Mapping the Zone: Improving Flood Map Accuracy (2009)

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Mapping Zone the Improving Flood Map Accuracy Committee on FEMA Flood Maps Board on Earth Sciences and Resources/Mapping Science Committee Water Science and Technology Board 

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 

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 advis- ing 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 



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  

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 vi 

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 L ­ aboratory (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 vii 

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 viii 

Preface F ederal Emergency Management Agency the committee conducted original analyses of variables (FEMA) Flood Insurance Rate Maps portray that influence flood map accuracy, such as elevation flood hazard areas, and they form the basis and flood flow. for setting flood insurance premiums and regulating The committee would like to thank the indi­ development in the floodplain. As such, they are an viduals who briefed the committee or provided data, important tool for individuals, businesses, commu- figures, or other input: Ken Ashe, Glenn Austin, Jerad nities, and government agencies to understand and Bales, Julio Cañon, Andy Carter, Tim Cohn, Todd deal with flood hazard and flood risk. Improving map Davison, David Divoky, Mary Erickson, Dean Gesch, accuracy is therefore not an academic question—better Mike Godesky, Susan ­ Greenlee, Ruth ­ Haberman, maps help everyone. Eric Halpin, Victor Hom, Marti Ikehara, Doo Sun This study was requested by managers of FEMA’s Kang, Larry Larson, Kevin Long, Doug Marcy, Kate Risk Analysis Division and the National Oceanic and Marney, Robert Mason, Gordon McClung, Sally Atmospheric Administration’s (NOAA’s) Coastal Ser- M ­ cConkey, Venkatesh Merwade, Mike Moya, Jim vices Center, supported by NOAA’s National Weather Nelson, Rick Neuherz, Edward ­ Pasterick, Kernell Service, National Geodetic Survey, and Coast Survey Ries, Dan Roman, Paul Rooney, Rick Sacbibit, Brett Development Laboratory. The Committee on FEMA Sanders, Eric Tate, ­ Ronnie ­Taylor, Patty Templeton- Flood Maps was established to examine the factors Jones, Gary Thompson, D. Phil Turnipseed, Gordon that affect flood map accuracy, assess the economic Wells, Bruce ­ Worstell, and Dave Zilkoski. Special benefits of more accurate flood maps, and identify ways thanks go to Thomas ­ Langan, Stephanie Dunham, to improve flood mapping, communication, and man- and Jerry Sparks, who carried out extensive hydrologic agement of flood-related data. Committee members and economic case studies for the committee. Their included academics and practitioners who collectively efforts greatly expanded the pool of data from which possessed expertise covering inland and coastal flood to draw conclusions about improving the accuracy of modeling and mapping, geospatial data management, flood maps. The committee also thanks the National flood hazard assessment, and economic and policy Academies staff who worked on this report: Lauren implications of flood map accuracy. Information on Alexander Augustine, Tonya Fong Yee, Jared Eno, and these topics was gathered from the literature, the Asso- particularly Anne Linn, the study director, who expertly ciation of State Floodplain Managers, discussions with guided the committee’s activities and contributed colleagues, and briefings at five committee meetings s ­ ignificantly to synthesizing our results. held between June 2007 and April 2008. In addition to these traditional means of gathering information, David R. Maidment Chair ix 



Acknowledgment of Reviewers T his report has been reviewed in draft form by William Nechamen, New York State Department individuals chosen for their diverse perspectives of Environmental Conservation, Albany and technical expertise, in accordance with James Plasker, American Society for procedures approved by the NRC’s Report Review Photogrammetry and Remote Sensing, Committee. The purpose of this independent review is Bethesda, Maryland to provide candid and critical comments that will assist Kerry Smith, Arizona State University, Tempe the institution in making its published report as sound Jery Stedinger, University of Cornell, Ithaca, as possible and to ensure that the report meets institu- New York tional standards for objectivity, evidence, and respon- siveness to the study charge. The review comments Although the reviewers listed above have provided and draft manuscript remain confidential to protect the many constructive comments and suggestions, they integrity of the deliberative process. We wish to thank were not asked to endorse the conclusions or recom- the following individuals for their participation in the mendations nor did they see the final draft of the report review of this report: before its release. The review of this report was over- seen by Michael Goodchild, University of California, Gregory Baecher, University of Maryland, Santa Barbara, and Robert Dalrymple, Johns Hopkins College Park University, Baltimore, Maryland. Appointed by the Richard Bernknopf, United States Geological National Research Council, they were responsible for Survey, Menlo Park, California making certain that an independent examination of this Christopher Jones, Christopher Jones and report was carried out in accordance with institutional Associates, Durham, North Carolina procedures and that all review comments were carefully William Kirby, United States Geological Survey, considered. Responsibility for the final content of this retired, Reston, Virginia report rests entirely with the authoring committee and Richard Luettich, University of North Carolina, the institution. Chapel Hill xi 



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 xiii 

xiv CONTENTS 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