FLOODS OF OCTOBER 1986 IN SOUTHCENTRAL ALASKA " » U.S. GEOLOGICAL SURVEY OPEN- FILE REPORT 87-391 REVISED 1988 Prepared in cooperation with the: ALASKA DEPARTMENT OF TRANSPORTATION AND PUBLIC FACILITIES ALASKA DIVISION OF EMERGENCY SERVICES FEDERAL HIGHWAY ADMINISTRATION FLOODS OF OCTOBER 1986 IN SOUTHCENTRAL ALASKA by Robert D. Lamke and Bruce P. Bigelow U.S. GEOLOGICAL SURVEY Open-File Report 87-391 REVISED 1988 Prepared in cooperation with the: ALASKA DEPARTMENT OF TRANSPORTATION AND PUBLIC FACILITIES ALASKA DIVISION OF EMERGENCY SERVICES FEDERAL HIGHWAY ADMINISTRATION Anchorage, Alaska 1988 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information Copies of this report can be write to: purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Books and Open-File Reports Section Water Resources Division Federal Center 4230 University Drive, Suite 201 Box 25425 Anchorage, Alaska 99508-4664 Denver, Colorado 80225 11 CONTENTS Page Abstract .............................................................. 1 Introduction .......................................................... 1 Purpose and scope ................................................ 1 Acknowledgements ................................................. 3 Precipitation ......................................................... 3 Discharge data ........................................................ 7 Peak stage and discharge table ................................... 7 Discharge data for October 1986 .................................. 7 Flood description ..................................................... 10 Kenai Peninsula .................................................. 10 Bradley Lake and Homer area streams ......................... 10 Seward area streams ......................................... 14 Kenai Lake tributaries ...................................... 17 Anchorage ........................................................ 19 Wasilla and lower Susitna Valley ................................. 24 Prince William Sound ............................................. 30 References cited ...................................................... 31 ILLUSTRATIONS Figure 1. Map showing index to location of peak-discharge measurement sites for flood of October 10-12, 1986 in Southcentral Alaska ..................................................... 2 2. Map showing selected rainfall measurement sites 1 to 43 and isohyets of total precipitation, October 9-11, 1986, for Southcentral Alaska ........................................ 5 3. Graphs showing accumulated precipitation at selected sites, October 9-11, 1986 ......................................... 6 4. Map showing rainfall measurement sites 9 to 11 and peak- discharge measurement sites 20 to 28, Bradley Lake to Homer. 11 5. Flood discharge hydrograph and concurrent 6-hour storm precipitation at selected sites in Kenai Peninsula ......... 13 6. Map showing rainfall measurement sites 7 to 8, 13 to 15 and peak-discharge measurement sites 6 to 19, 29 to 37, Seward to Kenai Lake .............................................. 15 7. Cross sections at selected sites in Kenai Peninsula, showing channel changes after flood of October 11, 1986 .... 18 8. Map showing rainfall measurement sites 4, 20 to 26, 35 and peak-discharge measurement sites 4, 40 to 45 in Anchorage area........................................................ 20 9. Flood discharge hydrographs, concurrent 6-hour storm precipitation, and air temperatures at selected sites in Anchorage area.............................................. 21 10. Map showing rainfall measurement sites 25 to 35, 38 to 40 and peak-discharge measurement sites 46 to 51, 54 to 65, 67 to 68 in Wasilla and lower Susitna Valley area........... 25 11. Flood discharge hydrographs and concurrent 6-hour storm precipitation at selected sites in Susitna River basin ..... 26 111 TABLES Page Table 1. Rainfall totals for October 9-11, 1986 storm in flood area 4 2. Summary of flood stages and discharges for the flood of October 10-12, 1986 ....................................... 8 3. Discharge data for Upper Bradley River during October 1986 12 4. Discharge data for Campbell and East Fork Eklutna Creeks during October 1986 ....................................... 22 5. Discharge data for three selected gaging stations in lower Susitna River basin during October 1986 ................... 27 CONVERSION TABLE For readers who may prefer to use metric (International System) units rather than inch-pound units, the conversion factors for the terms used in this report are listed below: Multiply by To obtain inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) square mile (mi2 ) 2.590 square kilometer (km2 ) cubic foot (ft3 ) 0.02832 cubic meter (m3 ) cubic foot per second (fts/s) 0.02832 cubic meter per second (ms/s) cubic foot per second per 0.01093 cubic meter per second per square mile [(fts/s)/mi2 ] square kilometer [(m3 /s)/km2 ] degree Fahrenheit (°F) 'C - 5/9 x (°F-32) degree Celsius (°C) Sea level: In this report "sea level" refers to the 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 "Mean Sea Level of 1929." IV FLOODS OF OCTOBER 1986 IN SOUTHCENTRAL ALASKA by Robert D. Lamke and Bruce B. Bigelow ABSTRACT Heavy precipitation associated with a large storm system resulted in major flooding in several areas of southcentral Alaska during October 10-12, 1986. Flooding was particularly severe in the Seward area of the Kenai Peninsula and in tributaries to Susitna River from Talkeetna downstream. Flood damage has been estimated at $20 million and the region was declared a Federal disaster area. The report includes a brief discussion of meteorological conditions that caused the unusual amounts of precipitation, a summary of flood stages and discharges, a comparison to prior floods, flood-frequency estimates, a brief description of flood areas, and hydrologic data for each area. INTRODUCTION Unusually large amounts of rainfall that began during mid-morning of October 9 and generally persisted until the evening of October 11 caused flooding in many areas in southcentral Alaska. Flooding generally began on the 10th and most streams peaked on the llth (with the exception of larger streams, which peaked later) . The areas hardest hit by the flooding were near Seward on the Gulf of Alaska and the lower Susitna River Valley -- west of Talkeetna, southward from Talkeetna to Willow, and then southwest toward Tyonek. The most extensive damage was in Seward and along the Susitna River tributaries that cross the Parks Highway. Total damage has been estimated to be in excess of $20 million; a large part of the damage was to the Alaska Highway system and the Alaska Railroad. Purpose and Scope This report presents a brief description of the floods, a summary of pertinent streamflow data, and estimated flood frequencies. Data on the magnitude and frequency of peak discharges and volumes of floodflows are useful to agencies involved in planning and design. The area covered in this report is shown in figure 1. The peak-discharge measurement sites shown in figure 1 (and more detailed maps of selected portions of the flood area) are in the downstream order system used by the U.S. Geological Survey. This revised report replaces the initial report of 1987. It incorporates additional data not previously available; some of the discharge values, originally called provisional, have been verified and others were changed after additional review. 162* 161* ISO* 148* er- SEE FIGUREI8 Prince William Sound . Cooper Laru Ing A 5 Peak-discharge measurement site GULF OF ALASKA Numbers refer to those in table 2 Figure 1. Index to location of peak-discharge measurement sites for flood of October 10-12,1986 in Southcentral Alaska. 2 Acknowledgements The data in this report were collected as part of the cooperative programs between the Geological Survey and various State of Alaska agencies: Department of Transportation and Public Facilities, Division of Geological and Geophysical Surveys of Department of Natural Resources, and Department of Fish and Game; and in cooperation with Kenai Peninsula Borough and the Municipality of Anchorage. Supplementary funding for the fieldwork and preparation of this report was furnished by the Alaska Division of Emergency Services, Alaska Department of Transportation and Public Facilities, and the Federal Highway Administration. Meteorological data were furnished by Gerald Nibler (1986) of the National Weather Service and an assessment of the flood was made by the Federal Emergency Management Agency (1986). The U.S. Forest Service and the U.S. Army Corps of Engineers each provide funds for two gaging stations used in the report. PRECIPITATION Precipitation totals for the 3-day storm period, October 9-11, are listed in table 1. An isohyetal map (fig. 2) shows that the rains were concentrated around Seward and in the lower Susitna River basin. Intense rainfall began first in the lower Kenai Peninsula and then moved eastward toward Seward. Precipitation data collected at Seward (fig. 3A) show that a period of heavy precipitation began the evening of October 9 and continued into the morning of October 11. The heaviest precipitation within a 6-hour period occurred on October 10 from 4 to 10 a.m., during which 6.00 in. of rain were recorded. The