Potential Sand Transport Research Articles

Impact of coastline orientation on the dynamics of foredune growth (Łeba Barrier, south Baltic Sea coast, Poland)

Abstract There are many factors controlling the aeolian sand transport rate on the beach and thus, variation in rates can impact foredune development. This study analyses coastline orientation as one such factor. The research was conducted at two sites on the Łeba Barrier, south Baltic Sea coast, Poland, located 4.2 km apart and which differed in coastline orientation by 20°. Based on wind data the potential aeolian sand transport rate for the period 2006–2017 was calculated. It appeared that taking into account the coastline orientation, the landward sand transport strongly exceeded the seaward and alongshore transport at one site, whereas the other site was dominated by alongshore sand transport. Topographic surveys carried out every 6 months showed that net sand deposition within the foredune at the coast dominated by landward transport was two and a half times greater than at the other site and resulted in significant dune growth.

Wind regime and aeolian landforms on the eastern shore of Qinghai Lake, Northeastern Tibetan Plateau, China

Wind regime is typically highly variable in space and in time, and the studies of aeolian geomorphology often use a single meteorological station as representative of a specific dune field due to the scarcity of meteorological stations in remote areas. However, the wind regime may vary greatly even between sites located just a few kilometers apart. In this study, to fully understand the relationship between wind regime and aeolian landforms of the dune field on the eastern shore of the Qinghai Lake, 1-year hourly recorded wind data from an average wind year 2012 from four stations, namely Gangcha Station, Eri Station, Wendu Station and Daotanghe Station was used to analyze the wind regime and resultant potential sand transport. The wind data analysis revealed that the drift potential (DP) at Gangcha, Eri, Wendu and Daotanghe stations is 144, 537, 829 and 431 VU, respectively. The resultant drift potential (RDP) at Gangcha, Eri, Wendu and Daotanghe stations is 112, 143, 544 and 125 VU, respectively. The high-energy wind environment, in addition to multidirectional and locally highly variable wind regime are the principal factors likely responsible for the coexistence of several types of aeolian landforms (e.g., Reversing transverse, barchanoid and stardunes).

An improved approach to estimate sand-driving winds

Wind resource assessment has important economic and ecological implications. Many developing countries in arid and semiarid areas are caught in a dilemma between the development of wind energy and the threat of sandy desertification. Sand-driving winds are momentous for both generating wind energy and aeolian research. The estimation for the probability of sand-driving winds has been rarely studied. The ability of the two-parameter Weibull distribution to estimate high wind velocities is questioned and parameters appear to have no practical significance. Drift potential (DP) is a way of evaluating potential sand transport volume by sand-driving winds, but the relationship between DP and the wind energy of sand-driving winds has been rarely studied. In this paper, a function was constructed from the expressions of DP and the two-parameter Weibull probability distribution to improve the estimation of sand-driving winds. This is simple and applicable to arid and semiarid areas worldwide. The mean wind velocity and the mean wind power density at a height of 10 m around the Badain Jaran Sand Sea in China, are in the range between 2.64 and 4.79 m·s−1, and 27.61–165.61 W·m−2, respectively. The mean wind power density of sand-driving winds accounted for half or more of the total mean wind power density. The relationship among DP, the wind power density of sand-driving winds and the probability of sand-driving winds, was proved theoretically. Gentle winds and strong winds are distinguished by the drift wind velocity at which potential sand transport activities (potential lost wind power density) reach their maximum. The scale parameter dominates the mean and median wind velocity, and represents the effect of convection. The shape parameter characterizes the atmospheric stability. These results provide a basis for the wind energy development, improve the understanding of aeolian activities, and have implications for sand control engineering.

An improved model to estimate annual sand transport rate by sand-driving winds

Some of the surface on Earth, Mars, Venus, and Titan is covered by ripples, dunes, and other features formed by sand-driving winds. Many developing countries in arid and semiarid areas are caught in a dilemma between the threat of sandy desertification and the development of wind energy. The estimation for the probability of sand-driving winds has been rarely studied. The ability of two-parameter Weibull probability distribution to estimate sand-driving winds is questioned. Sand transport rate is used only for the evaluation of short-term aeolian activities. Drift potential (DP) is an accessible way of evaluating potential sand transport volume by sand-driving winds. However, the relationship among annual sand transport volume, DP and wind energy has been rarely studied. Sand-driving winds can be divided into gentle winds and strong winds, but it is hard to distinguish them and to quantify their effects on aeolian geomorphology. A function was constructed from expressions of sand transport rate and the two-parameter Weibull probability distribution to improve the estimation of sand-driving winds. On this basis, annual sand transport volume was accurately evaluated by sand-driving winds. This is applicable to arid and semiarid areas worldwide. The relationship among annual sand transport volume, DP and wind energy was analyzed from the perspective of dimensional analysis. Gentle winds and strong winds are distinguished by transport wind velocity at which aeolian activities on a sandy underlying surface reach the erosion–deposition balance, and which corresponds to annual maximum sand transport rate. On a sandy underlying surface strong winds control deposition, while gentle winds dominate wind erosion. The transport wind velocity is nearly constant, weakly related to the probability distribution of sand-driving winds. These results provide a basis for an improved understanding of aeolian activities as an important planetary surface process, sand control engineering, and wind energy development.

Regional versus local wind speed and direction at a narrow beach with a high and steep foredune.

Dune growth and post-storm recovery of foredune systems is predominantly determined by the aeolian sand transport through the beach-dune interface. Potential sand transport rates, estimated with empirical transport equations using regionally representative wind conditions, are generally too high. This positive bias might be, at least partly, due to the effect of the beach and foredune topography on the regional airflow. Here, we investigate the relation between local (on the beach) and regional wind velocities and direction in front of the high (∼22 m) and steep (∼1:2.5) foredune partially vegetated with Marram grass at Egmond aan Zee, The Netherlands based on a dataset with a large variety in wind speeds spanning over all onshore wind directions. We observed that local 10-minute averaged wind speed and direction can differ from the regional wind conditions (here measured 15 km away from the study site) depending on the regional approach angle of the wind. The ratio of local over regional wind speed is smallest (∼0.39) when the wind direction is dune-normal. This ratio increases with increasing obliquity towards almost 1 for alongshore winds. Wind steering only happens at the dune foot and is the largest (∼13°) with oblique approaching winds of 40° from the dune normal. Perpendicular and nearly alongshore winds do not show any steering near the dune foot. The use of local rather than regional wind conditions in a potential transport equation reduces the predicted annual supply from 86 to 32 m3/m/y, substantially closer to the measured deposition of 15 m3/m/y. The drop in velocity was more important to the reduction in predicted supply than the alongshore steering.

Overview of the Monsoon-influenced Ayeyarwady River delta, and delta shoreline mobility in response to changing fluvial sediment supply

A morpho-sedimentary analysis of the Ayeyarwady delta shoreline was conducted based on a field mission in Myanmar in November 2016 and interpretation of satellite images spanning the period 1974–2019. These analyses were complemented by data on land-to-water conversion and vice versa within a 2 km-wide coastal fringe, and on MERIS-derived seasonal and decadal-scale evolution of suspended particulate matter (SPM) off the delta. The objectives were to: (1) characterize the 450 km-long delta shoreline and coastal sediment transport pathways, (2) define the shoreline status (stability, erosion, accretion), and (3) identify potential causes of shoreline change and the future outcome of this status in terms of delta vulnerability. The delta shoreline was characterized on the basis of qualitative alongshore tidal, wave-energy, and sediment grain-size patterns (muddy, sandy or a mixture of both), and morphology (sandy beaches and mudflats). The deltaic coast exhibits a mixed wave-and-tide-dominated morphology and comprises a western sector characterized by four of the five main distributary mouths separating inter-distributary plains bounded by low overwash-influenced beaches devoid of aeolian dunes. The eastern sector, in the Gulf of Martaban, is embayed, much less prograded and bounded by dominantly muddy shores. This simple shoreline dichotomy reflects the overarching alongshore sediment redistribution and storage patterns that have accompanied the growth of the delta, resulting in the two facies: sand dominantly retained in the western sector where the multiple distributary mouths have constrained potential alongshore sand transport by low- to moderate-energy monsoon-generated southwesterly waves, and mud transported by the regional monsoon-influenced coastal shelf circulation towards the eastern sector. The recent multi-decadal shoreline mobility in the Ayeyarwady delta points to the influence of fluvial sediment supply on these two facies. Between 1974 and 2019, 49% of the delta's shoreline underwent erosion, mainly affecting the sandy beaches in the western sector, whereas shoreline accretion is still prevalent along the large inter-distributary plain east of Yangon, where coastal mud preferentially accumulates in the Gulf of Martaban. We attribute erosion to reduced river sand supply generated by dams and by massive in-channel sand mining upstream of the delta, exacerbated by important channel dredging. Large-scale deforestation resulting from land-use changes (agriculture and mining) in the Ayeyarwady catchment are probably contributing to enhanced fine-grained sediment supply to the delta plain and the coast, as reflected by the relative stability of coastal SPM over a decade (2002–2012) and continued deposition in the accreting eastern sheltered part of the delta. The coastal sediment balance will be further impacted in the future if planned dams are constructed. Without proper sediment management, notably a significant reduction or prohibition of in-channel sand mining, the sandy beaches that armour the wave-exposed western sector of the delta will continue to erode, resulting in increased potential exposure of the delta to cyclones and sea-level rise.

Formation and development of dunes in the northern Qarhan Desert, central Qaidam Basin, China

The Qarhan Desert in the Qaidam Basin is a high‐elevation geomorphic depression characterized by low temperatures, resulting in an environment similar to those predicted to exist on Mars. As a result, this desert has attracted many aeolian researchers. However, relatively little research has addressed the factors that control dune formation in this region. In this paper, we used observational wind data and sediment characteristics to analyze the formation and development of 3 dune types. The spatial distribution of the Qarhan Desert dune types is controlled by both wind regime and surface characteristics (primarily surface barriers, i.e., yardangs). Linear dunes formed on the leeside of yardangs and became elongated under a bimodal wind regime. The elongation direction of the linear dunes (295°) was similar to the direction of potential sand transport (290° to 296°). Linear dune development experienced different erosion and deposition processes under different climate regimes, and the dunes can be divided into developed and developing linear dunes at present. The movement of developed linear dunes included elongation and lateral movement; however, the movement of developing linear dunes was restricted to elongation. The mean elongation movement rate of the developing and developed linear dunes was approximately 16.87 and 4.84 m/yr, respectively.

Field observations of sand transport over the crest of a transverse dune in northwestern China Tengger Desert

Dune movement processes are affected by various sand transport processes, especially at the dune crest; however, there are relatively few field measurements on this issue. In this paper, 76 field measurements were collected over a dune crest at the Shapotou Aeolian Experiment Site, northeastern Tengger Desert, China to measure the rate of sand transportation under filed conditions. The measured sand transport rate ranged from 0.2 to 121.2kgm−1h−1, and most (95%) were less than 67kgm−1h−1. The roughness length calculated from the wind profiles was much larger than the results of previous studies. A linear regression analysis indicated that the modified Charnock model gives the best relationship fit between the shear velocity and roughness length (Cm=0.432, n=76, p<0.0001, R2=0.997). The sand transport flux over the dune crest can be expressed as an exponential function and a Gaussian function. A correlation coefficient analysis of the complex relationship between the sand transport and wind conditions (mean velocity, maximum wind velocity, shear velocity, potential sand transport, and wind direction) and the surface properties (roughness length, fetch length) indicated that the sand transport rate over the dune crest is positively correlated with the mean velocity (n=76, p<0.0001, R2=0.58) and shear velocity (n=76, p< 0.0001, R2=0.57) and is negatively correlated with the wind direction (n=76, p<0.0001, R2=−0.37). The wind direction affects sand transport over the dune crest and the coefficients of the sand transport flux. The critical wind direction is 272°, which is approximately perpendicular to the line of the crest (37°).

Implications of surface properties for dust emission from gravel deserts (gobis) in the Hexi Corridor

Gravel deserts (gobis) may be primary dust sources in China, where they occupy almost the same area as sandy deserts. The obvious difference between gravel and sandy deserts is the dominant grain size in the surface sediments: fine in sandy deserts versus coarse in gravel deserts. Potential sand transport, gravel cover, and the mean grain size of the surface sediments of gravel deserts are the main factors that affect dust release. However, little data is available on these properties. In the present study, we estimated gravel cover in surface photographs using ImageJ software, determined the proportion of the total weight accounted for by gravel (diameter>2mm), and described the grain size distribution for study areas in the Hexi Corridor of northern China. We found statistically significant differences between the proportions of total weight as gravel among five sub-regions. This proportion ranged from 22 to 91% of the total (66±17%; mean±SD), and the proportions in most of the samples (73%) ranged from 40 to 80%. The gravel cover ranged from 15 to 87% (52±17%), which was within the range in previous research that produced maximum aerodynamic roughness. The sandy material in the surface sediments was mainly medium sand, which accounted for 52.5% of the total sample. Potential sand transport was >200vector units in most gravel deserts, and 75% of the study sites had a physical soil crust. The high gravel cover and frequency of surface crusts is likely to decrease dust emission from the gravel deserts of the Hexi Corridor.

The effect of the time interval used to calculate mean wind velocity on the calculated drift potential, relative drift potential, and resultant drift direction for sands from three deserts in northern China

Wind is the power behind many erosion processes and is responsible for many of the characteristics of arid zone geomorphology. Wind velocity is a key factor in determining the potential sand transport, but the nature of the wind velocity data can strongly affect assessments of the risk of blowing sand. In this study, we obtained real-time wind velocity data in a region of the Tengger Desert with shifting sands, in the Badain Jaran Desert, and in the Madoi desertification land, with the data obtained at 1-min intervals, and used the data to determine the influence of how the wind velocity was calculated (mean versus mid-point values and the averaging time used to calculate these values) on sand drift potential. In the three regions, for both the mean and the mid-point wind velocities, the estimated drift potential decreased with increasing averaging time. The relationships between velocities calculated using the different averaging time intervals and the value calculated using a 1-min interval could be expressed as linear functions. The drift potential calculated using the mid-point wind velocity was larger than that calculated using the mean wind velocity.

Evaluation of the efficiency of dune reconstruction techniques in Xangri-lá (Rio Grande do Sul, Brazil)

The effects arising from the inordinate use of the coastal zone are unfolding in a reduction or even elimination of the foredune systems. Therefore, rehabilitation programs of these systems are currently one of the priorities of the coastal managers. The proposal of this study is to assest the efficiency of the recuperation techniques of the foredune system, having as a starting point the performance of a stormwater runoff piping installation. The first stage of this work evaluated the methods utilized for the dune system recuperation through digital terrain models and the sediment volume variations in three distinct periods, discussing the unanticipated alterations due to the interventions performed. The second part established a correlation between the actual accumulation of sediments and the calculation of the potential aeolian sand transport, with the aim of choosing the empirical equation that best fits the aeolian sand transport in the Rio Grande do Sul State coast. The results obtained show the efficiency of Northeast and East-Northeast winds in the transport of sediments towards the foredunes, resulting, thus, in success in the dunes reconstruction. The alterations in the system by the interventions realized evidence the necessity for constant adaptation of the dunes recuperation project in function of the system responses. The equation of Bagnold (1941) is the most accurated to predict the potential aeolian sand transport to the study area.

Wind regimes and aeolian geomorphology in the western and southwestern Tengger Desert, NW China

Wind is the primary control on the formation of aeolian geomorphology. In this study, we combined wind regime data from automated weather stations in the western and southwestern Tengger Desert of the Inner Mongolia region in China with remote-sensing data to analyse the relationship between the wind energy environment and aeolian geomorphology. Tengger Desert is one of the main dust storm sources in northwestern China. Therefore, efforts aimed at controlling desertification and dust storm require a deeper understanding of the processes that govern the formation and subsequent evolution of dunes in this area. Wind speed was largest in the northwest (3.3 m/s in the Xiqu station) and smallest in the southeast (1.2 m/s in the Haizitan station). Potential sand transport was also largest in the northwest (195 in the Jiahe station) and smallest in the southeast (33 in the Tumen station). The sand-driving wind (5.92 m/s) directions were from the NW and SE quadrant across the study area, at >76% of all sand-driving wind, reaching 99% in the Tumen station. The sand-driving wind in the NW quadrant reached >48%, and in the SE quadrant, >12% of all sand-driving wind in all stations. In the study area, sand dunes included crescent, dune networks, transverse, and coppice dunes. Dune crest directions had similar trends from upwind to downwind, at 133° in the middle region, and 124° in the southwestern region. Mean dune spacing changed with dune patterns; the maximum spacing for crescent dunes was 147 m, for dune networks 118 m, and for transverse dunes it was 77 m. The mean crest length was 124 m (maximum) for crescent dunes in the northwest, 121 m for transverse dunes, and 84 m for dune networks. However, because of gullies in the southern region, the mean crest length was only 58 m (least) for the crescent dunes in that area. The defect density ranged from 0.007 to 0.014. The spatial differences in dune patterns reflected the evolution of the dune field, where older dunes had been formed upwind and younger downwind. Copyright © 2014 John Wiley & Sons, Ltd.

Increasing rainfall, decreasing winds, and historical changes in Santa Catarina dunefields, southern Brazil

ABSTRACTMorphological and vegetation changes on the Moçambique barrier dunefield system are examined for the period 1938–2002 from aerial photography, and a variety of factors are investigated as possible driving factors. Human factors include a decrease in grazing pressure and tree felling from the early 1960s onwards after 200 years of these activities, and fires. In the 1960s tree planting also took place. During the period 1963 to 1970 there was a marked decline in drift potential (DP – potential sand transport), and then a period of very low DPs (1970–1974). This period falls within the time interval when vegetation cover significantly increased by ~70% along the Moçambique barrier (from 1956 to 1978). During the 1960s to present, the rainfall increased. Analyses of other transgressive dunefields in Santa Catarina and Rio Grande do Sul states show similar trends so it is likely that climatic factors such as increasing rainfall and decreasing DPs are responsible for driving dunefield changes and vegetation colonization of the barriers. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Modern and late Holocene wind regimes over the Great Plains (central U.S.A.)

The Great Plains of the central United States contain numerous presently stabilized dune fields that were active during the late Holocene. The most recent period of reactivation occurred about 700–1000 years ago during the Medieval Climatic Anomaly, a period of frequent, severe drought recognized throughout the North American West. Dune orientations and internal bedding preserve information about paleowinds from this time period. We compared paleowind information with modern winds and potential sand transport analyzed from meteorological data. While both reconstructed paleowinds and modern winds include a strong northwesterly component over the northern to central portions of the Great Plains, a significant difference exists to the south. During the late Holocene, southwesterly winds had a strong influence on dune forms over the central and southern Great Plains, while modern potential sand transport is dominated by southerly to southeasterly winds that bring moisture to the region from the Gulf of Mexico. Hypothetical dune types forming under modern winds would be different and have different crestal orientations than those preserved from the late Holocene. In order for such a shift in southerly winds to take place it is likely that: 1) the typical position of the Bermuda High in summer was shifted either to the east or to the south resulting in weaker low-level winds from the south and southeast and greater extent of southwesterly winds on the Great Plains, and/or 2) drought, likely enhanced by evapotranspiration and soil moisture feedbacks, brought the normally higher-level southwesterly flow down to the land surface. Given our understanding of megadroughts over the Great Plains as well as recent climate predictions for increased warmth and drought over this area, future reactivation of the dunes seems likely.

Limited change in dune mobility in response to a large decrease in wind power in semi-arid northern China since the 1970s

The climatic controls on dune mobility, especially the relative importance of wind strength, remain incompletely understood. This is a key research problem in semi-arid northern China, both for interpreting past dune activity as evidence of paleoclimate and for predicting future environmental change. Potential eolian sand transport, which is approximately proportional to wind power above the threshold for sand entrainment, has decreased across much of northern China since the 1970s. Over the same period, effective moisture (ratio of precipitation to potential evapotranspiration) has not changed significantly. This “natural experiment” provides insight on the relative importance of wind power as a control on dune mobility in three dunefields of northern China (Mu Us, Otindag, and Horqin), although poorly understood and potentially large effects of human land use complicate interpretation. Dune forms in these three regions are consistent with sand transport vectors inferred from weather station data, suggesting that wind directions have remained stable and the stations adequately represent winds that shaped the dunes. The predicted effect of weaker winds since the 1970s would be dune stabilization, with lower sand transport rates allowing vegetation cover to expand. Large portions of all three dunefields remained stabilized by vegetation in the 1970s despite high wind power. Since the 1970s, trends in remotely sensed vegetation greenness and change in mobile dune area inferred from sequential Landsat images do indicate widespread dune stabilization in the eastern Mu Us region. On the other hand, expansion of active dunes took place farther west in the Mu Us dunefield and especially in the central Otindag dunefield, with little overall change in two parts of the Horqin dunes. Better ground truth is needed to validate the remote sensing analyses, but results presented here place limits on the relative importance of wind strength as a control on dune mobility in the study areas. High wind power alone does not completely destabilize these dunes. A large decrease in wind power either has little short-term effect on the dunes, or more likely its effect is sufficiently small that it is obscured by human impacts on dune stability in many parts of the study areas.

Responses of desertification to variations in wind activity over the past five decades in arid and semiarid areas in China

There have been significant variations in wind activity over the past five decades in arid and semiarid areas in China. High wind activity occurred from the 1960s to the 1970s, but wind activity has decreased continuously from the 1980s to the present; as a result, the potential sand transport during the latter period was only 20%–50% of the values during the 1960s and 1970s. Phases of high wind activity were highly consistent with the trends in desertification over the past five decades in arid and semiarid areas in China, but spring precipitation was also a significant factor: rapid desertification during the 1960s and 1970s was due to high wind activity, generally combining with low spring precipitation; subsequent rehabilitation since the 1980s has resulted from the combined effects of low wind activity and higher spring precipitation. Therefore, although modern desertification and rehabilitation processes are being more or less affected by human activities, both processes appear to be more strongly controlled by climate change.

Storm-generated morphological change and longshore sand transport in the intertidal zone of a multi-barred macrotidal beach

The morphology, hydrodynamics and sediment transport processes of a multi-barred macrotidal beach were surveyed over 21 semi-diurnal tides, the first eight of which were dominated by storm conditions with offshore wave heights ranging from 1.2 to nearly 3.5 m. The hydrodynamic conditions prevailing over the intertidal beach showed a rapid response to these storm conditions, notably through strong longshore currents that attained peak velocities of 1.6 m s − 1 , i.e. over three times the peak spring tide velocities of tide-generated longshore currents during calm conditions. The bars showed no cross-shore migration during the storm events. Morphological mapping of the beach, tracer dispersal patterns, and calculations of potential sand transport rates using the energetics-based model of Bailard [Bailard, J.A., 1981. An energetics total load sediment transport model for a plane sloping beach. J. Geophys. Res., 86, 10938–10954.] showed significant longshore sand transport, as well as longshore migration of an intertidal drainage channel. These longshore movements were bi-directional, responded to changes in incident wind and wave directions, and involved wave stirring, coupled with longshore advection of sand along the bar crests in particular. Strong trough and cross-shore channel flows also led to the longshore migration of crescentic dunes. The longshore sediment movements were embedded in a cross-shore gradient associated with increasing overall profile mobility from the lower to the upper beach in response to wave shoaling and post-breaking processes, assessed through examination of the relative wave height Hs h − 1 , based on the relationship between significant wave height (Hs) and water depth ( h). Longshore sediment transport is a characteristic feature of the long-term evolution of Wissant Bay, which is dominated by a net northeastward residual sand transport within a bay sediment cell characterised by significant updrift erosion and downdrift accumulation. Longshore transport should be considered as an essential element of the morphodynamics of bar-trough beaches subject to large tidal ranges and significant wind stress, but there is a need for a better understanding of the role of wind stress on longshore currents in the shallow intertidal zone of these beaches.

Movement of a small slipfaceless dome dune in the Namib Sand Sea, Namibia

The migration of a small slipfaceless dome dune close to the northern edge of the Namib Sand Sea has been measured by topographic survey. The dune dimensions are 45-m wide and 1-m high with a volume of 551 m 3 that has been calculated as the difference between the dune's surface elevation and an interdune surface extrapolated from measurements around the edge of the dune. The migration direction, 64°, and distance moved, 90 m, are measured against stakes set out in 1976. The dune has moved about 90 m between 1976 and 1999. This is an average linear migration rate of around 4 m year −1, and is equivalent to an annual sand transport rate of about 1.2 tonnes m −1 year −1. The calculated total potential sand flow in this part of the Namib Desert is 119 tonnes m −1 year −1, and the resultant potential sand flow is 63 tonnes m −1 year −1. The dune migration is therefore about 1% of the total potential sandflow and 2% of the resultant indicating that dune migration is only a small part of total potential sand transport. The results suggest that small slipfaceless dome dunes are very inefficient at trapping sand, and that winds blowing across the interdune in this area are undersaturated with sand.

Influence of high water levels on aeolian sand transport: upper beach/dune evolution on a macrotidal coast, Wissant Bay, northern France

Aeolian sand transport measurements and detailed topographic surveys were carried out during 1 year along a macrotidal upper beach/dune system experiencing rapid coastal retreat. Aeolian transport measurements, coupled with wind records, showed extreme spatial and temporal variability along a spatially limited upper beach sector. Analysis of wind velocity, rainfall, and water level showed that aeolian sand transport may occur all year long on the upper beach with the most energetic conditions occurring during winter and spring. Analysis of volume change revealed that aeolian sedimentation on the upper beach occurred only during the summer period, while the rest of the year was characterised by upper beach and dune scarp erosion. This erosion, due to storm events combined with high water levels, completely eliminated summer aeolian sand accumulation. Our results show that aeolian transport and upper beach/dune evolution on this macrotidal beach is strongly controlled by the magnitude and frequency of occurrence of high water levels. This study illustrates the fact that upper beach/dune evolution cannot be completely understood and satisfactorily modelled if only potential aeolian sand transport is considered.

Holocene tidal conditions and tide‐induced sand transport in the southern North Sea

A numerical model of Holocene tides and sand transport in the North Sea was used to establish potential tidal sand transport patterns in the Southern Bight and sand budgets for the Dutch and Belgian coast. The sand transport formulation yields magnitude and direction in addition to erosion/deposition values, in contrast to previous studies. All palaeobathymetries were obtained using the present bathymetry and sea level change results from an isostatic rebound model. The sea first invaded the Southern Bight through the Strait of Dover, and the microtidal range increased rapidly to mesotidal. The amphidromic system developed when the land bridge between Britain and the continent flooded and the northern tidal influence increased. Simultaneously, the current ellipses rotated from shore normal to shore parallel, while the flow velocities increased. The net tidal sand transport direction changed from toward the Dutch and Belgian shore to northward alongshore in the north and southward in the south. Until 7000 B.P. erosion dominated at sea, and deposition dominated near shore. Subsequently, erosion occurred in the south, and sedimentation occurred in the north. To the north of the Southern Bight, transport rates were low. Since 7500 B.P., sand has been transported mainly in suspension. Dominant transport by tidal asymmetry changed to transport by tidal residual currents and back in large areas. Comparison with previous studies illustrates that isostatic rebound is important for constructing palaeobathymetries. Most sediment was supplied to the Dutch and Belgian coast before 6000 BP. Substantial tidally induced mean sea level changes occurred in the early Holocene.