Dam Failure Mechanisms and Risk Assessment

Professor Limin Zhang, Hong Kong University of Science and Technology, ChinaLimin Zhang is currently Professor of Civil Engineering at the Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology. His research areas include embankment dams and slopes, geotechnical risk assessment and foundation engineering. Dr. Ming Peng, Hong Kong University of Science and Technology, ChinaMing Peng is a Post-doctoral Research Associate at the Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology. His research areas include risk analysis methodologies, flood vulnerability analysis and decision theory. Dr. Dongsheng Chang, Hong Kong University of Science and Technology, ChinaDongsheng Chang is a Post-doctoral Research Associate at the Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology. Dr. Chang is an expert in internal erosion and overtopping erosion of dams. He invented a laboratory device to test the internal erodibility of soils under complex stress conditions. Dr. Yao Xu, China Institute of Water Resources and Hydropower Research and Chinese National Committee on Large Dams, ChinaYao Xu recently joined China Institute of Water Resources and Hydropower Research and Chinese National Committee on Large Dams after working as a Post-doctoral Research Associate at the Department of Civil and Environmental Engineering.

• Foreword by Kaare Høeg xiiiForeword by Jinsheng Jia xivPreface xviAcknowledgements xviiiAbout the Authors xixPART I DAM AND DIKE FAILURE DATABASES 11 Dams and Their Components 31.1 Classification of Dams 31.2 Constructed Embankment Dams 41.3 Landslide Dams 71.4 Concrete Gravity Dams 71.5 Concrete Arch Dams 81.6 Dikes 102 Statistical Analysis of Failures of Constructed Embankment Dams 112.1 Database of Failures of Constructed Embankment Dams 112.2 Failure Modes and Processes 112.2.1 Overtopping 162.2.2 Internal Erosion 172.3 Common Causes of Embankment Dam Failures 192.4 Failure of Different Types of Embankment Dams 212.4.1 Analysis of Homogeneous and Composite Earthfill Dams 232.4.2 Analysis of Earthfill Dams with Corewalls 233 Statistical Analysis of Failures of Landslide Dams 253.1 Database of Failures of Landslide Dams 253.1.1 Locations of Landslide Dams 253.1.2 Formation Times of Landslide Dams 263.1.3 Triggers of Landslide Dams 263.1.4 Types of Landslide 263.1.5 Dam Heights and Lake Volumes 323.2 Stability, Longevity, and Failure Modes of Landslide Dams 333.2.1 Stability of Landslide Dams 333.2.2 Longevity of Landslide Dams 353.2.3 Failure Modes 363.3 Mitigation Measures for Landslide Dams 373.3.1 Stages of Landslide Dam Risk Mitigation 383.3.2 Engineering Mitigation Measures for Landslide Dams 393.3.3 Engineering Measures for the Landslide Dams Induced by the Wenchuan Earthquake 413.3.4 Mitigation Measures for the Tangjiashan Landslide Dam 514 Statistical Analysis of Failures of Concrete Dams 534.1 Database of Failures of Concrete Dams 534.2 Failure Modes and Processes 534.3 Common Causes of Concrete Dam Failures 555 Statistical Analysis of Failures of Dikes 575.1 Introduction 575.2 Database of Dike Breaching Cases 575.3 Evaluation of Dike Failure Mechanisms 595.3.1 Most Relevant Failure Mechanisms 595.3.2 Statistics of Observed Failure Mechanisms 62PART II DAM FAILURE MECHANISMS AND BREACHING PROCESS MODELING 676 Internal Erosion in Dams and Their Foundations 696.1 Concepts of Internal Erosion 696.2 Mechanisms of Initiation of Internal Erosion 726.2.1 Concentrated Leak Erosion 726.2.2 Backward Erosion 736.2.3 Contact Erosion 736.2.4 Suffusion 746.3 Initiation of Concentrated Leak Erosion Through Cracks 746.3.1 Causes of Concentrated Leak 756.3.2 Need for Studying Soil Erodibility for Concentrated Leak Erosion 806.3.3 Laboratory Tests on Concentrated Leak Erosion 816.3.4 Factors Affecting Concentrated Leak Erosion 836.3.5 Soil Dispersivity 846.4 Initiation of Backward Erosion 876.4.1 Susceptibility of a Dam or Dike to Backward Erosion 876.4.2 Methods for Assessing Backward Erosion 896.4.3 Formation of a Pipe due to Backward Erosion 926.5 Initiation of Contact Erosion 936.5.1 Fundamental Aspects of Contact Erosion Process 946.5.2 Laboratory Investigation on Contact Erosion 966.5.3 Threshold of Contact Erosion 1006.6 Initiation of Suffusion 1026.6.1 Control Parameters for Likelihood of Suffusion 1026.6.2 Laboratory Testing of Suffusion 1036.6.3 Geometrical Criteria for Internal Stability of Soils 1086.6.4 Critical Hydraulic Gradients for Suffusion 1156.7 Filter Criteria 1206.7.1 Functions of Filter 1206.7.2 Filter Criteria 1216.8 Continuation of Internal Erosion 1246.9 Progression of Internal Erosion 1256.10 Suggested Topics for Further Research 1267 Mechanics of Overtopping Erosion of Dams 1277.1 Mechanics of Surface Erosion 1277.1.1 Incipient Motion of Sediment 1287.1.2 Sediment Transport 1337.2 Determination of Erodibility of Soils 1447.2.1 Critical Erosive Shear Stress 1447.2.2 Coefficient of Erodibility 1457.2.3 Laboratory Tests 1477.2.4 Field Tests 1517.2.5 Classification of Soil Erodibility 1557.3 Characteristics of Overtopping Erosion Failure of Dams 1577.3.1 Homogeneous Embankment Dams with Cohesionless Materials 1577.3.2 Homogeneous Embankment Dams with Cohesive Materials 1587.3.3 Composite Embankment Dams 1597.4 Suggested Topics for Further Research 1598 Dam Breach Modeling 1618.1 Methods for Dam Breach Modeling 1618.2 Dam Breaching Data 1638.2.1 Embankment Dam Breaching Data 1638.2.2 Landslide Dam Breaching Data 1658.2.3 Dike Breaching Data 1658.3 Empirical Analysis Methods 1668.3.1 Multivariable Regression 1668.3.2 Empirical Breaching Parameters for Constructed Embankment Dams 1698.3.3 Empirical Breaching Parameters for Landslide Dams 1798.3.4 Empirical Breaching Parameters for Dikes 1878.3.5 Comparison of Breaching Parameters for Landslide Dams and Constructed Embankment Dams 1898.4 Numerical Simulation of Overtopping Erosion 1928.4.1 Simplified Physically Based Methods 1978.4.2 Detailed Physically Based Methods 2068.4.3 Case Studies 2118.5 Numerical Simulation of Internal Erosion 2158.5.1 Continuum Methods 2158.5.2 Particle Level Analysis 2188.5.3 Case Studies 2189 Analysis of Dam Breaching Flood Routing 2229.1 River Hydraulics 2229.1.1 One-dimensional Models 2239.1.2 Two-dimensional Models 2239.2 Numerical Models for Flood Routing Analysis 2249.2.1 One-dimensional Numerical Models 2249.2.2 Two-dimensional Numerical Models 2279.2.3 Coupling of 1D/2D Numerical Models 2299.3 Example – Tangjiashan Landslide Dam Failure 2299.3.1 Geometric Information 2299.3.2 Dam Breaching Simulation 2329.3.3 Boundary and Initial Conditions 2329.3.4 Flood Routing Analysis and Results 232PART III DAM FAILURE RISK ASSESSMENT AND MANAGEMENT 24110 Analysis of Probability of Failure of Dams 24310.1 Introduction 24310.2 Analysis Methods 24310.2.1 Failure Modes and Effects Analysis 24310.2.2 Event Tree 24410.2.3 Fault Tree 24610.2.4 First-order Reliability Method/First-order Second-moment Method 24710.2.5 Monte Carlo Simulation 25010.2.6 Bayesian Networks 25010.3 Examples of Probabilistic Analysis of Dam Failure 25310.3.1 Probabilistic Analysis of Chinese Dam Distresses 25310.3.2 Probabilistic Analysis of the Chenbihe Dam Distresses Using Bayesian Networks 26411 Vulnerability to Dam Breaching Floods 27311.1 Concepts of Vulnerability 27311.2 Human Vulnerability to Dam Breaching Floods 27311.2.1 Human Stability in Flood 27411.2.2 Influence Factors 27711.2.3 Methods for Evaluating Human Vulnerability Factor in a Flood 27811.2.4 Database of Fatalities in Dam/Dike Breaching or Other Floods 28311.3 Bayesian Network Analysis of Human Vulnerability to Floods 28411.3.1 Bayesian Networks 28411.3.2 Building the Bayesian Network for Human Vulnerability 28511.3.3 Quantifying the Networks 29111.3.4 Validation of the Model 29711.4 Damage to Buildings and Infrastructures 30011.4.1 Flood Action on Buildings 30011.4.2 Models for Building Damage Evaluation 30311.4.3 Relationship between Building Damage and Loss of Life 30511.5 Suggested Topics for Further Research 30612 Dam Failure Risk Assessment 30712.1 Risk and Risk Assessment 30712.1.1 Definition of Risk 30712.1.2 Risk Management 30812.2 Dam Failure Risk Analysis 31112.2.1 Scope Definition 31112.2.2 Hazards Identification 31112.2.3 Identification of Failure Modes 31212.2.4 Estimation of Failure Probability 31212.2.5 Evaluation of Elements at Risk 31312.2.6 Vulnerability Evaluation 31412.2.7 Risk Estimation 31412.3 Risk Assessment 31512.3.1 Risk Tolerance Criteria 31512.3.2 ALARP Considerations 31912.4 Suggested Topics for Further Research 32113 Dam Failure Contingency Risk Management 32213.1 Process of Contingency Risk Management 32213.1.1 Observation and Prediction 32313.1.2 Decision-making 32313.1.3 Warning 32413.1.4 Response 32513.1.5 Evacuation 32613.2 Decision-making Under Uncertainty 32813.2.1 Decision Tree 32913.2.2 Multi-phase Decision 33013.2.3 Influence Diagrams 33313.3 Dynamic Decision-Making 33413.3.1 Dam Failure Emergency Management 33613.3.2 Dynamic Decision-making Framework 33913.3.3 Time Series Models for Estimating Dam Failure Probability 34213.3.4 Evaluation of the Consequences of Dam Failures 34813.3.5 Features of DYDEM 35013.4 Suggested Topics for Further Research 35114 Case Study: Risk-based Decision-making for the Tangjiashan Landslide Dam Failure 35314.1 Timeline for Decision-making for the Tangjiashan Landslide Dam Failure 35314.2 Prediction of Dam Break Probability with Time Series Analysis 35514.2.1 Forecasting Inflow Rates 35514.2.2 Forecasting Lake Volume 35814.2.3 Prediction of Dam Failure Probability 35914.3 Simulation of Dam Breaching and Flood Routing 36114.3.1 Simulation of Dam Breaching and Flood Routing in Stage 1 36214.3.2 Simulation of Dam Breaching and Flood Routing in Stage 2 36314.3.3 Simulation of Dam Breaching and Flood Routing in Stage 3 36514.4 Evaluation of Flood Consequences 36514.4.1 Methodology 36614.4.2 Calculated Dam Break Flood Consequences 36714.5 Dynamic Decision-making 37014.5.1 Methodology 37014.5.2 Dynamic Decision-making in Three Stages 37114.6 Discussions 37414.6.1 Influence of the Value of Human Life 37414.6.2 Influence of Failure Mode 37414.6.3 Sensitivity of the Minimum Expected Total Consequence 375PART IV APPENDIXES: DAM FAILURE DATABASES 377Appendix A: Database of 1443 Cases of Failures of Constructed Dams 379Appendix B: Database of 1044 Cases of Failures of Landslide Dams 419References 452Index 474