Derivation of the nonlinear dependence of aeolian sand flux on wind speed

Quantitative prediction of aeolian sand transport rates on beaches is still a difficult task, mainly due to the large spatio-temporal variability inherent to this type of transport. In order to validate new approaches to calculate aeolian transport, in situ field measurements are needed, combined with the knowledge on how to interpret point measurements in this spatio-temporal varying transport field. Despite the various instruments available and techniques to convert measurements to sand fluxes, there is no consensus about which trapping-device or sensor is the optimal one for aeolian sand transport studies. Often, the results of deployments with electronic sensors (optical and impact sensors) and sand traps are not similar.During the last decade, laser particle counter sensors (Wenglor fork sensors) have been used in various studies to obtain rates of aeolian sand transport in the beach-dune environment. The sensor has been tested in wind tunnels and generally seemed to record aeolian transport properly, and field applications of the sensor reported in literature seemed to provide realistic results. However, some strongly deviating results in our own transport measurements by a co-located sand trap and Wenglor sensor array urged us to further look into the detectability of various grain sizes by the Wenglor sensor.Laboratory experiments were developed, to test the ability of the Wenglor sensor, to accurately count sand grains of various grain size classes and stainless steel beads. It was compared the count data collected by the Wenglor with images from a Highspeed camera which revealed the actual number of grains passing the laser beam. Also, a Silicon photodiode was used to record the laser intensity reduction induced by the sand grain passage through the laser beam to derive the minimal necessary reduction for the Wenglor to count grains, and thereby the minimal detectable grain size. The behaviour of Wenglor laser particle counter was tested in the field. Rates of aeolian sand transport were recorded using Wenglor sensors and co-located vertically stacked mesh sand traps collected sand transported at various elevations. The results show a large variability between fluxes calculated with the sand traps and those derived from the Wenglor counts at the corresponding elevation. The influence of the saturation of the Wenglor in the results was studied and a simulation model of the counting process is presented to look into the role of sediment concentration, sediment fluxes, particle speed and grain size in the mismatch. In the final part of this dissertation an approach for annual scale transport prediction from the intertidal beach is presented. In this approach the surface conditions of the intertidal beach are aggregated, in particular moisture content and roughness. Monitoring data on wind speed, wind direction, rain and water levels is used to calculate the annual onshore aeolian sand transport. To obtain the aggregated value for moisture content the calculated transport for various moisture content values is compared to the volume increase of the dune area obtained from topography. The approach to determine a characteristic moisture content value for aeolian transport gives surface moisture values of 1.2% to 3.2% for wind average and wind gust respectively, implying that to achieve the dune volume change a quite dry beach is necessary. This indicates that the main area for aeolian transport corresponds to the upper part of the intertidal source, most likely the region between mean high tide line and spring high tide line.

Aeolian sand transport and aeolian deposits on Venus: A review

Onshore wind is the primary driver of sediment transport allowing the construction of coastal dunes. In contrast, offshore winds result in the seaward export of sediment inducing a loss of the terrestrial beach sedimentary budget. Many parameters limit aeolian sand transport such as moisture, beach slope, beach length, vegetation and sediment characteristics. Although the impact of grain size on wind transport is well known, few studies have focused on its temporal variability. The temporal evolution of grain size characteristics is particularly important in microtidal environments where the relatively small tidal range minimises the mixing of the beach sand and winds have a strong impact on grain size sorting, resulting in the coarsening of the beach grain size. Leucate beach (SE, France) is a wave-dominated microtidal environment, subject to a strong offshore wind (72 % of the annual time and 17.5 % over 10m/s) which made this site suitable to this study. During the 19 months of meteorological surveys, 5 field measurements campaigns of 1 to 3 days were conducted. For this purpose, wind processes (intensity and direction); aeolian sand transport (24 runs), morphological variations of the beach-dune system and also many sub-surface sediment samples were collected.The results show a large temporal variability in beach grain size ranging from medium to very coarse sand in relation to wind and wave conditions. Aeolian processes produce a coarser beach grain size at days/months scale, whereas short marine storms (day) induce a mixing and finer  beach grain size, resulting  in very different  aeolian sediment transport values for  similar incident wind conditions. For example, with a wind speed of 10 to 14 m/s the measured sediment flux ranged from10 kg/m/h (coarse beach grain size) to 50 to 150 kg/m/h (medium beach grain size). Morphological variations of the upper beach surface are not significant when the sand is coarse but can cause lower by the upper beach surface by 0.5 m when the sand is composed of medium-sized. The time scale of the temporal beach grain size variations is closely related to the frequency and intensity of marine and wind storms. This study quantifies the effect of the beach grain size variability on the aeolian sand transport and thus the morphological changes of the beach. We conclude that because of their importance in the temporal variability of sediment size and the inherited sedimentological framework of the beach, it is crucial to take into account marine and aeolian processes to refine the predictions of wind transport rates. This confirms the need in a microtidal environment, to obtain beach grain size temporal data to better understand the aeolian sediment transport rates affecting a study site and not underestimated its impact when calculating transport rates with empirical formulas and numeric models.

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

10 - Aeolian (windblown) sand transport over beaches

Reevaluation of the aeolian sand flux from the Ulan Buh Desert into the upper Yellow River based on in situ monitoring

Aeolian sand transport is caused by wind erosion, which is the main cause of environmental problems such as land desertification, blown sand disasters and air pollution. Previous research results and engineering practices have confirmed that vegetation improvement has long‐term and comprehensive benefits for controlling aeolian sand transport. The role of vegetation in aeolian sand transport can be explained by the complexity of atmospheric, earth and biosphere systems. However, the lack of a universal model used to predict aeolian transport rate on vegetated surface results in different understandings about vegetation effects due to different experimental methods and materials. To clarify the effect of vegetation on aeolian sand transport, we used a large wind tunnel whose experimental section was 24.0 m‐long, 3.0 m‐wide and 2.0‐m high to provide enough length for aeolian transport and designed a set of novel experimental methods to improve the measurement accuracy of the aeolian transport rate. Based on experimental data, we developed a new equation for aeolian transport rate over bare and vegetated surfaces. We determined that models of aeolian transport rate over bare surface in the literature underestimated the aeolian transport rate measured in this study by 63–80%, 89%, 50% and 35%, confirming that the transport rate decreases exponentially as vegetation coverage increases. However, the power exponent is a variable related to particle size and wind velocity as opposed to a constant in the existing literature; thus, we proposed a dimensionless parameter to express the relationship between the power exponent with particle size and wind velocity and constructed the new equation for vegetated surfaces to be more widely applicable. These results would help to understand the effect of vegetation cover on aeolian sand transport process and to improve the effectiveness of vegetation prevention and control.

Aeolian sand transport is an important component of material circulation above terrestrial surfaces and can include processes of creep, saltation, and suspension. The complex movement of material and energy during aeolian transport has meant that these processes have previously been examined in isolation. Although a significant amount of research has been conducted on aeolian sand transport, this focused primarily on saltation. As a result, there are few data available on sand grain creep, primarily due to a lack of theoretical models and the difficulty of direct measurements. In this study, we present novel methods and instrumentations to accurately measure the transport mass and the velocity of creeping sand grains. Using data collected with above instrumentations and a ladder sampler at four friction velocities (u* = 0.26, 0.35, 0.47, and 0.56 m s−1) in a wind tunnel, we studied the transport mass of creeping sand grains and their movement velocities, as well as other key parameters of aeolian sand transport. Four major conclusions can be drawn from this study: (1) The transport mass (q0) of creeping sand grains increases with increasing frictional wind velocity (u*). The relationship between these variables is represented by the power function q0 = −0.053 + 9.195u*2.800. (2) The contribution of creep to total aeolian sand transport decreases with increasing frictional wind velocity. Creep contributed 57% of total aeolian transport at the lowest frictional wind velocity but only 19% at the highest velocity. (3) The threshold frictional wind velocity for entrainment was 0.158 m s−1. (4) The movement velocity of creeping sand grains ranged from 0 to 0.14 m s−1, but more than 70% of recorded velocities were less than 0.02 m s−1. Although the results of this study require further validation, they provide a strong basis for future research and help deepen our understanding of aeolian sand transport.

Dune sand transport as influenced by wind directions, speed and frequencies in the Ordos Plateau, China

Aeolian sand transport influenced by tide and beachface morphology

The wind transports sand from the beach to the dunes, causing dune growth. This process takes months to years. Predicting Aeolian sand transport on these time scales is difficult, as existing models only use wind speed as a variable under ideal conditions. Aeolian transport on narrow (<100 m) sandy beaches is influenced by several factors, causing the moments of predicted sand transport not always to match with reality. The main aim of this thesis is to clarify the timing of aeolian transport on a narrow beach and the associated key factors. This is done using a video monitoring system and weather data. A strong, cross-shore wind causes limited sand transport, whereby the expected transport based on the wind speed did not result in the expected sand transport on the video images. Unlimited transport only occurs in strong winds when they are alongshore. With unlimited transport with smaller wind speeds, the wind can also be cross-shore, which mainly occurs in summer.
\nThe beach width and wind direction are built into a new sand transport model. The model predicted moments of (un)limited sand transport, whose timing was reasonably in line with expectations. According to the model, unlimited transport is rare, but they can potentially make a significant contribution to transport to the dunes. The next step is to test the model at other locations and with in-situ measurements. This improves aeolian sand transport predictions towards the dunes, where it can contribute to protecting the land against marine flooding and to a diverse ecosystem.

Transport of beach sand to the foredune by wind is essential for dunes to grow. The aeolian sand transport rate is related to wind velocity, but wind-based models often overpredict this transport for narrow beaches (<100 m). To better predict aeolian sand transport, the fetch-based Aeolus model was developed. Here, we qualitatively test this model by comparing its transport-rate output to visual signs of aeolian transport on video imagery collected at Egmond aan Zee, the Netherlands, during a six-month winter period. The Aeolus model and the Argus images often agree on the timing of aeolian transport days, except when transport is small; that is not always visible on the Argus images. Consistent with the imagery (minimal signs of aeolian activity in strong winds), the Aeolus model sometimes predicts the actual transport to be smaller than the potential transport. This difference is largest when wind velocity is large, and its direction is cross-shore. Although transport limitations are not predicted to be common, the results suggest that their effect on the total transport in the study period was substantial. This indicates that the fetch distance should be taken into account when calculating aeolian transport for narrow beaches on longer timescales (>weeks).

Aeolian sand transport results from interactions between the wind and the ground surface, and is a key link between microscale mechanisms responsible for the movement of individual grains of sand and macroscopic mechanisms that govern the flow of windblown sand. As a result, many studies have been performed to study aeolian transport, but a few studies have simultaneously compared the effects of the surface characteristics on aeolian transport in the same region. In this study, we designed three experimental areas to provide a comparison of the transport rates under different types of boundary layer conditions (shifting sand, straw checkerboard, and gravel), with different sediment availability but under the same wind regime. The study was conducted at the Shapotou Aeolian Experiment Site, Chinese Academy of Sciences, where we studied the sand transport rate and mass-flux-density profiles using synchronous experimental data. The surface characteristics dramatically affected aeolian transport. The mass-flux-density profile above the open shifting sand surface and straw checkerboard reached its maximum value near the surface, and followed an exponential-decay function. For the gravel surface, the maximum transport occurred at a certain height above the ground, but above that height, followed an exponential-decay curve. The transport rate was greatest above the open shifting sand, reaching up to 465 times the rate above the straw checkerboard, with intermediate values above the gravel surface. The average saltation height was highest above the gravel-covered surface, and lowest above the straw checkerboard surface.

A fully predictive model for aeolian sand transport, part 2: Description and calibration of models and effect of moisture and coarse materials

The Nizzana area has experienced periods of heavy grazing which resulted in the destruction of a cryptogamic soil crust and vegetation cover which, in turn, led to increased aeolian sand movement (Tsoar and Moller 1986). The influence of cryptogamic soil crusts on deflation has been the focus of several studies mainly in drylands used or suitable for grazing (Harper and Marble 1988; West 1990; Eldridge and Greene 1994; Belnap 1995; Belnap and Gillette 1998; Leys and Eldridge 1998). The resistance of such crusts against wind erosion has been tested in wind tunnel studies (McKenna Neuman et al. 1996; McKenna Neuman and Maxwell 1999, 2002). The results of these studies show that intact crusts inhibit deflation of sandy soils, while a destruction (e.g. trampling by grazing animals) leads to a sharp increase of aeolian transport rates (Leys and Eldridge 1998; Allgaier 2005). Aeolian sand transport rates over crust-covered areas at the Nizzana site have been found to be negligible, and experimental studies showed that current wind energy at the site is not sufficient to destroy the crust (Allgaier 2005). Where no soil crust is present, vascular vegetation offers an increasing protection of the surface against wind action. It increases the aerodynamic roughness of a surface, thereby extracting energy from the airflow and reducing shear stress at the soil surface. Various studies have investigated the influence of vegetation on aeolian sand movement. Stockton and Gillette (1990) and Musick and Gillette (1990) have dealt with the relationship between plant cover and erodibility of surfaces, concentrating on the influence of vegetation density on the partition of shear stress between the vegetation and soil surface. Lancaster and Baas (1998) undertook field studies over areas with different vegetation cover. Wiggs (1993) reports a possible threshold cover of 14% for certain areas of the Kalahari Desert, above which no noteworthy aeolian sand transport occurs. However, he does not claim this to be a universal value. The value is consistent with the findings of Marshall (1973), who concluded that wind erosion will increase rapidly if vegetation cover decreases below 15% on level alluvial sand surfaces. However, Ash and Wasson (1983, p. 20) report sand movement on dune crests with a ground cover of up to 35%.