Airflow and sand transport variations within a backshore–parabolic dune plain complex: NE Graham Island, British Columbia, Canada

Geomorphology 77 (2006) 17 – 34 www.elsevier.com/locate/geomorph

Airflow and sand transport variations within a backshore–parabolic dune plain complex: NE Graham Island, British Columbia, Canada Jeffrey L. Anderson, Ian J. Walker ⁎ Boundary Layer Airflow and Sediment Transport (BLAST) Laboratory, Department of Geography, University of Victoria, PO Box 3050, Station CSC, Victoria, British Columbia, Canada, V8W 3P5 Received 17 December 2004; received in revised form 12 December 2005; accepted 22 December 2005 Available online 27 January 2006

Abstract Onshore aeolian sand transport beyond the beach and foredune is often overlooked in the morphodynamics and sediment budgets of sandy coastal systems. This study provides detailed measurements of airflow, sand transport (via saltation and modified suspension), vegetation density, and surface elevation changes over an extensive (325 × 30 m) “swath” of a backshore foredune–parabolic dune plain complex. Near-surface (30 cm) wind speeds on the backshore ranged from 4.3 to 7.3 m s− 1, gusting to 14.0 m s− 1. Oblique onshore flow is steered alongshore near the incipient foredune then landward into a trough blowout where streamline compression, flow acceleration to 1.8 times the incident speed, and increasing steadiness occur. Highest saltation rates occur in steady, topographically accelerated flow within the blowout. As such, the blowout acts as a conduit to channel flow and sand through the foredune into the foredune plain. Beyond the blowout, flow expands, vegetation roughness increases, and flow decelerates. Over the foredune plain, localized flow steering and acceleration to 1.6 times the incident speed occurs followed by a drop to 40% of incident flow speed in a densely vegetated zone upwind of an active parabolic dune at 250 m from the foredune. Sediment properties reflect variations in near-surface flow and transport processes. Well-sorted, fine skewed backshore sands become more poorly sorted and coarse skewed in the blowout due to winnowing of fines. Sorting improves and sands become fine skewed over the foredune plain toward the parabolic dune due to grainfall of finer sands winnowed from the beach and foredune. During the fall–winter season, significant amounts of sand (up to 110 kg m− 2) are transported via modified suspension and deposited as grainfall up to 300 m landward of the foredune. No distinct trend in grainfall was found, although most fell on the depositional lobe of the blowout and at 200 m near an isolated, active parabolic dune. Grainfall amounts may reflect several transporting events over the measurement period and the transport process is likely via localized, modified suspension from the crest of the foredune and other compound dune features in the foredune plain. This evidence suggests that the process of grainfall delivery, though often overlooked in coastal research, may be a key process in maintaining active dunes hundreds of metres from the shoreline in a densely vegetated foredune plain. The effectiveness of this process is controlled by seasonal changes in vegetation cover and wind strength as well as shorter term (e.g., tidally controlled) variations in sand availability from the beach. © 2006 Elsevier B.V. All rights reserved. Keywords: Aeolian; Dune; Grainfall; Saltation; Foredune; Parabolic dune; Coastal swath; Driftwood

⁎ Corresponding author. Tel.: +1 250 721 7347; fax: +1 250 721 6216. E-mail address: [email protected] (I.J. Walker). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.12.008

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1. Introduction Aeolian processes play a key role in the geomorphology of most sandy coastal systems by transporting sand delivered to the beach via littoral processes into the backshore. This sand is then stored within incipient and established dune systems and occasionally cycled back to the littoral system via coastal erosion and storm surges. Research on coastal aeolian dynamics to date has focused largely on several key areas including: (i) the influence of vegetation type and density on sediment entrainment and deposition (e.g., Hesp, 1981, 1983, 1984, 1989, 2002; Buckley, 1987; Sarre, 1989; Arens, 1996a; Arens et al., 2001a; Davidson-Arnott et al., 2003); (ii) incident wind angle and resultant beach fetch effect and sediment supply to coastal dunes (e.g., Jungerius et al., 1981; Nordstrom and Jackson, 1993; Arens et al., 1995; Davidson-Arnott, 1996; DavidsonArnott and Law, 1996; van der Wal, 1998; Jackson and Cooper, 1999; Bauer and Davidson-Arnott, 2003); (iii) topographic influences on near-surface wind speed and direction (e.g., Svasek and Terwindt, 1974; Rasmussen, 1989; Hesp and Hyde, 1996; Hesp and Pringle, 2001; Hesp et al., 2005; Walker et al., in press); and (iv) the effects of moisture content on rates of sand transport on beaches (e.g., Belly, 1964; Sarre, 1989; Kocurek et al., 1992; Namikas and Sherman, 1995; Arens, 1996b; van Dijk et al., 1996; Jackson and Nordstrom, 1997; Sherman et al., 1998; Wiggs et al., 2004). The influences of these factors on beach–dune sediment transport have been well documented, although largely independent of one another. However, with the exception of remotely sensed, GPR or topographic survey-based characterizations of morphological changes in coastal dune systems (e.g., Brown and Arbogast, 1999; Bristow et al., 2000; Andrews et al., 2002) and empirical and/or conceptual models of barrier island and beach–dune evolution and sediment balance (e.g., Armon and McCann, 1979; Hesp, 2002), relatively little research has been conducted on the process-response dynamics and controls of beach–dune systems at the “meso” scale (i.e., morphological responses at the landform assemblage scale over periods of hours to years) (Psuty, 2004; Sherman, 1995). Furthermore, sediment delivery well into the backshore (e.g., 10s to 100s of metres beyond the foredune) via suspended grainfall has received little attention in coastal research. To address this, the purpose of this study is to examine the influence of variations in vegetation and topography on airflow and sediment transport over a spatially extensive “swath” of a backshore foredune–parabolic

dune plain complex. The study site includes several distinct geomorphic units of driftwood jammed backshore, foredune, trough blowout and parabolic dune. Albeit limited in temporal scope, this study describes a typical onshore SE wind event representative of the formative winds in the study area. In addition, seasonal measurements of sediment deposition via suspended grainfall are presented, and implications for net sediment transport and dune maintenance are discussed. 2. Physical setting The study site is located 15 km south of Rose Spit on East Beach in Naikoon Provincial Park, NE Graham Island, Queen Charlotte Islands (Haida Gwaii) ∼80 km offshore of the central coast of British Columbia, Canada (54°N, 131°W, Fig. 1). The Naikoon Peninsula consists of a low plain of unconsolidated Quaternary glaciofluvial sediments (Clague et al., 1982) that, during a marine regression over the late Holocene, has been reworked by energetic littoral and aeolian processes (Barrie and Conway, 2002; Walker and Barrie, in press), leaving a series of relict shorelines and prograding foredune ridges (Fig. 2). Over the twentieth century, relative sea level has risen at a rate of + 1.6 mm a− 1 (Abeysirigunawardena and Walker, unpublished data) and the coastline of East Beach has retreated at 1 to 3 m a− 1 (Barrie and Conway, 2002) while the shores of North Beach have prograded at 0.3 to 0.6 m a− 1 (Harper, 1980). Littoral sediments are moved onshore by a highly competent wind regime dominated by strong SE winds in fall through winter and W–NW winds in summer (see wind rose in Fig. 1). Annual average wind speed is 8.5 m s− 1 with b1% calm conditions. For the period 1995– 1999, winds above the accepted sand transport threshold of 6 m s− 1 (Fryberger, 1979) occurred 67% of the time (Walker and Barrie, in press). Potential aeolian activity is high in the region with a total sand drift potential (DP) (per Fryberger, 1979) of 4566 vector units (VU) (Pearce, 2005). This is well above those documented for desert regions (80–489; Fryberger, 1979) and for parabolic dunes in the Canadian prairies (300–1600; Wolfe and Lemmen, 1999). The resultant drift potential vector (RDP) is 2967 VU aligned NW (316°), reflecting the dominant SE winds in the study area (Pearce, 2005). Despite a moist maritime (Cfb) climate and dense vegetation and forest cover, the high onshore sand supply in this wind regime maintains active parabolic dunes and foredunes (Figs. 2 and 3). East Beach is subject to semi-diurnal mixed tides ranging 5–7 m with HHWMT exceeding 7 m. Annual

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Fig. 1. Study site location on East Beach, Naikoon Provincial Park, NE Graham Island, Queen Charlotte Islands (Haida Gwaii), British Columbia. Wind rose (inset, upper right) derived from Environment Canada data from Rose Spit station (1995–1999) shows strong bimodal wind regime. Annual drift rose (inset, lower right) derived from the same data shows directional potential drift (DP) vectors and large resultant drift potential (RDP) vector toward the NW resulting from dominant, strong SE winds.

significant wave height (Hs) is 1.8 m and the peak period is 10 s. Higher values of Hs to 3.5 m occur in the shallower waters of Dogfish Banks along East Beach and prevail for 20–30% of the time during winter months (Eid et al., 1993; Thomson, 1981). Beaches in the study region are of the intermediate class (Masselink and Short, 1993) and are wave-tide-dominated (Anthony and Orford, 2002). Multiple, transverse nearshore bars occasionally weld to the shoreline (Figs. 2 and 3) and provide enhanced localized sand supply to the backshore (c.f., Anthony, 2000; Aagard et al., 2004). At low tide, as much as 250 m of exposed beach fetch that increases significantly to 500 m under oblique onshore SE winds (Fig. 3A). The study site is a 325 m deep × 30 m wide “swath” of the coastal landscape of East Beach. The site begins as a driftwood jammed backshore that extends ∼50 to 70 m landward from a 0.5-m high storm-cut scarp on the

beach to a low 0.3-m sparsely vegetated incipient foredune backed by a 5-m established foredune ridge (Figs. 3B and 5A). A 2.5-m deep trough blowout extends through the foredune for ∼30 m and adjoins a depositional lobe that continues 40 m onto a hummocky backshore foredune plain. An isolated, partly vegetated parabolic dune exists 175 m landward of the foredune with an approximate surface area of 0.2 ha. Vegetation cover in the study site ranges from 0% to 95% density and is dominated by three species of vegetation: Largeheaded sedge (Carex macrocephala), dune grass (Elymus mollis), and Pacific alkali grass (Puccinellia nutkaensis). Density of cover for these species is seasonally variable during the growth season from late April to late November. Tree species present include Sitka spruce (Picea sitchensis) and red alder (Alnus rubra). To date, few studies have considered the meso scale suite of geomorphic features and surface

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Fig. 2. Airphoto showing location of the study site on East Beach in the Naikoon Peninsula region of NE Graham Island, Haida Gwaii (Source: 1980 National Airphoto Library, photo #A25613-39).

3. Methods

grainfall trap (Fig. 4). Stations were installed in the beach backshore, foredune trough, depositional lobe, and foredune plain locations (Fig. 5B). Incident flow conditions are referenced to the backshore instrument station. Windspeed data from each ultrasonic station were normalized by measurements at the backshore station using

3.1. Airflow properties

U0:3 ¼ u0:3

characteristics encountered by onshore airflow and sediment in transport well into the backshore. This study examines flow and sand transport responses over a broader landform assemblage (i.e., a coastal swath) of dune form, surface roughness, and vegetation at both event-based and seasonal temporal scales.

Airflow properties were measured throughout the study site using two methods: (i) high frequency measurements from ultrasonic anemometers, and (ii) time-averaged flow vectors from precise handheld anemometers. High frequency wind speed and direction was measured from a transect of four Gill Windsonic anemometers at 30 cm above the surface (u0.3) sampled at 1 Hz. Co-located with each was a SAFIRE-type saltation probe (Baas, 2003), a Guelph-Trent wedge total flux trap (Nickling and McKenna Neuman, 1997), a surface elevation pin, and a suspended sediment

station x =u0:3 station 1

ð1Þ

where x is station location 1 to 4. As such, normalized U0.3 wind speeds provide a relative measure of flow acceleration and deceleration relative to incident flow conditions in the backshore. In addition, flow steadiness factor, Fs was derived for each station using the coefficient of variation Fs ¼ u0:3r =u0:3

mean

ð2Þ

where u0.3σ is the standard deviation of the wind speed dataset. As such, lower Fs values represent steadier flows.

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Fig. 3. Oblique airphotos of the study site showing the extent of the backshore and driftwood jam (A) and a closer view showing an outline of the study swath within the backshore–foredune–parabolic dune plain complex (B).

Time-averaged velocity vectors were measured at 50 cm (u0.5) at 39 locations over the study site (Fig. 5B) using Kestrel 1000 handheld anemometers. Average and maximum wind speeds (u0.5 mean and u0.5 max, respectively) were recorded from the instrument over a 20-s interval. Normalized U0.5 values for all locations were produced using Eq. (1) and u0.5 values. In that additional wind speed statistics were not available from the handheld instruments (e.g., σ), a flow gust factor (Fg) was calculated for each location using Fg ¼ u0:5

mean =u0:5 max

ð3Þ

As such, lower values of Fg represent gustier flow conditions. Flow vector direction was measured over the same interval from the alignment of flow streamers (flagging tape) attached to surface elevation pins at 50 cm using a Silva 2° surveying compass. Incident flow direction remained essentially constant (varying by only 5°) during this period. Given the relatively short measurement interval, not all frequencies of gusts may be captured in the gust factor. However, longer test intervals to 2 min at select locations revealed little difference in u0.5 values. Measurements were collected twice at all locations over the 5 h experiment and compared for representativeness.

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Fig. 4. Instrument stations showing “SAFIRE” style saltation sensors, grainfall trap, surface elevation pin, “Guelph-Trent Wedge” style total flux trap, and Gill Windsonic™ ultrasonic anemometer.

3.2. Sediment transport Two modes of sediment transport were measured in this study. First, quasi-instantaneous saltation intensity was measured using four SAFIRE-type saltation impact sensors (Baas, 2003), each inset 5 cm above the surface and co-located with an ultrasonic anemometer and total flux trap (Fig. 4). Saltation intensity was measured at the same frequency as wind speed (1 Hz). Second, grainfall of sediments in suspension was measured on staggered transects every 5 m inland from the foredune at 39 locations. Grainfall traps consisted of 10-cm sections of 10-cm I.D. PVC tubing attached to steel rods that also served as surface elevation monitoring pins (Figs. 4 and 5B). Traps were situated 1 m above the surface to exclude significant saltation inputs and a plastic sample bag was attached to the bottom of each trap to collect sediment. No grainfall was observed during the transport experiment in July, so traps were left in the field and were checked at 2 months (18 September 2003) and 7 months (15 February 2004). Only a very small amount of sediment (1 to 5 g) was observed in September at some locations, and traps were emptied in February. Amounts of sand captured in the 0.008 m2 sampling area were proportionally extrapolated to, and assumed to be representative of, the surrounding 1 m2 of surface

surrounding each trap over this period. Grainfall samples were dried at 130 °C for 72 h then weighed. Total dry weights were then converted to quantities of kilograms per square metre. 3.3. Vegetation density Vegetation density was measured at 61 plots within the study site, each 50 m2 (5 × 10 m) (Fig. 5B). For each plot, vegetation height, species and cover density were estimated. The cover of all species in each plot was to determine total density (i.e., vegetation cover by plant type was not distinguished). 3.4. Surface elevation change A network of 61 surface elevation monitoring pins was installed within the study area, in three separate and staggered transects, each ∼5 m apart (Fig. 5B). Pins consisted of a 1.5-m piece of 1/2″ steel rod with a zero mark set flush with the initial surface. Surface change after the 5-h experiment was measured from this line with a tape measure to a precision of 1 mm. A map of surface elevation change was produced initially using Surfer's® default inverse distance interpolation algorithm and then was modified manually in graphics software using point data to refine the map.

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Fig. 5. Digital elevation model of study site (A) showing distinct geomorphic regions within the study swath as well as sampling and instrument deployment layout (B).

3.5. Grain size variations Eight surface sediment grab samples were collected along the central axis of the study site in each

geomorphic region from the backshore to the parabolic dune (Fig. 5B). Samples were dried at 130 °C for 72 h then sieved at 1/4 ϕ intervals from − 1.0 to 4 ϕ. Grain size statistics (mean grain size, sorting, and

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skewness) were calculated from the frequency distribution of each sample in a spreadsheet program using the method of moments. 4. Results This study presents 5 h of airflow and sediment transport data measured during a SE storm on 19 July 2003. During this event, regional wind speed recorded at 5 m from the nearby BLAST02 met station (∼1 km north of the study site) was 5 to 10 m s− 1 (18 to 36 km h− 1) from the SE (125° to 130°). Near-surface wind speed measured at 30 cm on the beach backshore ranged from 4.3 to 7.3 m s− 1, gusting up to 14.0 m s− 1. Rain fell consistently during the experiment and measured amounts at the met station (11.8 mm total) are likely an underestimate because of under sampling of the rain gauge during high winds. Despite this, aeolian sediment transport was observed at some locations in the study site. 4.1. Near-surface flow vectors Normalized flow vectors (U0.3 and U0.5) throughout the study site are shown in Fig. 6A. Incident flow conditions averaged 5 to 10 m s− 1 from 125° to 130° during the study period. Backshore flow vectors show slight topographic acceleration (U0.5 approaching 1.1) and steering in the alongshore direction (N) in the vicinity of the incipient foredune. A minor deceleration occurs upwind of the established foredune to 0.9. After entering the trough blowout, flow accelerates from 1.3 to 1.8 times that of the incident flow at station 1, and flow vectors steer slightly up the north wall of the blowout. Beyond the blowout, flow decelerates to 0.6 due to flow expansion in the lee of the depositional lobe then accelerates gradually down the lobe to 1.1. Throughout the foredune plain, localized positive slope effects on a NE slope at 160 to 250 m cause slight northward topographic steering and acceleration from U0.5 = 1.1 to 1.6. As flow encounters the densely vegetated region upwind of the parabolic dune (40% to 95%, Fig. 6C) from ∼250 to 275 m, U0.5 values drop from 1.0 to 0.4. On the parabolic dune, U0.3 increases slightly to 0.6 on the stoss slope. 4.2. Flow gustiness (Fg) During the study period, incident wind speed on the backshore (u0.3 station 1) averaged 4.3 to 7.3 m s− 1 with gusts up to 14.0 m s− 1. An interpolated contour map of Fg derived from u0.5 values using Eq. (3) is presented in

Fig. 6B. Flow is slightly gusty at the dune toe (Fg = 0.78) and becomes steadier over the incipient foredune (0.88) and through the blowout (from 0.84 to 0.89). Flow becomes slightly gustier (0.80) at the break in slope from the blowout trough to the depositional lobe leeward of the foredune. Flow is gustier above the more densely vegetated surface of the foredune plain (0.80 to 0.72) while positive slope effects on the NE slope serve to increase steadiness (0.83 to 0.91). On the parabolic dune, flow is moderately gusty with Fg values from 0.79 to 0.82. 4.3. Vegetation density Vegetation density in the backshore ranges from 2% to 10% and increases to 20% on the incipient foredune (Fig. 6C). Vegetation cover on the established foredune increases from 20% to 30% at the toe to 50% at the crest. In contrast, vegetation density was low (∼2%) in the trough blowout. Both the depositional lobe and lower regions of the foredune plain within 50 m of the foredune crest have low vegetation cover (5% to 10%). Vegetation on the foredune plain generally increases in density from 25% downwind of the depositional lobe to a noticeable transition to N40% at 175 m. Higher hummocks and the NE slope in the foredune plain show higher vegetation densities from 30% to 60%. From 225 to 255 m, another distinct increase in vegetation cover occurs from 70 to 95, followed by a distinct drop in density toward the bare surface of the parabolic dune. The windward edge of the parabolic dune is bordered by several sparsely spaced Sitka spruce ranging 2 to 4 m high, while the lee side of the dune head is flanked by a dense (i.e., N85%) stand of Red alder (Figs. 3B and 7A). Spruce trees show significant abrasion damage and crown flagging by dominant transporting winds. Alder stands are sculpted on the parabolic dune head into a streamlined profile by abrading near-surface winds (Fig. 7B). Although not explored in this study, a distinct interaction exists between near-surface airflow and tree stands that exert some control on the morphodynamics of this parabolic dune. 4.4. Sediment transport and grainfall deposition Sand transport recorded from saltation sensors is plotted with normalized wind speed (U0.3) and flow steadiness (Fs) in Fig. 8. Over the period of the experiment, no saltation was recorded at stations 1 (backshore) or 4 (vegetated foredune plain). Station 2 at the mouth of the blowout recorded over 13,000 counts,

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Fig. 6. Normalized flow vectors throughout the study area (A), flow gust factor (Fg) derived from 50-cm wind speed measurements (B), and vegetation density map (C).

and station 3 had 2896 counts. Total flux traps captured insignificant amounts of transport, perhaps because of rainfall and the relatively small width of the trap opening.

Topographic forcing effects are evident in the nearsurface wind speed, flow steadiness, and sediment transport data. In general, an inverse relationship exists between wind speed and flow steadiness, and more

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Fig. 7. Oblique airphoto of vegetation associations with the parabolic dune including stands of Sitka spruce (Picea sitchensis) on the windward flanks and red alder (Alnus rubra) leeward of the head (A). Red alder stand on the head of the parabolic dune moves landward with the dune as saltation abrades the windward side of the stand (B), whilst avalanching and grainfall within the stand provide favourable conditions for growth on the leeward side.

sediment is moved in faster, steadier flows. For instance, as flow is forced toward and accelerates into the blowout at station 2, wind speed increases from U0.3 of 1.0 (station 1) to 1.6, flow becomes steadier (from Fs 0.3 to 0.22), and moves the most sediment. Downwind of the depositional lobe, flow at station 3 is slower and moderately gusty (Fs 0.27). Flow at station 4 is slower, but similar in gustiness to that on the backshore (Fs 0.29). No grainfall was observed during the experiment in July and only a very small amount (1 to 5 g) was observed in September at some locations. This suggests that the majority of grainfall at the site occurs during the fall–winter storm season. Between September 2003 and February 2004, at least two major storms occurred in Haida Gwaii — one in November 2003 and the second

on 24 December 2003 with SE winds approaching 110 km h− 1. In this wind regime, winds approaching 100 km h− 1 are relatively frequent and occur in most months on record (Walker and Barrie, in press) and are capable of transporting sand in suspension. Given the long duration of sampling and inability to identify the occurrence of transporting events, grainfall data are not normalized over a shorter time interval. Grainfall data are shown as an interpolated contour map in kilograms per square metre (Fig. 9B). Grainfall occurred only landward of the foredune, varying by two orders of magnitude from 1 to 110 kg m− 2 with an average of 12.69 kg m− 2. In general, no observable trend was found in grainfall deposition with distance from the foredune, though most sediment fell in two

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Fig. 8. Flow steadiness (Fs), normalized wind speed (U0.3), and saltation intensity (total impact counts) plotted against distance from the shoreline at stations 1 to 4.

locations. First, 108 kg m− 2 sediment fell in the immediate lee of the trough blowout (at the head of the depositional lobe) with quantities dropping rapidly to 11 kg m− 2 within 50 m of the foredune ridge. Only small amounts (3 to 5 kg m− 2) fell in the foredune plain region from ∼170 to 250 m. Second, nearing the parabolic dune (i.e., beyond 250 m) grainfall increases rapidly from 15 to 110 kg m− 2 between the arms of the dune just upwind of a small stand of Sitka spruce. On the exposed parabolic dune itself, grainfall was low (1 to 2 kg m− 2) and increased considerably to 84 kg m− 2 on the lee side slip-face within the Red alder stand. 4.5. Surface elevation change Changes in surface elevation were measured after the experiment at 61 monitoring pin locations that were initially set flush to the surface. An interpolated, spatially averaged contour plot of resulting areas of erosion (− values) and deposition (+ values) is shown in Fig. 9A. Quantities ranged from +9 to − 11 mm over the study area. In the backshore region, an average of 8 mm of deposition occurred, with slight erosion (− 2 mm) on the windward slope of the incipient foredune. In the lee of the incipient foredune, 8 mm of deposition occurred near the toe of the established foredune. Though little change in surface elevation occurred at the entry of the

blowout, most of the trough surface was erosional to a maximum of − 11 mm toward the head. On the depositional lobe, + 4 to +6 mm of deposition occurred to a distance of ∼50 m downwind of the blowout head. No measurable deposition was noted in the seaward region of the foredune plain from 160 to 200 m. In the densely vegetated region upwind of the parabolic dune (from 210 to 275 m), the surface was generally depositional although localized in amount. The southern (left) side of this zone experienced little change (± 1 mm), while the sloping north side showed + 9 mm at 210 m to − 8 mm on the stoss slope of a small dune feature with + 4 mm of deposition in the lee. Much of this surface change and deposition in this region is manifested in small (0.15 to 0.30 m high) shadow dune features (Fig. 10). At the toe of the parabolic dune slight erosion (− 2 mm) then deposition occurred on the upper stoss slope (+ 7 mm), erosion of − 5 mm at the crest, and deposition of +8 mm in the immediate lee. 4.6. Sediment properties Mean grain size (ϕ), sorting (σϕ), and skewness (sk) values from eight surface samples along the central axis of the study site are shown in Fig. 11. The upper beach sample is a poorly sorted (σφ = 1.367), fine skewed (sk = 0.212) medium sand (0.102 ϕ or 0.932 mm). All

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Fig. 9. Surface elevation change map derived from surface elevation pin data following the experiment (A) and spatially averaged grainfall derived from 7 months of collection during the fall–winter season (B).

aeolian sands, from the backshore to the parabolic dune, are medium in size (1.427 to 1.684 ϕ or 0.372 to 0.311 mm) and moderately well to well sorted (σϕ from 0.451

to 0.653). In general, as mean grain size increases, sorting (in ϕ units) becomes poorer. Sorting declines from the backshore sample 2 (σϕ = 0.484) into the

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Fig. 10. Small shadow dune features (0.15 to 0.30 m high) found in the densely vegetated backshore ∼225 m from the shoreline.

blowout throat (σϕ = 0.517) then improves slightly toward the parabolic dune, then declines to moderately well-sorted on the dune head. Skewness values progress from fine (positively) skewed on the beach to coarse (negatively) skewed in the foredune–trough blowout region perhaps from progressive winnowing of finer sands in this region. On the foredune plain, sediments

change from coarse skewed (σϕ = − 0.163) to symmetrical (σϕ = 0.052) then fine skewed on the parabolic dune (σϕ = 0.177). This shift to symmetrical and fine skewed with distance from the foredune may reflect grain fall deposits of finer sediments selectively winnowed and transported in modified suspension from the beach and foredune region.

Fig. 11. Variation in sediment properties with distance from the beach to the parabolic dune. A dashed line is provided to indicate 0 skewness (symmetrical).

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5. Discussion 5.1. Backshore The driftwood-laden backshore of East Beach poses an abrupt roughness on boundary layer airflow that extracts momentum and sediment during dominant oblique onshore SE winds. As such, driftwood jams act as a sink for coastal sediments in the backshore (Komar, 1976; Hesp, 1983, 1989; Walker and Barrie, in press). On the beach, wind action on typically coarser, poorly sorted sands winnows the finer (medium sand) fraction, which produces well-sorted backshore deposits. Aside from common supply- and transport-limiting factors that control the aeolian transport process (e.g., moisture content, vegetation cover), the rate of infilling of the driftwood matrix is also dependent on (i) the presence of shore-attached intertidal bars (Figs. 2 and 3A) that provide enhanced sediment supply (Anthony, 2000; Aagard et al., 2004) and (ii) tide stage during transporting events that controls effective fetch and potential sand transport (Wal and McManus, 1993; Jackson and Nordstrom, 1998; Hesp, 2002; Bauer and Davidson-Arnott, 2003). Although the rate of infilling is unknown, a considerable amount of sediment was transported into the backshore over the 7 months of the grainfall observations, completely filling some areas of the driftwood matrix. During this 7-month period, the storm cut scarp also retreated by ∼1 m in response to one of the highest storm surges on record (0.7 m) on 24 December 2003 (Abeysirigunawardena and Walker, unpublished data). This event occurred just after low tide, and the extent of wave runup at the site is unknown. Remnant scarps in the established foredune indicate that complete erosion of backshore driftwood jams by wave attack occurs on a longer timescale than our observations. Subsequently, driftwood and flotsam return, the roughness matrix rebuilds (in some areas nearing 2 to 3 m deep) and promotes the trapping of aeolian sediments. The erosion and rebuilding of sediment-laden driftwood jams is similar to Hesp's (1983, 1999) observations of incipient foredune rebuilding, although on a much larger scale; and in this case, vegetation is not required for dune growth within the matrix. This process is believed to be important for incipient dune formation, sediment cycling and storage on similar beaches in the NE Pacific (Walker and Barrie, in press). Such sediment stores may also serve as an important buffer against erosive winter storms and gradual sea-level rise. Airflow over the incipient foredune shows slight topographic forcing and acceleration. Nearing the

established foredune, flow is steered alongshore and decelerates in response to increasing vegetation cover (from 2% to 20%) and possible flow stagnation upwind of the established foredune. This promotes sediment deposition and growth of the incipient foredune as shown by net positive surface elevation changes and confirms other accounts from settings with little to no driftwood (e.g., Hesp, 1983, 1989; Rasmussen, 1989; Sarre, 1989; Arens, 1996b; Arens et al., 1995, 2001a). 5.2. Foredune–trough blowout As flow enters the blowout, it accelerates by as much as 1.8 times that of incident flow on the beach and is steered up to the wall of the blowout. Once flow enters the blowout, it becomes steadier and faster and promotes increasing saltation. This confirms similar observations by Hesp and Hyde (1996), Hesp and Pringle (2001) and suggests that blowouts act as transport “conduits” that channel flow and sediment from a variety of incident flow angles through the foredune into the foredune plain. In addition, flow speed in this region is inversely related to flow steadiness, which confirms Walker and Nickling's (2003) wind tunnel observations of accelerated flow toward the crest of an artificial dune. Furthermore, in the narrowest reach of the blowout where streamlines are most constricted, flow steadiness, speed, and surface deflation are greatest. These effects are reflected in the more poorly sorted, coarser (winnowed) sands in this region. Under drier conditions, enhanced sand transport toward the head of the blowout would occur, promoting continued erosion of the trough and increased sand delivery into the foredune plain. 5.3. Vegetated foredune plain Based on observations of as much as a 1000-fold decrease in near-surface wind speed, Arens (1996a) concluded that negligible amounts of sediment move beyond the crest of vegetated foredunes via saltation. However, the influence of vegetation on sand transport is clearly contingent upon plant density, distribution, morphology, and height as well as timing during the growth season (Hesp, 1989, 2002). In essence, the higher and denser the vegetation canopy, the greater the reduction in sediment transport. This study suggests however, that during the fall–winter season when vegetation density is lowest and storm winds are more frequent, significant amounts of sand are transported as far as 300 m beyond the foredune. It is unlikely that this results from full suspension of grains from the foredune crest over this distance. Rather, it is likely that grains are

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transported via localized, modified suspension from the crest of the foredune and other compound dune features further landward in the foredune plain. This evidence suggests that modified suspension at the landscape scale may play a key role in maintaining active dune forms on the foredune plain distant from the shoreline. Sediment channeled through the blowout is deposited initially on a sparsely vegetated depositional lobe downwind of the trough as flow expands and decelerates (Hesp and Hyde, 1996) to ∼60% of the incident flow. With distance down the depositional lobe, flow then gradually accelerates into the foredune plain. Local positive slope effects (e.g., the NE slope at 160 to 250 m) promote slight (northward) topographic steering and further acceleration of flow along the slope. Despite moderate to dense vegetation cover, sediment erosion and deposition occur causing surface elevation changes in some areas of the foredune plain in response to these topographically induced flow patterns. Localized jetting and erosion around vegetated dunes also occurs (see Fig. 3B) (c.f., Hesp, 2002) in the hummocky area between stations 3 and 4. Otherwise, as vegetation density increases from 40% to 95% deep into the foredune plain (beyond 200 m from the foredune), surface roughness increases thereby reducing near-surface windspeeds and sheltering the surface from wind action. 5.4. Parabolic dune In coastal environments, parabolic dunes often develop from the migration of depositional lobes of blowouts (Hesp, 1999, 2002). However, no evidence exists to suggest that this is the case for the parabolic dune at the study site. This dune appears to be maintained by sediment delivered via grainfall 150 to 200 m downwind of the foredune during highmagnitude events of moderate (seasonal) frequency. Its shape is partly controlled by a bio-geomorphic interaction between woody vegetation on the flanks and head of the dune that alters near-surface airflow and traps sediment on the dune (Fig. 7). In turn, the dune provides a distinct micro-environment with a different disturbance regime (i.e., wind and sand abrasion) and hydrology from that of the surrounding coastal plain. 5.5. Grainfall delivery Sediment transport via saltation has been researched extensively in beach and backshore settings over the past two decades (e.g., Hesp, 1983, 2002, 2003; Hesp and Hyde, 1996; Hesp and Pringle, 2001; Davidson-Arnott and Law, 1990, 1996; Arens, 1996a,b, 1997; Arens et

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al., 2001a,b, 2002; Jackson and Nordstrom, 1998; Davidson-Arnott et al., 2003). In contrast, sand delivery via grainfall in dune systems has received comparatively less attention, particularly in coastal research. Over transverse desert dunes, Hunter (1985) observed that the rate of decay in grainfall leeward of the crest could be described by a power function. Further work by Anderson (1988) found an exponential decay with deposition concentrated at 0.2 and 0.4 m from the brink on the lee slope. More recently, Nickling et al. (2002) also observed an exponential decline in grainfall deposition where up to 99% of total grainfall was deposited within 2 m of the crest for a variety of dune sizes and aspect ratios. Nickling et al. (2002) found, however, that Anderson's (1988) model, based on saltation trajectories, under-predicted grainfall rates by more than an order of magnitude. This was attributed to vertical lift and modified turbulent suspension of grains by secondary flows in the wake region that cause longer transport paths than those of true saltation, as observed by McDonald and Anderson (1995). Detailed wind tunnel measurements by Walker and Nickling (2002) confirm the presence of vertical lift and balanced vertical mixing in the immediate lee of transverse dunes. These recent observations indicate that saltation is not the sole mechanism for sediment delivery over and beyond dunes and that secondary lee-side airflow patterns (e.g., flow separation and reversal cells) have a significant effect on dune sedimentary dynamics. To date, very little research exists to document such effects over coastal dunes. No trend in grainfall deposition was found inland from the foredune, although these data reflect the influence of several transporting events over the period of study. For instance, large quantities farther inland may be the product of a few, less frequent storms, whilst high quantities in the lee of the foredune and blowout may reflect more frequent grainfall during lower magnitude events. McKenna Neuman et al. (2000) concluded that dune morphology is a product of the frequency and distribution of wind speeds above threshold as well as the nature of the regional wind regime. In this study, grainfall driven by higher magnitude (and relatively frequent) SE storm wind events contributes a great deal to sediment delivery and, thus, to dune maintenance throughout the foredune plain. During these events, nearsurface flow may separate from the foredune crest and from other compound dune features in the foredune plain to transport grains in modified suspension for hundreds of metres beyond the beach — an order of magnitude farther than Arens' (1996a,b) observations along the Dutch coast. In this environment, the amount of

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sediment delivered by this mechanism is appreciable and in the order of 10 to 100 kg m− 2 over a few months of the typically stormy fall–winter season. As the parabolic dune is located near a zone of high grainfall, it seems that modified suspension, coupled with flow–vegetation interactions (e.g., bluff body stagnation effects and sand trapping on the flanks of the dune), may be responsible for maintaining active dunes hundreds of metres from the shoreline in an otherwise vegetated foredune plain. 6. Conclusions This study presents extensive measurements of airflow vectors, vegetation density, sand transport, and seasonal grainfall delivery and surface elevation changes over a 325 × 30-m swath of a topographically complex backshore–foredune plain complex. The results indicate a clear need to consider onshore aeolian sediment transport well beyond the beach and foredune as potentially significant in the morphodynamics of sandy coastal systems. Key findings include: (i) Flow vectors are topographically steered and forced. Oblique onshore flow is steered alongshore over an incipient foredune, landward at the established foredune, and into a trough blowout. In the blowout, flow acceleration to 1.8 times the incident flow and increasing flow steadiness occur. Beyond the depositional lobe of the blowout, flow expands, vegetation roughness increases, and flow decelerates to 0.6 times the incident flow. Over the foredune plain, flow steering and acceleration to 1.6 times slight occur (due to positive slope effects) followed by a drop to 0.4 in a densely vegetated zone upwind of an active parabolic dune. (ii) Topographic forcing influences the relations between near-surface wind speed (U0.3), flow steadiness (Fs), and sand transport. High-frequency wind speed and transport intensity from saltation probes reveal an inverse relationship between U0.3 and Fs. More sediment is moved in faster, steadier flows within the trough blowout and, via topographic steering and flow acceleration, the blowout channels flow and sediment through the foredune into, and beyond, the foredune plain. (iii) Sediment properties (sorting and skewness) are influenced by topographic forcing and the mechanisms of sand transport. Well-sorted medium sands in the backshore become more poorly

sorted in the blowout throat, better sorted over the foredune plain, then more poorly sorted at the parabolic dune. Sands are fine (positively) skewed on the beach to coarse (negatively) skewed in the foredune–blowout region due to progressive winnowing of fines. On the foredune plain, sands are coarse skewed to symmetrical and progress to fine skewed toward the parabolic dune, perhaps because of increasing deposition of fines transported in modified suspension. (iv) Significant amounts of sand are transported in modified suspension and deposited (up to 110 kg m− 2) as far as 300 m beyond the foredune during the fall–winter season when vegetation density is low and storm winds are frequent. Although no observable trend in grainfall was found, most was deposited in two locations: (a) in the immediate lee of the foredune blowout and (b) upwind of an isolated, active parabolic dune ∼200 m landward of the foredune. This distribution may reflect the influence of several transporting events over the period of study and/ or localized separation and modified suspension from compound dune forms on the foredune plain. Thus, sand delivery via modified suspesion and grainfall coupled may be significant in maintaining active dunes hundreds of metres landward of the shoreline. Acknowledgements Thanks are extended to Kim Pearce for field assistance and to Dr. S.A. Wolfe for helpful contributions on research design and interpretation. Gratitude is also extended to the Council of the Haida Nation and to Naikoon Provincial Park staff Dan Bates and Lucy Stefanyk for access and logistical support. Support funding was provided by an NSERC operating grant and a Canadian Foundation for Innovation New Opportunities grant to IJW. Thorough reviews by Drs. Bernard Bauer and Karl Nordstrom also substantially improved this manuscript. References Aagard, T., Davidson-Arnott, R.G., Greenwood, B., Nielsen, J., 2004. Sediment supply from shoreface to dunes: linking sediment transport measurements and long-term morphological evolution. Geomorphology 60, 204–224. Abeysirigunawardena, D.S. and Walker, I.J., unpublished data. Sea level responses to climate variability and change in northern British Columbia, Canada. Manuscript in preparation. Anderson, R.S., 1988. The pattern of grainfall deposition in the lee of aeolian dunes. Sedimentology 35 (2), 175–188.

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