Bluff Recession in the Elwha and Dungeness Littoral Cells, Washington, USA · 2018-03-27 · Bluff Recession in the Elwha and Dungeness Littoral Cells, Washington, USA DAVID S. PARKS1

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-ning, Erosion, Landslides

ABSTRACT

The spatial distribution and temporal variability ofretreat rates of coastal bluffs composed of unconsolidatedglacial deposits are of interest to landowners who occupybluff-top properties as well as coastal resource managerswho are responsible for protecting marine habitats suchas forage fish spawning beaches that are dependent onbluff-derived sediments. Assessment of bluff retreat andassociated sediment volumes contributed to the nearshoreover time is the first step toward development of a coastalsediment budget for bluff-backed beaches using datasources including aerial photography (1939, 2001), GPS-based beach profile data (2010–2013), and airborneLiDAR (2001, 2012). These data are analyzed in contextto determine alongshore rates of bluff retreat andassociated volume change for the Elwha and Dungenesslittoral cells in Clallam County, WA. Recession ratesfrom 2001 to 2012 range from 0 to 1.88 m/yr in both driftcells, with mean values of 0.26 ± 0.23 m/yr (N = 152) inElwha and 0.36 ± 0.24 m/yr (N = 433) in Dungeness.Armored sections show bluff recession rates reduced by50 percent in Elwha and 80 percent in Dungeness, relativeto their respective unarmored sections. Dungeness bluffsproduce twice as much sediment per alongshore distanceas 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. Ratesderived from short timescales should not be used directlyfor predicting decadal-scale bluff recession rates formanagement purposes, as they tend to represent short-term localized events rather than long-term sustainedbluff retreat.

INTRODUCTION

Coastal bluffs are a dominant geomorphic featureof the shorelines of the Strait of Juan de Fuca,

Washington State, USA, and are the primary sourceof sediment contributed to mixed sand and gravelbeaches in the region (Schwartz et al., 1987;Shipman, 2004; Finlayson, 2006; and Johannessenand MacLennan, 2007). The spatial and temporaldistribution of bluff recession from wave-, wind-,precipitation-, and groundwater-induced erosion ispoorly understood and documented for the southernshore of the Strait of Juan de Fuca and has led tounderestimating the potential hazards to infrastruc-ture (e.g., roads, houses) posed by eroding bluffsover time (Figures 1 and 2). Efforts to protectinfrastructure and limit the rates of bluff erosionby constructing shoreline revetments have historical-ly ignored the physical and ecological effects ofsediment starvation of beaches caused by shorelinehardening (Shipman et al., 2010). The disruption ofsediment movement from bluffs to beaches hascaused the loss of suitable habitats for critical marinespecies, including forage fish and juvenile salmonids(Rice, 2006; Shipman et al., 2010; Shaffer et al., 2012;and Parks et al., 2013). The importance of under-standing the long-term littoral sediment budget hasbeen underscored by the recent removal of two damson the Elwha River and the subsequent introductionof approximately 6.4 3 106 m3 of sediment into thenearshore environment within the first 2 years(between September 2011 and September 2013) (Eastet al., 2014; Gelfenbaum et al., in review; andWarrick et al., in review).

Relatively few studies of coastal bluff recessionhave been completed for the shoreline areas of theStrait of Juan de Fuca, and the studies that have beencompleted have used a variety of methods, leading todifficulty in comparing results. In the Elwha littoralcell (herein referred to as ‘‘drift cell’’), the U.S. ArmyCorps of Engineers (USACE) completed an evalua-tion of bluff recession rates and sediment volumesupply to the nearshore environment as part of anenvironmental assessment for a shoreline armoringand beach nourishment project on Ediz Hook in PortAngeles (USACE, 1971). Using Government LandOffice and National Geodetic Survey shoreline maps,the USACE estimated a gradual reduction in bluffrecession rates from 1.5 m/yr (1850–1885) to 1.3 m/yr1Corresponding author email: [email protected].

Environmental & Engineering Geoscience, Vol. XXI, No. 2, May 2015, pp. 129–146 129

(1885–1926), decreasing to 1.1 m/yr (1926–1948) andthen to 0.2 m/yr (1948–1970). Each successivereduction in bluff recession rates since 1930 has beenattributed to construction and maintenance of a

multitude of shoreline armoring projects at the baseof the Elwha bluffs (USACE, 1971).

The USACE (1971) study also shows a reduction insediment volumes provided by the Elwha bluffs over

Figure 1. (A) Homes threatened by receding bluffs, Dungeness drift cell. (B) Seawall installed at bluff toe to protect Port Angeles CityLandfill from bluff retreat, Elwha drift cell.

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time. Prior to the construction of the Elwha Dam in1911, the estimated sediment supply from the bluffswas 2.22 3 105 m3/yr. After construction of the ElwhaDam and prior to construction of shoreline armoringalong the Elwha bluffs in 1929, the estimatedsediment supply from the bluffs was nearly the same,measuring 2.06 3 105 m3/yr. Between 1929 and 1961,when substantial shoreline armoring along the bluffswas installed and maintained, the bluff sedimentsupply decreased to 0.73 3 105 m3/yr. Following thecompletion of a major shoreline armoring projectalong the bluffs in 1961, bluff sediment supply wasestimated to have further declined to 0.31 3 105 m3/yr. The reduction of bluff-supplied sediment over thisentire time period, 1.91 3 105 m3/yr, represents an 85percent reduction in the coastal sediment supply toEdiz Hook (Galster, 1989), which is essentiallyequivalent to the pre-dam fluvial sediment supplyestimated by Randle et al. (1996).

Bluff erosion rates to the east of the Dungenessdrift cell along the Strait of Juan de Fuca wereevaluated through land-parcel surveys by Keuler(1988). Bluff recession rates of up to 0.30 m/yr andsediment production rates of 1–5 m3/m/yr wereobserved in areas exposed to wave attack associatedwith long fetches. On the west side of Whidbey Island,at the eastern limit of the Strait of Juan de Fuca,Rogers et al. (2012) determined long-term bluff

erosion rates of 0–0.08 m/yr using cosmogenic 10Beconcentrations in lag boulders to date shorelinepositions over time scales of 103–104 years.

In this study, estimates of short- and long-termbluff recession rates and associated sediment volumescontributed to the Elwha and Dungeness drift cellsalong the Central Strait of Juan de Fuca between1939 and 2012 are derived from historical aerialphotography, GPS beach profiles, and airborneLiDAR, and the relative contribution of bluff-derivedsediment supply to the nearshore, in the context of acoastal sediment budget recently rejuvenated by theremoval of two dams on the Elwha River, ispresented.

STUDY AREA

The study area is located on the southern shore ofthe Central Strait of Juan de Fuca near the city ofPort Angeles, WA (Figure 2). The study area isdivided into two distinct shoreline segments thatencompass separate but adjacent littoral cells withbluff-backed beaches: the Elwha bluffs extend alongthe central portion of the Elwha drift cell, and theDungeness bluffs extend along the western portion ofthe Dungeness drift cell (Figure 3). Each drift cellcontains an updrift segment of eroding coastal bluffsto the west that supply sediment via longshore littoral

Figure 2. Map of the study area showing direction of net alongshore sediment transport within the Elwha and Dungeness drift cells inClallam County, WA.

Coastal Bluff Recession


transport to long spits at the down-drift end to theeast.

The Elwha bluff segment is 4.9 km long andsupplies sediment to Ediz Hook. The Dungeness bluffsegment is 13.6 km long and supplies sediment to

Dungeness Spit. A fundamental difference betweenthe two drift cells is that the Elwha River dischargesinto the Strait of Juan de Fuca updrift of the Elwhabluffs, while the Dungeness River empties into theStrait of Juan de Fuca on the lee side of Dungeness

Figure 3. (A) Photograph of the Dungeness bluffs looking west from Dungeness Spit. (B) Photograph of the Elwha bluffs west from EdizHook. Note the armoring placed mid-beach in front of the bluffs in photograph B.

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Spit (Figure 2). Therefore, the Elwha drift cell iscomposed of both river- and bluff-derived sediments,while the Dungeness drift cell is composed of onlybluff-derived sediments.

The Strait of Juan de Fuca is a wind-dominatedmarine system that exhibits net easterly longshoresediment transport within the intertidal zone of thestudy area (Galster and Schwartz, 1989; Schwartz etal., 1989; Warrick et al., 2009; and Miller et al., 2011).Winds in the Central Strait of Juan de Fuca aredominantly west and northwesterly, with a minorcomponent of north and northeasterly winds (Milleret al., 2011). Therefore, both the Elwha and Dunge-ness drift cells exhibit net easterly littoral sedimenttransport (USACE, 1971; Galster and Schwartz,1989; and Schwartz et al., 1989).

The wave climate of the Central Strait of Juan deFuca is similarly dominated by west to northwestwind waves and west to northwest swells from thePacific Ocean. Maximum wave heights within thestudy area range up to 3 m, whereas average heightsare 0.5 m (USACE, 1971; Gelfenbaum et al., 2009;Warrick et al., 2009; and Miller et al., 2011).Gelfenbaum et al. (2009) have modeled the distribu-tion of significant wave heights within the CentralStrait of Juan de Fuca, and given a 2-m swell at theentrance to the Strait of Juan de Fuca, nearshorewave heights of 1 m are shown throughout the studyarea, but with significant alongshore variability inwave height due to wave focusing or sheltering and inwave direction due to refraction.

Tides within the Strait of Juan de Fuca are mixed-diurnal, with two high and low tides per day. Tidalelevations range between 21.0 m and +3.7 m inelevation (NAVD 88) (Zilkoski et al., 1992; NOAA,2013).

A precipitation gradient exists from west to eastwithin the study area as the result of a rain-shadoweffect of the Olympic Mountains. Average annualprecipitation (1971–2000) in the Elwha drift cell is660 mm vs. 406 mm in the Dungeness drift cell(Drost, 1986; NCDC, 2014). Maximum rainfallintensities within the Elwha drift cell are 117 mm/hrvs. 71 mm/hr in the Dungeness (Drost, 1986; NCDC,2014). Precipitation occurs primarily as rain, with thewettest months between October and April and aseasonal dry period between May and September.Freezing temperatures occur within the study areabetween October and May, and snowfall intermit-tently occurs in the period between November andApril.

The surficial geology of the study area is domi-nantly composed of Pleistocene continental glacialdeposits overlying pre-Fraser non-glacial sedimentsassociated with an Elwha River source (Schasse et al.,

2000; Polenz et al., 2004) and Eocene marinesedimentary rocks (Schasse et al., 2000; Schasse andPolenz, 2002; Schasse, 2003; and Polenz et al., 2004).Pleistocene glacial deposits occurring within the studyarea include recessional outwash, glaciomarine drift,and glacial till.

Groundwater recharge occurs along the OlympicMountains and discharges into the Strait of Juan deFuca. Local groundwater recharge occurs within low-elevation glacial landforms adjacent to the coastalbluffs and discharges at varying elevations on thebluffs controlled by local aquitards (i.e., beds of low-permeability materials composed of dense silt, clay,and till) (Drost, 1986; Jones, 1996).

The shoreline within the study area exhibits steeplysloping to vertical and overhanging coastal bluffs upto 80 m high created by changes in relative sea levelfrom post-glacial rebound following Cordilleranglacial retreat; erosion of the shoreline in the studyarea began around 5,400 years before the present time(Downing, 1983; Dethier et al., 1995; Booth et al.,2003; Schasse, 2003; Mosher and Hewitt, 2004; andPolenz et al., 2004).

Bluff recession within the study area is dominatedby shallow landsliding in the form of topples, debrisavalanches, flows, and slides (Varnes, 1978). Othertypes of gravitational failures are also present,including stress release fracturing (Bradley, 1963),cantilever, and Culmann-type (near-vertical planar)failures (Carson and Kirkby, 1972). These types ofshallow mass wasting processes are common in seacliffs composed of weakly lithified sediments (Hamp-ton, 2002). Aeolian erosion during dry periods (in theform of ravel) is also observed. Aerial-, boat-, andground-based surveys of the study area have deter-mined the absence of deep-seated (Varnes, 1978)landslides consistent with existing geologic mapping(Schasse et al., 2000; Schasse and Polenz, 2002;Schasse, 2003; and Polenz et al., 2004). Processesdriving shallow landsliding include over-steepeningand subsequent failure of bluffs from wave-inducederosion at the bluff-base and the development of highpore-water pressures within hillslopes during storms.

Land use above the bluffs varies throughout thestudy area from dense urban development in theElwha drift cell within the City of Port Angeles tonative second-growth forest within the Dungenessdrift cell. Vegetation within the study area rangesfrom dense stands of mature second- and third-growth Douglas fir forest to open grass associatedwith urban lawn-scapes.

The sediment budget of the Elwha drift cell hassubstantially declined as a result of human-inducedchanges. The construction of coastal revetmentsbegan in the Elwha drift cell shortly after the



construction of two dams on the Elwha River in theearly 20th century (Galster, 1989). In 1929, a coastalrevetment was installed between Dry Creek and EdizHook to protect an industrial waterline that suppliedwater from the Elwha River to paper mills on EdizHook. Within 6 years of the placement of coastaldefense works, Ediz Hook began to erode as a result ofthe reduction in sediment supply from bluffs (Galster,1989). Galster (1989) estimated that in the Elwha driftcell, 15 percent of the sediment supplying Ediz Hookoriginated from the Elwha River, and 85 percent wassupplied from coastal bluff erosion prior to construc-tion of Elwha River Dams and coastal revetments.Galster (1989) estimated that coastal armoring in theElwha drift cell resulted in an 89 percent reduction ofsediment volume supplied to Ediz Hook. In 1975, theUSACE and the City of Port Angeles armored theshoreline of Ediz Hook and began a program of beachnourishment that continues to the current time. In2005, the City of Port Angeles constructed a 122 m–long concrete, steel, and rock seawall at the PortAngeles Landfill. Currently, 68 percent of the Elwhabluffs are armored with rip-rap or constructedseawalls. In contrast, less than 1 percent of the lengthof the Dungeness bluffs is armored.

In 2012, the Elwha Dam on the Elwha River wascompletely removed, and, as of 2014, the GlinesCanyon Dam has also been completely removed,resulting in the delivery of 6.4 3 106 m3 ofpredominantly fine sediment to the nearshore of theElwha littoral cell within the first 2 years since damremoval began in September 2011 (East et al., 2014;Gelfenbaum et al., in review; and Warrick et al., inreview). This sediment volume represents approxi-mately 30 percent of the total sediment stored in bothreservoirs. It is estimated that within 7–10 yearsfollowing the complete removal of both Elwha RiverDams, the long-term annual sediment contributionfrom the Elwha River to the nearshore will beapproximately 2.5 3 105 m3/yr (Gilbert and Link,1995; Bountry et al., 2010).

Understanding the relative contribution of blufferosion to the overall sediment budget of the Elwhadrift cell will help with efforts to manage the long-term coastal environment once the reservoir sedi-ments released by dam removal have been transport-ed out of the fluvial network and into the Strait ofJuan de Fuca.

METHODS

Bluff-Face Change Mapping

Short- and long-term coastal bluff recession ratesfor the Elwha and Dungeness drift cells were

determined by analyzing data from historical aerialphotographs and existing airborne LiDAR data. Inorder to make comparisons of the bluffs between thetwo data types, two-dimensional cross-shore transectswere established in each drift cell at 30-m intervals,except where interrupted by coastal streams orravines (Figure 4). Transects extend across the beachand up the bluff face, to at least the bluff crest, alongwhich retreat distances could be calculated. Bluffretreat was measured between consecutive surveys atthe bluff crest for aerial photos and at selectedelevations across the bluff face for LiDAR data.

Long-Term Bluff Change

Bluff recession rates for 1939–2001 were deter-mined by calculating the distance between bluff crestpositions on geo-referenced historical aerial photo-graphs. Prior to analysis, aerial photographs werescanned, geo-referenced, and imported into ArcGISv. 10.1 (ESRI, Redlands, CA), and bluff crestpositions were digitized for study segment areasunobstructed by vegetation. Distances between the1939 and 2001 bluff crest positions were measured ateach transect location.

Recession rates for 2001–2012 were determinedfrom the differences in horizontal position of selectedelevations on bluff-face profiles extracted from digitalelevation models (DEMs) available from recentairborne LiDAR data sets using methods outlinedin Hapke (2004), Young and Ashford (2009) andYoung et al. (2009, 2010, 2011). For this analysis, weused a 2001 bare earth DEM (2-m grid) from thePuget Sound LiDAR Consortium (PSLC, 2001) thatcovered the entire survey area, 2012 Clallam CountyLiDAR (1-m grid; Yotter-Brown and Faux, 2012) forthe Dungeness drift cell, and 2012 LiDAR data (0.5-m grid) from the U.S. Geological Survey (Woolpert,2013) for the Elwha drift cell. DEMs were importedinto ArcGIS and evaluated using the 3D Analysttoolset. At each transect location a two-dimensionaltopographic profile from the mid-beach to the bluffcrest was extracted from each DEM. The nethorizontal distance between the two profiles wasmeasured at 6-m vertical intervals between thebottom and top of the bluff face. The difference intotal cross-sectional area between the 2001 and 2012topographic profiles was measured and multiplied bya unit width to estimate a volume of sediment lostbetween the two DEMs.

Statistical evaluation of the data for bluff recessionand sediment volume contributions from the airborneLiDAR DEMs was performed using exploratory dataanalysis methods (Schuenemeyer and Drew, 2011).Bluff recession distance values were tested for spatial

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Figure 4. Map showing bluff and beach transect locations for the Elwha bluffs (A) and Dungeness bluffs (B5west, C5east).



trend and normalized using a lognormal transforma-tion. Summary statistics were then computed usingthe de-trended values. Sources of error includeinternal error in the LiDAR data acquisition andprocessing technique as well as differences in grid sizeof the LiDAR-derived DEMs.

Beach Profile Change Monitoring

To assess general trends in beach elevation change(m/yr) and to estimate rates of sediment flux on thebeaches (m3/m/yr), two-dimensional, cross-shore to-pographic beach profiles at 12 locations, eight alongthe Dungeness bluffs and four along the Elwha bluffs,were surveyed between 2010 and 2013 with a Pro-Mark 800 and 200 Real-Time Kinematic GlobalPositioning System (RTK-GPS). Elwha and Dunge-ness drift cell beach profiles were collected in allseasons. Profiles were oriented normal to the slope ofthe beach, extending from the base of coastal bluffs tothe low water limit. Elevation measurements wererecorded along each transect at horizontal intervals ofapproximately 1.5 m. RTK-GPS measurement accu-racy ranged from 1 to 5 cm based on repeatmeasurements of fixed control points across the studyarea.

Sediment volume changes were calculated using theupper 20 m of each profile, which was the extent ofoverlap between all surveys. The elevation differencebetween each pair of profiles was calculated every0.5 m, with a linear interpolation between the original

1.5-m data point spacing. The difference values alongthe entire transect were averaged to yield a singlevalue of average elevation change per transect. Theaverage elevation change was multiplied by the 20-mlength of the profile and an alongshore unit width of1 m to yield a volume change per alongshore meter(m3/m) for the 20 m of upland beach.

RESULTS

Bluff-Face Change

Long-Term Bluff Change

Observed rates of coastal bluff recession are highlyvariable across both drift cells (Figures 5–7). Table 1provides data results from sections of each drift cellwith unobstructed views of the bluff edge in aerialphotography from 1939 and 2001 and includesidentical shoreline reaches used for a comparison ofrates derived from airborne LiDAR from 2001 and2012. The data show a recent decrease in meanrecession rates in the Elwha drift cell (20.22 m/yr)and a slight increase in mean recession rates in recentyears in the Dungeness drift cell (+0.1 m/yr).

Table 2 provides data results that extend alongthe full length of the bluffs in each drift cell. Themaximum observed rate of recession between 2001and 2012 in both drift cells was 1.88 m/yr, associatedwith housing development in the Dungeness drift cell(Figure 1A) and erosional hotspots along the Port

Figure 5. Maximum observed bluff recession rates (m/yr) in the Dungeness drift cell for the time periods 1939–2001 (derived from aerialphotography) and 2001–2012 (derived from airborne LiDAR).

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Figure 6. Maximum observed bluff recession rates (m/yr) in the Elwha drift cell for the time periods of 1939–2001 (derived from aerialphotography) and 2001–2012 (derived from airborne LiDAR).

Figure 7. Box plot of recession rates (m/yr) by drift cell and shoreline type (created in ABOXPLOT; Bikfalvi, 2012). The central line withinthe box represents the sample median, while the circle represents the sample mean. The upper and lower limits of the box represent the 50thpercentile of the population and the whiskers the 75th percentile. Dots beyond the upper and lower whiskers represent outliers ofthe population.



Angeles landfill revetment in the Elwha drift cell(Figure 1B). The mean recession rate in the Dunge-ness was 0.36 m/yr vs. 0.26 m/yr for the Elwha driftcell (Table 2) for the 2001–2012 period.

In both drift cells, armored sections of bluffsshowed significantly lower rates of recession thandid unarmored sections: 80 percent less in theDungeness drift cell and 50 percent less in the Elwhadrift cell (Table 2 and Figure 7). Unarmored bluffsections demonstrated very similar mean rates ofrecession between drift cells: 0.37 m/yr for Dungenessand 0.40 m/yr for Elwha (Table 2 and Figure 7).Unarmored sections of bluffs directly down-drift andadjacent to armored sections experienced the highestrates of bluff recession in the Elwha drift cell (1.88 m/yr) and higher than mean rates (1.0 m/yr) in theDungeness drift cell (Figure 7).

Sediment volumes eroded from bluffs in the Dunge-ness drift cell were almost double those observed in theElwha drift cell per transect (Table 3 and Figures 8–10). The mean sediment production rate in theDungeness drift cell was 25.4 m3 per transect vs.13.8 m3 per transect in the Elwha drift cell. Rates ofsediment production from unarmored sections ofbluffs were similar between drift cells. Mean valuesfor sediment production from unarmored sections ofbluffs in the Dungeness drift cell were 25.8 m3 pertransect vs. 22.0 m3 per transect for the Elwha drift cell(Table 3). Sediment production rates for armoredsections of bluffs were twice as high in the Elwha driftcell (11.9 m3 per transect) compared to the Dungenessdrift cell (5.8 m3 per transect) (Table 3 and Figure 10).

At the drift cell scale, the Dungeness bluffsproduced approximately five times the volume of

sediment of the Elwha bluffs, on average (1.03 3

105 m3/yr vs. 2.0 3 104 m3/yr, respectively), on anannual basis over the 2001–2012 period (Table 4).When normalized for length, the Dungeness bluffscontributed approximately 55 percent more sedimentthan did the Elwha bluffs to the nearshore (7.5 m3/m/yr vs. 4.1 m3/m/yr, respectively) on an annual basisfor the 2001–2012 period (Table 5).

Beach Sediment Volume Changes

Annual beach sediment volume changes as well asthe net 3-year change at the 12 transect locations(eight along the Dungeness bluffs; four along theElwha bluffs) are shown in Figure 11 and Tables 6and 7. With the exception of transect EB-1 (where theeffects of sediment supply from the Elwha River areevident), the general trend in beach sediment volumehas been one of net loss over the 3-year periodoccurring between 2010 and 2013.

In the Elwha drift cell, annual beach transectelevation changes ranged from 20.72 (net loss) to+1.19 m/yr (net gain) (mean 5 20.13 6 0.52 m/yr). Thegreatest loss at all Elwha transects occurred during the2010–2011 period. In the Dungeness drift cell, annualbeach transect elevation changes ranged from 21.05 m/yr to +0.22 m/yr (mean 5 20.19 6 0.29 m/yr).

DISCUSSION

Bluff Recession Rates

Rates of bluff recession observed in this study inthe Elwha drift cell generally agree with rates

Table 1. Recession rates (m/yr) from aerial photography (1939–2001) and airborne LiDAR (2001–2012) for unobstructed bluff-edge reachesof each drift cell.

Drift Cell PeriodMinimum

(m/yr)Mean(m/yr)

Maximum(m/yr)

Standard Deviation(m/yr)

No. ofTransects Length (m)

Dungeness 1939–2001 0.0 0.40 1.00 0.20 181 5,6392001–2012 0.1 0.50 0.90 0.17 181 5,639

Elwha 1939–2001 0.2 0.42 0.60 0.10 75 2,4692001–2012 0.0 0.20 0.55 0.10 75 2,469

Table 2. Recession rates (m/yr) by drift cell and shoreline type, 2001–2012.

Drift Cell Shoreline TypeMinimum

(m/yr)Mean(m/yr)

Maximum(m/yr)

Standard Deviation(m/yr)


Dungeness Unarmored 0.0 0.37 1.88 0.79 423 13,320Armored 0.0 0.08 0.46 0.40 10 305All 0.0 0.36 1.88 0.24 433 13,625

Elwha Unarmored 0.0 0.40 1.88 1.30 60 1,829Armored 0.0 0.21 0.58 0.40 92 3,048All 0.0 0.26 1.88 0.23 152 4,877

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measured by the USACE (USACE, 1971) in theElwha drift cell in the decade between 1960 and1971,but they are elevated over those observed by Keuler(1988) in the Dungeness drift cell and are substan-tially higher than the long-term rates observed byRogers et al. (2012) for Eastern Strait of Juan de Fucashorelines. Rates of bluff erosion documented in thisstudy are also consistent with rates observed along thewest coast of the United States exposed to open-oceanwave climates (Collins and Sitar, 2008; Pettit et al.,2014). Rates of bluff recession observed between 2001and 2012 may represent higher-than-average erosionrates due to high storm frequency and intensityoccurring during this period: two time intervals, thewinters of 2007 and 2009, represent two of the wettestand windiest periods on record for this location(NCDC, 2014). Additionally, the 2001–2011 periodexperienced four high-tide events that exceeded the50-year recurrence interval for extreme high waterlevels in the Central Strait of Juan de Fuca (NOAA,2013).

Bluff recession rates observed in the Dungeness andElwha drift cells in this study have immediateapplication to land-use planning for residential and

commercial construction activities adjacent to thecoastal bluffs. Given a typical design life of a singlefamily home of 100 years, applying the observedmean bluff recession rates (Table 1) provides aminimum setback distance between a structure andthe edge of the bluff of 42 m in the Elwha drift celland of 40 m in the Dungeness drift cell, based onmean long-term (1939–2001) recession rates. It shouldbe noted that these rates of observed bluff recessionfall closely in line with estimates of 0.47 m/yrpublished for the Elwha drift cell by Polenz et al.(2004) and likely represent the long-term post-glacialaverage bluff recession rate for glacial deposits on thesouth shore of the Central Strait of Juan de Fuca.

Extending past observed bluff recession rates intothe future is likely a simplistic and inaccurate methodto determine future bluff recession (Hapke and Plant,2010). Probabilistic methods of predicting blufferosion (Lee et al., 2001; Walkden and Hall, 2005;and Hapke and Plant, 2010) that accommodatespatial and temporal variability could be applied tothe Dungeness and Elwha drift cells and would likelybe more accurate than using hindcast observationsof bluff recession to predict future erosion rates.

Table 3. Sediment volume contribution per transect (m3) by drift cell and shoreline type, 2001–2012.

Drift Cell Shoreline Type Minimum (m3) Mean (m3) Maximum (m3)Standard

Deviation (m3) No. of Transects Length (m)

Dungeness Unarmored 0.0 25.8 163.3 24.3 423 13,320Armored 0.0 5.8 9.6 3.8 10 305All 0.0 25.4 124.8 31.7 433 13,625

Elwha Unarmored 0.0 22.0 143.6 30.1 60 1,829Armored 0.0 11.9 41.2 7.9 92 3,048All 0.0 13.8 159.9 35.9 152 4,877

Figure 8. Sediment volume (m3) per transect in the Dungeness drift cell (2001–2012).



Figure 9. Sediment volume (m3) per transect in the Elwha drift cell (2001–2012).

Figure 10. Box plot of sediment volume contributions (m3/transect) by drift cell and shoreline type (created in ABOXPLOT; Bikfalvi,2012). The central line within the box represents the sample median, while the circle represents the sample mean. The upper and lower limitsof the box represent the 50th percentile of the population and the whiskers the 75th percentile. Dots beyond the upper and lower whiskersrepresent outliers of the population.

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However, the data necessary to employ these proce-dures (e.g., wave and tidal height distributions alongthe bluffs) are not currently available.

Sediment Volume Change

Annual sediment volume contributions within theElwha drift cell from this study (2.0 3 104 m3/yr;Table 4) are consistent with the flux of 3.1 3 104 m3/yr determined by USACE (1971). The calculatedlength-normalized rate of 4.1 m3/m/yr for the Elwhadrift cell is substantially less (255 percent) than therate observed (7.5 m3/m/yr) for the Dungeness driftcell, which is consistent with a previous study byKeuler (1988) that measured sediment contributionrates for the exposed areas of the Strait of Juan deFuca ranging between 6.0 and 12.0 m3/m/yr.

Bluff-supplied sediment volume estimates for theElwha drift cell from this study can help refine thecoastal sediment budget post–dam removal. Sinceshore-protection works in the Elwha drift cell willremain after the Elwha Dams have been removed, asignificant component of the Elwha drift cell sedimentbudget will remain impaired after the sediment supplyfrom the Elwha River has been restored.

Randle et al. (1996) estimates that the pre-damfluvial sediment contribution to the Strait of Juan deFuca was about 1.9 3 105 m3/yr. In the Elwha driftcell, the current upper estimate of annual sedimentvolume contribution to the nearshore from blufferosion is approximately 2.0 3 104 m3/yr–4.9 3

104 m3/yr (Table 4), or about 11–26 percent of thepre-dam annual sediment contribution from theElwha River. The current annual sediment volumecontribution from bluff erosion in the Elwha drift cellrepresents a 90 percent reduction from the 1911 pre-armoring estimate (2.2 3 105 m3/yr; USACE, 1971)but is roughly approximate to the 1960 post-armoringestimate (3.1 3 104 m3/yr; Galster, 1989).

Comparing the sediment production rates betweenthe Dungeness and Elwha bluffs demonstrates thelevel of impairment within the Elwha drift cell. Whennormalized for drift cell length, the Elwha bluffsproduce 56 percent less sediment volume than do theDungeness bluffs on an annual basis. Comparing themeasured rates of sediment production from bluffs(Table 5) versus sediment volume change in beachtransects (Tables 6 and 7 and Figure 11) demon-strates the imbalance in the sediment supply relativeto available sediment transport. In most years, theamount of available sediment volume contributedfrom bluffs to the beach is substantially less than theaverage rate of sediment loss, leading to beachlowering and resulting in accelerated bluff erosion.

Management Implications

Bluff recession rates were shown to vary dependingon the time of measurement and length of timeobserved. It is not appropriate to extrapolate short-term measurements into long-term rates, especially ifthe length of measurement is less than the time spanof the rate being reported (e.g., producing an annualrate from ,1 year of observation). For instance, ameasurement taken over a month when there was alarge bluff failure could result in large overestimatesof bluff recession on an annual basis if there was nofurther change for the remainder of the year.Moreover, using the maximum measured recessiondistance to calculate an annual recession rate willresult in an even-greater overestimate and could givea false impression of how much the bluff is actuallyretreating. The maximum recession distance is mea-sured for a specific point along the bluff and may notrepresent the trends observed over the larger area. Itwould be more correct to calculate a mean bluffrecession distance for a given area measured over along period of time (i.e., years to decades). The long-term rates should then be qualified with the amountof recession that may occur during a given event (e.g.,the average maximum recession distance). As anexample, for land-use management, it would be moreappropriate to use a long-term mean recession rateover the horizon of interest to obtain a setbackdistance, with an added buffer based on event-scalerecession.

Table 4. Annual sediment volume contribution (m3/yr) by driftcell, 2001–2012.

Drift CellMean

(m3/yr)Mean + 1 StandardDeviation (m3/yr)


Dungeness 103,000 232,000 433 13,625Elwha 20,000 49,000 152 4,877

Table 5. Annual length-normalized sediment contribution (m3/m/yr) by drift cell, 2001–2012.

Drift Cell Mean (m3/m/yr)Mean + 1 StandardDeviation (m3/m/yr) Maximum (m3/m/yr) No. of Transects Length (m)

Dungeness 7.5 17.0 11.3 433 13,625Elwha 4.1 10.0 14.5 152 4,877



It should be emphasized that the bluff recessiondistances reported in this study are derived fromselected elevations across the bluff-face profile,which may not be seen by the homeowner at thebluff top. While the trends are not likely tosignificantly change, results will differ according tothe methods used to analyze bluff-face change.Other methods of calculating bluff recession dis-tances (e.g., contour change analysis, volume changeanalysis) are expected to provide different resultsthan the profile-based methods used herein, and thepotential to produce alongshore averaging of bluffrecession rates over appropriate alongshore lengthscales may result in less spatially variable rates that

are more conducive to land-use zoning, buffers, anddevelopment setbacks. The bluff-face profile methodhas the potential to accentuate the localized erosionsignals due to a lack of continuity along the bluff toenable alongshore averaging commensurate with theobserved signals of change obtained at finer scalealong the bluff face.

While land-use planners and coastal managers arein need of long-term erosion rates for prudentresource management, property owners experiencelocalized erosion and tend to be most interested inand concerned about the magnitude of bluff recessionoccurring along relatively small increments of spacealong their bluff-top property boundary.

Figure 11. Length-normalized sediment volume change (m3/m) in the highest 20 m of each beach topographic profile during four winter-to-winter time intervals. EB-1 through BL-1 were winter surveys; BL-2 through DB-4 were summer surveys. Note that intervals 1–3 are annual,whereas interval 4 spans 3 years.

Table 6. Beach topographic profile sediment volume changes for the Elwha drift cell. Note that the right-most column is net change between2010 and 2013, while all others are annual intervals.

2010–2011 2011–2012 2012–2013 2010–2013

ProfileVolume Change

(m3/m)Change Rate

(m/yr)Volume Change

(m3/m)Change Rate

(m/yr)Volume Change

(m3/m)Change Rate

(m/yr)Volume Change

(m3/m)Change Rate

(m/yr)

EB-1 213.54 20.69 2.42 0.12 24.89 1.19 13.77 0.23EB-2 27.77 20.40 20.17 20.01 5.82 20.28 213.77 20.23EB-3 212.33 20.66 1.87 0.09 22.06 20.11 212.66 20.22EB-4 211.88 20.72 0.41 0.02 21.52 20.08 212.98 20.23Average 211.38 20.62 1.13 0.06 3.87 0.18 26.41 20.11

Parks


Chronic sediment supply deficits in the Elwha driftcell due to shoreline armoring have resulted insignificant habitat impairment on intertidal beachesfor forage fish and juvenile salmonids (Shaffer et al.,2012; Parks et al., 2013). The removal of the twoElwha River Dams will restore a significant compo-nent of the Elwha littoral cell sediment supply. It iscurrently unknown to what degree and over whattimescale Elwha River sediments will be stored onintertidal beaches within the Elwha drift cell. Localshoreline managers have an unprecedented opportu-nity to optimize storage of Elwha River sediments onintertidal beaches through implementation of selectedshoreline armoring removal and large-woody debrisplacement strategies prior to the complete delivery ofElwha River reservoir sediments into the intertidalenvironment over the next 5–7 years.

In contrast to the impaired habitat function ofElwha drift cell due to sediment starvation fromshoreline armoring and Elwha River Dams, theDungeness drift cell exhibits less than 1 percent bylength armored shoreline and highly functioningforage fish spawning habitat (Shaffer et al., 2012;Parks et al., 2013). The intact littoral sedimentsupply processes from coastal bluff erosion withinthe Dungeness drift cell are maintaining suitableforage fish habitat (Parks et al., 2013) and expandingthe Dungeness Spit through sediment deposition(Schwartz et al., 1987).

CONCLUSIONS

Rates of coastal bluff recession in the Dungenessand Elwha drift cells over the 1939–2012 period werehighly variable in space and time and ranged between0.31 m/yr and 1.88 m/yr. Differences betweenmaximum near-term bluff erosion rates observedfrom 2001–2012 LiDAR and long-term (1939–2001)observations from digitized historical photography

were the result of individual medium-scale landslides.The presence of shoreline armoring is a controllingfactor on the rate of bluff recession, with armoredbluffs showing a reduced recession rate comparedwith unarmored bluffs. The volume of sedimentproduced by a unit length of unarmored bluffshoreline is greater than that of armored bluffs byfactors of two (Elwha) and five (Dungeness), respec-tively.

Sediment volumes contributed by bluffs in theElwha drift cell between 2001 and 2012 represent 11–29 percent of the estimated fluvial sediment contri-bution to the nearshore from the Elwha River prior todam construction in 1911. Annual sediment volumescontributed by bluffs in the Elwha drift cell between2001 and 2012 represent approximately 8–20 percentof the current estimate (Gilbert and Link, 1995;Bountry et al., 2010) of the long-term, post-damremoval annual fluvial sediment contribution to thenearshore from the Elwha River of about 2.5 3

105 m3/yr.This study confirms that alteration to bluffs, in this

case armoring, drastically affects bluff recession ratesand sediment volume contributions to the nearshore.Armored sections of bluffs showed significantly lowerrates (280 percent, Dungeness; 253 percent, Elwha)of recession than did unarmored sections. Unarmoredsections of bluffs directly down-drift and adjacent toarmored sections experienced the highest rates ofbluff recession in the Elwha drift cell (1.88 m/yr) andhigher than mean rates (1.0 m/yr) in the Dungenessdrift cell.

It was beyond the scope of this study to determinewhy there was a difference in sediment productionrates between the Elwha and Dungeness drift cells.Geology, groundwater effects, wave-approach angle,wave energy, and land use are all possible factorsexplaining the observed differences, and these shouldbe further investigated in future studies.

Table 7. Beach topographic profile sediment volume changes for the Dungeness drift cell. Note that the right-most column is net changebetween 2010 and 2013, whereas all others are annual intervals.

2010–2011 2011–2012 2012–2013 2010–2013

ProfileVolume Change

(m3/m)Change Rate

(m/yr)Volume Change

(m3/m)Change Rate

(m/yr)Volume Change

(m3/m)Change Rate

(m/yr)Volume

Change (m3/m)Change Rate

(m/yr)

BC-1 23.87 20.20 25.37 20.24 22.63 20.16 211.86 20.20BC-2 2.83 0.15 27.85 20.34 1.02 0.06 24.01 20.07BL-1 210.47 20.43 28.57 20.38 23.67 20.23 222.72 20.36BL-2 24.38 20.22 25.22 20.29 4.73 0.20 24.88 20.08DB-1 28.56 20.43 22.07 20.12 21.22 20.05 211.84 20.20DB-2 1.11 0.06 24.30 20.24 0.76 0.03 22.42 20.04DB-3 212.23 20.79 0.48 0.03 2.02 0.08 29.73 20.17DB-4 219.44 21.05 21.67 20.09 3.10 0.14 218.01 20.31Average 26.88 20.36 24.32 20.21 0.51 0.01 210.68 20.18



While wave run-up and erosion at the base ofcoastal bluffs is a dominant driving factor of erosionthroughout both drift cells, portions of each drift cellalso exhibited erosion in the upper one-third of thebluff profile driven by a combination of precipitation,local groundwater discharge, and relatively permeableglacial strata overlying impermeable glacial strata.The observed upper-bluff erosion driven by ground-water and precipitation appears to be spatially andtemporally isolated from wave erosion, especiallywhere shore protection works are in place, and thistrend will continue whether the shoreline is armoredor not.

At present, it remains challenging to make reliableprojections of bluff recession that may guide devel-opment setback distances for the future, given thecoarse resolution of a multi-decadal interval (aerialphotos for 1939–2001) and only one higher-resolutiondecadal interval (airborne LiDAR data for 2001–2012). The combination of chronic recession rates andevent-based erosion magnitudes is important fordecision makers, and the most reliable rates willcome from a longer-term high-resolution data set thatmust be developed over time.

The results of this study provide estimates forminimum setback distances between structures andbluff edges based on long-term mean recession ratesmeasured over the scale of an entire drift cell. Thistype of information provides the scientific basis thatland-use planners and government regulators needin order to develop sound long-term managementpolicies for bluff development.

Recession distances measured for a specific pointalong the bluff may not represent the trends observedover the larger drift-cell area and over a longer periodof time. It would be more correct to calculate a meanbluff recession distance for a given area measuredover a long period of time (i.e., years to decades). Thelong-term rates should then be qualified with theamount of recession that may occur during a givenevent (e.g., the average maximum recession distance).As an example, for land-use management, it would bemore appropriate to use a long-term mean recessionrate over the horizon of interest to obtain a setbackdistance with an added buffer based on event-scalerecession.

Repeat surveys performed at relatively shortintervals would enable a better determination of therelative importance of a variety of mechanismscontributing to bluff erosion, such as surface runoff(and associated land-clearing and development prac-tices), wind, precipitation, groundwater discharge,soil saturation, wave height and direction, total waterlevel, beach width and elevation, and littoral sedimentsupply. All of these factors play a role in bluff retreat

dynamics, and measurement of these parameterscombined with high-resolution bluff-face topographyand differences over time will enable the developmentof improved process-based bluff erosion models (Leeet al., 2001; Castedo et al., 2012).

ACKNOWLEDGMENTS

This study benefited from discussions with AnneShaffer (Coastal Watershed Institute), Jon Warrick(USGS), and Hugh Shipman (WDOE) on coastalprocesses and sediment budgets along the CentralStrait of Juan de Fuca. Jesse Wagner and WadeRaynes (Western Washington University) and ClintonStipek (University of Washington) provided field andtechnical support. Western Washington Universityand Peninsula College provided field equipment andstudent interns. Anne Shaffer (Coastal WatershedInstitute) provided vital overall support, coordination,and integration with other project components.

Diana McCandless, Washington Department ofEcology Coastal Mapping Program, provided analy-sis of beach erosion data. Heather Baron, MattBrunengo, Kerry Cato, Wendy Gerstel, AmandaHacking, Michael W. Hart, George Kaminsky, andKeith Loague provided helpful reviews.

We want to sincerely thank Ruth Jenkins, JohnWarrick, Chris Saari, Paul Opionuk, Pam Lowry,Connie and Pat Schoen, Hearst Cohen, MalcolmDudley, Nippon Paper, and the Lower ElwhaS’Klallam Tribe for access across private property.Dungeness National Wildlife Refuge personnel andvolunteers provided access and transportation.

Student interns were funded by the U.S. Environ-mental Protection Agency under grant number PC-00J29801-0 awarded to the Washington Departmentof Fish and Wildlife (contract number 10-1744) andmanaged by the Coastal Watershed Institute. Fund-ing for student interns and GPS equipment used tocollect beach profiles were provided by the ClallamCounty Marine Resources Committee and by theEnvironmental Protection Agency grant listed above.

Any opinions, findings and conclusions, or recom-mendations expressed in this material are those of theauthor and do not necessarily reflect the views of theEnvironmental Protection Agency or the WashingtonDepartment of Natural Resources.

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Bluff Recession in the Elwha and Dungeness Littoral Cells, Washington, USA · 2018-03-27 · Bluff Recession in the Elwha and Dungeness Littoral Cells, Washington, USA DAVID S. PARKS1