Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington Technical Report August 2016 Prepared by Department of Natural Resources and Parks Water and Land Resources Division River and Floodplain Management Section 201 South Jackson Street, Suite 600 Seattle, WA 98104 Alternate Formats Available 206-477-4800 TTY Relay: 711 Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington Technical Report Submitted by: Department of Natural Resources and Parks Water and Land Resources Division River and Floodplain Management Section 201 South Jackson Street, Suite 600 Seattle, WA 98104 Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington FINAL Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington This document was prepared under the supervision and direction of the undersigned, whose seals as licensed geologists affixed below: John Bethel, WA LEG King County Department of Natural Resources and Parks Water and Land Resources Division River and Floodplain Management Section Date Sevin Bilir, WA LHG King County Department of Natural Resources and Parks Water and Land Resources Division Science and Technical Support Section Date King County August2016 Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington FINAL Acknowledgements Primary contributions to the technical work for this investigation were provided by King County Department of Natural Resources and Parks (DNRP) geologists licensed in the State of Washington. These staff include John Bethel in the River and Floodplain Management Section (RFMS), Water and Land Resources Division (WLR Division), and Sevin Bilir in the Science and Technical Support Section, WLR Division. Other WLR Division staff provided technical, inventory, mapping and field support (Fred Lott and Evelyn Conrado). Mapping information for this investigation was coordinated with the King County Department of Permitting and Environmental Review (DPER). Technical editing was provided by Jennifer Vanderhoof. Sevin Bilir served as the project manager. Jeanne Stypula, PE, was the project supervisor. The King County Department of Natural Resources and Parks wishes to specifically acknowledge the Technical Review Committee members who provided expert advice and direction in the consideration of the mapping methodologies used in the preparation of the river corridor landslide hazard mapping. The Committee members are expert practitioners in Washington State in landslide science, geotechnical engineering, local geology and landslides, and landslide mapping, and include: . Chris Brummer, PE, WA LEG, King County Senior Engineer, DNRP WLR Division RFMS . Alan Corwin, PE, King County Supervising Material Engineer, Department of Transportation, Road Services Division . William Laprade, WA LEG, Senior Vice President, Shannon & Wilson, Inc. Stephen Slaughter, WA LEG, Landslide Hazards Program Coordinator, Washington State Department of Natural Resources, Division of Geology and Earth Resources (WA DNR DGER) . Kathy Troost, WA LG., Senior Lecturer, University of Washington, College of the Environment Citation King County. 2016. Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington. Prepared by River and Floodplain Management Section, Water and Land Resources Division, Department of Natural Resources and Parks. Seattle, WA. August. King County ii August 2016 Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington FINAL Table of Contents Executive Summary.......................................................................................................................................... viii 1.0 Introduction and Background............................................................................................................ 1 1.1 Previous Landslide Mapping ......................................................................................................... 1 1.2 Project Background ........................................................................................................................... 1 1.3 Study Limits ......................................................................................................................................... 2 1.4 Expert Review and Consultation .................................................................................................. 3 1.5 Conventions Used in this Report .................................................................................................. 3 2.0 Geologic Setting ....................................................................................................................................... 5 3.0 Landslide Activity in King County .................................................................................................... 7 3.1 Types of Landslides in King County ............................................................................................ 7 3.1.1 Deep-Seated Landslides ............................................................................................................ 7 3.1.2 Shallow Debris Slides ...............................................................................................................13 3.1.3 Depositional Fans .......................................................................................................................14 3.1.4 Rock Fall ........................................................................................................................................18 3.1.5 Rock Avalanches .........................................................................................................................20 3.2 Influences on Landsliding .............................................................................................................20 3.2.1 Human Activities ........................................................................................................................20 3.2.2 Seismicity ......................................................................................................................................20 3.2.3 Climate Change ...........................................................................................................................21 4.0 Methodologies For Mapping Hazards ...........................................................................................22 4.1 Map Resources ..................................................................................................................................22 4.2 Deep-Seated Landslides ................................................................................................................23 4.3 Shallow Debris Slides .....................................................................................................................28 4.4 Depositional Fans.............................................................................................................................30 4.4.1 Identifying Fans ..........................................................................................................................31 4.4.2 Rationale for Excluding Large Fans ....................................................................................32 4.4.3 Alpine Fans and Debris Flows ...............................................................................................33 4.4.4 Puget Lowland Fans ..................................................................................................................36 4.5 Rock Fall ..............................................................................................................................................37 4.6 Rock Avalanche Deposits ..............................................................................................................39 King County iii August 2016 Mapping of Potential Landslide Hazards along the River Corridors of King County, Washington FINAL 4.7 Fieldwork Supporting this Project ............................................................................................40 5.0 Products ...................................................................................................................................................41 5.1 Landslide Map ...................................................................................................................................41 5.2 Landslides Inventory ......................................................................................................................41 5.2.1 Sources and Type of Data Reviewed ...................................................................................41 5.2.2 Attribute Table ............................................................................................................................43 5.3 Landslide Mapping Webtool ........................................................................................................45 6.0 Future Considerations ........................................................................................................................48 7.0 Disclaimer ................................................................................................................................................49 8.0 References ...............................................................................................................................................51 Figures