Dune Toe Research Articles

Effects of sand fences on coastal dune vegetation distribution

Sand fences are important human adjustments modifying the morphology of developed shores. The effects of sand fences on sediment transport and deposition in their initial stages have been well studied, but little is known about the effect of deteriorated sand fences that have become partially buried low scale barriers within the dune, potentially benefiting vegetation growth by protecting it from onshore stress. Data on vegetation, topography and fence characteristics were gathered at three dune sites in Ocean City, New Jersey on September 2007 and March 2008 to evaluate the effect of fences within the dune on vegetation distribution. Variables include: distance landward of dune toe, degree of sheltering from onshore stressors, net change in surface elevation (deposition or erosion), vegetation diversity and density, presence of remnant fence, and distance landward of fence. Results for the studied environment reveal that 1) vegetation diversity or density does not increase near remnant fences because most remnants are lower than average vegetation height and can not provide shelter; but 2) vegetation distribution is related to topographic variables, such as degree of sheltering, that are most likely the result of sand accretion caused by fence deployment. Fence deployment that prioritizes the creation of topographically diverse dunes within a restricted space may increase the diversity and density of the vegetation, and the resilience and value of developed dunes. Managers should consider the benefits of using sand fences on appropriately wide beaches to create a protective dune that is also diverse, functional and better able to adapt to change.

A CFD study of wind patterns over a desert dune and the effect on seed dispersion

In the Namib Desert seed distribution is greatly influenced by wind patterns. Existing literature regarding wind patterns over dunes focuses on two-dimensional simulations of flow over simplified dune structures. The three-dimensional geometries of the sand dunes suggests far more complex flow features exist, which are not captured by two-dimensional simulations. Computational fluid dynamics (CFD) was used to reproduce the three-dimensional near surface wind patterns around a dune with the aim to learn more about seed distribution. Field work included terrain mapping, wind speed, direction and temperature metering. The CFD results show the expected two-dimensional flow features of high pressure at the dune toe, low pressure at the crest and flow acceleration up windward slope. Also observed are some three-dimensional flow features such as a spiral vortex near the crest and transverse flow due to crest-line curvature of the dune. It was also observed how the wall shear stress differs due to the three-dimensional shape of the dune. The wall shear stress suggests that seed accumulation is more likely to occur behind trailing (down-wind) crest edges. Particle tracking showed how seeds tend to move over the dune crest and recirculate towards the crest on the lee-side. The study showed that adding the third dimension makes the simulations more complex, adds to computational requirements and increases simulation time but also provides vital flow information which is not possible with two-dimensional simulations.

A field study of mean and turbulent flow characteristics upwind, over and downwind of barchan dunes

ABSTRACTField‐measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes. Turbulence was sampled using a high frequency sonic anemometer, deriving near‐surface Reynolds shear and normal stresses. Measurements upwind of and over a crest‐brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe. The mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress. This field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements.Downwind, the effect of different dune profiles on flow re‐attachment and recovery was apparent. With transverse incident flow, a re‐attachment length between 2·3 and 5·0h (h is dune brink height) existed for a crest‐brink separated dune and 6·5 to 8·6h for a crest‐brink coincident dune. The lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes, and a decrease in turbulence with distance characterized flow recovery. Recovery of mean velocity for the crest‐brink separated dune occurred over a distance 6·5h shorter than the crest‐brink coincident form.As the application of sonic anemometers in aeolian geomorphology is relatively new, there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle. This paper demonstrates the effect on Reynolds stresses of mathematically correcting data to the local streamline over varying dune slope. Where the streamline angle was closely related to the surface (windward slope), time‐averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines. In the close lee, however, the angle of downwardly projected (separated) flow was not aligned with the flat ground surface. Here, shear stress appeared to be underestimated by streamline correction, and corrected shear stress values were less than half of those uncorrected. Copyright © 2011 John Wiley & Sons, Ltd.

Sea turtle responses to barriers on their nesting beach

We experimentally studied the nesting behavior and nest-site choice of loggerhead sea turtles ( Caretta caretta) that emerged to nest at night on a beach in southern Brevard County, Florida USA. Emerging turtles were divided between two paired treatments: wall and control. Intercepted turtles in the wall treatment were presented a portable wall that blocked their ascent of the beach. The position of the portable wall was shore-parallel and midway between the recent wrack line and the dune toe. Intercepted turtles in the control treatment ascended the beach and did not encounter a wall. We observed individual turtles in both wall and control treatments so that their nesting behavior could be measured. We observed 44 female loggerheads (22 paired samples). In the control treatment, 15 of 22 turtles completed a nest, and in the wall treatment, 14 of 22 turtles completed a nest. There was no significant difference in nesting success (nests/attempts) between treatments (Chi-square = 0.10, p = 0.75). However, effects from the wall resulted in wall-treatment nests being 3.5 m closer to the surf than control nests, on average. Although it was possible for turtles to nest within 0.5 m of the wall, mean distance of nests from the wall was 3.2 m (SD ± 2.0 m). Although the wall affected the location where nesting occurred, there was no observable effect on the effort turtles made to prepare the nest site, dig an egg chamber, fill it with eggs, cover the eggs, or camouflage the nest site. Egg survivorship did not differ significantly between treatments; however, the sample size was not large enough to detect biologically significant differences in hatching success amidst the high variation in the data. An additional factor that made our test of hatching success less rigorous was the absence of significant beach erosion during the study period. Erosion has been shown to be the principal cause of mortality for nests low on the beach and can cause 100% mortality in the region of beach where the wall-treatment nests were located.

Investigation of three‐dimensional wind flow behaviour over coastal dune morphology under offshore winds using computational fluid dynamics (CFD) and ultrasonic anemometry

ABSTRACTThe behaviour of offshore‐directed winds over coastal dune and beach morphology was examined using a combination of modelling (3‐D computational fluid dynamics (CFD)) and field measurement. Both model simulations and field measurements showed reversal of offshore flows at the back beach and creation of an onshore sediment transport potential. The influence of flow reversals on the beach‐dune transport system and foredune growth patterns has previously received little attention. Detailed wind flow measurements were made using an extensive array of mast‐mounted, 3‐D ultrasonic anemometers (50 Hz), arranged parallel to the dominant incident wind direction. Large eddy simulation (LES) of the offshore wind flow over the dune was conducted using the open‐source CFD tool openFOAM. The computational domain included a terrain model obtained by airborne LiDAR and detailed ground DGPS measurements. The computational grid (~22 million cells) included localized mesh refinement near the complex foredune terrain to capture finer details of the dune morphology that might affect wind flows on the adjacent beach.Measured and simulated wind flow are presented and discussed. The CFD simulations offer new insights into the flow mechanics associated with offshore winds and how the terrain steering of wind flow impacts on the geomorphological behaviour of the dune system. Simulation of 3‐D wind flows over complex terrain such as dune systems, presents a valuable new tool for geomorphological research, as it enables new insights into the relationship between the wind field and the underlying topography. The results show that offshore and obliquely offshore winds result in flow reversal and onshore directed winds at distances of up to 20 m from the embryo dune toe. The potential geomorphological significance of the findings are discussed and simple calculations show that incoming offshore and obliquely offshore winds with mean velocities over 13 m s−1 and 7 m s−1, respectively, have the potential to create onshore‐directed winds at the back beach with mean velocities above 3.3 m s−1. These are above the threshold of movement for dry sand and support previous conclusions about the significance of offshore winds in dune and beach budget calculations. Copyright © 2011 John Wiley & Sons, Ltd.

Quantifying thresholds for significant dune erosion along the Sefton Coast, Northwest England

Field and model hindcast data are used to establish a critical dune erosion threshold for the Sefton Coast (NW England). Events are classified as causing significant erosion if they result in: (a) a mean dune retreat along the entire study area of >2m; (b) a dune retreat of ≥5m along a coastal segment ≥2km in length; and (c) an eroded area ≥20,000m2. For the period 1996 to 2008, individual storms were characterised using hindcast results from a POLCOMS-WAM model and measured data from the Liverpool Bay Coastal Observatory. Results show that combined extreme surge levels (>1.5m) and wave heights (>4m), or tidal water levels above 9.0m Chart Datum (CD), do not always result in significant dune erosion. Evidence suggests that erosion is more likely to occur when wave heights are >2.6m, peak water level is >10.2m CD at Liverpool and when consecutive tidal cycles provide 10h or more of water levels above 9.4m CD. However, lower water levels and wave heights, and shorter events of sustained water levels, can cause significant erosion in the summer. While the return period for events giving rise to the most severe erosion in the winter is >50years, significant erosion in the summer can be caused by events with return periods <1year. It is suggested that this may be attributable to a known reduction in the mean dune toe elevation c. 30cm. Although the study shows it might be possible to characterise objectively storm events based on oceanographic conditions, the resultant morphological change at the coast is demonstrated to depend on the time and duration of events, and on other variables which are not so easy to quantify. Further investigation is needed to understand the influence of alongshore and seasonal variability in beach/dune morphology in determining the response to the hydrodynamic and meteorological conditions causing significant erosion. Improved monitoring pre- and post-storm of changes in beach/dune morphology is required to develop reliable proxies that can be used to establish early warning systems to mitigate the impacts of erosion and flooding in the future.

Aeolian Sediment Transport Across Beach Wrack

Abstract This field study identifies the short-term (hours) effect of freshly deposited wrack on aeolian transport and surface elevation changes on the backshore and foredune of a barrier island at Avalon, New Jersey, USA. Storm wave uprush reached the dune toe on 21 October 2008 and deposited a line of vegetative wrack about 2 m wide (cross-shore) and 70 mm high near the dune toe (upper wrack) and a line about 0.3–0.5 m wide and 30 mm high about 5 m from the dune toe (lower wrack). Fourteen cylindrical sediment traps were deployed 23 October 2008 when the wind blew onshore at an angle of 8 deg to the trend of the dune. Wind speed at 1 m elevation at the dune toe averaged 6.3 m s−1. The lower wrack line was nearly covered with sand about an hour after initiation of transport, diminishing its effect as a barrier to cross-shore transport. The upper wrack line caused greater reduction in trapping rates, with downwind traps collecting only 3.2% and 12.7% of upwind amounts. Data from erosion pins revealed the ...

Variability and correlations of shoreline and dunes on the southern Baltic coast (CRS Lubiatowo, Poland)

The paper analyses the results of field investigations into the evolution of the shoreline and dune toe positions in a multi-bar, dissipative coastal zone. The correlations between the changes in the shoreline and the dune toe range from −0.4 to 0.8. It is most often the case that the dune toe is stable while the shoreline moves. Consistent cross-shore migration is slightly more likely to happen than the divergent or convergent movements of both lines. Shoreline retreat and advance attain respective rates of 0.7m day−1 and 0.4m day−1. Deep-water wave energy of about 50kJ m−1 constitutes the boundary between shore accumulation and erosion.

Aeolian transport rates across raked and unraked beaches on a developed coast

AbstractWind characteristics and aeolian transport were measured on a naturally evolving beach and dune and a nearby site where the beach is raked and sand‐trapping fences are deployed. The beaches were composed of moderately well sorted to very well sorted fine to medium sand. The backshore at the raked site was wider and the foredune was more densely vegetated and about 1 m higher than at the unraked site. Wind speeds were monitored using anemometers placed at 1 m elevation and sand transport was monitored using vertical traps during oblique onshore, alongshore and offshore winds occurring in March and April 2009. Inundation of the low backshore through isolated swash channels prevented formation of a continuously decreasing cross‐shore moisture gradient. The surface of the berm crest was dryer than the backshore, making the berm crest the greatest source of offshore losses during offshore winds. The lack of storm wrack on the raked beach reduced the potential for sediment accumulation seaward of the dune crest during onshore winds, and the higher dune crest reduced wind speeds and sediment transport from the dune to the backshore during offshore winds. Accretion at wrack seaward of the dune toe on the unraked beach resulted in a wider dune field and higher, narrower backshore. Although fresh wrack is an effective local trap for aeolian transport, wrack that becomes buried appears to have little effect as a barrier and can supply dry sand for subsequent transport. Aeolian transport rates were greater on the narrower but dryer backshore of the unraked site. Vegetation growth may be necessary to trap sand within zones of buried wrack in order to allow new incipient foredunes to evolve. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Dynamic morphology of small south-eastern Mediterranean river mouths: a conceptual model

River mouths along the Israeli Mediterranean coast are characterized by a dynamic morphology as their channels migrate hundreds of meters along the coast. This study examines the dynamic morphology of seven such river mouths. It offers a conceptual model aimed at generalizing and describing their spatial and temporal morphological patterns, and the environmental factors influencing them. The study methodology comprised a detailed monitoring and mapping by GIS techniques, with quantitative data derived from historic aerial photographs, river discharge records, wave measurements, and a digital elevation model. These data were incorporated into a homogenous database and subsequently applied in the investigation of the morphological patterns of these mouths, and the analysis of their influencing factors. River mouths in this study occur in two distinctive topographic settings. In one setting (here termed barrier topography) the river mouth is deflected alongshore by a sandy barrier. In the second setting (termed funnel topography) the river mouth is confined to a funnel-shaped topographic depression perpendicular to the coast. The behavior of river mouths in these two settings is quite distinctive. Barrier mouths usually migrate over larger distances, as they tend to deflect along a sand barrier and establish semi-permanent channels along the dune toe. This enables the wide range migration of semi-permanent channels over decades. Funnel topography mouths deflect over shorter distances and they rapidly migrate within the funnel boundaries. This study concludes that the topographic setting of the beach, a constant element in the temporal scale of this study, is the primary influencing factor on the morphology of the mouths studied. The influence of other factors on the morphology of these mouths differs in space and time and depends on the topographic settings.

The response of three main beach types to tropical storms in South China

Based on the observational data of 6 storms and 8 different beaches on the coasts of southern China, the paper is concerned with the characteristics of storm-effects on beaches with different morphodynamic types in South China. A dissipative beach is very stable and has a weak response to tropical storms. The storm-induced morphological changes for this type of beach are characterized by erosion at the dune toe and minor changes on the beach face. A low-tide terrace beach has a strong response to storms, with marked changes in the upper profile. A barred beach usually experiences berm erosion, swash bar smoothening, and seaward bar migration during storms. Overwash may occur for the beach without backshore dune or protection. In this paper, backshore changes and the alteration of beach gradient in response to tropical storms are discussed. With taking into account tidal influences, a new morphodynamic parameter ( ε T ) is proposed in order to highlight the beach response intensity to storm action. Results show that the inherent morphodynamic properties have significant effects on the beach response to storm. The more dissipative beach has a smaller response intensity and shows a moderate change in beach profile during a storm, while a more reflective beach has a greater response intensity and changes markedly under storm conditions.

Coastline evolution at Balneario Parque Mar Chiquita, Argentina

This study determined coastline evolution and retreat rates at Balneario Parque Mar Chiquita (Argentina) between 1949 and 2007. The dune toe was considered the coastline indicator, and it was determined based on historical maps, aerial photographs, a satellite image, and field measurements. The analysis showed that the intensity of the erosion process has declined though it continues to be meaningful. The annual rate has decreased from 17.71 to 1.41 m yr–1, corresponding to a total retreat of 299.01 m in the last 58 years. The temporal variations of the phenomenon mainly result from changes in beach nourishment by the littoral drift, which has been affected by sand mining and coastal defenses built to the south of the study area. Different spatial behaviours were also registered in the town, associated with geomorphological characteristics and dune conservation state. Sudestada storms have played a significant role in the coastline retreat. The results contribute to the understanding of the magnitude and dynamics of the erosion process, and should be of use in the preparation of coastal management programs and risk evaluations.

Temporal and spatial variabilities in the surface moisture content of a fine-grained beach

This study examined spatial and temporal variations in the surface moisture content of a fine-grained beach at Padre Island, Texas, USA. Surface moisture measurements were collected on a 27 × 24 m grid that extended from the dune toe to the upper foreshore. The grid was surveyed at 2 to 4 h intervals for two tidal cycles, generating 17 maps of the spatial distribution of surface moisture. Simultaneous measurements of air temperature and humidity, wind speed and direction, tidal elevation, and water table elevation were used to interpret observed changes in surface moisture. It was found that the spatial distribution of surface moisture was broadly characterized by a cross-shore gradient of high to low content moving landward from the swash zone. The distribution of surface moisture was conceptualized in terms of three zones: saturated (> 25%), intermediate or transitional (5–25%), and dry (< 5%). The position of the saturated zone corresponded to the uppermost swash zone and therefore shifted in accordance with tidal elevation. Moisture contents in the intermediate and dry zones were primarily related to variation in water table depth (which was in turn controlled by tidal elevation) and to a lesser extent by evaporation. Signals associated with atmospheric processes such as evaporation were muted by the minimal degree of variation in atmospheric parameters experienced during most of the study period, but were apparent for the last few hours. The observed spatial and temporal variations in moisture content correspond reasonably well with observations of key controlling processes, but more work is needed to fully characterize this process suite.

Decadal-scale variation in dune erosion and accretion rates: An investigation of the significance of changing storm tide frequency and magnitude on the Sefton coast, UK

Monitoring of frontal dune erosion and accretion on the Sefton coast in northwest England over the past 50 years has revealed significant spatial and temporal variations. Previous work has shown that the spatial variations primarily reflect longshore differences in beach and nearshore morphology, energy regime and sediment budget, but the causes of temporal variations have not previously been studied in detail. This paper presents the results of work carried out to test the hypothesis that a major cause of temporal variation is changes in the frequency and magnitude of storms, surges and resulting high tides. Dune toe erosion/accretion records dating from 1958 have been compared with tide gauge records at Liverpool and Heysham. Relatively high dune erosion rates at Formby Point 1958–1968 were associated with a relatively large number of storm tides. Slower erosion at Formby, and relatively rapid accretion in areas to the north and south, occurred during the 1970's and 1980's when there were relatively few major storm tides. After 1990 rates of dune erosion at Formby increased again, and dunes to the north and south experienced slower accretion. During this period high storm tides have been more frequent, and the annual number of hours with water levels above the critical level for dune erosion has increased significantly. An increase in the rate of mean sea-level rise at both Liverpool and Heysham is evident since 1990, but we conclude that this factor is of less importance than the occurrence of extreme high tides and wave action associated with storms. The incidence of extreme high tides shows an identifiable relationship with the lunar nodal tidal cycle, but the evidence indicates that meteorological forcing has also had a significant effect. Storms and surges in the eastern Irish Sea are associated with Atlantic depressions whose direction and rate of movement have a strong influence on wind speeds, wave energy and the height of surge tides. However, preliminary analysis has indicated only a modest relationship between dune erosion/accretion rates and the North Atlantic Oscillation index.

Coastal flooding hazard related to storms and coastal evolution in Valdelagrana spit (Cadiz Bay Natural Park, SW Spain)

Mapping of coastal inundation hazard related to storms requires the combination of multiple sources of information regarding meteorological, morphological and dynamic characteristics of both the area at risk and the studied phenomena. Variables such as beach slope, storm wave height or wind speed have traditionally been used, but detailed geomorphological features of the area as well as long-term shoreline evolution trends must also be taken into account in order to achieve more realistic results. This work presents an evaluation of storm flooding hazard in Valdelagrana spit and marshes (SW Spain), considering two types of storm that are characteristic of the area: a modal storm with 1 year of recurrence interval (maximum wave height of 3.3 m), and an extreme storm with 6–10 years of recurrence interval (maximum wave height of 10.6 m), both approaching the coast perpendicularly. After calculating theoretical storm surge elevation, a digital terrain model was made by adjusting topographic data to field work and detailed geomorphological analysis. A model of flooding extent was subsequently developed for each storm type, and then corrected according to the rates of shoreline change in the last decades, which were assessed by means of aerial photographs taking the dune toe as shoreline indicator. Results show that long-term coastline trend represents an important factor in the prediction of flooding extent, since shoreline retreat causes the deterioration of natural coastal defences as dune ridges, thus increasing coastal exposure to high-energy waves. This way, it has been stated that the lack of sedimentary supply plays an important role in spatial variability of inundation extent in Valdelagrana spit. Finally, a hazard map is presented, where calculated coastal retreat rates are employed in order to predict the areas that could be affected by future inundation events.

Beach-Dune Interactions in Nourishment Areas along the Dutch Coast

The impact of beach nourishment on the development of coastal dunes was studied along the Dutch coast. A database of annual cross-shore profiles was analysed to derive volumetric changes associated with aeolian and hydrodynamic processes. The database covered a period of 15 years. Beach nourishment projects that were carried out arbitrarily within this time-span were selected, and the volumetric changes occurring on the selected sites were statistically related to the number of years following nourishment. An overall negative sand budget was found for the supratidal zone of the nourished sites. A substantial part of the sand was blown to the foredunes. One year after nourishment, this amount increased significantly. At the same time, the supratidal beach was eroded more. In the second and third year after nourishment, the erosion of the higher parts of the nourishment decreased. In the foredune, dune toe erosion due to storm surges was usually negligible until the fourth year following beach nourishment. Thus, beach nourishment temporarily protected the adjacent foredunes from being eroded by periodic wave attack, and also temporarily enlarged the aeolian sand transport rate to the dunes.

Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry

A Computational Fluid Dynamics (CFD) model (PHOENICS™ 3.5) previously validated for wind tunnel measurements is used to simulate the streamwise and vertical velocity flow fields over idealized transverse dunes of varying height ( h) and stoss slope basal length ( L). The model accurately reproduced patterns of: flow deceleration at the dune toe; stoss flow acceleration; vertical lift in the crest region; lee-side flow separation, re-attachment and reversal; and flow recovery distance. Results indicate that the flow field over transverse dunes is particularly sensitive to changes in dune height, with an increase in height resulting in flow deceleration at the toe, streamwise acceleration and vertical lift at the crest, and an increase in the extent of, and strength of reversed flows within, the lee-side separation cell. In general, the length of the separation zone varied from 3 to 15 h from the crest and increased over taller, steeper dunes. Similarly, the flow recovery distance ranged from 45 to >75 h and was more sensitive to changes in dune height. For the range of dune shapes investigated in this study, the differing effects of height and stoss slope length raise questions regarding the applicability of dune aspect ratio as a parameter for explaining airflow over transverse dunes. Evidence is also provided to support existing research on: streamline curvature and the maintenance of sand transport in the toe region; vertical lift in the crest region and its effect on grainfall delivery; relations between the turbulent shear layer and downward forcing of flow re-attachment; and extended flow recovery distances beyond the separation cell. Field validation is required to test these findings in natural settings. Future applications of the model will characterize turbulence and shear stress fields, examine the effects of more complex isolated dune forms and investigate flow over multiple dunes.

Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel

AbstractTopographic interactions generate multidirectional and unsteady airflow that limits the application of velocity profile approaches for estimating sediment transport over dunes. Results are presented from a series of wind tunnel simulations using Irwin‐type surface‐mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp‐crested model dunes. Findings complement existing theories on secondary airflow effects on stoss transport dynamics and provide new information on the influence of lee‐side airflow patterns on dune morphodynamics.For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a significant drop in time‐averaged measurements of streamwise shear stress at this location. This effect is believed sufficient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and flow acceleration cause an increase in flow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60–70 per cent greater than at flow reattachment on the lower stoss of closely spaced dunes. Streamwise flow accelerations, rather than turbulence, have greater influence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed flow within the separation cell generates significant surface shear (30–40 per cent of maximum values) for both spacings. This supports field studies that suggest reversed flow is competent enough to return sediment to the dune directly or in a deflected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time‐averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and flow steadiness increase within 6 h (h = dune height) of reattachment and approach upwind values by 25 h. A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to fluvial estimates.The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost‐effective design shows promise for validating these findings in natural dune settings. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Wind-blown sand on beaches: an evaluation of models

Five models for predicting rates of aeolian sand transport were evaluated using empirical data obtained from field experiments conducted in April, 1994 at a beach on Inch Spit, Co. Kerry, Republic of Ireland. Measurements were made of vertical wind profiles (to derive shear velocity estimates), beach slope, and rates of sand transport. Sediment samples were taken to assess characteristics of grain size and surface moisture content. Estimates of threshold shear velocity were derived using grain size data. After parsing the field data on the basis of the quality of shear velocity estimation and the occurrence of blowing sand, 51 data sets describing rates of sand transport and environmental conditions were retained. Mean grain diameter was 0.17 mm. Surface slopes ranged from 0.02 on the foreshore to about 0.11 near the dune toe. Mean shear velocities ranged from 0.23 m s −1 (just above the observed transport threshold) to 0.65 m s −1. Rates of transport ranged from 0.02 kg m −1 h −1 to more than 80 kg m −1 h −1. These data were used as input to the models of Bagnold [Bagnold, R.A., 1936. The Movement of Desert Sand. Proc. R. Soc. London, A157, 594–620], Kawamura [Kawamura, R., 1951. Study of Sand Movement by Wind. Translated (1965) as University of California Hydraulics Engineering Laboratory Report HEL 2–8, Berkeley], Zingg [Zingg, A.W., 1953. Wind tunnel studies of the movement of sedimentary material. Proc. 5th Hydraulics Conf. Bull. 34, Iowa City, Inst. of Hydraulics, pp. 111–135], Kadib [Kadib, A.A., 1965. A function for sand movement by wind. University of California Hydraulics Engineering Laboratory Report HEL 2–8, Berkeley], and Lettau and Lettau [Lettau, K. and Lettau, H., 1977. Experimental and Micrometeorological Field Studies of Dune Migration. In: K. Lettau and H. Lettau (Eds.), Exploring the World's Driest Climate. University of Wisconsin-Madison, IES Report 101, pp. 110–147]. Correction factors to adjust predictions of the rate of transport to account for the effects of slope and moisture content were calculated using the models of Bagnold [Bagnold, R.A., 1973. The nature of saltation and ‘bed-load’ transport in water. Proc. R. Soc. London, Ser. A, 332, 473–504] and Belly [Belly, P.-Y., 1964. Sand movement by wind. U.S. Army Corps Eng. CERC. Tech. Mem. 1, Washington D.C., 38 pp.], respectively. None of the models was able to produce a strong correspondence between measured and predicted rates of transport. Best results were obtained using the Bagnold and Zingg models, and the Kadib model was the least viable of this group. The influence of sediment moisture content appeared to be the critical factor in degrading model viability. Overall, none of the models is adequate for general applications to coastal-aeolian environments where moisture content complications tend to override the predictive competence of the simple transport formulations.

Sediment flux and airflow on the stoss slope of a barchan dune

Measurements of sediment flux on the windward slope of an isolated barchan using an array of 30 sand traps provide new data that can constrain models of dune dynamics. The data show that at low wind incident speeds, flux increases up the dune exponentially, whereas at higher wind speeds the increase with distance approaches linearity. Wind profile measurements, conducted at the same time as the flux measurements, indicate that, although wind speed at a given height increases by 1.2 times from dune toe to brinkline, wind shear velocity derived from the profile data decreases up the dune and is in many cases below transport threshold values. This demonstrates that conventional wind profiles, derived from anemometry on dunes, do not measure the part of the boundary layer that is significant for sediment transport.