The Ledgewood-Bonair Landslide toe and where did it go? A case study in littoral sediment budgets from a deep-seated landslide in Island County, Washington

On the morning of March 27th, 2013, a small portion of a much larger landslide complex failed on the western shoreline of central Whidbey Island, Island County, Washington. This landslide, known as the Ledgewood-Bonair Landslide (LB Landslide), mobilized as much as 150,000 cubic meters of unconsolidated glacial sediment onto the coastline of the Puget Sound (Slaughter et al., 2013, Geotechnical Engineering Services, 2013). This study aims to determine how sediment from the Ledgewood-Bonair Landslide has acted on the adjacent beaches 400 meters to the north and south, and specifically to evaluate the volume of sediment contributed by the slide to adjacent beaches, how persistent bluff-derived accretion has been on adjacent beaches, and how intertidal grain sizes changed as a result of the bluff-derived sediment, LiDAR imagery from 2013 and 2014 were differenced and compared to beach profile data and grain size photography. Volume change results indicate that of the 41,850 cubic meters of sediment eroded at the toe of the landslide, 8.9 percent was redeposited on adjacent beaches within 1 year of the landslide. Of this 8.9 percent, 6.3 percent ended up on the north beach and 2.6 percent ended up on the south beach. Because the landslide deposit was primarily sands, silts, and clays, it is reasonable to assume that the remaining 91.1 percent of the sediment eroded from the landslide toe was carried out into the waters of the Puget Sound. Over the course of the two-year study, measurable accretion is apparent up to 150 meters north and 100 meters south of the landslide complex. Profile data also suggests that the most significant elevation changes occurred within the first two and half months since the landslides occurrence. The dominant surficial grain size of the beach soon after the landslide was coarse-sand; in the years following the landslide, 150 meters north of the toe the beach sediment became finer while 100 meters south of the toe the beach sediment became coarser. Overall, the LB Landslide has affected beach profile and grain size only locally, within 150 meters of the landslide toe.

The effect of shore armoring on beach slope in mixed sand-and-gravel beaches

Senior thesis written for Oceanography 445

Geology and landslide geomorphology of the Burpee Hills, Skagit County, Washington, USA

Landforms within the Skagit Valley record a complex history of land evolution from Late Pleistocene to the present. Late Pleistocene glacial deposits and subsequent incision by the Skagit River formed the Burpee Hills terrace. The Burpee Hills comprises an approximately 205-m-thick sequence of sediments, including glacio-lacustrine silts and clays, overlain by sandy advance outwash and capped by coarse till, creating a sediment-mantled landscape where mass wasting occurs in the form of debris flows and deep-seated landslides (Heller, 1980; Skagit County, 2014). Landslide probability and location are necessary metrics for informing citizens and policy makers of the frequency of natural hazards. Remote geomorphometric analysis of the site area using airborne LiDAR combined with field investigation provide the information to determine relative ages of landslide deposits, to classify geologic units involved, and to interpret the recent hillslope evolution. Thirty-two percent of the 28-km2 Burpee Hills landform has been mapped as landslide deposits. Eighty-five percent of the south-facing slope is mapped as landslide deposits. The mapped landslides occur predominantly within the advance outwash deposits (Qgav), this glacial unit has a slope angle ranging from 27 to 36 degrees. Quantifying surface roughness as a function of standard deviation of slope provides a relative age of landslide deposits, laying the groundwork for frequency analysis of landslides on the slopes of the Burpee Hills. The south-facing slopes are predominately affected by deep-seated landslides as a result of Skagit River erosion patterns within the floodplain. The slopes eroded at the toe by the Skagit River have the highest roughness coefficients, suggesting that areas with more frequent disturbance at the toe are more prone to sliding or remobilization. Future work including radiocarbon dating and hydrologic-cycle investigations will provide a more accurate timeline of the Burpee Hills hillslope evolution, and better information for emergency management and planners in the future.