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The 22 March 2014 Oso Landslide, Snohomish County, Washington

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The 22 March 2014 Oso Landslide, Snohomish County, Washington Geotechnical Extreme Events Reconnaissance Turning Disaster into Knowledge Sponsored by the National Science Foundation July 22, 2014 The 22 March 2014 Oso Landslide, Snohomish County, Washington Jeffrey R. Keaton, AMEC Americas (Team Leader – GEER Steering Committee) Joseph Wartman, University of Washington (Team Leader) Scott Anderson, Federal Highway Administration (GEER Steering Committee) Jean Benoît, University of New Hampshire (GEER Member) John deLaChapelle, Golder Associates, Inc. Robert Gilbert, University of Texas, Austin (GEER Steering Committee) David R. Montgomery, University of Washington THE 22 MARCH 2014 OSO LANDSLIDE, SNOHOMISH COUNTY, WASHINGTON List of Tables and Figures ..........................................................................................iii 1. Introduction ...........................................................................................................1 2. Geographic and Geologic Setting...................................................................6 2.1 Physiography..................................................................................................6 2.2 Geologic Setting ............................................................................................7 2.3 Groundwater Setting ......................................................................................8 2.4 Vegetation History .........................................................................................10 3. CLIMATE AND PRECIPITATION ......................................................................26 3.1 Climate and Average Precipitation .................................................................26 3.2 March 2014 Precipitation ................................................................................28 3.3 Flow in North Fork Stillaguamish River .........................................................33 4. OSO LANDSLIDE BACKGROUND ....................................................................48 4.1 History of Landsliding along the North Fork Stillaguamish River Valley ....48 4.2 History of Slope Stability at the Oso (Hazel) Landslide Site .........................49 4.3 Prior Slope Stability Analyses .......................................................................51 4.4 Subsurface Characterization ..........................................................................52 4.5 Land-Use and Risk ........................................................................................53 5. DATA COLLECTION FOR THE 22 MARCH 2014 LANDSLIDE ...................69 5.1 Site stratigraphy .............................................................................................69 5.2 Field Observations .........................................................................................71 5.3 Discussion of Observations ...........................................................................80 5.4 Seismic Signals Generated by the Landslide .................................................84 5.5 Eyewitness Accounts .....................................................................................86 5.6 Impacts on the built environment ..................................................................90 6. HYPOTHESIZED MECHANISMS OF THE MARCH 22 LANDSLIDE ...........128 6.1 Introduction ...................................................................................................128 6.2 Stage 1 ...........................................................................................................128 6.3 Stage 2 ..........................................................................................................133 6.4 Summary ......................................................................................................134 7. DISCUSSION ......................................................................................................141 7.1 Historical Context ........................................................................................141 7.2 Empirical Predictions of Runout ..................................................................142 7.3 Influence of Forest Practices on Groundwater Levels and Slope Stability .145 i 7.4 Influence of Renewed Riverbank Erosion on Slope Stability .....................147 7.5 Benchmarks for Risk .....................................................................................148 8. CONCLUSIONS AND RECOMMENDATONS ..................................................159 8.1 The 22 March Oso, Washington Landslide ...................................................159 8.2 Recommendations ..........................................................................................159 9. ACKNOWLEDGMENTS ......................................................................................165 10. REFERENCES .....................................................................................................166 APPENDIX A: Field reconnaissance ii LIST OF TABLE AND FIGURES TABLES Table 3.1.1 Location information for weather stations and stream gauge shown in Figure 3.1.1. Table 3.2.1 Cumulative rainfall for March 1 to 22, 2014. Table 3.2.2 Cumulative precipitation for periods in March 2014 at nearby gauges and at the Oso Landslide. Table 5.2.1: Tree characteristics in areas 1 through 10 as defined in Figure 5.2.4 Table 5.3.2: Summary of Atterberg Limit Testing Table 5.3.3: Tabulated summary of sieve analysis results. Table 5.3.4 USCS Soil Classification Table 5.3.5 Radiocarbon dates for material collected from exposures in the eastern lateral scarp of the 2014 Oso landslide. Table 5.5.1 Summary of Eyewitness Accounts Table 7.2.1 Regression coefficients for Equation 7-2 (adapted from Hungr et al. 2005). Table 7.2.2 Parameters for empirical relationships of Legros (2002) in Equation 7-4. Table 7.2.3 Predicted runout distances for 2014 Oso Landslide based on estimated total volume of 7.6 million cubic meters. FIGURES Figure 1.1 Site and vicinity map Figure 1.2 The 22 March 2014 Oso, Washington Landslide Figure 2.1.1 Physiographic provinces of Washington. Figure 2.1.2 Topography in the vicinity of the Oso Landslide. Figure 2.1.3 Slope inclination in the area around the Oso Landslide. iii Figure 2.1.4 Valley width indexed to distance between the 200-m elevation contours. Figure 2.2.1 Location of the 2014 landslide superimposed on geologic map of the Oso Landslide area. Figure 2.2.2 Hillshade map of 2003 ground conditions. Base image from 2003 lidar data. Figure 2.2.3 View of Oso Landslide and geologic exposures in scarps. Figure 2.4.1 Aerial photograph of Oso Landslide area taken in 1933. Figure 3.1.1 Map of the Oso Landslide region showing locations of selected weather stations and a stream gauge. Figure 3.1.2 Monthly average precipitation data for four NWS cooperative weather stations and monthly precipitation values for January 2012 to March 2014. Figure 3.1.3 Monthly precipitation values for three RAWS gauges for January 2012 to March 2014. Figure 3.1.4 Daily and cumulative annual precipitation at three RAWS gauges in the vicinity of the Oso Landslide for 2012, 2013, and the winter months of 2014. Figure 3.2.1 Cumulative precipitation preceding March 22, 2014, and associated return period at the Darrington Ranger Station gauge. Figure 3.2.2 Daily and cumulative monthly precipitation recorded at Darrington and the three RAWS gauges during the first three months of 2014. Figure 3.2.3 Exponential regression predicting rainrate (Rr) as a function of composite Doppler radar reflectivity (dBZ). Figure 3.2.4 NEXRAD Reflectivity at the Oso Landslide on 19 March 2014 from 12:29 to 13:42 PDT. Figure 3.2.5 Example of NEXRAD composite reflectivity showing highly variable conditions. Figure 3.2.6 Composite precipitation diagram showing radar rainfall and gauge precipitation for the Oso Landslide and four gauge locations. Figure 3.3.1 Discharge hydrograph of North Fork Stillaguamish River near Arlington, WA, January 2012 through March 2014. iv Figure 3.3.2 Hydrograph of North Fork Stillaguamish River, January through March 2014. Figure 3.3.3 North Fork Stillaguamish River stage showing effects of Oso Landslide dam. Figure 3.3.4 Stillaguamish River stage showing effects of Oso Landslide dam and tidal influence from Puget Sound. Figure 4.1.1 North Fork Stillaguamish River Valley (2003 lidar map). Figure 4.1.2: Relative age classes of pre-2014 landslides in the immediate vicinity of the 2014 Oso Landslide. Figure 4.1.3: Oso Landslide area 2003 lidar map. Figure 4.1.4: Oso Landslide area 2013 lidar map. Figure 4.1.5: Elevation differences between 2013 and 2003 lidar data sets. Figure 4.1.6: Oso Landslide area 2014 lidar map. Figure 4.1.7: Elevation differences between 2014 and 2013 lidar data sets. Figure 4.1.8: Topographic Profile Section A-A'. Figure 4.2.1 Historic Oso/Hazel Landslide disturbances and scarps from 1958 to 2013. Figure 4.4.1 Reproduced boring logs from Shannon (1952). Figure 4.5.1: Land-use at Time of Slide. Figure 4.5.2: Land classified as geologically hazardous areas in vicinity of the slide. Figure 5.2.1 Locations of key observations of GEER team during site reconnaissance. Figure 5.2.2. Seeps along western (right) margin of the landslide. Figure 5.2.3: Interpreted landslide zone map. Figure 5.2.4 Photograph of Zone A1, recessional outwash and gray glacial till exposed in landslide headscarp. Figure 5.2.5a: Photograph of Zone A2 as seen from top of landslide headscarp; view to south. v Figure 5.2.5b: Photograph of Zone A2 as seen from top of landslide headscarp; view to southwest.
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