GEOLOGIC HAZARDS OF THE ZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA, WASHINGTON AND KANE COUNTIES, UTAH by William R. Lund, Tyler R. Knudsen, and David L. Sharrow SPECIAL STUDY 133 UTAH GEOLOGICAL SURVEY a division of UTAH DEPARTMENT OF NATURAL RESOURCES 2010 GEOLOGIC HAZARDS OF THE ZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA, WASHINGTON AND KANE COUNTIES, UTAH by William R. Lund, Tyler R. Knudsen, and David L. Sharrow Cover Image: Dump truck crushed by rock fall on November 23, 1947, in Zion National Park (photo courtesy of the National Park Service). ISBN 978-1-55791-833-8 SPECIAL STUDY 133 UTAH GEOLOGICAL SURVEY a division of UTAH DEPARTMENT OF NATURAL RESOURCES 2010 STATE OF UTAH Gary R. Herbert, Governor DEPARTMENT OF NATURAL RESOURCES Michael Styler, Executive Director UTAH GEOLOGICAL SURVEY Richard G. Allis, Director PUBLICATIONS contact Natural Resources Map & Bookstore 1594 W. North Temple Salt Lake City, UT 84116 telephone: 801-537-3320 toll-free: 1-888-UTAH MAP Web site: mapstore.utah.gov email: [email protected] UTAH GEOLOGICAL SURVEY contact 1594 W. North Temple, Suite 3110 Salt Lake City, UT 84116 telephone: 801-537-3300 Web site: geology.utah.gov Although this product represents the work of professional scientists, the Utah Department of Natural Resources, Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use. The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product. CHAPTER CONTENTS Chapter 1 - Introduction ABSTRACT .....................................................................................................................................................................................3 PURPOSE ........................................................................................................................................................................................ 3 BACKGROUND ............................................................................................................................................................................. 4 SCOPE OF WORK ..........................................................................................................................................................................5 SETTING ......................................................................................................................................................................................... 6 GEOLOGY ...................................................................................................................................................................................... 7 RELATIVE IMPORTANCE OF GEOLOGIC HAZARDS IN THE STUDY AREA ....................................................................11 ADDITIONAL INFORMATION AND GUIDELINES ................................................................................................................ 12 ACKNOWLEDGMENTS ............................................................................................................................................................. 12 REFERENCES .............................................................................................................................................................................. 12 Chapter 2 – Flood and Debris-Flow Hazards INTRODUCTION ......................................................................................................................................................................... 16 SOURCES OF INFORMATION ................................................................................................................................................... 16 FLOOD TYPES ............................................................................................................................................................................. 16 Riverine Floods ......................................................................................................................................................................... 16 Flash Floods ............................................................................................................................................................................. 17 Debris Flows ............................................................................................................................................................................. 17 Sheetfloods ................................................................................................................................................................................18 Unintentional Water Release from Water-Retention Structures ................................................................................................18 Flooding Associated with Rock-Fall or Landslide Dams ......................................................................................................... 20 FLOOD DISCHARGE AND FREQUENCY ESTIMATES ........................................................................................................ 20 FLOOD-PRONE-AREA CLASSIFICATION ...............................................................................................................................20 National Flood Insurance Program 100-Year Flood Map .........................................................................................................20 National Park Service “Floodplain” Map ................................................................................................................................. 21 Summary of Flood Hazard to Existing National Park Service Facilities in Zion Canyon ....................................................... 21 Other Flood-Prone Areas .......................................................................................................................................................... 21 USING THE MAP ......................................................................................................................................................................... 23 MAP LIMITATIONS ..................................................................................................................................................................... 23 HAZARD REDUCTION ...............................................................................................................................................................23 REFERENCES .............................................................................................................................................................................. 25 Chapter 3 – Rock-Fall Hazard INTRODUCTION ......................................................................................................................................................................... 30 SOURCES OF INFORMATION ................................................................................................................................................... 30 ROCK-FALL SOURCES............................................................................................................................................................... 30 ROCK- FALL-HAZARD CLASSIFICATION ............................................................................................................................. 32 USING THE MAP ......................................................................................................................................................................... 33 MAP LIMITATIONS ..................................................................................................................................................................... 33 HAZARD REDUCTION ...............................................................................................................................................................33 REFERENCES .............................................................................................................................................................................. 34 Chapter 4 – Landslide Hazard INTRODUCTION ......................................................................................................................................................................... 38 SOURCES OF INFORMATION ................................................................................................................................................... 38 LANDSLIDE CAUSES .................................................................................................................................................................38 LANDSLIDE TYPES AND PROCESSES ................................................................................................................................... 39 LANDSLIDES IN THE ZION NATIONAL PARK GEOLOGIC-HAZARD STUDY AREA ..................................................... 41 Sentinel Landslide .....................................................................................................................................................................41 Zion–Mount Carmel Highway Switchbacks Landslide ............................................................................................................ 42 Kolob Canyons Scenic Drive Landslides ................................................................................................................................