Influence of lithology on erosion along the Dungeness Bluffs

Erosion along the Dungeness Bluffs, located along the Strait of Juan de Fuca, poses a serious hazard to landowners living above. Concurrently, species such as forage fish depend on sediment input from bluff erosion to replenish the beaches in which they spawn. Between 2001 and 2012, the Dungeness bluffs eroded an average of 0.36 m/y, ranging from 0.0 to 1.88 m/y (Kaminsky et al., 2014; Parks, 2015). The steep bluffs in the area are ~30-80 m tall and are composed of glacial and interglacial sediment that vary in composition and strength. Failure styles range from steady erosion due to raveling, to more significant events including falls, topples, slides, and flows (Varnes, 1978). In this study, I investigate sites along the Dungeness Bluffs with different failure styles to understand how each erode in relation to lithologic composition and characteristics. To do this, I use field mapping methods combined with point cloud differencing derived from structure from motion and the Washington State Department of Ecology Coastal Monitoring & Analysis Program's boat-based LiDAR datasets collected in 2015 and 2021. Preliminary findings suggest that glacially overridden materials are more resistant to erosion, fluctuations in groundwater and climate may control larger failure events, and grain size-distribution controls cohesion and erosivity. Given this, detailed geologic characterization of bluffs is necessary for predicting relative erosion rates along with other factors, such as wave energy and groundwater discharge. Understanding the mechanisms of bluff erosion is of increasing importance as sea levels continue to rise and weather becomes more extreme. This information is essential for land use planning, habitat evaluation, hazard recognition, and hazard mitigation along the Salish Sea.