Magnitude and Frequency of Floods in Alaska and Conterminous Basins of Canada U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 93-4179 Prepared in cooperation with the STATE OF ALASKA DEPARTMENT OF TRANSPORTATION AND PUBLIC FACILITIES and FEDERAL HIGHWAY ADMINISTRATION Cover map. The map of Alaska and conterminous basins of Canada on the cover is a Molleweide projection, which is an equal-area projection of the Earth within an ellipse. The Molleweide projection is used for very large regions because only two points on the Molleweide are completely free of distortion unless the projection is interrupted (Snyder, 1987, p. 249). Magnitude and Frequency of Floods in Alaska and Conterminous Basins of Canada By Stanley H. Jones and Charles B. Fahl U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 93-4179 Prepared in cooperation with the STATE OF ALASKA, DEPARTMENT OF TRANSPORTATION AND PUBLIC FACILITIES and FEDERAL HIGHWAY ADMINISTRATION Anchorage, Alaska 1994 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY ROBERT M. HIRSCH, Acting Director For additional information write to: Copies of this report may be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Earth Science Information Center 4230 University Drive, Suite 201 Open-File Reports Section Anchorage, Alaska 99508-4664 Box 25286, MS 517 Denver Federal Center Denver, Colorado 80225 CONTENTS Abstract .................................................................. 1 Introduction................................................................ 1 Purpose and scope ...................................................... 2 Methods............................................................... 2 Acknowledgments. ...................................................... 3 Estimation of flood magnitude and frequency...................................... 6 Sites on ungaged streams ................................................. 6 Sample problems for sites on ungaged streams................................. 14 Sites on gaged streams.................................................... 19 Sample problems for sites on gaged streams................................... 21 Accuracy and limitations.................................................. 24 Flood-frequency analysis...................................................... 26 Drainage basin characteristics.............................................. 26 Flood characteristics ..................................................... 27 Maximum known floods .................................................. 28 Magnitude and frequency of floods at gaged sites .............................. 28 Regional regression analysis................................................ 30 Criteria ........................................................... 30 Generalized skew coefficients ......................................... 31 Multiple regression analysis ............................................... 32 Precipitation map ........................................................... 34 Summary................................................................... 34 References cited............................................................. 34 Glossary .................................................................. 37 PLATES [Plates are in pocket] 1. Map showing location of gaging stations and crest-stage partial-record sites having at least 8 years of peak-flow data, and mean minimum January temperature for Alaska and conterminous basins of Canada 2. Map showing mean annual precipitation for Alaska and conterminous basins of Canada Contents FIGURES 1-2. Maps showing: 1. Hydrologic regions of Alaska and conterminous basins of Canada used in the flood-frequency analysis ........................................ 4 2. Areas used to develop flood-frequency estimating equations. ................ 5 3-7. Graphs showing peak discharge as a function of drainage area and envelope curve for rainfall and snowmelt runoff floods in: 3. Flood-frequency area 1. ............................................. 12 4. Flood-frequency area 2. ............................................. 12 5. Flood-frequency area 3. ............................................. 13 6. Flood-frequency area 4. ............................................. 13 7. Flood-frequency area 5. ............................................. 14 8. Map showing locations of sites for sample problems ......................... 15 9-10. Graphs showing: 9. Magnitude and frequency curve for Nicolet Creek near Cordova............. 17 10. Relation of discharge to drainage area for selected recurrence intervals along the Yukon River................................................... 19 TABLES 1. Statistics of logarithms of annual peaks in Alaska through 1990 and conterminous basins of Canada through 1984 ............................................ 39 2. Equations for estimating magnitude and frequency of floods in Alaska and conterminous basins of Canada............................................ 7 3. T-year peak discharges at gaging stations and partial-record sites in Alaska through 1990 and conterminous basins of Canada through 1984.................. 49 4. Values of unbiased station skew, generalized skew, standard error of generalized skew, and standard deviation of generalized skew ............................ 20 5. Basin characteristics of gaging stations and crest-stage partial-record sites in Alaska and conterminous basins of Canada ........................................ 77 6. Maximum known floods in Alaska through 1990 and conterminous basins of Canada through 1984 ................................................... 90 IV Contents CONVERSION FACTORS AND VERTICAL DATUM Multiply By To Obtain inch (in.) 25.4 millimeter foot (ft) 0.3048 meter mile (mi) 1.609 kilometer square mile (mi2) 2.590 square kilometer foot per mile (ft/mi) 0.1894 meter per kilometer cubic foot per second (ft3/s) 0.02832 cubic meter per second cubic foot per second per square mile [(ft3/s)/mi2] 0.01093 cubic meter per second per square kilo- meter degree Fahrenheit (°F) °C = 5/9 x (°F-32) degree Celsius (°C) National Geodetic Vertical Datum of 1929 (NGVD of 1929): A geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929. A "Glossary" of technical terms used in this report starts on page 37. Contents V Magnitude and Frequency of Floods in Alaska and Conterminous Basins of Canada By Stanley H. Jones1 and Charles B. Fahl2 ABSTRACT Equations for estimating the magnitude and frequency of floods at ungaged sites on streams in Alaska and conterminous basins of Canada were developed using multiple- regression analyses of basin climatic and physical characteristics and peak-flow statistics from 260 gaged locations in Alaska and 72 gaged locations in Canada. Methods are pre­ sented for estimating flood magnitude and frequency at sites on gaged streams. Flood-fre­ quency data based on observed peaks and basin physical and climatic characteristics are given for 332 gaged locations on streams with natural flow. The State of Alaska and con­ terminous basins of Canada were divided into five flood-frequency areas having similar flood characteristics on the basis of statistical cluster analyses and regional regression analyses. Generalized skew coefficients were determined for each of the five flood-fre­ quency areas using at-site unbiased skew coefficients computed for 82 stations in Alaska having 22 or more annual peaks through the 1987 water year and 31 stations in Canada hav­ ing 22 or more annual peaks through the 1984 calendar year. A set of equations for estimat­ ing peak discharge having recurrence intervals of 2,5,10,25, 50,100, 200, and 500 years was developed for each flood-frequency area. Significant basin characteristics in the equa­ tions are drainage area, mean annual precipitation, percentage of lakes and ponds, mean minimum January temperature, mean basin elevation, and percentage of forest. Drainage basin sizes range from 1.02 to 321,000 square miles. Average standard errors of prediction for the equations range from 26 to 77 percent. A regionalized mean annual precipitation map for the climatic normal period of 1951-80 was developed for Alaska west of longitude 141° and modified from published maps for southeastern Alaska and conterminous basins of Canada. Maximum known floods at 722 sites in Alaska and conterminous basins of Canada are tabulated. INTRODUCTION Floods in Alaska and conterminous basins of Canada result from rainfall, snowmelt runoff, a combination of rain on snow, rapid melting of snow and ice during eruptions of glacier-clad volca­ noes, and the sudden release of water stored behind natural dams dams created by glaciers, river ice, snow (avalanches), and rock and unconsolidated materials (landslides and debris flows). Infor­ mation about the probable magnitude and frequency of floods, whatever their cause, is necessary 'U.S. Geological Survey, Anchorage, Alaska 2Alaska Pacific University, Anchorage, Alaska INTRODUCTION 1 for the design of culverts, bridges, and other hydraulic structures that must withstand or "accom­ modate" those floods, and is useful for flood-plain management. A data-collection program designed to define flood magnitudes and frequency on small streams those with drainage areas of about 50 mi or less-throughout Alaska was begun in 1962 by the U.S. Geological Survey in cooperation with the State of Alaska Department