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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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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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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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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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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
