LANDSLIDE ROUGHNESS TO IDENTIFY AND RELATIVELY DATE LANDSLIDES IN GLACIAL DEPOSITS, SKAGIT VALLEY, WASHINGTON, USA

Landforms within the Skagit Valley record a complex history of landscape evolution from Late Pleistocene to the present. Late Pleistocene glacial deposits and subsequent incision by the Skagit River and its tributaries, Baker River and Grandy Creek formed the Burpee Hills terrace. The Burpee Hills comprises a more than 200-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. Landslide location and probability are necessary metrics for informing citizens and policy makers of the frequency of natural hazards. Remote geomorphometric analysis of the 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, including eighty-five percent of the south-facing slope overlooking the town of Concrete (WA). The identified landslides are primarily within the advance outwash deposits; this glacial unit has a slope angle ranging from 27 to 36 degrees and stratigraphically overlies a fine-grained advance glacio-lacustrine deposit. 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 likely 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 could provide a more accurate timeline of the Burpee Hills hillslope evolution, and better-constrained information for emergency management and planners.