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Mapping the Zone: Improving Flood Map Accuracy MAPPING THE ZONE IMPROVING FLOOD MAP ACCURACY Committee on FEMA Flood Maps Board on Earth Sciences and Resources/Mapping Science Committee Water Science and Technology Board NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES THE NATIONAL ACADEMIES PRESS Washington, D.C. www.nap.edu
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Mapping the Zone: Improving Flood Map Accuracy THE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This study was supported by the Federal Emergency Management Agency under Award No. HSFE03-06-C-0030 and by the National Oceanic and Atmospheric Administration under Award No. DG133R07SE2691. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the organizations or agencies that provided support for this project. International Standard Book Number-13: 978-0-309-13057-8 (Book) International Standard Book Number-10: 0-309-13057-3 (Book) International Standard Book Number-13: 978-0-309-13058-5 (PDF) International Standard Book Number-10: 0-309-13058-1 (PDF) Library of Congress Control Number: 2009924737 Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu. Cover: June 2008 flooding in Cedar Rapids, Iowa (foreground), and a portion of the FEMA Flood Insurance Rate Map in the same region (background). This part of the downtown is within the Special Flood Hazard Area (zone A), which is subject to a 1 percent or greater chance of flooding in any given year. Photograph courtesy of Stephen Mally. Used with permission. Map extract from FEMA’s Map Service Center. Cover design by Van Nguyen. Copyright 2009 by the National Academy of Sciences. All rights reserved. Printed in the United States of America
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Mapping the Zone: Improving Flood Map Accuracy THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Charles M. Vest is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council. www.national-academies.org
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Mapping the Zone: Improving Flood Map Accuracy COMMITTEE ON FEMA FLOOD MAPS DAVID R. MAIDMENT, Chair, University of Texas, Austin DAVID S. BROOKSHIRE, University of New Mexico, Albuquerque J. WILLIAM BROWN, City of Greenville, South Carolina JOHN DORMAN, State of North Carolina, Raleigh GERALD E. GALLOWAY, University of Maryland, College Park BISHER IMAM, University of California, Irvine WENDY LATHROP, Cadastral Consulting, LLC, Bala Cynwyd, Pennsylvania DAVID MAUNE, Dewberry, Fairfax, Virginia BURRELL E. MONTZ, Binghamton University, New York SPENCER ROGERS, North Carolina Sea Grant, Wilmington KAREN L. SCHUCKMAN, Pennsylvania State University, University Park Y. PETER SHENG, University of Florida, Gainesville JUAN B. VALDES, University of Arizona, Tucson National Academies Staff ANNE M. LINN, Study Director, Board on Earth Sciences and Resources LAUREN E. ALEXANDER, Senior Program Officer, Water Science and Technology Board, and Director, Disasters Roundtable JARED P. ENO, Research Associate, Board on Earth Sciences and Resources TONYA E. FONG YEE, Senior Program Assistant, Board on Earth Sciences and Resources
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Mapping the Zone: Improving Flood Map Accuracy MAPPING SCIENCE COMMITTEE KEITH C. CLARKE, Chair, University of California, Santa Barbara ROBERT P. DENARO, NAVTEQ Corporation, Chicago, Illinois GEORGE F. HEPNER, University of Utah, Salt Lake City JOHN R. JENSEN, University of South Carolina, Columbia NINA S.-N. LAM, Louisiana State University, Baton Rouge LEWIS A. LAPINE, South Carolina Geodetic Survey, Columbia MARY L. LARSGAARD, University of California, Santa Barbara XAVIER R. LOPEZ, Oracle Corporation, Nashua, New Hampshire ROBERT B. MCMASTER, University of Minnesota, Minneapolis SHASHI SHEKHAR, University of Minnesota, Minneapolis EUGENE TROBIA, Arizona State Land Department, Phoenix National Academies Staff ANNE M. LINN, Senior Program Officer JARED P. ENO, Research Associate
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Mapping the Zone: Improving Flood Map Accuracy BOARD ON EARTH SCIENCES AND RESOURCES GEORGE M. HORNBERGER, Chair, Vanderbilt University, Nashville GREGORY B. BAECHER, University of Maryland, College Park KEITH C. CLARKE, University of California, Santa Barbara DAVID J. COWEN, University of South Carolina, Columbia WILLIAM E. DIETRICH, University of California, Berkeley ROGER M. DOWNS, The Pennsylvania State University, University Park JEFF DOZIER, University of California, Santa Barbara KATHERINE H. FREEMAN, The Pennsylvania State University, University Park RUSSELL J. HEMLEY, Carnegie Institution of Washington, Washington, D.C. MURRAY W. HITZMAN, Colorado School of Mines, Golden LOUISE H. KELLOGG, University of California, Davis CLAUDIA INÉS MORA, Los Alamos National Laboratory, New Mexico BRIJ M. MOUDGIL, University of Florida, Gainesville V. RAMA MURTHY, University of Minnesota (retired), Minneapolis CLAYTON R. NICHOLS, Idaho National Engineering and Environmental Laboratory (retired), Sandpoint RAYMOND A. PRICE, Queen’s University, Kingston, Ontario, Canada BARBARA A. ROMANOWICZ, University of California, Berkeley JOAQUIN RUIZ, University of Arizona, Tucson WILLIAM W. SHILTS, Illinois State Geological Survey, Champaign RUSSELL STANDS-OVER-BULL, Anadarko Petroleum Corporation, Billings, Montana TERRY C. WALLACE, JR., Los Alamos National Laboratory, New Mexico HERMAN B. ZIMMERMAN, National Science Foundation (retired), Portland, Oregon National Research Council Staff ANTHONY R. DE SOUZA, Director ELIZABETH A. EIDE, Senior Program Officer DAVID A. FEARY, Senior Program Officer ANNE M. LINN, Senior Program Officer SAMMANTHA L. MAGSINO, Program Officer CAETLIN M. OFIESH, Associate Program Officer JENNIFER T. ESTEP, Financial and Administrative Associate JARED P. ENO, Research Associate NICHOLAS D. ROGERS, Research Associate COURTNEY R. GIBBS, Program Associate TONYA E. FONG YEE, Senior Program Assistant ERIC J. EDKIN, Program Asssistant
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Mapping the Zone: Improving Flood Map Accuracy WATER SCIENCE AND TECHNOLOGY BOARD CLAIRE WELTY, Chair, University of Maryland, Baltimore County JOAN G. EHRENFELD, Rutgers University, New Brunswick, New Jersey GERALD E. GALLOWAY, University of Maryland, College Park SIMON GONZALEZ, National Autonomous University of Mexico CHARLES N. HAAS, Drexel University, Philadelphia, Pennsylvania KENNETH R. HERD, Southwest Florida Water Management District, Brooksville JAMES M. HUGHES, Emory University, Atlanta, Georgia THEODORE L. HULLAR, Private Consultant, Tucson, Arizona KIMBERLY L. JONES, Howard University, Washington, DC G. TRACY MEHAN III, The Cadmus Group, Inc., Arlington, Virginia DAVID H. MOREAU, University of North Carolina, Chapel Hill THOMAS D. O’ROURKE, Cornell University, Ithaca, New York DONALD I. SIEGEL, Syracuse University, New York SOROOSH SOROOSHIAN, University of California, Irvine HAME M. WATT, Independent Consultant, Washington, DC JAMES L. WESCOAT, JR., University of Illinois, Urbana-Champaign National Research Council Staff STEPHEN D. PARKER, Director JEFFREY W. JACOBS, Scholar LAURA J. EHLERS, Senior Staff Officer STEPHANIE E. JOHNSON, Senior Staff Officer LAURA E. HELSABECK, Associate Staff Officer M. JEANNE AQUILINO, Financial and Administrative Associate ELLEN A. DE GUZMAN, Research Associate ANITA A. HALL, Senior Program Associate MICHAEL J. STOEVER, Senior Program Assistant STEPHEN T. RUSSELL, Project Assistant
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Mapping the Zone: Improving Flood Map Accuracy Preface Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps portray flood hazard areas, and they form the basis for setting flood insurance premiums and regulating development in the floodplain. As such, they are an important tool for individuals, businesses, communities, and government agencies to understand and deal with flood hazard and flood risk. Improving map accuracy is therefore not an academic question—better maps help everyone. This study was requested by managers of FEMA’s Risk Analysis Division and the National Oceanic and Atmospheric Administration’s (NOAA’s) Coastal Services Center, supported by NOAA’s National Weather Service, National Geodetic Survey, and Coast Survey Development Laboratory. The Committee on FEMA Flood Maps was established to examine the factors that affect flood map accuracy, assess the economic benefits of more accurate flood maps, and identify ways to improve flood mapping, communication, and management of flood-related data. Committee members included academics and practitioners who collectively possessed expertise covering inland and coastal flood modeling and mapping, geospatial data management, flood hazard assessment, and economic and policy implications of flood map accuracy. Information on these topics was gathered from the literature, the Association of State Floodplain Managers, discussions with colleagues, and briefings at five committee meetings held between June 2007 and April 2008. In addition to these traditional means of gathering information, the committee conducted original analyses of variables that influence flood map accuracy, such as elevation and flood flow. The committee would like to thank the individuals who briefed the committee or provided data, figures, or other input: Ken Ashe, Glenn Austin, Jerad Bales, Julio Cañon, Andy Carter, Tim Cohn, Todd Davison, David Divoky, Mary Erickson, Dean Gesch, Mike Godesky, Susan Greenlee, Ruth Haberman, Eric Halpin, Victor Hom, Marti Ikehara, Doo Sun Kang, Larry Larson, Kevin Long, Doug Marcy, Kate Marney, Robert Mason, Gordon McClung, Sally McConkey, Venkatesh Merwade, Mike Moya, Jim Nelson, Rick Neuherz, Edward Pasterick, Kernell Ries, Dan Roman, Paul Rooney, Rick Sacbibit, Brett Sanders, Eric Tate, Ronnie Taylor, Patty Templeton-Jones, Gary Thompson, D. Phil Turnipseed, Gordon Wells, Bruce Worstell, and Dave Zilkoski. Special thanks go to Thomas Langan, Stephanie Dunham, and Jerry Sparks, who carried out extensive hydrologic and economic case studies for the committee. Their efforts greatly expanded the pool of data from which to draw conclusions about improving the accuracy of flood maps. The committee also thanks the National Academies staff who worked on this report: Lauren Alexander Augustine, Tonya Fong Yee, Jared Eno, and particularly Anne Linn, the study director, who expertly guided the committee’s activities and contributed significantly to synthesizing our results. David R. Maidment Chair
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Mapping the Zone: Improving Flood Map Accuracy Acknowledgment of Reviewers This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the NRC’s Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report: Gregory Baecher, University of Maryland, College Park Richard Bernknopf, United States Geological Survey, Menlo Park, California Christopher Jones, Christopher Jones and Associates, Durham, North Carolina William Kirby, United States Geological Survey, retired, Reston, Virginia Richard Luettich, University of North Carolina, Chapel Hill William Nechamen, New York State Department of Environmental Conservation, Albany James Plasker, American Society for Photogrammetry and Remote Sensing, Bethesda, Maryland Kerry Smith, Arizona State University, Tempe Jery Stedinger, University of Cornell, Ithaca, New York Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations nor did they see the final draft of the report before its release. The review of this report was overseen by Michael Goodchild, University of California, Santa Barbara, and Robert Dalrymple, Johns Hopkins University, Baltimore, Maryland. Appointed by the National Research Council, they were responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.
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Mapping the Zone: Improving Flood Map Accuracy Contents SUMMARY 1 1 INTRODUCTION 7 Committee Charge and Approach, 7 Case Studies, 9 Organization of the Report, 11 2 FLOOD MAPPING AND FLOOD INSURANCE 13 Flood Insurance Rate Maps, 13 Flood Map Production, 16 FEMA’s Map Modernization Program, 19 Map Maintenance, 20 Flood Map Information Technology, 22 Flood Data and a National Hydrologic Information System, 22 3 ELEVATION AND HEIGHT DATA 25 Establishing a Reference Surface, 25 Establishing Base Surfaces, 29 Measuring and Monitoring Water Surface Elevations, 32 Surveying Structure Elevations, 34 Impact of Elevation Uncertainties in a Flood Study, 36 Conclusions, 38 4 INLAND FLOODING 41 Uncertainty of the Base Flood Elevation at Stream Gages, 42 Determining the Flood Discharge, 46 USGS Regional Regression Equations, 49 Hydraulic Models, 55 North Carolina Flood Mapping Case Study, 59 Shallow Flooding, 65 Conclusions, 66
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Mapping the Zone: Improving Flood Map Accuracy 5 COASTAL FLOODING 67 Flood Hazards in Coastal Systems, 67 FEMA Coastal Flood Modeling Methodology, 68 From Models to Maps: Developing the Next Generation of Coastal Flood Models, 72 Conclusions, 77 6 BENEFITS AND COSTS OF ACCURATE FLOOD MAPPING 79 Benefits and Costs, 79 FEMA Benefit-Cost Analyses, 82 North Carolina Case Study, 82 Conclusions, 86 7 MAPPING AND RISK COMMUNICATION: MOVING TO THE FUTURE 89 Improving Coastal Flooding Designations, 89 Mapping Flood Risk, 91 Conclusions, 98 REFERENCES 101 APPENDIXES A Methods for Estimating Base Flood Elevations in Approximate Studies 107 B Biographical Sketches of Committee Members 109 C Glossary 113 D Acronyms and Abbreviations 121