Aeolian fetch distance and secondary airflow effects: the influence of micro-scale variables on meso-scale foredune development

Aeolian fetch distance and secondary airflow effects 991

Copyright © 2007 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 33, 991–1005 (2008)DOI: 10.1002/esp

Aeolian fetch distance and secondary airflow effects:the influence of micro-scale variables on meso-scaleforedune developmentKevin Lynch,1* Derek W. T. Jackson2 and J. Andrew G. Cooper2

1 Department of Geography National University of Ireland, Galway, Ireland2 Centre for Coastal and Marine Research, School of Environmental Sciences, University of Ulster, Coleraine, Co. Derry, UK

AbstractUnsuccessful attempts to use process-scale models to predict long-term aeolian sedimenttransport patterns have long been a feature of aeolian research. It has been proposed thatone approach to overcome these problems is to identify micro-scale variables that are im-portant at longer timescales. This paper assesses the contribution of two system variables(secondary airflow patterns and fetch distance) to medium-term (months to years) dunedevelopment. The micro-scale importance of these variables had been established during pre-vious work at the site (Magilligan Strand, Northern Ireland). Three methods were employed.First, sand drift potentials were calculated using 2 years of regional wind data and a sedi-ment transport model. Second, wind data and large trench traps (2 m length ××××× 1 m width ×××××1·5 m depth) were used to assess the actual sediment transport patterns over a 2-monthperiod. Third, a remote-sensing technique for the identification of fetch distance, a saltationimpact sensor (Safire) and wind data were utilized to gauge, qualitatively, sediment trans-port patterns over a 1-month period. Secondary airflow effects were found to play a majorrole in the sediment flux patterns at these timescales, with measured and predicted ratesmatching closely during the trench trap study. The results suggest that fetch distance is anunimportant variable at this site. Copyright © 2007 John Wiley & Sons, Ltd.

Keywords: aeolian fetch distance; secondary airflow effects; sediment budget; scale

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 33, 991–1005 (2008)Published online 26 September 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1582

Introduction

Conventional methodologies applied in coastal aeolian research, and geomorphology in general, are usually scaledependent (Carter and Woodroffe, 1994; Phillips, 1999; Sherman, 1995). This presents a problem when attempting toextrapolate findings from one scale to another (either temporal or spatial) (Cooper and Pilkey, 2004). In an aeoliancontext, up-scaling through the use of process-based sediment transport models has been shown to produce results thatdiffer by several orders of magnitude (Sherman et al., 1998). Two reasons have been proposed for this situation: first,an inability to fully specify, measure and parameterize the transport system (Bauer et al., 1996), and second, as aconsequence of the inherent non-linearity, a system with many degrees of freedom may evolve to more than one finalstate as a result of a single forcing condition (Phillips, 2003). Alternatively, the same final state may result from amultitude of initial conditions (Beven, 2006).

A number of methodological approaches have been proposed to overcome the problem of scale linkage, for exam-ple embedding process-scale models in meso-scale sediment budget type approaches (Phillips, 1999; Sherman, 1995)or integrating field experiment methodologies across more than one scale (see, e.g., Aagaard et al., 2003). Anotherapproach is to identify particular micro-scale processes relevant to longer-term landscape development (Horn, 2002).These may then be used in specifying meso-scale models, essentially accepting scale independence; i.e., foreduneevolution is not predictable through the application of process-scale models.

Two primary controls on aeolian sediment transport at the study site, Magilligan Strand (Northern Ireland), areconsidered to be secondary airflow patterns (primary wind speed and direction altered by dune topography) and fetchdistance (the distance required from the leading edge of erodible material to a point downwind where sediment

*Correspondence to: KevinLynch, Department ofGeography, National Universityof Ireland, Galway, Ireland.E-mail: [email protected]

Received 4 December 2006;Revised 14 June 2007;Accepted 28 June 2007

992 K. Lynch, D. W. T. Jackson and J. A. G. Cooper


transport has reached a maximum value). The aim of this paper is to investigate these two variables with respect totheir relevance to meso-scale foredune development (moisture conditions were also monitored but are not discussedin detail here). Earlier work by the present authors established the micro-scale influence of these variables on sedi-ment transport at the study site; here, their relationship to meso-scale geomorphological evolution is assessed. Threemethods were employed. First, sand drift potentials were calculated using two years of regional wind data and asediment transport model. Second, wind data and large trench traps were used to assess the actual sediment transportpatterns over a two-month period. Third, a remote-sensing technique was utilized to gauge, qualitatively, sedimenttransport patterns over a one-month period.

Secondary Airflow PatternsOn Magilligan Strand secondary leeside airflows are particularly relevant; because the dominant wind is offshore, thesediment available for transport is nearly completely on the leeside, seaward of the foredune crest.

It was recognized at an early stage that understanding of sediment transport over flat surfaces might not easilytranslate to aeolian dunes (Bagnold, 1941). Study of airflow patterns over dunes and the implications for sedimenttransport has been largely carried out on desert dunes and in wind tunnel studies (e.g. Frank and Kocurek, 1996a,1996b; Lancaster et al., 1996; Neuman et al., 1997; Tsoar, 1983; van Boxel et al., 1999; van Dijk et al., 1999; Walker,1999; Wang et al., 2003). The basis for the majority of this work has been the Jackson and Hunt (JH) model (1975) offlow over gently sloping hills. More recently, advances in computational fluid dynamics (CFD) have been utilized inmodelling approaches (see, e.g., Parsons et al., 2004a, 2004b; Schatz and Herrmann, 2006). Field investigations ofairflow over coastal dunes are much less numerous.

Wind that encounters a dune, or hill, is expected to lose velocity as a result of pressure build-up at the base of theobstruction. As the air flows up the stoss slope, streamlines are compressed and a declining pressure gradient results inflow acceleration. The accelerated flow along the stoss slope can extend up to, and over, the crest. If flow becomesseparated at the crest a flow re-circulation cell may develop in the lee of the dune (Lancaster, 1995; Walker andNickling, 2002). Secondary airflow patterns in the lee of aeolian dunes are thought to be controlled primarily by duneshape (described as an aspect ratio: height/width) and wind approach angle (Sweet and Kocurek, 1990). Leesideairflow has been categorized into three distinct types (Figure 1): (a) attached and undeflected, (b) attached anddeflected and (c) separated (Sweet and Kocurek, 1990; Walker and Nickling, 2002). The first category occurs whereincident wind approaches at angles less than 10° and for lee slope angles below 20°, while incident wind angles above70° for dunes with aspect ratios greater than 0·2 experience separated flow. Unstable thermal atmospheric conditionscan also promote separation. Incident angles between 10 and 70°, or 70 to 90° for aspect ratios below 0·2 withatmospheric stability, promote attached and deflected secondary leeside airflow (Sweet and Kocurek, 1990, p. 1033;Walker and Nickling, 2002, p. 66). Characteristically leeside airflow exhibits velocity deceleration and an increase inthe variability of both wind speed and direction. This variability is taken as an indication of turbulent motion (Arenset al., 1995; Walker, 1999; Walker et al., 2003).

Leeside airflow patterns can play an important role in dune development. Proto-dune survival and growth rateshave been shown to correspond to the evolution of secondary airflow patterns on the lee faces. This involves thetransition from attached to separated airflow as dune size increases, with the dune acting more efficiently as a sandtrap as flow separation becomes more pronounced (Kocurek et al., 1992). Similarly, the maintenance of the elongatedshape of linear dunes has been shown to be dependent on lee flank sediment flux rates and flow deflection to crest-parallel (Wang et al., 2003). On the east coast of New Zealand the occurrence of large climbing dunes, in the lee ofhigh bedrock cliffs, has been directly related to large-scale leeside re-circulation cells. Locally, the prevailing offshorewinds are reversed, moving sediment in an onshore direction and supplying the dunes (Hesp, 2005). Recent work by

Figure 1. Secondary leeside airflow patterns are categorised into three types a) attached and undeflected, b) attached anddeflected and c) separated, with flow reversal within a recirculation cell possible.



the authors has shown that sharp-crested dune topography influences offshore flow to such an extent that secondaryflows in the lee of the dune, on the sub-aerial beach, no longer blow offshore. Offshore-directed flow becomesdeflected or reversed in the lee of the dune depending on the approach angle of the incident wind. This results insediment movement alongshore or onshore. The work has recorded foredune accretion and scarp infilling under theseoffshore conditions (Lynch et al., 2007).

Fetch distanceThe fetch effect is similar to the avalanching of snow; each grain entrained by the wind saltates a certain length beforereturning to the surface and dislodging more than one additional grain. The increase in saltation continues at anexponential rate downwind until a saturation point has been reached (Chepil, 1957; Chepil and Milne, 1939). At thispoint ‘sand movement near the ground carries all the vertical momentum flux from the wind’ (Gillette et al., 1996,p. 641). The distance required for this to occur is termed the critical fetch, with the total expanse of sediment availablefor transport termed the maximum fetch. If critical fetch exceeds maximum fetch, the sediment transport system doesnot operate at its full potential (Bauer and Davidson-Arnott, 2002).

Numerous studies have sought to quantify the critical fetch distance and have found it to range from seven to tensof metres (Bagnold, 1941; Davidson-Arnott et al., 2005; Nordstrom and Jackson, 1992; Svasek and Terwindt, 1974).Under certain conditions, any of the variables controlling sediment transport (e.g., moisture, surface lag deposits) canaffect the critical fetch, increasing the distance required for the transport rate to reach a maximum value (Davidson-Arnott et al., 2005; van der Wal, 1998). It could be argued that the role these additional variables play is to limit thesediment available for transport and therefore it is a sediment supply effect rather than the fetch effect that iscontrolling the transport rate. Jackson and Cooper (1999) found that critical fetch is an unimportant constraint onaeolian sediment transport when sediment supply is abundant. Over longer timescales maximum fetch has been shownto be a more accurate long-term indicator of sediment supply to foredunes than estimates based on wind velocity(Davidson-Arnott and Law, 1990, 1996).

Other factors can also influence the relationship between fetch distance and transport rate. During strong onshorewinds wave set-up reduces the source width of beaches and thus the fetch distance. Conversely, more moderateoblique onshore or alongshore winds that have greater fetches and may experience higher transport rates (Arens,1996). Oblique onshore wind also encounters lower gradient slopes than directly onshore winds, further enhancing thetransport flux (Bauer and Davidson-Arnott, 2002).

Sediment budget analysisOnly one medium-term study has thus far included the effects of secondary airflow patterns in sediment budgetcalculations. Bórowka and Rotnicki (2001) established relationships between nine local measurement stations and aregional weather station for wind speed and direction. They used this information to adjust annual budget calculationsbased on the regional data and found that these adjustments resulted in estimates that were close to the measuredquantity of sediment transport.

Illenberger and Rust (1988) utilized two methods to calculate a sand budget for the Alexandria coastal dunefield.One was based on dune migration rates and the other employed regional wind data and a sediment transport equation.The latter approach produced a resultant drift potential vector as defined by Fryberger (1979), giving an annualdirection and magnitude of transport potential that was successfully extrapolated to account for the present dayvolume of the dunefield.

Sarre (1989) used a sediment transport equation and onshore wind data to calculate monthly potential transportrates, which were compared with measured volumes. The results showed that actual transport quantities were signifi-cantly smaller that those predicted. Goldsmith et al. (1990) also compared measured and predicted rates of sedimenttransport along the Israeli coast. They corrected regional wind data based on measured local wind speeds, while usingjust four wind directions (not adjusted). Again, measured and predicted rates did not compare favourably but themeasured annual budget was similar to quantities at other sites in the USA and South Africa.

Study Site

As with many aeolian sediment transport studies a conscious decision was made to choose a site that exhibited verylittle variation within as many variables as possible. Variables exhibiting little spatial or temporal variability couldthen be reasonably assumed to be insignificant in controlling the variability of measured sediment transport rates.



Figure 2. Study site at Magilligan Strand and weather station location.

Furthermore, carrying out both the micro- and meso-scale studies at one site would reduce spatial variability anduncertainty. Magilligan foreland has long been used as a research site in coastal dynamics, and sediment size andsorting is consistently reported as being well sorted fine-grain sand. Small-scale variations do occur, but the sedimentwithin the coastal cell is essentially homogeneous (Carter and Stone, 1989; Carter and Wilson, 1990; Jackson andCooper, 1999). Vegetation at the site is usually confined to the foredune but on occasion can appear on the back beach.Other sand trapping debris (strandlines etc.) is relatively sparse. The vegetation in the foredunes is of such a densityand height that the vast majority of sediment entering the dune is trapped, to be released again only by wave inducederosion.

The study site was located on Magilligan Strand, Co. Derry, Northern Ireland (Figure 2). The beach is approxi-mately 6 km in length and extends from Magilligan Point, at its north-western extremity, to Benone Strand to thesouth-east. It has a north-eastern orientation on the margin of the Atlantic Ocean. The beach forms the seaward edgeof a large beach ridge plain that was formed during the mid- to late Holocene (Wilson et al., 2004).

The beach has experienced alternating periods of accretion and erosion over the last number of decades. The tallestseaward dune crest is a remnant of an erosional scarp formed during a large storm in 1980 (Carter and Stone, 1989).It runs nearly the entire length of the beach and follows a 130–310° bearing at the study site. The site itself is at thecentre of a segment about 1 km long that appears to have been accreting since 1980. The dune morphology comprisesa uniformly sloping landward side, densely vegetated with Ammophila arenaria (~1·5 m height). There is a sharpbreak of slope at the crest, 11·4 m ordnance datum (OD) Belfast. The seaward slope follows a stepped profile down tothe beach, with an accreting section, about 5 m wide and 6–7 m high, leading to a dune ramp. A large storm duringthe winter of 2004/05 formed a 4 m high foredune scarp. The recovery of this dune ramp, by aeolian processes, wascomplete by the summer of 2006.

The beach is up to 100 m wide at low tide, planar and dissipative. The surface is usually unvegetated with littledebris. Sediments consist of very well sorted quartz sands with a mean grain size of 0·17 mm and a calcium carbonatecontent less than 5%. Nearshore sand bars periodically weld onto the foreshore, supplying the main sediment sourcefor dune building (Carter and Wilson, 1990). The high energy coast is dominated by Atlantic swell waves and has atidal range of ~1·60 m (Jackson et al., 2005). The wind climate at the site is dominated by south-westerly winds.

Methods

Wind climate and sand drift potentialA weather station, situated at Magilligan Point, measured wind speed and direction at 14·5 m elevation (OD). Velocitywas recorded with a three-cup anemometer (Delta-T AN4) at 1 min intervals (an average of ten measurements overeach minute), while a Delta-T WD4 wind vane recorded the wind direction, also at 1 min intervals, to a precision of 1°.

Following Illenberger and Rust (1988), the potential sand drift was calculated using the Bagnold (1941) formula:

q = C(d/D)1/2(ρ/g)(u*)3 (1)



where q is the sediment transport rate in kg m−1, C is an empirical coefficient (1·8 for well sorted dune sands), d is thesediment grain diameter (0·17 mm), D is the diameter of ‘standard’ sand (0·25 mm), ρ is the density of air (1·23 kg m−3),g is gravitational acceleration (9·81 m s−2) and u* is the shear velocity. Shear velocity was calculated using theBagnold (1941) formula:

u* = (uz − ut)/5·75 log(z /k) (2)

where uz is the wind speed at height z (14·5 m), ut is the threshold wind velocity at which transport is assumed tobegin (calculated from instantaneous studies at the site to be 4·7 m s−1 at 14·5 m), 5·75 is a factor of proportionalitybetween u* and the rate of increase in wind speed with log height (Bagnold, 1941) and k is the roughness factor(10 mm for a rippled sand surface).

For each month the wind data were segregated into 2 m s−1 bins, starting with a 4·00 –5·99 m s−1 bin then 6·00–7·99 m s−1 and so on. Within each bin, sediment transport was calculated for each 1° direction increment based onEquation (1), with uz equal to the actual mean wind speed for each velocity bin. The calculated quantities were totalledto give a drift potential (DP) for the month and vectorially resolved to give a resultant drift potential (RDP) andresultant drift direction (RDD).

Drift quantities and sand roses were calculated for each month of the study period, June 2004 to May 2006. Annualsediment budgets were calculated by totalling the monthly values. Sand drift potential was calculated for two condi-tions. In the first, the findings from Lynch et al. (2007) were utilized to adjust offshore wind velocities and directionsto allow for secondary airflow effects. Oblique offshore winds (131°–202° and 253°–309°) were corrected to 130 and310° (alongshore airflow) respectively, with no velocity changes. Directly offshore winds (203°–252°) were correctedto 40° (onshore airflow), with wind speeds reduced to 36% of their crestal velocities. In the second calculation, alloffshore winds were excluded from the analysis (the common approach in the literature). The sand drift potentialmethod does not factor in any other variables, such as vegetation or moisture.

Trench trap studyA 2-month (June, July 2004) study was undertaken to quantify the total amount of sediment in transport on the beachand moving into the foredunes. A large integrating trench trap (TT) (constructed of plywood walls and with an opentop, 2 m length × 1 m width × 1·5 m depth) was placed within the foredune (at 6·5 m OD), being 2 m landward of thetop of the dune ramp (Figure 3). Another trench trap was placed at the foot of the dune ramp (2·4 m OD) 27 m to thenorth-west (to avoid depriving the dune trap of sediment supply). The content of both traps was emptied out every 1–3 days (depending on accessibility). The volume of the trapped sediment was measured using a graduated container; asample was analysed for density and the total weight was calculated using the formula:

weight = (volume × density).

Although the inflexible nature of large trench traps is considered a limitation, the trapping efficiency is consideredclose to unity if sediment is not lost from the trap through remobilization by wind (Wang and Kraus, 1999). Sedimentremobilization was not observed at either of the trench traps used. Because of the positioning of the beach trap on thelower dune ramp it is not known whether the trapped sediment was destined for the dunes or was in transit throughthe ramp area while moving alongshore. Conversely, the trap located in the foredune was well constrained by thickvegetation landward and in the shore parallel directions. Only sediment moving up the dune ramp from the beachcould therefore enter the trap. Over the course of the experiment no scour was noted on the upwind side of the traps;minor scouring was present on some days on the downwind side of the beach TT, presumably due to sediment supplybeing cut off by the trap itself.

Remote-sensing sediment response to wind forcingThis study, conducted over a 28 day period, consisted of the following three components. (1) Environmental variableswere measured at the Magilligan Point weather station. (2) A record of saltation activity, was recorded using a Safire1·4 impact sensor (Baas, 2004). The instrument was placed at the crest of an accreting dune ramp, with the sensitivering 2 cm above the sediment bed, at the base of the scarped dune. A logger (Phillips 8055 micro-controller with staticRAM storage) recorded the total number of grain impacts in each 30 second period. The logger was downloaded everythree to four days, at which point the position of the sensitive ring could be repositioned at 2 cm (depending onwhether erosion or deposition of the sediment surface had occurred). These data were amalgamated into daily values



Figure 3. a) Trench trap locations – June, July 2004 b) Instrument locations and scarped duneline September, October 2005.

of the total number of impacts and the number of minutes for which saltation was recorded. (3) An estimation of fetchdistance was made utilizing a remote sensing technique; the position of the wet–dry boundary (detectable on a geo-rectified image) was combined with wind direction data to identify the available fetch for each day of the study. Acomplete description of this technique, with particular reference to the dataset collected for this study, is available inthe work of Lynch et al. (2006).

Topographic surveysFive shore-normal surveying profiles were established at 50 m intervals along the beach. Repeat surveys were con-ducted intermittently over the two years. While a total of 16 surveys were carried out, only three individual surveysare presented here – 21 July 2004, 3 March 2005 and 6 July 2006 (Figure 4). These data were taken from profile 2, theclosest profile to the trench traps. For the volume estimate, using these data, it was assumed that each surveyed pointis representative of a 1 m shore parallel length of beach. The basement level for the volume analysis was set at 0 mOD. Topography was surveyed with a Trimble 4400 differential global positioning system, to an accuracy of ±2 cm.

Results

The changes in the shore-normal profile at the site give a good indication of the beach-dune state over the time periodof the studies. The initial gently sloping profile (July 2004) was eroded in January/February 2005, giving a scarpedprofile with a lowering of beach elevation (March 2005). This was followed by a period of sustained accretion untilthe dune ramp was re-emplaced and the beach elevation had recovered (July 2006) (Figure 4). These changes in thebeach profile equated to quantities of 2700 kg m2 for July 2004, 1950 kg m2 for March 2005 and 2850 kg m2 for July2006.

Sand Drift PotentialThe average wind speed for the first year of the study was 5·8 m s−1, with 33% of the winds recorded exceeding thethreshold for transport (4·7 m s−1). The maximum velocity for the period was 29·0 m s−1. Values for the second yearwere lower, at 5·2 m s−1, 30% and 23·3 m s−1 respectively. The vast majority of these winds were offshore and because



Figure 4. Changes in Profile 2 reveal the extent of erosion and recovery over the course of study (9x vertical exaggeration).

of this would normally be disregarded when compiling a sediment budget. Figures 5 and 6 show how this can lead tolarge disparities in sand drift potential; inclusion of topographically altered airflows increases the predicted quantitiesfrom 110% (November 2005) to 6160% (August 2005) compared with predictions that exclude the offshore winds.The annual sand roses show that offshore winds that result in flow reversal (from 203 to 252°) did not contributegreatly to the drift potential; it was the winds that were deflected alongshore that had the most significant impact.From June 2004 to May 2005 the predominant south-westerlies produced a drift potential that was directed slightlyonshore and to the south-east (Figure 5). This may contribute to the documented foredune stability of Benone Strandto the south-east (Carter, 1991). The following 12 months (June 2005–May 2006) experienced a slight shift in thedistribution of wind directions; while the dominant south-westerlies remained prevalent, a stronger southerly compo-nent was evident when compared with the 2004 –2005 data (Figure 6). Consequently, the deflected airflows counter-acted each other and had little effect on the resultant drift potential and direction. It is to be noted that the northerlywind component of the wind rose diagrams coincided with wave-induced erosion at the site rather than enhancedonshore aeolian movement of sediment. For example, the large DP values for January and February 2005 (Figure 5)were actually associated with the foredune scarping evident in the profile data (Figure 4).

Trench trap studyA slight adjustment was made to the June and July 2004 DP totals to allow comparison with quantities measured bythe trench traps; three days of wind data that coincided with periods for which no TT data were available wereomitted. The adjusted drift potential for June was 1873 kg m−1 compared with the measured 1724 kg m−1, while theJuly totals were 455 and 519 kg m−1 respectively. These surprisingly accurate predictions involved no tuning ofcoefficients, or other parameter values.

The drift potentials were also calculated at weekly intervals and compared with the measured amounts (Figure 7).The most conspicuous feature of these sand roses is that the weeks (2, 7 and 8) that contain little or no onshorecomponent did experience sand movement. This transfer is comparable to that predicted when deflected offshorewinds are accounted for in the analysis. As noted earlier the retention of sediment versus its loss to the south-west canbe strongly influenced by the secondary airflow patterns at the site. Weeks 2 and 4 have similar drift potentials (401and 257 kg m−1 respectively) and similar measured quantities (481 and 438 kg m−1 respectively). Nevertheless, theRDP for Week 2 is 381 kg m−1 with an RDD of 130° (sediment moved to the south-west), while the RDP for Week 4is just 11 kg m−1 with an RDD of 257° (sediment retained at the site).

When it is considered that alongshore winds have near unlimited fetch, it would be expected that periods withalongshore winds would experience the highest sediment transport rates. This is not the case; the week with thehighest measured quantity of sediment (Week 3) was the week with mainly onshore winds and therefore the shortestfetch. The drift potential for this week was, however, also considerably higher than the other weeks. In fact, theshorter fetch distance may be a factor in the measured quantity for this week not meeting the potential drift available,as it does for all the other weeks with the exception of Week 8.



Figure 5. The annual (top line) and monthly sand roses for 2004/2005. The annual wind rose is top left. Sand roses show vectors ofpotential transport from particular directions and the resultant drift potential towards the direction of movement. Drift Potential(DP) 1 includes secondary airflow effects DP 2 excludes all offshore winds. The closed-headed arrows represent the resultant driftpotential including secondary airflow effects; the open-headed arrows represent resultant drift potential excluding offshore winds.

Two factors may have influenced the balance between predicted and measured quantities. First, the highest deliveryof sediment to the foredune was during Week 2, when the dune trench trap collected 23 kg m−1 of sediment. Thisquantity was trapped over a 2-day period with flow reversal of offshore winds. This secondary airflow is not repre-sented in the sand rose diagram and suggests that the degree of deceleration allowed for in the sand drift analysis isexcessive. If this were adjusted, an increase in drift potential values would be expected. This is countered by theprobable under-representation of the actual amount of sediment in transport by the placement of the beach trench trapon the dune ramp; shore-parallel sediment transport commonly occurred exclusively along the mid-beach with littlemovement close to the foredune at the trap position. That said, the method used did result in a strongly correlated



Figure 6. The annual (top line) and monthly sand roses for 2005/2006. The annual wind rose is top left. Sand roses show vectorsof potential transport from particular directions and the resultant drift potential towards the direction of movement. DriftPotential (DP) 1 includes secondary airflow effects DP 2 excludes all offshore winds. The closed-headed arrows represent theresultant drift potential including secondary airflow effects; the open-headed arrows represent resultant drift potential excludingoffshore winds.

(r2 = 0·81) pair of datasets for the two months. Differences between observed and predicted quantities might alsorelate to variations in other variables such as moisture content.

Looking purely at measured quantities from the foredune trench trap, the importance of offshore winds to dunemaintenance and development can be shown. As noted earlier, a 2-day offshore wind event during Week 2 contributed23 kg m−1 of sediment to the foredunes. This event supplied more sand to the foredunes than the combined contribu-tion of all the onshore events during the 8 weeks of the study.



Figure 7. The weekly sand roses for June/July 2004. Sand roses show vectors of potential transport from particular directions andthe resultant drift potential towards the direction of movement. Drift Potential (DP) 1 includes secondary airflow effects DP 2excludes all offshore winds. The closed-headed arrows represent the resultant drift potential including secondary airflow effects;the open-headed arrows represent resultant drift potential excluding offshore winds.

Remote-sensing sediment response to wind forcingThe position of the wet–dry boundary was identified from rectified images taken every 2 h, with the intention of estimatingthe fetch distance for each 2 h period. Much of this information, however, was not required, as onshore winds prevailedfor only four of the 28 days studied. Fetch distance is not applicable to offshore winds experiencing reverse flow at



Figure 8. An oblique onshore wind results in along shore sediment transport, 1 October 2005. Note sediment surface is wet withdry deposits overlain.

this site, because downward vertical wind motion can occur above the leading edge of erodible material (Lynch et al.,2007), while deflected offshore airflow travels alongshore with a relatively unlimited fetch. The results presented inTable I again reveal, similarly to the trench trap study, that the day (1 October) with one of the shortest fetches (15 m)experienced the most saltation activity. A rectified image from 1 October, 3:30 pm, shows that although the prevailingwind for this event was obliquely onshore, resulting in a fetch distance of 15 m, the crest alignment of depositionalfeatures on the sub-aerial beach was distinctly shore normal, indicating an alongshore transport direction (Figure 8).Shore-parallel deflection of the onshore winds (previously noted by Arens et al., 1995) may have been enhanced by thescarped nature of the foredune. Overall, no particular relationship between fetch distance and sediment transport is evident.

It is abundantly clear that offshore winds make a significant contribution to the transport quotient for the period.Irrespective of the value used, total impact count or number of minutes with saltation activity, the percentage contribu-tion from offshore winds was considerable: 59% for the former and 74% for the latter. The days when flow reversalwas prevalent contributed 11% of the total saltation activity for the 28 days. Surface elevation changes, measured atthe impact sensor, show a net accretion of 4·5 cm, with all of the changes associated with offshore winds.

The remote-sensing technique also provided qualitative information on the moisture state of the sediment surface,backing up the quantitative rain gauge measurements from Magilligan Point. Saltation activity occurred on 14 of the17 days in which rainfall was recorded; surface conditions were considered at least partially wet on all but one ofthese days. Aeolian sediment transport during rainfall events has been noted by other authors (e.g. Jackson andNordstrom, 1998), with saltation ceasing after a certain time from the start of the event. During this study prolongedperiods of rainfall did not seem to unduly hinder aeolian transport (e.g. 23 Sept–1 Oct 2005; Table I).

Discussion

While there was a surprisingly good correspondence between measured and predicted sediment transport quantities forthe two months of the trench trap study, the differences between estimates that included or omitted secondary airfloweffects were of particular interest. The results suggest that omission of secondary airflow effects can result in largeunder-estimates of potential transport when employing process-scale models to predict long-term trends. Bearing inmind that the study site is located on a coast with predominant offshore winds, the results suggest that fetch distancein this instance did not play an important role in longer-term transport rates. Therefore, scale-up models for this coastcould probably omit fetch distance as a variable.



Table I. Results of 28-day qualitative study (September 2005–October 2005)

Impact Impact Surface Ave. wind Incidentsensor sensor height speed wind Fetch Rainfall

Date (count) (minutes) (cm) (m s−1) direction (m) Airflow Surface state (mm)

19 Sep 0 1 0 4 W >100 TS alongshore wet 0.220 Sep 0 1 – 4 SW N/A flow reversal wet with dry patches 1.021 Sep 300 21 0 6 S >100 TS alongshore wet with dry patches 0·422 Sep 17 500 174 – 7 S >100 TS alongshore dry 0·023 Sep 11 800 85 2 8 NW >100 TS alongshore wet with depositional features 1·824 Sep 600 105 – 5 S >100 TS alongshore wet with depositional features 5·025 Sep 100 19 – 4 SW N/A flow reversal wet with dry patches 3·826 Sep 29 500 252 2 8 S >100 TS alongshore wet with dry patches 4·227 Sep 2 200 82 – 9 SW >100 TS alongshore wet with dry patches 0·828 Sep 5 900 141 1 8 SW N/A flow reversal wet 3·029 Sep 100 15 – 5 W >100 TS alongshore wet with dry patches 8·230 Sep 19 500 101 0 8 SW N/A flow reversal wet with dry patches 0·01 Oct 93 700 570 – 10 NW 15 onshore wet with depositional features 2·82 Oct 0 3 – 5 W >100 TS alongshore wet with dry patches 0·03 Oct 0 0 0 3 SW N/A flow reversal wet with dry patches 0·04 Oct 0 1 – 4 S >100 TS alongshore dry 0·05 Oct 0 0 – 4 S >100 TS alongshore dry 0·06 Oct 100 5 – 6 S >100 TS alongshore dry 0·07 Oct 800 15 1 8 S >100 TS alongshore dry 1·68 Oct 200 10 – 6 W >100 TS alongshore wet 2·09 Oct 33 500 436 – 9 S >100 TS alongshore wet with dry patches 0·2

10 Oct 6 900 178 0·5 6 S >100 TS alongshore wet with dry patches 12·411 Oct 0 0 – 4 N 12 onshore wet with dry patches 2·612 Oct 100 4 – 5 N 12 onshore wet 0·413 Oct 0 0 – 2 NW >100 TS alongshore dry with wet patches 0·014 Oct 100 4 0 3 SE >100 TS alongshore dry with wet patches 0·015 Oct 4 400 37 – 5 SE >100 TS alongshore dry with wet patches 0·016 Oct 0 2 – 5 SE >100 onshore dry with wet patches 0·0

Some aspects of the sand drift potential method require further consideration. First, the representation of offshorewinds in the drift analysis is misleading and was noted earlier. The reverse flow component is known to returnsediment back to the dune and was under-represented in the analysis. In addition, the alongshore movement is notsimply unidirectional; the transition between reverse flow and shore parallel deflected flow is gradual at wind speedsjust above threshold. Therefore, sediment movement may be directed into the foredunes and result in accumulation, aswas recorded during the remote-sensing study, where alongshore winds contributed most to the growth of the embry-onic dune ramp.

Second, the transport threshold employed here involved amalgamating 1 Hz data to 1 min intervals and correlatingthese data with 1-min records taken at the weather station. The y-intercept of the best-fit line was set to zero and windspeed at this point was read as the transport threshold. At 4·7 m s−1 the threshold was fairly representative, roughly atthe midpoint between the lowest value for which sediment transport was recorded and the highest value for which nosediment transport was recorded. The data used in this analysis were taken from days where offshore winds prevailed.Establishing a threshold for offshore winds was not possible, as little or no correlation was evident using the 1-mintime step. A feature of these data was that the lowest wind speed at which sediment transport was recorded wasgenerally about 2 m s−1 lower than days of onshore winds, supporting the notion that turbulent conditions (experiencedby secondary airflow patterns in the lee of the foredune under offshore winds) enhance sediment entrainment (Wiggset al., 1996). The threshold used in the analysis should have been lower for offshore winds, and therefore transportrates would have been higher than those reflected by the DP method.

Third, the Fryberger (1979) method is a very generalized technique to assess the energy being supplied by the wind,and is not a process-scale sediment transport model. Therefore additional variables, such as moisture or sediment size,are not included. Omission of the effects of vegetation is, however, a fundamental drawback in the applicability of themethod in a beach-dune setting. Although vegetation was discounted as an important variable at this study site it is



important in the context of the Fryberger technique. Vegetated foredunes have strong sediment trapping abilities.Consequently, any vector of movement that crosses the dune line may result in the sediment being trapped in theforedune, rendering it unavailable for re-mobilization at a later stage (by a wind from a different direction). Take, forexample, the May 2006 sand rose (Figure 6), in which the DP including topographically steered winds was 2285 kg m−1. If,however, the onshore wind occurred before the alongshore component, 1470 kg m−1 (the drift potential for the onshorewinds only) could have been trapped by the foredune, i.e. lost from the beach aeolian sediment transport system. Thesediment available for alongshore movement would then amount to 815 (2285 − 1470) kg m−1. In other words, thereare two separate resultant vectors rather than the single resultant drift potential produced by a simple summation of allthe input vectors. Any vector analysis for beach dune systems will experience this to some degree. The magnitude ofthis effect is reduced by the fact that (1) not all onshore movement crosses the dune line (unfulfilled transport potentialdue to slope, moisture etc.) and (2) the dune will not trap 100% of the sediment.

Conclusions

A number of different approaches were employed to assess the importance of two micro-scale variables, secondaryairflow effects and fetch distance, on medium-term foredune development. The results suggest that fetch distance is anunimportant variable at this site. At the Magilligan Strand site secondary airflow effects on offshore winds had asignificant influence on sediment transport rates. Results show a considerable correlation (r2 = 0·81) between meas-ured and predicted quantities over a 2-month period when offshore winds are included in predicted estimates. A morequalitative 1-month remote-sensing study at the site supported this finding. Predominant offshore winds, deflected inthe lee of the foredune, were associated with the majority of the saltation activity recorded at the seaward foot of theforedune. It may be concluded that secondary airflow patterns should be considered a key variable when linkingmicro- and meso-scale sediment transport studies, both temporally and spatially.

Acknowledgements

The authors would like to thank Colin Anderson, Nigel Macauley, Sam Smyth and Robert Stewart for their valuable field assistanceand technical support. We would also like to thank the Ministry of Defence staff at Magilligan Point Firing Range, especially BillyNichol. This work is part of a Vice Chancellor’s Research Scholarship funding for PhD studies at the Centre for Coastal and MarineResearch, School of Environmental Sciences, University of Ulster. We are grateful to Professor Karl F. Nordstrom and an anony-mous reviewer for a critical and helpful review of this paper.

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Aeolian fetch distance and secondary airflow effects: the influence of micro-scale variables on meso-scale foredune development