Safety Assessment and Retrofit Techniques of Quake Lakes
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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.