ALLUVIAL FAN FLOODING

Committee on Alluvial Fan Flooding

Water Science and Technology Board

Commission on Geosciences, Environment, and Resources

National Research Council

NATIONAL ACADEMY PRESS
Washington, D.C. 1996



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ALLUVIAL FAN FLOODING Committee on Alluvial Fan Flooding Water Science and Technology Board Commission on Geosciences, Environment, and Resources National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1996

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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 report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. Support for this project was provided by the U.S. Federal Emergency Management Agency under Contract Agreement EMW-94-C-4550. Library of Congress Catalog Card Number 96-69351 International Standard Book Number 0-309-05542-3 Additional copies of this report are available from: National Academy Press 2101 Constitution Ave., NW Box 285 Washington, DC 20055 800-624-6242 202-334-3313 (in the Washington Metropolitan Area) http://www.nap.edu Cover credit: Alluvial fan flooding at Magnesia Spring Canyon in July 1979 caused one death and more than $7 million in damage. Photograph taken from 1989 FEMA Document 165, Alluvial Fans: Hazards and Management. Copyright 1996 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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COMMITTEE ON ALLUVIAL FAN FLOODING STANLEY A. SCHUMM, Chair, Colorado State University, Fort Collins, Colorado VICTOR R. BAKER, University of Arizona, Tucson MARGARET (PEGGY) F. BOWKER, Nimbus Engineers, Reno, Nevada JOSEPH R. DIXON, U.S. Army Corps of Engineers, Phoenix, Arizona THOMAS DUNNE, University of California, Santa Barbara DOUGLAS HAMILTON, engineering consultant, Irvine, California HJA DOROTHY MERRITTS, Franklin & Marshall College, Lancaster, Pennsylvania Staff CHRIS ELFRING, Study Director ANGELA BRUBAKER, Research Assistant ETAN GUMERMAN, Research Associate ROSEANNE PRICE, Consulting Editor

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WATER SCIENCE AND TECHNOLOGY BOARD DAVID L. FREYBERG, Chair, Stanford University, Stanford, California BRUCE E. RITTMANN, Vice-Chair, Northwestern University, Evanston, Illinois LINDA ABRIOLA, University of Michigan, Ann Arbor PATRICK L. BREZONIK, University of Minnesota, St. Paul, through June 30, 1996 JOHN BRISCOE, The World Bank, Washington, D.C. WILLIAM M. EICHBAUM, The World Wildlife Fund, Washington, D.C. WILFORD R. GARDNER, University of California (emeritus), Berkeley EVILLE GORHAM, University of Minnesota, St. Paul THOMAS M. HELLMAN, Bristol-Myers Squibb Company, New York, New York CHARLES D. D. HOWARD, Charles Howard & Associates, Ltd., Victoria, British Columbia, Canada CAROL JOHNSTON, University of Minnesota, Duluth WILLIAM M. LEWIS, JR., University of Colorado, Boulder JOHN W. MORRIS, J. W. Morris, Ltd., Arlington, Virginia CAROLYN H. OLSEN, Brown and Caldwell, Pleasant Hill, California, through June 30, 1996 CHARLES R. O'MELIA, The Johns Hopkins University, Baltimore, Maryland REBECCA T. PARKIN, American Public Health Association, Washington, D.C. IGNACIO RODRIGUEZ-ITURBE, Texas A&M University, College Station HENRY VAUX, JR., University of California, Oakland Staff STEPHEN D. PARKER, Staff Director SHEILA D. DAVID, Senior Staff Officer CHRIS ELFRING, Senior Staff Officer GARY D. KRAUSS, Staff Officer JACQUELINE A. MACDONALD, Senior Staff Officer M. JEANNE AQUILINO, Administrative Associate ETAN GUMERMAN, Research Associate ANITA A. HALL, Administrative Assistant ANGELA F. BRUBAKER, Research Assistant ELLEN DE GUZMAN, Project Assistant MARY BETH MORRIS, Senior Project Assistant

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COMMISSION ON GEOSCIENCES, ENVIRONMENT, AND RESOURCES M. GORDON WOLMAN, Chair, The Johns Hopkins University, Baltimore, Maryland PATRICK R. ATKINS, Aluminum Company of America, Pittsburgh, Pennsylvania JAMES P. BRUCE, Canadian Climate Program Board, Ottawa, Ontario, Canada WILLIAM L. FISHER, University of Texas, Austin JERRY F. FRANKLIN, University of Washington, Seattle GEORGE M. HORNBERGER, University of Virginia, Charlottesville DEBRA S. KNOPMAN, Progressive Foundation, Washington, D.C. PERRY L. MCCARTY, Stanford University, California JUDITH E. MCDOWELL, Woods Hole Oceanographic Institute, Massachusetts S. GEORGE PHILANDER, Princeton University, New Jersey RAYMOND A. PRICE, Queen's University at Kingston, Ontario, Canada THOMAS C. SCHELLING, University of Maryland, College Park ELLEN K. SILBERGELD, University of Maryland Medical School, Baltimore STEVEN M. STANLEY, The Johns Hopkins University, Baltimore, Maryland VICTORIA J. TSCHINKEL, Landers and Parsons, Tallahassee, Florida Staff STEPHEN RATTIEN, Executive Director STEPHEN D. PARKER, Associate Executive Director MORGAN GOPNIK, Assistant Executive Director GREGORY SYMMES, Reports Officer JAMES MALLORY, Administrative Officer SANDI FITZPATRICK, Administrative Associate SUSAN SHERWIN, Project Assistant

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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. Bruce M. Alberts 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. William A. Wulf is interim 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. Kenneth I. Shine 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. Bruce M. Alberts and Dr. William A. Wulf are chairman and interim vice chairman, respectively, of the National Research Council.

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Preface People have long elected to build in flood-prone areas—whether because they sought easy access to the waterways that were once our main transportation routes, because they offer relatively flat building sites, or because of their aesthetic appeal. As the population increases and people search for desirable locations to live, they sometimes come into conflict with those who regulate construction on s. In the western United States, some of the most intense conflicts revolve around development on alluvial fans, which can be susceptible to a particularly catastrophic type of flooding. Controversy over alluvial fan flooding issues led the Federal Emergency Management Agency (FEMA) to ask the National Research Council (NRC) for help. As a result, the NRC established the Committee on Alluvial Fan Flooding with a membership composed of eight engineers and earth scientists, all of whom have experience with alluvial fan morphology and processes. The committee was charged to revise the existing definition of alluvial fan flooding, to develop criteria to determine if an area is subject to alluvial fan flooding, and to provide examples of the application of the definition and the criteria used. The committee recognized immediately that in addition to "alluvial fan flooding," there exists a broader category termed "uncertain flow path flooding" that requires further consideration by FEMA. Confusion caused by linking two aspects of the flood hazard (i.e., land form type and uncertainty in flood processes) is part of the reason for the controversy on this subject. This committee cannot claim to have the final word on what it considers to be a complex technical and regulatory issue, hence we may not have achieved everything desired by FEMA. It has, however, provided significant guidance for characterizing how floods occur on alluvial fans and describing how FEMA might more consistently administer the National Flood Insurance Program on such landforms, which comprise large areas of the western United States and elsewhere. In order to more fully understand the problems associated with alluvial fan flooding, the committee met at three locations in Arizona, California, and Utah, where different alluvial fans could be visited and evaluated in the field. The examples ranged from typical large alluvial fans in Arizona and California to small debris flow fans in Utah. Fans ranged from fully active, where flooding or debris flows could occur anywhere on the fan, to incised, where the bulk of the fan is not subject to flooding. Hence, not only was the varied expertise of the committee brought to bear on the problem, but the members were exposed in the field to new and different situations.

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The committee benefited greatly from presentations and guidance in the field from the following people: Gary Christiansen and Mike Lowe, Utah Geological Survey; Fred Campbell, EIS Engineering, Salt Lake City; Sidney Smith, Davis County Public Works, Utah; Jeffrey Keaton, AGRA Earth and Environment, Salt Lake City; Joseph Tram, Maricopa County Flood Control District, Arizona; Terri Miller, Arizona Department of Water Resources; Philip Pearthree, Arizona Geological Survey; Joseph Hill, San Diego County Department of Public Works; Stuart McKibbin, Riverside County, California; Robert Mussetter, Mussetter Engineering, Fort Collins, Colorado; James Slosson, Slosson and Associates, California; and Joe Cook, Coachella Valley Water District. We also appreciate the support provided by FEMA personnel and contractors—especially Frank Tsai, Karl Mohr, and Ed Mifflin—who helped us understand the issues and how FEMA currently operates. We believe the hands-on perspective that all these people contributed was essential to the evolution of our thinking. In addition, the committee would like to thank the staff of the Water Science and Technology Board for their invaluable guidance to the committee, especially the insights provided by study director Chris Elfring and support from her associates Angela Brubaker and Etan Gumerman. Our thanks also to Tamera Benson for the preparation of the graphics. Stanley Schumm, Chair Committee on Alluvial Fan Flooding

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Contents     SUMMARY   1 1   INTRODUCTION   6     Origin of the Problem   8     The Committee's Response   13     The NFIP Definition of Alluvial Fan Flooding   14     Implications of Alluvial Fan Flooding as a Distinct Type of Flooding   18     Implications of Alluvial Fan Flooding for Floodplain Management   23     The Committee's Definition of Alluvial Fan Flooding   25     BOX Selecting the Site Visit Locations   9 2   FLOODING PROCESSES AND ENVIRONMENTS ON ALLUVIAL FANS   29     Formation and Nature of Alluvial Fans   29     Flooding Processes on Alluvial Fans   39     Change Over Time   43     Reports of Flooding on Alluvial Fans   45 3   INDICATORS FOR CHARACTERIZING ALLUVIAL FANS AND ALLUVIAL FAN FLOODING   51     Stage 1: Recognizing and Characterizing Alluvial Fan Landforms   54     Stage 2: Defining the Nature of the Alluvial Fan Environment and Identifying the Location of Active Erosion and Deposition   60     Stage 3: Defining and Characterizing Areas of 100-Year Alluvial Fan Flooding   72     Available Methods of Analysis   75     Summary   80     BOX "Time" in the Context of Alluvial Fan Flooding   53

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4   APPLYING THE INDICATORS TO EXAMPLE FANS   83     Henderson Canyon, California   83     Thousand Palms Wash, California   93     Lytle Creek, California   96     Tortolita Mountains, Arizona   100     Carefree, Arizona   102     Rudd Creek, Utah   111     Humid Region Alluvial Fans   115     Summary   125     BOX When It Is Not a Fan, But It Acts Like One   128 5   CONCLUSIONS AND RECOMMENDATIONS   133     APPENDIXES     A   Characteristics and Hazards Reported in Published and Unpublished Accounts of Alluvial Fan Flooding   139 B   Sources of Data   153 C   Biographical Sketches of Committee Members   164 D   Glossary and List of Acronyms   167