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Geomorphic Response of the North Fork Stillaguamish River to the State Route 530 Landslide Near Oso, Washington

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Geomorphic Response of the North Fork Stillaguamish River to the State Route 530 Landslide Near Oso, Washington Geomorphic Response of the North Fork Stillaguamish River to the State Route 530 Landslide near Oso, Washington Scientific Investigations Report 2017–5055 U.S. Department of the Interior U.S. Geological Survey Cover: Photograph showing North Fork Stillaguamish River and SR530 Landslide near Oso Washington; flow is from right to left. Photograph by Scott W. Anderson, U.S. Geological Survey, Janury 29, 2015. Geomorphic Response of the North Fork Stillaguamish River to the State Route 530 Landslide near Oso, Washington By Scott W. Anderson, Mackenzie K. Keith, Christopher S. Magirl, J. Rose Wallick, Mark C. Mastin, and James R. Foreman Prepared in cooperation with the Federal Emergency Management Administration and Snohomish County, Washington Scientific Investigations Report 2017–5055 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S. Geological Survey William H. Werkheiser, Acting Director U.S. Geological Survey, Reston, Virginia: 2017 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Anderson, S.W., Keith, M.K., Magirl, C.S., Wallick, J.R., Mastin, M.C., and Foreman, J.R., 2017, Geomorphic response of the North Fork Stillaguamish River to the State Route 530 landslide near Oso, Washington: U.S. Geological Survey Scientific Investigations Report 2017–5055, 85 p., https://doi.org/10.3133/sir20175055. ISSN 2328-0328 (online) iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................2 Purpose and Scope ..............................................................................................................................4 Units, Linear Referencing, and Study Interval .................................................................................5 Description of Study Area ............................................................................................................................6 Geography and Geology ......................................................................................................................6 River Characteristics and Geomorphic Study Reaches .................................................................6 Hydrology .............................................................................................................................................10 Erosion of the State Route 530 Landslide Deposit ..................................................................................13 Data Collection and Analysis Methods ...........................................................................................13 Pre-Existing Data .......................................................................................................................13 Structure from Motion Photogrammetry ...............................................................................13 Analysis of Channel Geometry ...............................................................................................13 Assessing Volumetric Erosion .................................................................................................14 Characterizing Sediment Characteristics of the Landslide Deposit .................................14 Evolution of Channel Geometry ........................................................................................................15 Sediment Characteristics and Erosion Processes ........................................................................23 Sediment Delivery from the Landslide ............................................................................................26 Sediment Loads at Streamgages ..............................................................................................................28 Sediment Sampling and Load Estimation Methods ......................................................................28 Suspended Sediment Measurements ....................................................................................28 Bedload Measurements ..........................................................................................................29 Sediment Loads and Uncertainty Estimates ..................................................................................29 Fraction of Landslide Sediment in Stanwood Suspended-Sediment Load ......................30 Sediment Loads at Streamgages .....................................................................................................32 Swede Heaven ...........................................................................................................................32 Whitman .....................................................................................................................................37 Suspended-Sediment Load .............................................................................................37 Bedload...............................................................................................................................40 Fraction of Total Load Carried as Bedload ...................................................................40 Grain-Size Distributions of Sediment Load ...................................................................42 Stanwood ....................................................................................................................................44 Bedload-Transport Modeling .....................................................................................................................46 Model Inputs and Development .......................................................................................................46 Selected-Sediment Transport Equations ...............................................................................46 Inputs and Methods for Bedload Transport Modeling ........................................................47 Surficial Grain Size ...........................................................................................................47 Uncertainty in Grain-Size Measurements ...........................................................48 Hydraulic Parameters from One-Dimensional Hydraulic Modeling .........................48 Implementation of Transport Equations and Methods for Load Estimation .............................49 Model Results ......................................................................................................................................52 Longitudinal Trends in Bedload Transport Capacity ............................................................52 Magnitude of Estimated Bedload Transport Capacities ......................................................55 iv Contents—Continued Downstream Channel Responses to Landslide Sediment ....................................................................55 Downstream Channel Change and Grain Size Response Methods ...........................................55 Assessing Local Vertical Channel Change ............................................................................55 Assessing Surficial Grain-Size Distributions ........................................................................56 Geomorphic Change Just Downstream of the Landslide ............................................................56 Downstream Channel Change ..........................................................................................................58 Changes in Surficial Grain Size ........................................................................................................64 Integrated Interpretation of Geomorphic Responses ...........................................................................66 Erosion of the Landslide Deposit .....................................................................................................66 Ambient and Landslide-Derived Fluvial Sediment Loads ............................................................67 Downstream Geomorphic Responses .............................................................................................69 Transport Modeling Results ..............................................................................................................69 Anticipated Trajectory ........................................................................................................................69 Landslide Channel and Sediment Delivery ............................................................................69 Downstream Geomorphic Responses ....................................................................................70 Conclusions...................................................................................................................................................70
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