

• A comprehensive study on the safety assessment and retrofit techniques of Quake lakes are presented. The following conclusions can be made: 11. It is necessary to improve and amend the related technique standards and guidelines based on the experience accumulated from the improvement and protection of Quake lakes created by Wenchuan earthquake. 22. The health status of Quake lakes changes with time. Therefore the improvement and protection measures should not be carried out once for all; it is not economical and practical to attempt to fix them at one time, either. Consistent long-term health status monitoring and proper measures implemented in steps are necessary.

July 13,2009 6 Conclusions

33. It is recommended that key problems including landslide dam break modes and related hydraulic problems be studied based on more than one hundred Quake lakes created by Wenchuan earthquake, and back-analysis of breaches of typical Quake lakes in history. 44. The river system in earthquake-affected areas suffered severe damage including large amount of landslide debris blockages in river channel. It is suggested that comprehensive improvement and recovery procedures be investigated and implemented to heal the damaged river systems.

July 13,2009 THANKS !

July 13,2009 Wenjiagou For example, two large-size mud-rock flows, i.e. Wenjiagou and Yinxinggou, occurred at 1.0 km upstream of Mianyuan River in Qingping County, and blocked the river channel.

As a result of earthquake,Yinxinggou the discharge capacity of this river As a result of earthquake, channel is far less than 1420 the discharge capacity of this m3/s which is the flood flow river channel is far less than rate occurring once every 20 1420 m3/s which is the flood years. flow rate occurring once every 20 years.

July 13,2009 Use of hydrology data • There are hydrological stations at the river mouth of Baisha River, Mianyuan River, etc. However, a majority of Quake lakes are located on the upper and middle reaches of rivers; hydrology data must be corrected for Quake lake conditions. • Besides, it was found that the largest historical flood of a few rivers in the earthquake-affected area was caused by Quake lake burst. • For instance, in 1964, a flood which appears once every 200 years in Baisha river was caused by overtopping of dam and self-burst of Jiaozigou Quake lake. It is therefore necessary to select proper hydrology data according to investigation of historical flooding.

July 13,2009 typical case of different breach modes Typical case Breach mode Formation, Quak lake Location River Consequences Breach time Overtopping Yuanbao City, 8/3/1934, 2nd largest flood in the drainage Mianyuan global Mountain 10 min. later area on record breach Mugang , 6/1/1786, Loss of more than ten Dadu Overtopping Mountain Sichuan 6/10/1786 thousands of lives deep breach , 6/8/1967, Highway and bridges damages Tanggudong Yalong Sichuan 7/17/1968 along downstream Overtopping 8/25/1933, Damaging 17 rope suspension , shallow Daqiao Min 6/15/1986& bridges, 11 small hydropower Sichuan breach Aug. 1990 plants and 21 irrigation facilities Drain global Bomi County, 4/9/2000, Highway and bridge damages Yigong Zangmu breach Tibet Jul. 2000 along downstream Drain deep Mianzhu City, 5/12/2008, Max. flood in the drainage area Xiaogangjian Mianyuan breach Sichuan 6/12/2008 on record Natural Five lakes from Mao County, 8/25/1933, group Songpinggou to Min loss of lives Sichuan 10/9/1933 breach Diexi Five lakes from Beichuan, 5/12/2008, weathered through flood season Tangjiashan to Tongkou Sichuan 6/10/2008 without damage/loss Drain group Xinjiekou breach Four lakes from Mianzhu City, 5/12/2008, Xiaogangjian to Mianyuan None Sichuan 6/12/2008 Yibadao July 13,2009 Overtopping global breach

• Overtopping global breach often occurs in small Quake lakes formed by landslide dams of soils without measures for water diversion. It is less likely to occur to medium and large Quake lakes with water diversion or drain measures. • The Yuanbaoshan Lake on Mianyuan River in Mianzhu City is classified as small II lake. The dam mainly consists of soils. When heavy rainstorm hit Mianyuan River drain basin, water level in the reservoir rose rapidly, leading to overtopping and global breach in about 10 min.

July 13,2009 Overtopping deep breach • Overtopping deep breach often occurs in medium and large Quake lakes. Direct cause is that reservoir water level rise rapidly, causing overtopping and deep breach. Because the dam is tall and reservoir is large, the breach impacts a large area in the downstream, which may cause great damage and loss of lives. • On June 1, 1786, a deadly earthquake that measured at 7.0 Ms occurred in Moxi, Luding County, Sichuan Province. This earthquake caused a landslide along the right bank of in Mugang Ridge, forming a Quake lake. In June 10, the landslide dam burst by aftershocks, causing flood that affected an area of 1400 km2 in the downstream; river water level surged 25 m even at the Waying and Yangsiying, , about 150 km downstream of the Quake lake site. Consequently about ten thousands of people were killed, which were far more than the live loss due to the earthquake. This disaster is probably the most deadly dam breach in human history. • Tanggudong Lake on Yanlong River in Yajiang County had a water storage of 6.8x108 m3, a dam height of 175 m. Fortunately, the forecast and prediction were accurate so that the dam breach did not cause any loss of life. However, many bridges and highways were destroyed due to deep dam breach, since no engineering measures were taken due to lack of road access. Even the water level at Yibing along the Jinsha River rose rapidly by 3 m.

July 13,2009 Overtopping shallow breach

• The overtopping shallow breach often occurs in landslide dams of global stability which lacks local shallow stability. The disposal of Heidongya Quake Lake on Mianyuan River shows that breach risk can be lowered or eliminated by removal of potential unstable materials or construction of sluice channel. • Diexi Earthquake hit Mao County on August 25, 1933, forming Minjiangdaqiao Quake lake. It first burst on October 9, 1933. On June 15, 1986, a 12 m deep breach occurred on the top of the dam due to persistent rainstorm, causing 6 m-high flood rushing to the downstream; seventeen rope-suspension bridges, eleven small hydropower stations and twenty-one irrigation facilities were severely damaged. It burst again in August 1990. A sluice channel was constructed in the dam in 1993, and there has been no breach since.

July 13,2009 Drain global breach

• Drain global breach often occurs in Quake lakes with landslide dams of soils and large volume, a huge reservoir capacity, limited construction period or intensity and rapidly rising water level. • Yigong Quake Lake held approximately 6x109 m3 of water. The landslide dam, of which 70% is fine soils, is 60~100 m tall, about 2500 m wide, 2500 m long, and has a volume of about 2.8~3.0x108 m3. A sluice channel of 150 m wide, 850 m long and 24.1 m deep was excavated to lower water level by18.16 m and reduce reservoir water by 1.0x109 m3. The maximum discharge flow rate was1.22x105 m3/s. However, the flow rate could reach 2.4x105 m3/s due to dam breach and the consequences would be hard to imagine if the sluice channel were not constructed to mitigate the risks.

July 13,2009 Drain deep breach

• The major cause for drain deep breach is excessive lowering of water level at a single time. Lowering water level in disposal of Quake lakes should be done at multiple times with a strictly controlled amount of water drained each time. • Xiaogangjian Quake Lake held approximately 1.2×108 m3 of water. The landslide dam, mainly composed of soils, is about 70~100 m high, about 200 m wide, and 220 m long. Due to damage to access road, there was no means to bring in construction equipment; water level rose rapidly. Blast had to be used to form a sluice channel to lower water level and reduce breach risk. However, blast formed a sluice channel which was deeper than designed, causing a breach over 30 m deep and forming the max. flood in the drainage area on record (Peak discharge rate of 3980m3/s by estimation). Fortunately there was no loss of life thanks to the forecast and regular drills for disaster prevention .

July 13,2009 Natural group breach

• The risk of Quake lake group natural breach, especially those trigged by overtopping global breach of Quake lakes located in the upstream, is extremely high. Thus, safety assessment of Quake lakes should not only focus on the size of the lake, risk of landslide dam and loss caused by dam breach, but also pay attention to whether there are Quake lakes and reservoirs in the downstream, and if there is any possibility of breach. • On October 9, 1933 five Quake lakes from Songpinggou to Diexi in Mao County, on Min River or its tributary, burst one after the other. This group breach started as overtopping deep breach of Songpinggou Middle Quake lake on Songpinggou River, a tributary of Min River, due to aftershocks and rainstorm, which led to overtopping global breach of Songpinggou Lower Quake lake. The surge wave pounded the landslide dam of Diexi Quake lake in Min River, causing overtopping deep breach; the top 90 m of the landslide dam was washed away. The flood later caused overtopping deep breach in the small and big Haizi Quake lakes.

July 13,2009 Drain group breach

• For large-size Quake lakes in the upstream, it is effective to use sluice channel to drain the lake in the upstream, and utilize subsequently formed flood to gradually breach Quake lakes in the downstream. It is crucial to strictly control the flood flow rate so as not to cause disaster in the downstream. • Breach of five Quake lakes from Tangjiashan to Xinjiekou on Tongkou River in Beichuan - In the evening of June 9, 2008, measures including “removing blocks” and “expanding capacity” were taken on the sluice channel. The discharge flow rate increased to about 80m3/s at around 6 am on June 10, causing overtopping shallow breach in four Quake lakes in the downstream of Kuzhuba dam in Tongkou River. As the discharge rate of Tangjiashan Quake Lake increased, the breach mouth grew due to water erosion; flood with a flow rate of 6,800 m3/s passed the river safely. It is noted that the pre-earthquake once- in-50-year flood had a flow rate of 5,900 m3/s. Thanks to the proper procedures applied in Tangjiashan Quake Lake sluice channel, its discharge rate grew gradually, leading to the gradual removal of 5 Quake lakes in the downstream without causing any loss. This is a miracle in the disposal of Quake lakes. • Breach of four Quake lakes from Xiaogangjian to Yibadao on Mianyuan River - on June 12, 2008, blast was used to mitigate the risks of Xiaogangjian Upper Quake Lake. However, due to a heavy precipitation, the breach depth reached over 30 m; the consequently formed flood overtopped and breached Xiaogangjian Middle and Lower Quake Lake, and Yibadao Quake Lake in the farthest downstream.

July 13,2009 • Dabachang Quake Lake, located on Mianyuan River in Mianzhu City, has a water storage capacity of approximately 7.0×105 m3. The landslide dam is 60 m high, and has a downstream slope ratio of 1:0.7. • However, boulders and rocks were found in the middle and lower parts of downstream Thus there is no need to river channel. dispose it. • It is likely that the dam is composed of rocks and Rather, it could be preserved boulders with good anti- to form a waterfall landscape erosion ability and stability. for development of tourism.

July 13,2009 • For example, the lowest elevation of the top of Tangjiashan landslide dam was 752 m high, and the bottom of sluice channel inlet was 740 m high. Anticipated elevation of the inlet was 720 m after erosion. • After interviewing local residents, it was found that the origin of the landslide was Dashui Village, and there were more than 350 residents before the earthquake, and there existed a thick layer of fertile soils. In-situ investigation found that the thick layer of soils slid to the right abutment of the dam, where the sluice channel is coincidently designed. It is therefore concluded that the sluice channel could be eroded to enlarge its discharge capacity if the rock and boulders blocking the original sluice channel are removed to expand its cross section. • After taking these measures, survey results show that the discharge flow rate reached 6,800 m3/s at the sluice channel, and the bottom elevation of the sluice channel inlet had been lowered to 713.5 m.

July 13,2009 Tangjiashan The lowest elevation of the top of Tangjiashan landslide dam was 752 m high, and the bottom of sluice channel inlet was 740 m high. Anticipated elevation of the rock and boulders blocking the inlet was 720 m after erosion. original sluice channel

rock and boulders blocking the The sluice channel original sluice channel could be eroded to enlarge its discharge capacity if the rock and boulders blocking the original sluice channel are removed to expand its cross section. July 13,2009 Quake lake group effect

• Quake lake group effect mainly includes two aspects: “water adding” effect and flood detention effect. Which effect dominate depends on the size, distribution and breach mode of Quake lake group (including reservoirs and other water retention facilities). • On June 10, 2008, after removal of blockage and scouring in the sluice channel of Tangjiashan Quake Lake, its discharge flow rate rose gradually. The discharge flow passed through 4 breached Quake lakes and three reserviors of hydropower plants in the downstream, and reached to urban area of City without attenuation.

July 13,2009 River channel characteristic and parameter selection

• It is imperative to consider the river channel characteristic in the selection of hydraulic gradient and momentum correction coefficient in one-dimensional unsteady flow model, and the selection of the maximum average flow velocity of cross section and coefficient of empirical formula. • The average gradient of Baisha River in Dujiangyan is 49‰. In July 1964, overtopping deep breach occurred in Heshangqiao Quake Lake, about 20 km upstream of Hongkou. This lake held less than 1.0x105 m3 of water at the time of breach; it, however, formed the maximum flood of this drainage area in history. This shows that breach of Quake lakes in mountainous rivers has prominent “water adding” effect.

July 13,2009