Dam Failure Mechanisms and Risk Assessment

Professor Limin Zhang, Hong Kong University of Science and Technology, China
Limin 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, China
Ming 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, China
Dongsheng 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, China
Yao 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 xiii

Foreword by Jinsheng Jia xiv

Preface xvi

Acknowledgements xviii

About the Authors xix

PART I DAM AND DIKE FAILURE DATABASES 1

1 Dams and Their Components 3

1.1 Classification of Dams 3

1.2 Constructed Embankment Dams 4

1.3 Landslide Dams 7

1.4 Concrete Gravity Dams 7

1.5 Concrete Arch Dams 8

1.6 Dikes 10

2 Statistical Analysis of Failures of Constructed Embankment Dams 11

2.1 Database of Failures of Constructed Embankment Dams 11

2.2 Failure Modes and Processes 11

2.2.1 Overtopping 16

2.2.2 Internal Erosion 17

2.3 Common Causes of Embankment Dam Failures 19

2.4 Failure of Different Types of Embankment Dams 21

2.4.1 Analysis of Homogeneous and Composite Earthfill Dams 23

2.4.2 Analysis of Earthfill Dams with Corewalls 23

3 Statistical Analysis of Failures of Landslide Dams 25

3.1 Database of Failures of Landslide Dams 25

3.1.1 Locations of Landslide Dams 25

3.1.2 Formation Times of Landslide Dams 26

3.1.3 Triggers of Landslide Dams 26

3.1.4 Types of Landslide 26

3.1.5 Dam Heights and Lake Volumes 32

3.2 Stability, Longevity, and Failure Modes of Landslide Dams 33

3.2.1 Stability of Landslide Dams 33

3.2.2 Longevity of Landslide Dams 35

3.2.3 Failure Modes 36

3.3 Mitigation Measures for Landslide Dams 37

3.3.1 Stages of Landslide Dam Risk Mitigation 38

3.3.2 Engineering Mitigation Measures for Landslide Dams 39

3.3.3 Engineering Measures for the Landslide Dams Induced by the Wenchuan Earthquake 41

3.3.4 Mitigation Measures for the Tangjiashan Landslide Dam 51

4 Statistical Analysis of Failures of Concrete Dams 53

4.1 Database of Failures of Concrete Dams 53

4.2 Failure Modes and Processes 53

4.3 Common Causes of Concrete Dam Failures 55

5 Statistical Analysis of Failures of Dikes 57

5.1 Introduction 57

5.2 Database of Dike Breaching Cases 57

5.3 Evaluation of Dike Failure Mechanisms 59

5.3.1 Most Relevant Failure Mechanisms 59

5.3.2 Statistics of Observed Failure Mechanisms 62

PART II DAM FAILURE MECHANISMS AND BREACHING PROCESS MODELING 67

6 Internal Erosion in Dams and Their Foundations 69

6.1 Concepts of Internal Erosion 69

6.2 Mechanisms of Initiation of Internal Erosion 72

6.2.1 Concentrated Leak Erosion 72

6.2.2 Backward Erosion 73

6.2.3 Contact Erosion 73

6.2.4 Suffusion 74

6.3 Initiation of Concentrated Leak Erosion Through Cracks 74

6.3.1 Causes of Concentrated Leak 75

6.3.2 Need for Studying Soil Erodibility for Concentrated Leak Erosion 80

6.3.3 Laboratory Tests on Concentrated Leak Erosion 81

6.3.4 Factors Affecting Concentrated Leak Erosion 83

6.3.5 Soil Dispersivity 84

6.4 Initiation of Backward Erosion 87

6.4.1 Susceptibility of a Dam or Dike to Backward Erosion 87

6.4.2 Methods for Assessing Backward Erosion 89

6.4.3 Formation of a Pipe due to Backward Erosion 92

6.5 Initiation of Contact Erosion 93

6.5.1 Fundamental Aspects of Contact Erosion Process 94

6.5.2 Laboratory Investigation on Contact Erosion 96

6.5.3 Threshold of Contact Erosion 100

6.6 Initiation of Suffusion 102

6.6.1 Control Parameters for Likelihood of Suffusion 102

6.6.2 Laboratory Testing of Suffusion 103

6.6.3 Geometrical Criteria for Internal Stability of Soils 108

6.6.4 Critical Hydraulic Gradients for Suffusion 115

6.7 Filter Criteria 120

6.7.1 Functions of Filter 120

6.7.2 Filter Criteria 121

6.8 Continuation of Internal Erosion 124

6.9 Progression of Internal Erosion 125

6.10 Suggested Topics for Further Research 126

7 Mechanics of Overtopping Erosion of Dams 127

7.1 Mechanics of Surface Erosion 127

7.1.1 Incipient Motion of Sediment 128

7.1.2 Sediment Transport 133

7.2 Determination of Erodibility of Soils 144

7.2.1 Critical Erosive Shear Stress 144

7.2.2 Coefficient of Erodibility 145

7.2.3 Laboratory Tests 147

7.2.4 Field Tests 151

7.2.5 Classification of Soil Erodibility 155

7.3 Characteristics of Overtopping Erosion Failure of Dams 157

7.3.1 Homogeneous Embankment Dams with Cohesionless Materials 157

7.3.2 Homogeneous Embankment Dams with Cohesive Materials 158

7.3.3 Composite Embankment Dams 159

7.4 Suggested Topics for Further Research 159

8 Dam Breach Modeling 161

8.1 Methods for Dam Breach Modeling 161

8.2 Dam Breaching Data 163

8.2.1 Embankment Dam Breaching Data 163

8.2.2 Landslide Dam Breaching Data 165

8.2.3 Dike Breaching Data 165

8.3 Empirical Analysis Methods 166

8.3.1 Multivariable Regression 166

8.3.2 Empirical Breaching Parameters for Constructed Embankment Dams 169

8.3.3 Empirical Breaching Parameters for Landslide Dams 179

8.3.4 Empirical Breaching Parameters for Dikes 187

8.3.5 Comparison of Breaching Parameters for Landslide Dams and Constructed Embankment Dams 189

8.4 Numerical Simulation of Overtopping Erosion 192

8.4.1 Simplified Physically Based Methods 197

8.4.2 Detailed Physically Based Methods 206

8.4.3 Case Studies 211

8.5 Numerical Simulation of Internal Erosion 215

8.5.1 Continuum Methods 215

8.5.2 Particle Level Analysis 218

8.5.3 Case Studies 218

9 Analysis of Dam Breaching Flood Routing 222

9.1 River Hydraulics 222

9.1.1 One-dimensional Models 223

9.1.2 Two-dimensional Models 223

9.2 Numerical Models for Flood Routing Analysis 224

9.2.1 One-dimensional Numerical Models 224

9.2.2 Two-dimensional Numerical Models 227

9.2.3 Coupling of 1D/2D Numerical Models 229

9.3 Example – Tangjiashan Landslide Dam Failure 229

9.3.1 Geometric Information 229

9.3.2 Dam Breaching Simulation 232

9.3.3 Boundary and Initial Conditions 232

9.3.4 Flood Routing Analysis and Results 232

PART III DAM FAILURE RISK ASSESSMENT AND MANAGEMENT 241

10 Analysis of Probability of Failure of Dams 243

10.1 Introduction 243

10.2 Analysis Methods 243

10.2.1 Failure Modes and Effects Analysis 243

10.2.2 Event Tree 244

10.2.3 Fault Tree 246

10.2.4 First-order Reliability Method/First-order Second-moment Method 247

10.2.5 Monte Carlo Simulation 250

10.2.6 Bayesian Networks 250

10.3 Examples of Probabilistic Analysis of Dam Failure 253

10.3.1 Probabilistic Analysis of Chinese Dam Distresses 253

10.3.2 Probabilistic Analysis of the Chenbihe Dam Distresses Using Bayesian Networks 264

11 Vulnerability to Dam Breaching Floods 273

11.1 Concepts of Vulnerability 273

11.2 Human Vulnerability to Dam Breaching Floods 273

11.2.1 Human Stability in Flood 274

11.2.2 Influence Factors 277

11.2.3 Methods for Evaluating Human Vulnerability Factor in a Flood 278

11.2.4 Database of Fatalities in Dam/Dike Breaching or Other Floods 283

11.3 Bayesian Network Analysis of Human Vulnerability to Floods 284

11.3.1 Bayesian Networks 284

11.3.2 Building the Bayesian Network for Human Vulnerability 285

11.3.3 Quantifying the Networks 291

11.3.4 Validation of the Model 297

11.4 Damage to Buildings and Infrastructures 300

11.4.1 Flood Action on Buildings 300

11.4.2 Models for Building Damage Evaluation 303

11.4.3 Relationship between Building Damage and Loss of Life 305

11.5 Suggested Topics for Further Research 306

12 Dam Failure Risk Assessment 307

12.1 Risk and Risk Assessment 307

12.1.1 Definition of Risk 307

12.1.2 Risk Management 308

12.2 Dam Failure Risk Analysis 311

12.2.1 Scope Definition 311

12.2.2 Hazards Identification 311

12.2.3 Identification of Failure Modes 312

12.2.4 Estimation of Failure Probability 312

12.2.5 Evaluation of Elements at Risk 313

12.2.6 Vulnerability Evaluation 314

12.2.7 Risk Estimation 314

12.3 Risk Assessment 315

12.3.1 Risk Tolerance Criteria 315

12.3.2 ALARP Considerations 319

12.4 Suggested Topics for Further Research 321

13 Dam Failure Contingency Risk Management 322

13.1 Process of Contingency Risk Management 322

13.1.1 Observation and Prediction 323

13.1.2 Decision-making 323

13.1.3 Warning 324

13.1.4 Response 325

13.1.5 Evacuation 326

13.2 Decision-making Under Uncertainty 328

13.2.1 Decision Tree 329

13.2.2 Multi-phase Decision 330

13.2.3 Influence Diagrams 333

13.3 Dynamic Decision-Making 334

13.3.1 Dam Failure Emergency Management 336

13.3.2 Dynamic Decision-making Framework 339

13.3.3 Time Series Models for Estimating Dam Failure Probability 342

13.3.4 Evaluation of the Consequences of Dam Failures 348

13.3.5 Features of DYDEM 350

13.4 Suggested Topics for Further Research 351

14 Case Study: Risk-based Decision-making for the Tangjiashan Landslide Dam Failure 353

14.1 Timeline for Decision-making for the Tangjiashan Landslide Dam Failure 353

14.2 Prediction of Dam Break Probability with Time Series Analysis 355

14.2.1 Forecasting Inflow Rates 355

14.2.2 Forecasting Lake Volume 358

14.2.3 Prediction of Dam Failure Probability 359

14.3 Simulation of Dam Breaching and Flood Routing 361

14.3.1 Simulation of Dam Breaching and Flood Routing in Stage 1 362

14.3.2 Simulation of Dam Breaching and Flood Routing in Stage 2 363

14.3.3 Simulation of Dam Breaching and Flood Routing in Stage 3 365

14.4 Evaluation of Flood Consequences 365

14.4.1 Methodology 366

14.4.2 Calculated Dam Break Flood Consequences 367

14.5 Dynamic Decision-making 370

14.5.1 Methodology 370

14.5.2 Dynamic Decision-making in Three Stages 371

14.6 Discussions 374

14.6.1 Influence of the Value of Human Life 374

14.6.2 Influence of Failure Mode 374

14.6.3 Sensitivity of the Minimum Expected Total Consequence 375

PART IV APPENDIXES: DAM FAILURE DATABASES 377

Appendix A: Database of 1443 Cases of Failures of Constructed Dams 379

Appendix B: Database of 1044 Cases of Failures of Landslide Dams 419

References 452

Index 474

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