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Assessing the Consequences of Dam Failures. a How-To Guide

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Assessing the Consequences of Dam Failures. a How-To Guide This document was prepared by Risk Assessment, Mapping, and Planning Partners 8401 Arlington Boulevard Fairfax, Virginia 22031 Table of Contents ACRONYMS AND ABBREVIATIONS ................................................................................ iii SECTION ONE INTRODUCTION ..............................................................................................1-1 1.1 What This Guide Provides.................................................................... 1-1 1.2 Purpose of This Guide .......................................................................... 1-1 1.3 Limitations of the Guide....................................................................... 1-1 1.4 Audience for This Guide....................................................................... 1-2 1.5 How This Guide Is Organized .............................................................. 1-2 1.6 Further Application of Recommended Procedures ................................ 1-3 SECTION TWO OVERVIEW OF ASSESSMENT METHODOLOGY ...........................2-1 2.1 Define the Inundation Area................................................................... 2-1 2.2 Create an Inventory of Assets ............................................................... 2-1 2.3 Estimate the Potential Consequences .................................................... 2-1 2.4 Determine the Impacts.......................................................................... 2-1 2.5 Limitations of the Recommended Approach ......................................... 2-2 SECTION THREE DEFINE DAM FAILURE SCENARIO ......................................................3-1 3.1 Characteristics of the Dam.................................................................... 3-1 3.2 Dam Failure Scenario ........................................................................... 3-1 3.3 Community .......................................................................................... 3-1 SECTION FOUR IDENTIFY INUNDATION AREA ...............................................................4-1 4.1 Methods for identifying inundation area ............................................... 4-1 4.2 Guidelines for conducting a Hydrologic and Hydraulic study ............... 4-1 4.3 Importance of Depth and Velocity ........................................................ 4-2 SECTION FIVE INVENTORY ASSETS.....................................................................................5-1 5.1 Economic, Social, and Environmental Assets ....................................... 5-1 5.2 Critical Infrastructure and Resources .................................................... 5-4 SECTION SIX IDENTIFY CONSEQUENCES .....................................................................6-1 6.1 Determine Anticipated Level of Damage .............................................. 6-1 6.2 Determine Potential for Loss of Life..................................................... 6-2 6.3 Determine other Potential Consequences .............................................. 6-6 SECTION SEVEN ASSESS CONSEQUENCES ...........................................................................7-1 7.1 Economic Impacts ................................................................................ 7-1 7.2 Social Impacts ...................................................................................... 7-9 7.3 Environmental Impacts........................................................................7-11 SECTION EIGHT IDENTIFY FOLLOW-UP ACTIVITIES ...................................................8-1 SECTION NINE SOURCES OF INFORMATION ..................................................................9-1 FIGURES Figure 1: Assessment Process.................................................................................................. 2-2 Figure 2: Inundation Area for Example Scenario ..................................................................... 5-4 i Table of Contents TABLES Table 1: Assets in the Inundation Area .................................................................................... 5-3 Table 2: Critical Infrastructure and Resources Beyond the Inundation Area ............................ 5-5 Table 3: Anticipated Level of Damage .................................................................................... 6-2 Table 4: Occupancy of Inundation Area .................................................................................. 6-4 Table 5: Method for Estimating Loss of Life ........................................................................... 6-5 Table 6: Anticipated Consequences of Dam Failure ................................................................ 6-6 Table 7: Anticipated Consequences for Critical Infrastructure and Resources Beyond the Inundation Area ...................................................................................................... 6-7 Table 8: Potential Economic Impacts Steps 1 through 3 .......................................................... 7-2 Table 9: Potential Economic Impacts Steps 4 through 7 .......................................................... 7-4 Table 10: Potential Economic Impacts Steps 8 through 11....................................................... 7-5 Table 11: Potential Economic Impacts Steps 12 through 15..................................................... 7-6 Table 12: Potential Economic Impacts Steps 16 and 17 ........................................................... 7-7 Table 13: Potential Social Impacts .........................................................................................7-10 Table 14: Potential Economic and Social Impacts for Critical Infrastructure and Resources...............................................................................................................7-11 Table 15: Environmental Impacts...........................................................................................7-12 Table 16: Potential Environmental Impacts for Critical Infrastructure and Resources .............7-13 APPENDICES Appendix A Assets Worksheet Appendix B Critical Infrastructure and Resources Worksheet ii Acronyms and Abbreviations EAP Emergency Action Plan FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map GIS Geographic Information System H&H Hydrologic and Hydraulic Hazus-MH Hazards U.S. – Multi-Hazard (Software) NFIP National Flood Insurance Program PMP Probable Maximum Precipitation USACE U.S. Army Corps of Engineer USDA U.S. Department of Agriculture iii This page intentionally left blank. iv Introduction SECTION ONE INTRODUCTION 1.1 WHAT THIS GUIDE PROVIDES The Federal Emergency Management Agency (FEMA) developed Assessing the Consequences of Dam Failure: A How-To Guide to provide community officials and interested stakeholders with a process for assessing the potential economic, social, and environmental impacts of dam failure. For this guide, the word dam is defined as any barrier, wall, or embankment, along with its abutments and appurtenant works, constructed for the purpose of storing water or other liquid material either temporarily or permanently. The term dam failure means that the dam is overtopped or fails to operate in the manner for which it was designed. This may include catastrophic failure whereby a breach would allow the dam’s reservoir to suddenly drain. Failure may also refer to a dam’s inability to operate correctly resulting in uncontrolled releases; for example, a dam with a flood pool that cannot be used because of structural concerns could increase flooding downstream during storm events. Dam failure can occur with little to no warning, or it may be an anticipated event. This guide presents an analytical method particularly appropriate for a community-level examination of the potential damage posed by the tens of thousands of dams in the United States. Information developed as a result of the assessment can be included in the description of potential impacts of a dam failure hazard in a local hazard mitigation plan or Emergency Operation Plan. 1.2 PURPOSE OF THIS GUIDE The purpose of this guide is to outline a procedure for identifying and assessing the potential consequences of dam failure at the community level using readily available information. A good assessment will establish the potential short- and long-term economic, social, and environmental effects of dam failure that will inform planning efforts. 1.3 LIMITATIONS OF THE GUIDE This guide provides information for a cursory assessment of the potential consequences of dam failure. Some investment of time and resources is required to conduct the assessment. Communities can use available resources, which can vary from paper maps and tax assessor records to the results of hydrologic and hydraulic (H&H) studies or geographic information system (GIS) data. Limitations include: The consequence assessment methodology described in this guide does not address the probability of a dam failure. Dam failure is typically considered a low-probability, high- consequence event. FEMA encourages the determination of consequences regardless of the probability of dam failure. 1-1 Introduction The consequence assessment methodology is not a substitute for detailed engineering or academic analyses. Therefore, the results should not be considered sufficient for use in a benefit-cost analysis of mitigation or rehabilitation measures. This guide does not provide all of the information necessary for developing an Emergency Action Plan (EAP). An EAP is typically developed by dam owners and outlines procedures
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