Bluff Recession in the Elwha and Dungeness Littoral Cells, Washington, USA

Bluff Recession in the Elwha and Dungeness Littoral Cells, Washington, USA DAVID S. PARKS1 Washington State Department of Natural Resources, 311 McCarver Road, Port Angeles, WA 98362 Key Terms: Environmental Geology, Land-Use Plan- Washington State, USA, and are the primary source ning, Erosion, Landslides of sediment contributed to mixed sand and gravel beaches in the region (Schwartz et al., 1987; Shipman, 2004; Finlayson, 2006; and Johannessen ABSTRACT and MacLennan, 2007). The spatial and temporal distribution of bluff recession from wave-, wind-, The spatial distribution and temporal variability of precipitation-, and groundwater-induced erosion is retreat rates of coastal bluffs composed of unconsolidated poorly understood and documented for the southern glacial deposits are of interest to landowners who occupy shore of the Strait of Juan de Fuca and has led to bluff-top properties as well as coastal resource managers underestimating the potential hazards to infrastruc- who are responsible for protecting marine habitats such ture (e.g., roads, houses) posed by eroding bluffs as forage fish spawning beaches that are dependent on over time (Figures 1 and 2). Efforts to protect bluff-derived sediments. Assessment of bluff retreat and infrastructure and limit the rates of bluff erosion associated sediment volumes contributed to the nearshore by constructing shoreline revetments have historical- over time is the first step toward development of a coastal ly ignored the physical and ecological effects of sediment budget for bluff-backed beaches using data sediment starvation of beaches caused by shoreline sources including aerial photography (1939, 2001), GPS- hardening (Shipman et al., 2010). The disruption of based beach profile data (2010–2013), and airborne sediment movement from bluffs to beaches has LiDAR (2001, 2012). These data are analyzed in context caused the loss of suitable habitats for critical marine to determine alongshore rates of bluff retreat and species, including forage fish and juvenile salmonids associated volume change for the Elwha and Dungeness (Rice, 2006; Shipman et al., 2010; Shaffer et al., 2012; littoral cells in Clallam County, WA. Recession rates and Parks et al., 2013). The importance of under- from 2001 to 2012 range from 0 to 1.88 m/yr in both drift standing the long-term littoral sediment budget has cells, with mean values of 0.26 ± 0.23 m/yr (N = 152) in been underscored by the recent removal of two dams Elwha and 0.36 ± 0.24 m/yr (N = 433) in Dungeness. on the Elwha River and the subsequent introduction 6 3 Armored sections show bluff recession rates reduced by of approximately 6.4 3 10 m of sediment into the 50 percent in Elwha and 80 percent in Dungeness, relative nearshore environment within the first 2 years to their respective unarmored sections. Dungeness bluffs (between September 2011 and September 2013) (East produce twice as much sediment per alongshore distance et al., 2014; Gelfenbaum et al., in review; and as do the Elwha bluffs (average, 7.5 m3/m/yr vs. 4.1 m3/m/ Warrick et al., in review). yr, respectively). Historical bluff recession rates (1939– Relatively few studies of coastal bluff recession 2001) were comparable to those from 2001–2012. Rates have been completed for the shoreline areas of the derived from short timescales should not be used directly Strait of Juan de Fuca, and the studies that have been for predicting decadal-scale bluff recession rates for completed have used a variety of methods, leading to management purposes, as they tend to represent short- difficulty in comparing results. In the Elwha littoral term localized events rather than long-term sustained cell (herein referred to as ‘‘drift cell’’), the U.S. Army bluff retreat. Corps of Engineers (USACE) completed an evalua- tion of bluff recession rates and sediment volume supply to the nearshore environment as part of an INTRODUCTION environmental assessment for a shoreline armoring and beach nourishment project on Ediz Hook in Port Coastal bluffs are a dominant geomorphic feature Angeles (USACE, 1971). Using Government Land of the shorelines of the Strait of Juan de Fuca, Office and National Geodetic Survey shoreline maps, the USACE estimated a gradual reduction in bluff 1Corresponding author email: [email protected]. recession rates from 1.5 m/yr (1850–1885) to 1.3 m/yr Environmental & Engineering Geoscience, Vol. XXI, No. 2, May 2015, pp. 129–146 129 Parks Figure 1. (A) Homes threatened by receding bluffs, Dungeness drift cell. (B) Seawall installed at bluff toe to protect Port Angeles City Landfill from bluff retreat, Elwha drift cell. (1885–1926), decreasing to 1.1 m/yr (1926–1948) and multitude of shoreline armoring projects at the base then to 0.2 m/yr (1948–1970). Each successive of the Elwha bluffs (USACE, 1971). reduction in bluff recession rates since 1930 has been The USACE (1971) study also shows a reduction in attributed to construction and maintenance of a sediment volumes provided by the Elwha bluffs over 130 Environmental & Engineering Geoscience, Vol. XXI, No. 2, May 2015, pp. 129–146 Coastal Bluff Recession Figure 2. Map of the study area showing direction of net alongshore sediment transport within the Elwha and Dungeness drift cells in Clallam County, WA. time. Prior to the construction of the Elwha Dam in erosion rates of 0–0.08 m/yr using cosmogenic 10Be 1911, the estimated sediment supply from the bluffs concentrations in lag boulders to date shoreline was 2.22 3 105 m3/yr. After construction of the Elwha positions over time scales of 103–104 years. Dam and prior to construction of shoreline armoring In this study, estimates of short- and long-term along the Elwha bluffs in 1929, the estimated bluff recession rates and associated sediment volumes sediment supply from the bluffs was nearly the same, contributed to the Elwha and Dungeness drift cells measuring 2.06 3 105 m3/yr. Between 1929 and 1961, along the Central Strait of Juan de Fuca between when substantial shoreline armoring along the bluffs 1939 and 2012 are derived from historical aerial was installed and maintained, the bluff sediment photography, GPS beach profiles, and airborne supply decreased to 0.73 3 105 m3/yr. Following the LiDAR, and the relative contribution of bluff-derived completion of a major shoreline armoring project sediment supply to the nearshore, in the context of a along the bluffs in 1961, bluff sediment supply was coastal sediment budget recently rejuvenated by the estimated to have further declined to 0.31 3 105 m3/ removal of two dams on the Elwha River, is yr. The reduction of bluff-supplied sediment over this presented. entire time period, 1.91 3 105 m3/yr, represents an 85 percent reduction in the coastal sediment supply to STUDY AREA Ediz Hook (Galster, 1989), which is essentially equivalent to the pre-dam fluvial sediment supply The study area is located on the southern shore of estimated by Randle et al. (1996). the Central Strait of Juan de Fuca near the city of Bluff erosion rates to the east of the Dungeness Port Angeles, WA (Figure 2). The study area is drift cell along the Strait of Juan de Fuca were divided into two distinct shoreline segments that evaluated through land-parcel surveys by Keuler encompass separate but adjacent littoral cells with (1988). Bluff recession rates of up to 0.30 m/yr and bluff-backed beaches: the Elwha bluffs extend along sediment production rates of 1–5 m3/m/yr were the central portion of the Elwha drift cell, and the observed in areas exposed to wave attack associated Dungeness bluffs extend along the western portion of with long fetches. On the west side of Whidbey Island, the Dungeness drift cell (Figure 3). Each drift cell at the eastern limit of the Strait of Juan de Fuca, contains an updrift segment of eroding coastal bluffs Rogers et al. (2012) determined long-term bluff to the west that supply sediment via longshore littoral Environmental & Engineering Geoscience, Vol. XXI, No. 2, May 2015, pp. 129–146 131 Parks Figure 3. (A) Photograph of the Dungeness bluffs looking west from Dungeness Spit. (B) Photograph of the Elwha bluffs west from Ediz Hook. Note the armoring placed mid-beach in front of the bluffs in photograph B. transport to long spits at the down-drift end to the Dungeness Spit. A fundamental difference between east. the two drift cells is that the Elwha River discharges The Elwha bluff segment is 4.9 km long and into the Strait of Juan de Fuca updrift of the Elwha supplies sediment to Ediz Hook. The Dungeness bluff bluffs, while the Dungeness River empties into the segment is 13.6 km long and supplies sediment to Strait of Juan de Fuca on the lee side of Dungeness 132 Environmental & Engineering Geoscience, Vol. XXI, No. 2, May 2015, pp. 129–146 Coastal Bluff Recession Spit (Figure 2). Therefore, the Elwha drift cell is 2000; Polenz et al., 2004) and Eocene marine composed of both river- and bluff-derived sediments, sedimentary rocks (Schasse et al., 2000; Schasse and while the Dungeness drift cell is composed of only Polenz, 2002; Schasse, 2003; and Polenz et al., 2004). bluff-derived sediments. Pleistocene glacial deposits occurring within the study The Strait of Juan de Fuca is a wind-dominated area include recessional outwash, glaciomarine drift, marine system that exhibits net easterly longshore and glacial till. sediment transport within the intertidal zone of the Groundwater recharge occurs along the Olympic study area (Galster and Schwartz, 1989; Schwartz et Mountains and discharges into the Strait of Juan de al., 1989; Warrick et al., 2009; and Miller et al., 2011). Fuca. Local groundwater recharge occurs within low- Winds in the Central Strait of Juan de Fuca are elevation glacial landforms adjacent to the coastal dominantly west and northwesterly, with a minor bluffs and discharges at varying elevations on the component of north and northeasterly winds (Miller bluffs controlled by local aquitards (i.e., beds of low- et al., 2011).

Load more