FLOOD HAZARD ASSESSMENT OF THE HOH RIVER AT OLYMPIC NATIONAL PARK RANGER STATION, WASHINGTON By David L. Kresch With a section on Debris Flow and Landslide Hazard Assessment, by Thomas C. Pierson U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 86-4198 Prepared in cooperation with the NATIONAL PARK SERVICE Tacoma, Washington 1987 DEPARTMENT OF THE INTERIOR DONALD PAUL MODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information Copies of this report write to: can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Books and Open-File Reports 1201 Pacific Avenue - Suite 600 Box 25425, Federal Center, Bldg. 810 Tacoma, Washington 98402 Denver, Colorado 80225 Telephone (303) 236-7476 CONTENTS Page Abstract-- -------------------------------------------------------- 1 Introduction------------------------------------------------------ 3 Description of study area----------------------------------------- 3 Methods of study-------------------------------------------------- 6 Flood-frequency analysis-------------------------------------- 6 Step-backwater analysis--------------------------------------- 9 Flood inundation-- -------------------------------------------- 16 Flood-hazard assessment-- ----------------------------------------- 17 Debris flow and landslide hazard assessment, by Thomas C. Pierson- 18 Summary and conelus ions ------------------------------------------- 21 References -------------------------------------------------------- 22 ILLUSTRATIONS [Plate in back cover pocket] PLATE 1. Map showing boundaries of inundation for a 100-year flood on the Hoh River at the ranger station complex. FIGURE 1. Map showing locations of Hoh River stream gaging stations and study area-------------------------------- 4 2-5. Plots of Hoh River cross-sections A through L------------ 11-14 6. Profile of 100-year flood elevations on the Hoh River in the vicinity of the ranger station complex------------- 15 7. Map showing location of debris flow fans, landslide scars and ridge-top depressions in the vicinity of the Hoh River ranger station complex---------------------------- 19 TABLES TABLE 1. Comparison of flood-frequency discharges estimated by regional regression equations for two Hoh River gaging stations with those estimated by log-Pearson type III analysis of the annual peak discharges at the gages---- 2. Water-surface elevations and mean flow velocities at Hoh River cross sections for 10-, 50-, and 100-year flood discharges --------------------------------------------- 10 iii CONVERSION FACTORS For the convenience of readers who may prefer to use metric (Inter­ national System) units rather than the Inch-pound units used In this report, values may be converted by using the following factors: Multiply Inch-pound unit By To obtain metric unit inch (In.) 25.4 millimeter 2.540 centimeter (cm) 0.0254 meter (m) foot (ft) 0.3048 meter (m) yard (yd) 0.9144 meter (m) mile (ml) 1.609 kilometer (km) square mile (ai2 ) 2.590 square kilometer (km2 ) cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) 28.32 liter per second CL/s) cubic yard (yd3 ) 0.7646 cubic meter (m3 ) National Geodetic Vertical Datum of 1929 fNGVD 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. iv FLOOD HAZARD ASSESSMENT OF THE HOH RIVER AT OLYMPIC NATIONAL PARK RANGER STATION, WASHINGTON With a Section on Debris Flow and Landslide Hazard Assessment--by Thomas C. Pierson By David L. Kresch ABSTRACT Federal regulations require buildings and public facilities on Federal land to be located beyond or protected from inundation by a 100-year flood. Flood elevations, velocities and boundaries were determined for the occurrence of a 100-year flood through a reach, approximately 1 mile long, of the Hoh River at the ranger station complex in Olympic National Park. Water-surface elevations and flow velocities were also determined for 10- and 50-year floods. Peak 10-, 50-, and 100-year flood discharges of 24,000, 31,000, and 35,000 cubic feet per second, respectively, were estimated from regional regression equations using drainage area and mean annual precipitation as independent parameters. Flood elevations, estimated by step-backwater analysis of the 100-year flood discharge through 14 channel and flood-plain cross sections of the Hoh River, indicate that the extent of flooding in the vicinity of buildings or public facilities at the ranger station complex is likely to be limited mostly to two historic meander channels that lie partly within loop A of the public campground and that average flood depths of about 2 feet or less would be anticipated in these channels. Mean flow velocities at the cross sections, corresponding to the passage of a 100-year flood, ranged from about 5 to over 11 feet per second. Flooding in the vicinity of either the visitors center or the residential and maintenance areas is unlikely unless the small earthen dam at the upstream end of Taft Creek were to fall. Debris flows with volumes on the order of 100 to 1,000 cubic yards could be expected to occur in the small creeks that drain the steep valley wall north of the ranger station complex. Historic debris flows in these creeks have generally traveled no more than about 100 yards out onto the valley floor. The potential risk that future debris flows in these creeks might reach developed areas within the ranger station complex is considered to be small because most of the developed areas within the complex are situated more than 100 yards from the base of the valley wall. Landslides or rock avalanches originating from the north valley wall with volumes potentially much larger than those for debris flows could have a significant impact on the ranger station complex. The probability that such landslides or avalanches may occur is unknown. Inspection of aerial photographs of the Hoh River valley revealed the apparent presence, along the ridge crest of the north valley wall, of ridge-top depressions--geologic features that are sometimes associated with the onset of deep-seated slope failures. However, evaluation of the potential landslide hazard associated with these depressions would require an onsite examination of the area by trained personnel. Such an effort was outside the scope of this study. INTRODUCTION Buildings and public facilities either constructed on Federal lands or financed with Federal funds are required by Federal regulation to be located beyond or protected from damage by a 100-year flood. The Hoh River ranger station complex, located in Olympic National Park in northwestern Washington, has many buildings and public facilities that are subject to this Federal regulation. Located within the ranger station complex are a visitors center, a residential area that contains several housing units, a maintenance area with workshop and utility buildings, and a public campground that contains nearly 100 campsites. The U.S. Geological Survey, in cooperation with the National Park Service, began a study to determine flood elevations, extent of inundation, and mean flow velocities that may result at the ranger station complex from a 100-year flood on the Hoh River. In addition to the study of a 100-year flood, water-surface elevations and flow velocities were also determined for 10- and 50-year floods on the Hoh River for use by the National Park Service in planning for potential mitigation measures and maintenance needs that might arise from the occurrence of these intermediate-sized floods. This report presents the results of this study and the results of a separate reconnaissance made to assess the potential hazards at the ranger station complex from debris flows and landslides originating from the north wall and ridge of the Hoh River valley. DESCRIPTION OF STUDY AREA The Hoh River flows in a generally westwardly direction from its headwaters at Mount Olympus in the Olympic Mountains to its mouth at the Pacific Ocean. The study area, located about 6 miles inside the western boundary of Olympic National Park (fig. 1), consists of a reach of the Hoh River channel and flood plains about 1 mile long, that extends both upstream and downstream from the ranger station (pi. 1). The north bank flood plain, which is where the ranger station is located, has been developed within the study area; however, the south bank flood plain is undeveloped and uninhabited, and there is no road or bridge access to it. The Hoh River channel in the vicinity of the ranger station complex has an average width of 700 feet and an average depth of 5 feet. During low- flow periods the channel is braided and the average total width of all flowing channels averages about 150 feet. The bed material consists mostly of gravel, cobbles and small boulders. The major topographic feature on the north bank flood plain throughout the upstream two-thirds of the study area is a steep-faced terrace with heights ranging from about 4 feet to over 10 feet. The downstream portion of the north bank flood plain appears to contain a series of small, old meander channels. 124° 123' f 48° 30' o * " * . , Sek'u Mm . >,.. s .v ( .,i N ^e " < ^ i~ p=^_ ^ r- -Sjfweil Peak nl<.: /£ -V-punee, ^D,i.v National; Park 48° 00' 47 s 30' 5i^AT,aNAlLi ;; '-^y-"--s rJ i V(.,< .Ml ««shingtnn I / / . 5^55 __j_______ i^. Base from U. S. Geological Survey 10 20 30 MILES State base map 1:500,000; 1962 i i i i Inset 10 20 30 40 KILOMETERS FIGURE 1.--Location of Hoh River stream gaging stations and study area. Flood plains on each side of the river, except within some of the developed areas along the north bank, are covered.with dense forests consisting primarily of Sitka spruce and western hemlock. Large numbers of deciduous trees, including vine maple, bigleaf maple, red alder, black cottonwood and Scouler willow, are found in parts of the study area where the density of spruce and hemlock is somewhat less. Red alder and Scouler willow are especially plentiful along the rocky gravel bars at the river's edge.