South Fork Skykomish River Channel Migration Study June 2017 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 South Fork Skykomish River Channel Migration Study June 2017 Submitted by: John Bethel, LEG, and Sevin Bilir King County Water and Land Resources Division Department of Natural Resources and Parks Funded by: King County Flood Control District South Fork Skykomish River Channel Migration Study Acknowledgements Fred Lott participated in fieldwork during study preparation and provided support on hydrologic analyses. Jennifer Vanderhoof provided technical editing of this report. Terry Butler participated in fieldwork and provided technical review. Jeanne Stypula supervised the study and map preparation. Citation King County. 2017. South Fork Skykomish River channel migration study. Prepared by John Bethel, Fred Lott, and Sevin Bilir. King County Department of Natural Resources and Parks, Water and Land Resources Division. Seattle, Washington. King County River and Floodplain Management i June 2017 South ForkSkykomish River Channel Migration Study South Fork Skykomish River Channel Migrat¡on Study This document was prepared under the supervision and direction of the undersigned, whose seal as licensed geologist is affixed below: John Bethel, WA LEG King County 1 300 Department of Natural Resources and Parks Water and Land Resources Division Bethel John River and Floodplain Management Section u\4 ( L1Vzrft Date King County River and Floodplain Management ¡¡ June 2017 South Fork Skykomish River Channel Migration Study King County River and Floodplain Management iii June 2017 South Fork Skykomish River Channel Migration Study Table of Contents Executive Summary ................................................................................................................................................................. x Introduction ................................................................................................................................................................ 1 1.1 Statement of Purpose ......................................................................................................................................... 1 1.2 Report Layout ....................................................................................................................................................... 2 1.3 Timeframes Relevant to this Study .............................................................................................................. 2 1.4 Effect of Landslides on Channel Migration ............................................................................................... 2 1.5 Conventions Used in this Report ................................................................................................................... 2 Methods ........................................................................................................................................................................ 4 2.1 Review of Existing Studies ............................................................................................................................... 4 2.2 New Field Data Collection ................................................................................................................................ 4 2.3 Historical and Current Information used in Report .............................................................................. 4 2.4 Channel Migration Zone Components......................................................................................................... 6 2.5 Mapping Criteria and Methods ...................................................................................................................... 6 2.5.1 Historical Migration Zone ...................................................................................................... 7 2.5.2 Avulsion Hazard Zone ............................................................................................................. 7 2.5.3 Erosion Hazard Area/Erosion Setback ............................................................................ 8 2.5.4 Erosion Hazard Area/Geotechnical Setback ............................................................... 10 2.5.5 Disconnected Migration Area ............................................................................................ 11 Study Area Characteristics ................................................................................................................................. 12 3.1 Regional Setting ................................................................................................................................................ 12 3.2 Human Activity and Built Features ........................................................................................................... 15 3.2.1 Present Day Land Use ........................................................................................................... 15 3.2.2 Flood Protection Facilities .................................................................................................. 15 3.3 Geology and Sediment .................................................................................................................................... 17 3.3.1 Geology ....................................................................................................................................... 17 3.3.2 Sediment Characteristics .................................................................................................... 21 3.4 Geomorphology ................................................................................................................................................. 24 3.4.1 Mainstem Channel and Floodplain Characteristics .................................................. 24 3.4.2 Floodplain Channels .............................................................................................................. 25 3.4.3 Tributaries to the South Fork Skykomish River ........................................................ 28 3.5 Flood Hydrology ............................................................................................................................................... 32 3.6 Large Wood ......................................................................................................................................................... 35 King County River and Floodplain Management iv June 2017 South Fork Skykomish River Channel Migration Study Characteristics of Channel Migration in the Study Area ........................................................................ 38 4.1 Channel Migration Processes ...................................................................................................................... 38 4.2 Morphology of the Study Reaches ............................................................................................................. 40 4.3 Lateral Channel Migration Rates ................................................................................................................ 43 4.4 Spatial Variation in Channel Migration ................................................................................................... 46 4.5 Temporal Changes in Channel Migration ............................................................................................... 47 Channel Migration Hazards on the South Fork Skykomish River...................................................... 49 5.1 Delineation of Channel Migration Hazard Areas ................................................................................. 49 5.1.1 Historical Migration Zone ................................................................................................... 49 5.1.2 Avulsion Hazard Zone .......................................................................................................... 49 5.1.3 Erosion Hazard Area/Erosion Setback ......................................................................... 51 5.1.4 Erosion Hazard Area/Geotechnical Setback ............................................................... 52 5.1.5 Disconnected Migration Area ............................................................................................ 52 5.2 Channel Migration Hazard Maps ................................................................................................................ 53 5.3 Summary and Conclusions ........................................................................................................................... 53 5.4 Limitations .......................................................................................................................................................... 55 References ................................................................................................................................................................ 56 Appendix A ............................................................................................................................................................... 60 King County River and Floodplain Management v June 2017 South Fork Skykomish River Channel Migration Study Figures Figure 1. Plan view schematic of channel migration zone (CMZ) components (modified from Rapp and Abbe 2003). ...................................................................................................................................