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

Wind 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 & Sons, Ltd.     

Offshore aeolian sediment transport across a human‐modified foredune

Concepts derived from previous studies of offshore winds on natural dunes are evaluated on a dune maintained for shore protection during three offshore wind events. The potential for offshore winds to form a lee‐side eddy on the backshore or transfer sediment from the dune and berm crest to the water are evaluated, as are differences in wind speed and sediment transport on the dune crest, berm crest and a pedestrian access gap. The dune is 18–20 m wide near the base and has a crest 4.5 m above backshore elevation. Two sand‐trapping fences facilitate accretion. Data were obtained from wind vanes on the crest and lee of the dune and anemometers and sand traps placed across the dune, on the beach berm crest and in the access gap. Mean wind direction above the dune crest varied from 11 to 3 deg from shore normal. No persistent recirculation eddy occurred on the 12 deg seaward slope. Wind speed on the berm crest was 85–89% of speed at the dune crest, but rates of sediment transport were 2.27 times greater during the strongest winds, indicating that a wide beach overcomes the transport limitation of a dune barrier. Limited transport on the seaward dune ramp indicates that losses to the water are mostly from the backshore, not the dune. The seaward slope gains sand from the landward slope and dune crest. Sand fences causing accretion on the dune ramp during onshore winds lower the seaward slope and reduce the likelihood of detached flows during offshore winds. Transport rates are higher in access gaps than on the dune crest despite lower wind speeds because of flatter slopes and absence of vegetation. Transport rates across dunes and through gaps can be reduced using vegetation and raised walkover structures. Copyright © 2017 John Wiley & Sons, Ltd.     

Aeolian sediment transport on a human‐altered foredune

Changes in wind speed and sediment transport are evaluated at a gap and adjacent crest of a 2 to 3 m high, 40 m wide foredune built by sand fences and vegetation plantings on a wide, nourished fine sand beach at Ocean City, New Jersey. Anemometer masts, cylindrical sand traps and erosion pins were placed on the beach and dune during two obliquely onshore wind events in February and March 2003. Results reveal that: (1) changes in the alongshore continuity of the beach and dune system can act as boundaries to aeolian transport when winds blow at an angle to the shoreline; (2) oblique winds blowing across poorly vegetated patches in the dune increase the potential for creating an irregular crest elevation; (3) transport rates and deflation rates can be greater within the foredune than on the beach, if the dune surface is poorly vegetated and the beach has not had time to dry following tidal inundation; (4) frozen ground does not prevent surface deflation; and (5) remnant sand fences and fresh storm wrack have great local but temporary effect on transport rates. Temporal and spatial differences due to sand fences and wrack, changes in sediment availability due to time‐dependent differences in surface moisture and frozen ground, combined with complex topography and patchy vegetation make it difficult to specify cause–effect relationships. Effects of individual roughness elements on the beach and dune on wind flow and sediment transport can be quantified at specific locations at the event scale, but extrapolation of each event to longer temporal and spatial scales remains qualitative. Copyright © 2006 John Wiley & Sons, Ltd.     

Aeolian sediment transport on a recovering storm‐eroded foredune with sand fences

Studies of sediment transport on developed coasts provide perspective on how human adjustments alter natural processes. Deployment of sand‐trapping fences is a common adjustment that changes the characteristics of the dune ramp and its role in linking sediment transfers from the backshore to the foredune. Fence effects were evaluated in the field using anemometer arrays and vertical sediment traps placed across a beach and dune at Seaside Park, New Jersey, USA during onshore and longshore winds. The foredune is 18 m wide and 4.5 m above the backshore. The mean speed of onshore winds at 0.5 m elevation decreased by 17% from the berm crest to the upper ramp and 36% in the lee of a fence there. Sediment transport during mean wind speeds up to 8.0 m s^?1 at 0.5 m elevation was < 0.06 kg m^?1 h^?1 on the berm crest and backshore where fetch distances were < 45 m and surface sediment was relatively coarse (0.74–0.85 mm) but increased to 5.63 kg m^?1 h^?1 on the upper ramp aided by the longer fetch distances (up to 82 m) and finer grain size of the source sediment there (0.52 mm). Sediment transport along the berm crest and backshore during longshore winds, where fetch distances were > 200 m, was up to 58.69 kg m^?1 h^?1, about three orders of magnitude greater than during the onshore winds. Fences can displace the toe of the ramp farther seaward than would occur under natural conditions. They can create a gentler slope and change the shape of the ramp to a more convex form. A fence on the ramp can cut off a portion of sediment supply to the upper slope. Decisions about fence placement thus should consider these morphologic changes in addition to the effects on dune volume. Copyright © 2017 John Wiley & Sons, Ltd.     

Morphological records of storm floods exemplified by the impact of the 1872 Baltic storm on a sandy spit system in south‐eastern Denmark

Beach‐ridge systems are important geo‐archives providing evidence for past wave climate including catastrophic storm flood events. This study investigates the morphological impacts of the 1872 Baltic storm flood on a beach‐ridge system (sandy spit) in south‐eastern Denmark and evaluates the frequency of extreme storm flood events in the area over a longer time perspective. This paper combines field studies of morphology and sedimentary deposits, studies of historical maps, digital terrain model, ground‐penetrating radar profiles, and luminescence dating. Sea water reached 2.8 m above mean sea level (amsl) during peak inundation and, based on studies of the morphological impacts of the 1872 storm flood, the event can be divided into four phases. Phase 1: increasing mean water levels and wave activity at the beach brought sediments from the beach (intertidal bars and normal berm) higher up in the profile and led to the formation of a storm‐berm. Phase 2: water levels further increased and sediment in the upper part of the profile continued to build up the storm‐berm. Phase 3: water levels now reached the top of the dune ridge and were well above the storm‐berm level. Sea water was breaching the dune ridge at several sites and wash‐over fans were generated until a level where the mean water level had dropped too much. Phase 4: the non‐vegetated wash‐over fans functioned as pathways for aeolian sand transport and relatively high dunes were formed in particular along the margins of the fan where aeolian sand was trapped by existing vegetation. The studied beach‐ridge system records about 4500 years of accumulation; the storm flood sediments described are unique suggesting that the 1872 Baltic storm flood event was an extreme event. Thus studies of beach‐ridge systems form a new source for understanding storm surge risk. Copyright © 2013 John Wiley & Sons, Ltd.     

Berm formation and dynamics on a gently sloping beach; the effect of water level and swash overtopping

Berm formation and morphological development of the beach face have been observed during a neap–neap tidal cycle on the gently sloping and accreting beach at Vejers, Denmark. During the field campaign, an intertidal bar migrated onshore and stabilized as a berm on the foreshore. A new intertidal bar occurred on the lower beach face, migrated onshore on the rising tide and finally merged with the pre‐existing berm. As the tide continued to rise, the new berm translated further onshore as an intertidal bar to the uppermost part of the foreshore. The sediment transport during the berm transition was onshore directed in the upper swash and offshore directed in the lower swash. This berm development can be described through both the neap‐berm, ridge‐and‐runnel and berm‐ridge development concepts proposed by Hine (Sedimentology 1979; 26: 333–351), and all three stages were observed during only three tidal cycles. The main factors controlling this fast transformation were the gentle slope of the cross‐shore profile, rapid water level translation rates, substantial swash overtopping of the berm, and low infiltration rates. Despite the onshore migration of intertidal bars and berm formation, no net foreshore accretion took place during the field campaign. This was largely due to the formation of rip channels with strong rip currents cutting through the intertidal bars and the berm, which acted as a sediment drain in the profile. Copyright © 2009 John Wiley & Sons, Ltd.     

Simple infrared techniques for measuring beach surface moisture

Knowledge of surface moisture on beaches is vital for aeolian process studies because moisture increases transport thresholds and limits mass flux. A number of beach surface moisture measurement techniques have been employed in the field, including sample extraction, commercial soil moisture sensors, and remote sensing techniques. Each method has significant limitations in the context of aeolian process studies. This study was designed to test infrared optic techniques for measuring beach surface moisture. A simple infrared sensor (narrow‐band radiometer) was developed to measure beach surface moisture content. The accuracy and practical usability of the narrow‐band radiometer were assessed in comparison to a commercial handheld spectroradiometer. Field calibrations conducted at Cape San Blas, Florida and Padre Island, Texas indicated that the narrow‐band radiometer performed quite well. The R² values exceeded 0·98 in each case, and the standard error averaged about 1% moisture content compared with gravimetric moisture contents determined from 1·5 mm deep surface scrapes. The performance of the two instruments was found to be comparable, with the narrow‐band radiometer slightly outperforming the spectroradiometer. In practical applications, the narrow‐band radiometer also has logistical advantages and is better suited to measure large numbers of points. Copyright © 2012 John Wiley & Sons, Ltd.     

Aeolian sand strip mobility and protodune development on a drying beach: examining surface moisture and surface roughness patterns measured by terrestrial laser scanning

Ephemeral aeolian sand strips are commonplace on beaches. Their formation during high energy sand transport events often precedes the development of protodunes and their dynamics present interesting feedback mechanisms with surface moisture patterns. However, due to their temporary nature, little is known of their formation, mobility or the specifics of their interaction with beach surface characteristics. Similarly surface moisture has an important influence on sediment availability and transport in aeolian beach systems, yet it is difficult to quantify accurately due to its inherent variability over both short spatial and temporal scales. Whilst soil moisture probes and remote sensing imagery techniques can quantify large changes well, their resolution over mainly dry sand, close to the aeolian transport threshold is not ideal, particularly where moisture gradients close to the surface are large. In this study we employed a terrestrial laser scanner to monitor beach surface moisture variability during a three and a half hour period after a rain event and investigated relationships between bedform development, surface roughness and surface moisture. Our results demonstrate that as the beach surface dries, sand transport increases, with sediment erosion occurring at the wet/dry surface boundary, and deposition further downwind. This dynamic structure, dependent upon changing surface moisture characteristics, results in the formation of a rippled sand strip and ultimately a protodune. Our findings highlight dynamic mobility relationships and confirm the need to consider transient bedforms and surface moisture across a variety of scales when measuring aeolian transport in beach settings. The terrestrial laser scanner provides a suitable apparatus with which to accomplish this. Copyright © 2010 John Wiley & Sons, Ltd.     

The role of wind direction in eolian transport on a narrow sandy beach

Comparison of eolian transport during five high-velocity wind events over a 29 day period on a narrow estuarine beach in Delaware Bay, New Jersey, USA, reveals the temporal variability of transport, due to changes in direction of wind approach. Mean wind speed measured 6 m above the dune crest for the five events ranged from 8·5 to 15·9 ms^?1. Mean wind direction was oblique to the shoreline (63° from shore-normal) during one event but was within 14° of shore-normal during the other events. Eolian transport is greatest during low tide and rising tide, when the beach source area is widest and when drying of surface sediments occurs. The quantity of sediment caught in a vertical trap for the five events varied from a total of 0·07 to 113·73 kgm^?1. Differences in temperature, relative humidity and moisture and salt content of surficial sediments were slight. Mean grain sizes ranged from 0·33 to 0·58 mm, causing slight differences in threshold shear velocity, but shear velocities exceeded the threshold required for transport during all events. Beach width, measured normal to the shoreline, varied from 15·5 to 18·0 m; beach slope differed by 0·5°. The oblique wind during one event created a source width nearly double the width during other days. Beach slope, measured in the direction of the wind, was less than half as steep as the slope measured normal to the shoreline. The amount of sand trapped during the oblique wind was over 20 times greater than any other event, even those with higher shear velocities. The ability of the beach surface to supply grains to the air stream is limited on narrow beaches, but increased source width, due to oblique wind approach, can partially overcome limitations of surface conditions on the beach.     

Small‐scale variability in surface moisture on a fine‐grained beach: implications for modeling aeolian transport

Small‐scale variations in surface moisture content were measured on a fine‐grained beach using a Delta‐T Theta probe. The resulting data set was used to examine the implications of small‐scale variability for estimating aeolian transport potential. Surface moisture measurements were collected on a 40 cm × 40 cm grid at 10 cm intervals, providing a total of 25 measurements for each grid data set. A total of 44 grid data sets were obtained from a representative set of beach sub‐environments. Measured moisture contents ranged from about 0% (dry) to 25% (saturated), by weight. The moisture content range within a grid data set was found to vary from less than 1% to almost 15%. The magnitude of within‐grid variability varied consistently with the mean moisture content of the grid sets, following an approximately normal distribution. Both very wet and very dry grid data sets exhibited little internal variability in moisture content, while intermediate moisture contents were associated with higher levels of variability. Thus, at intermediate moisture contents it was apparent that some portions of the beach surface could be dry enough to allow aeolian transport (i.e. moisture content is below the critical threshold), while adjacent portions are too wet for transport to occur. To examine the implications of this finding, cumulative distribution functions were calculated to model the relative proportions of beach surface area expected to be above or below specified threshold moisture levels (4%, 7%, and 14%). It was found that the implicit inclusion of small‐scale variability in surface moisture levels typically resulted in changes of less than 1% in the beach area available for transport, suggesting that this parameter can be ignored at larger spatial scales. Copyright © 2009 John Wiley & Sons, Ltd.     

Aeolian dynamics over a coastal foredune,Prince Edward Island,Canada

Near‐surface airflow over a morphologically simple, vegetated, 8 m high foredune with a small wave‐cut scarp was measured for onshore to oblique‐onshore conditions during a low‐moderate (5–6 m s^‐1 ) wind event and a high velocity (11–18 m s^‐1) sand‐transporting gale event. Flow across the foredune was characterized by significant flow compression and acceleration up and across the foredune during both events. During the gale, a pronounced jet (speed bulge) developed at the foredune crest, which increased in magnitude with increasing wind speed. The vertical (W) velocity component of the 3D flow field was positive (upwards) across the stoss slope under low wind conditions but negative (downwards) during gale wind conditions, with upslope acceleration. During the low velocity event, there was speed‐down within the vegetation canopy, as would be expected for a porous roughness cover. During the strong wind event there was speed‐up in the lower portion of the vegetation canopy, and this was found up the entire stoss slope. Sediment transport during the gale force event was substantial across the beach and foredune despite the moderate vegetation cover and minimum fetch. Aeolian suspension was evident in the lee of the dune crest. The observations presented herein show that strong storm winds are an effective mechanism for translating sediment landwards across a high vegetated foredune, contributing sediment to the stoss slope, crest and leeward slopes of the foredune and backing dunes. Copyright © 2013 John Wiley & Sons, Ltd.     

The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach

This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta‐T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two‐dimensional sonic anemometer. Transport intensity was measured at a height of 2–4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10–20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1–2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance. Copyright © 2007 John Wiley & Sons, Ltd.     

Tide-induced variability in beach surface moisture: Observations and modelling

The moisture content w_s of a beach surface strongly controls the availability of sand for aeolian transport. Our predictive capability of the spatiotemporal variability in w_s, which depends to a large extent on water table depth, is, however, limited. Here we show that water table fluctuations and surface moisture content observed during a 10-day period on a medium-grained (365μm) planar (1:30) beach can be predicted well with the nonlinear Boussinesq equation extended to include run-up infiltration and a soil–water retention curve under the assumption of hydrostatic equilibrium. On the intertidal part of the beach the water table is observed and predicted to continuously fall from the moment the beach surface emerges from the falling tide to just before it is submerged by the incoming tide. We find that on the lower 30% of the intertidal beach the water table remains within 0.1–0.2 m from the surface and that the sand is always saturated (w_s≈20%, by mass). Higher up on the intertidal beach, the surface can dry to about 5% when the water table has fallen to 0.4–0.5 m beneath the surface. Above the high-tide level the water table is always too deep (>0.5 m) to affect surface moisture and, without precipitation, the sand is dry (w_s < 5 − 8%). Because the water table depth on the emerged part of the intertidal beach increases with time irrespective of whether the (ocean) tide falls or rises, we find no need to include hysteresis (wetting and drying) effects in the surface-moisture modelling. Model simulations suggest that at the present planar beach only the part well above mean sea level can dry sufficiently (w_s < 10%) for sand to become available for aeolian transport. ©2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Wind direction and complex sediment transport response across a beach–dune system

Evidence from a field study on wind flow and sediment transport across a beach–dune system under onshore and offshore conditions (including oblique approach angles) indicates that sediment transport response on the back‐beach and stoss slope of the foredune can be exceedingly complex. The upper‐air flow – measured by a sonic anemometer at the top of a 3·5 m tower located on the dune crest – is similar to regional wind records obtained from a nearby meteorological station, but quite different from the near‐surface flow field measured locally across the beach–dune profile by sonic anemometers positioned 20 cm above the sand surface. Flow–form interaction at macro and micro scales leads to strong modulation of the near‐surface wind vectors, including wind speed reductions (due to surface roughness drag and adverse pressure effects induced by the dune) and wind speed increases (due to flow compression toward the top of the dune) as well as pronounced topographic steering during oblique wind approach angles. A conceptual model is proposed, building on the ideas of Sweet and Kocurek (Sedimentology 37 : 1023–1038, 1990), Walker and Nickling (Earth Surface Processes and Landforms 28 : 111–1124, 2002), and Lynch et al. (Earth Surface Processes and Landforms 33 : 991–1005, 2008, Geomorphology 105 : 139–146, 2010), which shows how near‐surface wind vectors are altered for four regional wind conditions: (a) onshore, detached; (b) onshore‐oblique, attached and deflected; (c) offshore, detached; and (d) offshore‐oblique, attached and deflected. High‐frequency measurements of sediment transport intensity during these different events demonstrate that predictions of sediment flux using standard equations driven by regional wind statistics would by unreliable and misleading. It is recommended that field studies routinely implement experimental designs that treat the near‐surface wind field as comprising true vector quantities (with speed and direction) in order that a more robust linkage between the regional (upper air) wind field and the sediment transport response across the beach–dune profile be established. Copyright © 2012 John Wiley & Sons, Ltd.     

Aeolian sediment transport and morphologic change on a managed and an unmanaged foredune

Sediment transport and short‐term morphologic change were evaluated at a site where sand fences are deployed and the beach is raked (Managed Site) and a site where these human adjustments are not practiced (Unmanaged Site). Data were gathered across the seaward portion of a low foredune when winds blew nearly shore‐normal at mean speeds 8.9 to 9.3 m s^‐1. Data from traps revealed sediment transport rates at unvegetated portions of the foredune crest (40.2 to 43.5 kg m^‐1 h^‐1) were greater than on the backshore (4.9 to 11.2 kg m^‐1 h^‐1) due to onshore decreases in surface moisture and speed‐up of the wind passing over the foredune. Data from erosion pins indicate sediment input to the dune was 1.48 m³ m^‐1 alongshore at the Managed Site and 1.25 m³ m^‐1 at the Unmanaged Site. The Unmanaged Site had deposition at the dune toe, erosion at mid‐slope, and deposition at the crest. Deposition occurred at mid‐slope on the Managed Site near a partially buried (0.58 m high) fence with a porosity of about 65%. Deposition at partially buried wrack on the upper backshore and dune toe at the Unmanaged Site was about twice as great as deposition in this zone at the Managed Site. Results indicate that: (1) the seaward slope of the foredune can be a more important source of sand to the lee of the crest than the beach; (2) wrack near the toe can decrease transport into the foredune; (3) a scour zone can occur on the foredune slope above the wrack line; (4) a fence placed in this location can promote deposition and offset scour, but fences can restrict delivery of sediment farther inland. Evaluation of alternative configurations of fences and strategies for managing wrack is required to better determine the ways that humans modify foredunes. Copyright © 2012 John Wiley & Sons, Ltd.     

Measurement and modeling of moisture content above an oscillating water table: implications for beach surface moisture dynamics

This study examined the influence of tidally‐induced oscillations of the beach water table in regulating beach surface moisture dynamics. A series of laboratory experiments were conducted to assess the influence of hysteresis and transient flow effects on surface moisture variability. The experimental apparatus utilized a column of well‐sorted fine sand partially immersed in a reservoir of water. The water level in the reservoir was raised and lowered via a diaphragm‐metering pump to simulate tidally induced fluctuations of the water table, and the moisture content profile within the column was monitored using an array of Delta‐T probes. Moisture contents at specific elevations within the column were utilized as proxies to represent various ‘surface’ elevations (relative to the high water table). Results indicate that surface moisture content behaves in a distinctly hysteretic manner. Examination of water flow scanning curves illustrated that for all surface elevations considered, higher moisture contents for a given pressure head occurred during the drying cycle than during the wetting cycle. This observation is particularly evident with shallow surface elevations (i.e. water table close to the surface) where the Haines Jump phenomenon was found to have a significant influence on moisture content dynamics. Additionally, an assessment of the accuracy of hysteretic and non‐hysteretic models to predict the measured moisture contents demonstrated that hysteretic simulations consistently provide a better representation of the observed moisture contents than non‐hysteretic simulations. A time lag was found between the respective maxima and minima in water table elevation surface moisture content. At the near surface water table positions the time lag ranged between 30 and 100 minutes, and it increased to 240 minutes (four hours) with the high water table at 60 cm below the surface. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of Time-dependent Moisture Content of Surface Sediments on Aeolian Transport Rates Across a Beach,Wildwood, New Jersey,U.S.A.

A one-day field investigation on an unvegetated backbeach documents the importance of surface sediment drying to aeolian transport. Surface sediments were well sorted fine sand. Moisture content of samples taken in the moist areas on the backbeach varied from 2·9 to 9·2 per cent. Lack of dry sediment inhibited transport prior to 08:50. By 09:10 conspicuous streamers of dry sand moved across the moist surface. Barchan-shaped bedforms, 30 to 40 mm high and composed of dry sand (moisture content <0·10 per cent), formed where sand streamers converged. The surface composed of dry sand increased from 5 per cent of the area of the backbeach at 09:50 to 90 per cent by 12:50 Mean wind speeds were beetween 5·6 and 8·6 m s⁻¹ at 6 m above the backbeach. Corresponding shear velocities were always above the entrainment threshold for dry sand and below the threshold for the moist sand on the backbeach. Measured rates of sand trapped (by vertical cylindrical traps) increased during the day relative to calculated rates. The measured rate of sand trapped on the moist foreshore was higher than the rate trapped on the backbeach during the same interval, indicating that the moist foreshore (moisture content 18 per cent) was an efficient transport surface for sediment delivered from the dry portion of the beach upwind. Measured rates of sand trapped show no clear relationship to shear velocities unless time-dependent surface moisture content is considered. Results document conditions that describe transport across moist surfaces in terms of four stages including: (1) entrainment of moist sediment from a moist surface; (2) in situ drying of surface grains from a moist surface followed by transport across the surface; (3) entrainment and transport of dry sediment from bedforms that have accumulated on the moist surface; and (4) entrainment of sand from a dry upwind source and transport across a moist downwind surface. © 1997 John Wiley & Sons, Ltd.     

High‐frequency sediment transport responses on a vegetated foredune

Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave‐cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten‐minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd.     

Reynolds stress and sand transport over a foredune

Reynolds shear stress (RS = –u′ w′) and sand transport patterns over a vegetated foredune are explored using three‐dimensional velocity data from ultrasonic anemometers (at 0 · 2 and 1 · 2 m) and sand transport intensity from laser particle counters (at 0 · 014 m). A mid‐latitude cyclone on 3–4 May 2010 generated storm‐force winds (exceeding 20 m s^–1) that shifted from offshore to obliquely alongshore. Quadrant analysis was used to characterize the spatial variation of RS quadrant components (Q1 through Q4) and their relative contributions were parameterized using the flow exuberance relation, EX_FL = (Q1 + Q3)/(Q2 + Q4). The magnitudes of RS and sand transport varied somewhat independently over the dune as controlled by topographic forcing effects on flow dynamics. A ‘flow exuberance effect’ was evident such that Q2 (ejection‐like) and Q4 (sweep‐like) quadrants (that contribute positively to RS) dominated on the beach, dune toe, and lower stoss, whereas Q1 and Q3 (that contribute negatively to RS) dominated near the crest. This exuberance effect was not expressed, however, in sand transport patterns. Instead, Q1 and Q4, with above‐average streamwise velocity fluctuations (+u′), were most frequently associated with sand transport. Q4 activity corresponded with most sand transport at the beach, toe, and stoss locations (52, 60, 100%). At the crest, 25 to 86% of transport was associated with Q1 while Q4 corresponded with most of the remaining transport (13 to 59%). Thus, the relationship between sand transport and RS is not as straightforward as in traditional equations that relate flux to stress in increasing fashion. Generally, RS was poorly associated with sand transport partly because Q1 and Q4 contributions offset each other in RS calculations. Thus, large amounts of transport can occur with small RS. Turbulent kinetic energy or Reynolds normal stresses (u′², w′²) may provide stronger associations with sand transport over dunes, although challenges exist on how to normalize and compare these quantities. Copyright © 2013 John Wiley & Sons, Ltd.