Engineering And Design Provisions To Set Final Levee Grade For Projects Formulated Using Risk Based Analysis

This engineer technical letter provides guidance for establishing the final top of levee grade for flood control projects and flood damage reduction studies formulated using risk-based analysis.

The Federal Emergency Management Agency's (FEMA) Federal Insurance and Mitigation Administration (FIMA) manages the National Flood Insurance Program (NFIP), which is a cornerstone in the U.S. strategy to assist communities to prepare for, mitigate against, and recover from flood disasters. The NFIP was established by Congress with passage of the National Flood Insurance Act in 1968, to help reduce future flood damages through NFIP community floodplain regulation that would control development in flood hazard areas, provide insurance for a premium to property owners, and reduce federal expenditures for disaster assistance. The flood insurance is available only to owners of insurable property located in communities that participate in the NFIP. Currently, the program has 5,555,915 million policies in 21,881 communities3 across the United States. The NFIP defines the one percent annual chance flood (100-year or base flood) floodplain as a Special Flood Hazard Area (SFHA). The SFHA is delineated on FEMA's Flood Insurance Rate Maps (FIRM's) using topographic, meteorologic, hydrologic, and hydraulic information. Property owners with a federally back mortgage within the SFHAs are required to purchase and retain flood insurance, called the mandatory flood insurance purchase requirement (MPR). Levees and floodwalls, hereafter referred to as levees, have been part of flood management in the United States since the late 1700's because they are relatively easy to build and a reasonable infrastructure investment. A levee is a man-made structure, usually an earthen embankment, designed and constructed in accordance with sound engineering practices to contain, control, or divert the flow of water so as to provide protection from temporary flooding. A levee system is a flood protection system which consists of a levee, or levees, and associated structures, such as closure and drainage devices, which are constructed and operated in accordance with sound engineering practices. Recognizing the need for improving the NFIP's treatment of levees, FEMA officials approached the National Research Council's (NRC) Water Science and Technology Board (WSTB) and requested this study. The NRC responded by forming the ad hoc Committee on Levee and the National Flood Insurance Program: Improving Policies and Practices, charged to examine current FEMA treatment of levees within the NFIP and provide advice on how those levee-elated policies and activities could be improved. The study addressed four broad areas, risk analysis, flood insurance, risk reduction, and risk communication, regarding how levees are considered in the NFIP. Specific issues within these areas include current risk analysis and mapping procedures behind accredited and non-accredited levees, flood insurance pricing and the mandatory flood insurance purchase requirement, mitigation options to reduce risk for communities with levees, flood risk communication efforts, and the concept of shared responsibility. The principal conclusions and recommendations are highlighted in this report.

A workshop on Risk-based Analysis for Flood Damage Reduction Studies was held on 20-22 October 1997 at the Asilomar Conference Center in Pacific Grove, CA. Policy issues, case example applications of procedures, a risk-based analysis computer program, and levee certification criteria for regulatory flood plain management actions were covered in the sessions. The workshop provided a forum for exchange of ideas and perspectives on ongoing risk-based analysis for flood damage reduction studies. It also provided an opportunity to assess the progress made since the Montecello, MN. Riverine Levee Freeboard Workshop, held in 1991. The objectives of the workshop were to: (1) review the present policy and procedures for performing risk-based analysis studies; (2) identify key issues and discuss their means of resolution; and (3) define and discuss Corps procedures and requirements for levee certification. The workshop proceedings are contained herein.

The U.S. Army Corps of Engineers software HEC-FDA is a tool that uses Risk-based Analysis to compute Expected Annual Damage (EAD) for flood damage reduction studies. EAD reduction is computed as the difference between EAD with and without alternative projects, a quantity used to aid in flood damage reduction project selection. Depending on the time and money spent in the collection and preparation of the input data sets, relatively lesser or greater accuracy can be achieved in EAD computation. Sets of numerical experiments on synthetic large and small drainage basins show that EAD is differently sensitive to the major input functions to HEC-FDA and their associated uncertainties depending on drainage area and the shape of the damage-stage curve. The results from the synthetic data sets are verified by two case studies on the Blue River, Missouri and the Chippewa River, Georgia. The relative sensitivity of EAD to its input functions and associated uncertainties can be used in federal investment decision-making for flood damage reduction projects.

The purpose of this manual is to present basic principles used in the design and construction of earth levees. This manual applies to all Corps of Engineers Divisions and Districts having responsibility for the design and construction of levees. This manual is approved for public release; distribution is unlimited. This manual is intended as a guide for designing and constructing levees and not intended to replace the judgment of the design engineer on a particular project.

Introductory technical guidance for civil engineers and other professional engineers and construction managers interested in planning, design and construction of levees for flood protection and water resources development projects. Here is what is discussed: 1. INTRODUCTION, 2. FIELD INVESTIGATIONS, 3. SUBSURFACE EXPLORATION, 4. FIELD TESTING, 5. LABORATORY TESTING.

The "Red Book" presents a background to conventional foundation analysis and design. The text is not intended to replace the much more comprehensive 'standard' textbooks, but rather to support and augment these in a few important areas, supplying methods applicable to practical cases handled daily by practising engineers and providing the basic soil mechanics background to those methods. It concentrates on the static design for stationary foundation conditions. Although the topic is far from exhaustively treated, it does intend to present most of the basic material needed for a practising engineer involved in routine geotechnical design, as well as provide the tools for an engineering student to approach and solve common geotechnical design problems.

Flooding often threatens riverine and coastal areas, particularly urbanized flood-prone areas that are densely populated and high-valued, which causes damages to life, property, society and the economy. Upstream flood reservoir operations and downstream levee construction are two common ways to protect from flooding. Most traditional risk-based analyses for optimal levee design focus primarily on overtopping failure, and few risk analysis studies explicitly include the more frequently observed intermediate geotechnical failures. This study first develops a risk-based optimization model for single levee designs given two simplified levee failure modes: overtopping and overall intermediate geotechnical failures. The optimization minimizes the annual expected total cost, which sums the expected annual damage cost and annualized construction cost. This optimization model is then extended to examine a common simple levee system with levees on opposite riverbanks, allowing flood risk transfer across the river. The economic optimality of asymmetric levee system is demonstrated mathematically and analytically, for overtopping failure, overall intermediate geotechnical failure and a combination of failure modes. Where residual flood risk is completely transferred to the low-valued riverbank at economic optimality, individuals may be compensated for the transferred flood risk to guarantee and improve outcomes for all parties. Such collaborative designs of the two levee system are economically optimal for the whole system. However, rational and self-interested land owners that control levees on each river bank separately often tend to independently optimize their levees. By applying game theory to the simple levee system, the cooperative game with a system-wide economically optimal design and the single-shot non-cooperative Nash equilibrium are identified, and the successive repeated non-cooperative reversible and irreversible games are examined. Compensation for the transferred flood risk can be determined by comparing different types of games and implemented with land owners' agreements on allocations of flood risk and benefits. The resulting optimized flood risks to a downstream leveed area would further affect the upstream reservoir's operation in optimizing flood hedging pre-releases, which would create a small flood downstream by pre-storm release to reduce the likelihood of a larger more damaging flood in the future. Overall damages from flood pre-release decisions must be convex for flood hedging to be optimal. Some theoretical conditions for optimal flood hedging are explored: the fundamental one is that the current marginal damages from pre-releases equals the future marginal expected damages from storm releases. Any additional economic water supply lost from pre-releases tends to reduce the use of hedging pre-release for flood management.

The purpose of this manual is to present basic principles used in the design and construction of earth levees. This manual applies to all Corps of Engineers Divisions and Districts having responsibility for the design and construction of levees. This manual is approved for public release; distribution is unlimited. This manual is intended as a guide for designing and constructing levees and not intended to replace the judgment of the design engineer on a particular project.