TABLE OF CONTENTS Page C-1 Consequences of Dam or Levee Failure .............................................. C-1-1 C-1.1 Introduction .......................................................................... C-1-1 C-1.2 Life Loss Estimation Methodologies and Agency Perspectives...................................................................... C-1-2 C-1.3 Summary of Historic Flooding Events ................................ C-1-3 C-1.3.1 Teton Dam (1976) .............................................. C-1-3 C-1.3.2 Francis Dam (1928) ........................................... C-1-7 C-1.3.3 Baldwin Hills Dam (1963) ............................... C-1-11 C-1.3.4 Laurel Run Dam (1977) ................................... C-1-14 C-1.3.5 New Orleans – Hurricane Katrina (2005) ........ C-1-17 C-1.3.6 Quail Creek Dike (1989) .................................. C-1-20 C-1.4 General Loss of Life Methodology Overview ................... C-1-22 C-1.4.1 Life Loss Estimation: Selecting Scenarios ..... C-1-22 C-1.5 Flood Inundation Modeling ............................................... C-1-24 C-1.5.1 One-Dimensional and Two-Dimensional Hydraulic Modeling for Flood Inundation Analysis ................................... C-1-26 C-1.5.2 Breach Modeling .............................................. C-1-30 C-1.5.3 Inundation Modeling, Terrain Data ................. C-1-40 C-1.5.4 Inundation Modeling Outputs .......................... C-1-42 C-1.6 Estimation of Downstream Population at Risk .................. C-1-43 C-1.7 Warning and Evacuation .................................................... C-1-45 C-1.8 Intensity of Flooding and Fatality Rate ............................. C-1-48 C-1.9 References .......................................................................... C-1-51 Tables Page C-1-1 Summary Table of Teton Dam ...................................................... C-1-6 C-1-2 Summary Table of St. Francis Dam ............................................... C-1-9 C-1-3 Summary Table of Baldwin Hills Dam ....................................... C-1-14 C-1-4 Summary Table of Laurel Run Dam ............................................ C-1-17 C-1-5 Summary Table of Quail Creek Dike .......................................... C-1-22 C-1-6 Breach Modeling Methods ........................................................... C-1-34 C-1-7 Empirical Breach Formulations (DSO-99-04 and HEC-RAS) .... C-1-35 C-1-8 Example Material Erodibility Rates............................................. C-1-37 C-1-9 Grouping Soil Materials into Erodibility Categories ................... C-1-38 C-1-10 Modeled Processes Available in Common Breach Models ......... C-1-39 C-1-11 Allowable Hydraulic Boundary Conditions................................. C-1-40 C-1-i July 2019 Figures Page C-1-1 Teton Dam failure. ......................................................................... C-1-4 C-1-2 Flooding and evacuation at Rexburg, Idaho. ................................. C-1-5 C-1-3 Flood wave propagation across farmland. ..................................... C-1-5 C-1-4 Flooding aftermath at Rexburg. ..................................................... C-1-6 C-1-5 St. Francis Dam before failure. ...................................................... C-1-7 C-1-6 The breached St. Francis Dam. ...................................................... C-1-8 C-1-7 Powerhouse No. 2 before its collapse. (University of Southern California Digital Archive (c)2004). ........................ C-1-10 C-1-8 Location of Powerhouse No. 2, area swept clean after flooding. ................................................................................... C-1-10 C-1-9 Aftermath of flooding at the Edison construction camp. ............. C-1-11 C-1-10 Baldwin Hills Dam. ..................................................................... C-1-12 C-1-11 Flooding downstream of Baldwin Hills Dam. ............................. C-1-13 C-1-12 Flooding immediately downstream of Baldwin Hills Dam. ........ C-1-13 C-1-13 Laurel Run Dam location map. .................................................... C-1-15 C-1-14 Remains of Laurel Run Dam. ...................................................... C-1-16 C-1-15 Flooding aftermath at Tanneryville. ............................................ C-1-16 C-1-16 Levee failure caused by Hurricane Katrina. ................................ C-1-18 C-1-17 Flooding from Hurricane Katrina. ............................................... C-1-18 C-1-18 View of the breached Quail Creek Dike. ..................................... C-1-21 C-1-19 Example reservoir level fluctuation plot. ..................................... C-1-23 C-1-20 Example of a 1D inundation study where a 2D study would be most appropriate. ................................................................. C-1-25 C-1-21 1D hydraulic model layout. River centerline shown in blue and cross sections in red. ......................................................... C-1-26 C-1-22 1D hydraulic model maximum water surface profile output. ...... C-1-27 C-1-23 TIN surface and interpolated maximum inundation boundary. ................................................................................. C-1-28 C-1-24 2D flood inundation example. ...................................................... C-1-29 C-1-25 Breach hydrograph showing phases of breach. ............................ C-1-31 C-1-26 Census block/inundation overlay. ................................................ C-1-43 C-1-27 Probability density function example fatality distribution for small PAR. ......................................................................... C-1-50 C-1-28 Histogram example fatality distribution for rapid failure with large PAR. ........................................................................ C-1-50 C-1-ii July 2019 C-1 CONSEQUENCES OF DAM OR LEVEE FAILURE C-1.1 Introduction Flood water can be one of the most destructive forces on earth, especially if caused by an event that unexpectedly overwhelms an existing flood defense or by catastrophic breach of a dam or levee. Relatively recent events, such as flooding caused by Hurricane Katrina (2005) and the tsunami in Japan (2011), have caused thousands of people to lose their life and unknown billions of dollars in damages. By the same token, dozens of floods (some from similarly unexpected events like a dam or levee breach) occur every year with no resulting loss of life and relatively minimal property damage. Although flooding can have many types of severe consequences, including economic, social, cultural, and environmental, the primary objective of Bureau of Reclamation’s (Reclamation) dam safety program and United States Army Corps of Engineers’ (USACE) dam and levee safety programs are to manage the risk to the public who rely on those structures to keep them reasonably safe from flooding. Thus, reducing the risk associated with loss of life is paramount. The safety programs of both agencies treat life loss separately from economic and other considerations. Decisions as to whether to invest in dam or levee improvements are based primarily on risk to life by applying the concept of tolerable risks. Since informed decisions based on tolerable risk require estimates of loss of life for potential flood events, the focus of this chapter is on estimating loss of life. Estimation of the magnitude of life loss resulting from a flood requires consideration of the following factors: • Understanding of the population at risk (PAR) in the potentially impacted area • Warning and evacuation assumptions for that PAR • Flood characteristics including extents, depths, velocities, and arrival time (can be heavily influence by failure mode and breach parameters) • Estimation of fatality rates The full consideration of all these factors is a complex problem that requires detailed modeling of the physical processes (breach characteristics and flood routing), human responses, and the performance of technological systems (such as warning and evacuation systems, transportation systems and buildings under flood loading). This chapter describes a range of practical approaches to this complex problem that can provide life-loss estimates for use in risk analysis. C-1-1 July 2019 Chapter C-1 Consequences of Dam or Levee Failure C-1.2 Life Loss Estimation Methodologies and Agency Perspectives The USACE and Reclamation perform risk analysis for the dams or USACE levees (to assist with risk informed decision making on flood defense infrastructure within each agency’s portfolio. While the basic concept of using life loss estimates to help quantify risk is similar between each agencies, the methodologies employed by the two agencies have differences. This chapter is intended for use by both agencies, and is structured in a way that presents general information on life loss estimation. USACE applies the Hydrologic Engineering Center (HEC)-LifeSim model to estimate life loss as well as direct economic damage. LifeSim is an agent-based simulation model that tracks the movement of people and their interaction with flooding through time. It includes an integrated transportation simulation algorithm to model the evacuation process, and evaluates loss of life based on location of people when the water arrives and important factors related to building, vehicle, and human stability. Fatalities are estimated by grouping people into high or low hazard “zones.” Each zone has a corresponding fatality rate, which were developed based on an extensive review and analysis of historic flood events.