Abstract

The spatial distribution and temporal variability of retreat rates of coastal bluffs composed of unconsolidated glacial deposits are of interest to landowners who occupy bluff-top properties as well as coastal resource managers who are responsible for protecting marine habitats such as forage fish spawning beaches that are dependent on bluff-derived sediments. Assessment of bluff retreat and associated sediment volumes contributed to the nearshore over time is the first step toward development of a coastal sediment budget for bluff-backed beaches using data sources including aerial photography (1939, 2001), GPS-based beach profile data (2010–2013), and airborne LiDAR (2001, 2012). These data are analyzed in context to determine alongshore rates of bluff retreat and associated volume change for the Elwha and Dungeness littoral cells in Clallam County, WA. Recession rates from 2001 to 2012 range from 0 to 1.88 m/yr in both drift cells, with mean values of 0.26 ± 0.23 m/yr (N = 152) in Elwha and 0.36 ± 0.24 m/yr (N = 433) in Dungeness. Armored sections show bluff recession rates reduced by 50 percent in Elwha and 80 percent in Dungeness, relative to their respective unarmored sections. Dungeness bluffs produce twice as much sediment per alongshore distance as do the Elwha bluffs (average, 7.5 m3/m/yr vs. 4.1 m3/m/yr, respectively). Historical bluff recession rates (1939–2001) were comparable to those from 2001–2012. Rates derived from short timescales should not be used directly for predicting decadal-scale bluff recession rates for management purposes, as they tend to represent short-term localized events rather than long-term sustained bluff retreat.

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