Aeolian Sand Transport Research Articles

Consequences of a Storm Surge for Aeolian Sand Transport on a Low-Gradient Beach

Wind-blown beach sand is the primary source for the volume growth of the most seaward dune, the foredune. Strong wind events can potentially dominate long-term aeolian supply but in reality do not contribute considerably because they often coincide with a storm surge. The aim of this paper is to further our understanding of how a storm surge prevents or severely restricts aeolian supply. Using field data collected on the 1:50 sloping Egmond beach (Netherlands) in the aftermath of a 1-m storm surge, we show that the ground water in the upper beach rose to well above normal levels during the surge, which resulted in the development of a seepage face during falling tide and hence persistent saturation of the emerging beach. Using a fetch-based model, we predicted aeolian supply during the 2-day surge period to be about 66% of the potential supply. Fetch limitations imposed by the surge-induced inundation and the continuous saturation of the sand on the emerging beach both contributed to the predicted supply limitation. Our results quantitatively support earlier studies that suggested surges to be the primary condition that causes predictions of long-term potential foredune growth to overestimate measured growth.

Aeolian Creep Transport: Theory and Experiment

AbstractAeolian sand transport drives geophysical phenomena, such as bedform evolution and desertification. Creep plays a crucial, yet poorly understood, role in this process. We present a model for aeolian creep, making quantitative predictions for creep fluxes, which we verify experimentally. We discover that the creep transport rate scales like the Shields number to the power 5/2, clearly different from the laws known for saltation. We derive this 5/2 power scaling law from our theory and confirm it with meticulous wind tunnel experiments. We calculate the creep flux and layer thickness in steady state exactly and for the first time study the relaxation of the flux toward saturation, obtaining an analytic expression for the relaxation time.

Effects of an oasis protective system on aeolian sediment deposition: a case study from Gelintan oasis, southeastern edge of the Tengger Desert, China

Desert-oasis ecotones are boundary areas between oases and desert ecosystems. Large efforts to control sediment and stabilize these boundaries depend on understanding sedimentary processes, especially aeolian transport and deposition. Previous studies on aeolian sediment deposition have focused primarily on a single land surface type or a single engineering approach. Few studies have considered deposition in a multi-layer oasis protective system. A complete oasis protective system consists of an outer bare sand area, a sand barrier zone, a shrub and herbaceous plant zone, and a farmland shelter zone. This study used sedimentary analysis to quantify grain-size characteristics in samples from the four land surfaces under different types of weather conditions in the Gelintan oasis of the Tengger Desert, the fourth largest desert in China. The results showed that aeolian sediment deposition decreased from the outer bare sand area through the oasis protective system and into the interior. The four land surface types showed significant differences in deposition volume (P < 0.05). Deposited sediment showed gradual decrease in dominant grain-size from sand to silt, but sediment deposited during dust weather contained a larger coarse-grained fraction. From the outer desert to the inner oasis, transport mechanisms shifted from saltation (sand) to suspension (silt and smaller) in non-dust weather. During dust weather, deposition primarily occurs from near-surface aeolian sand transport with saltation. Sediment sorting decreased from exterior to interior zones of the protective system while skewness and kurtosis showed no significant change (P < 0.05). These results can help inform strategies for stabilizing and protecting desert-oasis ecotones in this region and other localities.

Optimal design of sand blown wind tunnel

This work investigates the airflow driven by dual axial-flow fans in an atmospheric boundary layer (ABL) wind tunnel and the expected entrainment of sand movement together. The present study is conducted via 3D numerical simulation based on modelling the entire wind tunnel, including the power fan sections. Three configurations of dual fans in the tunnel are proposed. Simulation results show that the airflow in the tunnel with dual-fan configuration can satisfy the logarithmic distribution law for ABL flows. The airflow driven by the dual fans placed together at the tunnel outlet is highly similar to that in the tunnel with single fans. Although the boundary layer thickness is reduced, the maximum airflow velocity (53.393 m/s) and turbulence intensity (12.02%), which are respectively 1.75 and 1.49 times higher than those under the single-fan configuration, can be reached when dual fans are separately placed at the tunnel inlet and outlet. The simulation and experiment manifest that the separated arrangement of dual fans in the tunnel should be suitable for the experimental study of aeolian sand transport. Some measures, such as wind tunnel construction adjustment and optimal roughness element arrangement, are necessary to guarantee the required boundary layer thickness in the wind tunnel.

The effect of wind speed averaging time on the calculation of sand drift potential: New scaling laws

Drift potential (DP) is a key factor used to quantify aeolian sand transport, and it is often used for predicting wind erosion and dune mobility under climate change scenarios. It is proportional to the mean of the cube of the wind speed at a 10-m height when that wind speed is above 6.2 m/s (12 knots), a speed threshold above which sand can be transported by the wind. DP depends on the temporal resolution and the averaging time of the wind velocity measurements. Usually, either a 10-min or 1-h mean wind speed is used to calculate the DP, although different studies have used different averaging periods for calculating the mean wind speed and, consequently, the DP. Here, we perform a comprehensive analysis of an extensive, long-term wind speed database from different locations around the world and from reanalysis data sources, and show that DP decreases with the averaging time of the wind speed. The decrease of DP versus the averaging time interval can be approximated by a stretched exponential function. In addition, the relative decrease of DP, as a function of the maximum DP, is smaller for high wind speed sites. This result is a consequence of the fact that high DP sites are characterized by longer clusters of successive wind speed events above the threshold velocity; they are thus less affected by the time averaging process. Interestingly, the clusters' lengths decay exponentially with wind speed and faster for low DP sites. Our new results have important implications for calculating the effective DP and for better estimating the biases that result from using wind data with different averaging times, which is a common problem in aeolian research. We also found an approximate linear relation between DP and the wind power that is close to the analytic approximation for the relation between the two. This new finding has many important applications since it will allow one to calculate DP from wind power data and vice versa. In addition, it implies that the same scaling laws found for DP also exist for the wind power.

Early-Stage Aeolian Dune Development and Dynamics on the Upper-Beach

Montreuil, A-L.; Chen, M.; Brand, E.; De Wulf, A.; De Sloover, L.; Dan, S., and Verwaest, T., 2020. Early-stage aeolain dune development and dynamics on the upper-beach. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 336–340. Coconut Creek (Florida), ISSN 0749-0208.Early-stage aeolian protodunes, or ephemeral dunes, are ubiquitous features on the upper-beach, acting as requisite precursors to fully developed dunes. Their formation during high energy aeolian sand transport events plays an important role in the beach morphodynamics and sediment budget. However, little is documented about their life cycle from generation to degeneration as well as their interactions with environmental factors. This study takes the opportunity of the advance in high resolution terrestrial laser scanning technique to acquire morphological changes at an appropriate frequency and spatial resolution to characterize ephemeral dune dynamics. A 1.5 days set of hourly laser scans indicates the development of a field of protodunes with a height ranging from 0.15 to 0.42 m concentrated on the upper-beach. They develop in a few hours under oblique alongshore winds above 7 m/s. The results also show that the protodunes migrate relatively fast with an average rate up to 1.1 cm/hour. Spatial and temporal variability of their morphological characteristics occur across the upper-beach. The morphological changes reflect the pattern of the development and behaviour of the protodune field which seem to be self-organized within a complex system controlled by the interactions between the bedforms themselves, environmental factors and aeolian sediment transport. Our findings highlight the importance of quantifying processes at spatio-temporal scales for monitoring protodune morphology.

Fetch effect on the developmental process of aeolian sand transport in a wind tunnel

As the sand mass flux increases from zero at the leading edge of a saltating surface to the equilibrium mass flux at the critical fetch length, the wind flow is modified and then the relative contribution of aerodynamic and bombardment entrainment is changed. In the end the velocity, trajectory and mass flux profile will vary simultaneously. But how the transportation of different sand size groups varies with fetch distance is still unclear. Wind tunnel experiments were conducted to investigate the fetch effect on mass flux and its distribution with height of the total sand and each size group in transportation. The mass flux was measured at six fetch length locations (0.5, 1.2, 1.9, 2.6, 3.4 and 4.1 m) and at three free-stream wind velocities (8.8, 12.2 and 14.5 m/s). The results reveal that the total mass flux and the mass flux of each size group with height can be expressed by q=aexp(−bh), where q is the sand mass flux at height h, and a and b are regression coefficients. The coefficient b represents the relative decay rate. Both the relative decay rates of total mass flux and each size group are independent of fetch length after a quick decay over a short fetch. This is much shorter than that of mass flux. The equilibrium of the relative decay rate cannot be regarded as an equilibrium mass flux profile for aeolian sand transport. The mass fluxes of 176.0, 209.3 and 148.0 μm size groups increase more quickly than that of other size groups, which indicates strong size-selection of grains exists along the fetch length. The maximal size group in mass flux (176.0 μm) is smaller than the maximal size group of the bed grains (209.3 μm). The relative contribution of each size group to the total mass flux is not monotonically decreasing with grain size due to the lift-off of some small grains being reduced due to the protection by large grains. The results indicate that there are complex interactions among different size groups in the developmental process of aeolian sand transport and more attention should be focused on the fetch effect because it has different influences on the total mass flux, the mass flux profile and its relative decay rate.

Understanding wind-blown sand: Six vexations

The study of aeolian geomorphological systems over the last half century has advanced along several important fronts, supported by developments in technologies and analytic methods and the evolution of modeling approaches. Remote sensing has led to a wealth of knowledge concerning the distribution and dynamics of dunes on Earth and elsewhere, and wind tunnels have elucidated the roles of aeolian processes in reshaping surfaces. Improvements in laboratory and field instrumentation allow measurement of aeolian sand transport systems at levels of detail unimaginable when the Binghamton Geomorphology Symposium began. We have seen, for example, the advent of large, sophisticated field campaigns and advanced modeling approaches using cellular automata and computational fluid dynamics methods. Select avenues of development, especially concerned with depositional environments, are briefly summarized. Advances in many fields have not been matched with proportional advances in our ability to predict rates of sand transport in natural environments, one of the most fundamental challenges. At least eight, commonly cited, sand transport-rate models do not agree with each other and do not replicate field observations even under near-ideal conditions. This review focuses on select reasons why that might be so, and categorizes those reasons into six, principal theoretical and empirical vexations that have made sand transport an intractable problem: predicting rates of transport; characterizing grain size; characterizing fluid and sand densities; estimating shear velocity; predicting the threshold of motion, including the effects of sand moisture content; and understanding granular electrification. The examination of potential sources of error in the rate of transport and threshold of motion models leads to a point-by-point consideration of our current assumptions and protocols (or lack thereof). The objective is to help us identify the hard targets and the soft targets for future research.

Using wind run to predict sand drift

AbstractConventional aeolian sand transport models relate mass transport rate to wind speed or shear velocity, usually expressed and empirically tested on a 1‐s time scale. Projections of total sand delivery over long time scales based on these models are highly sensitive to any small bias arising from statistical fitting on empirical data. We analysed time series of wind speed and sand transport rate collected at 14 independent measurement stations on a beach during a prior field experiment. The results show that relating total sand drift to cumulative above‐threshold wind run yields models which are more statistically robust when fitted on empirical data, generating smaller prediction errors when projected to longer time scales. Testing of different power exponents indicates that a linear relationship between sand drift and above‐threshold wind run yields the best results. These findings inspire a speculative novel phenomenological model relating the mass flow of air in the boundary layer to the mass transport of sand over the surface. © 2020 John Wiley &amp; Sons, Ltd.

Space-borne radar observation of near-surface wind effect on anomalously highly-directional backscattering of radio waves from Aeolian processes of sand and dust transporting in desert regions

Aeolian process of sand and dust transporting is known to form the near-ground surface structures over vast territories and fill the atmosphere up with suspended aerosols-like dust particles which are spread then by winds over long distances. The presence of atmospheric dust in the planet's environment is one of the factors affecting the temperature and climatic conditions of vast regions of the Earth. A number of publications (Ivanov et al., 2015; Ivanov et al., 2016; Ivanov et al., 2016; Ivanov et al., 2018) analyze the revealed effect of anomalously highly(narrow) directed backscattering of radio waves which manifests itself in radar remote sensing (in range of local irradiation angles θ ≈ 31°÷32°) in areas covered with deep sand. At the same time, there is no specific data available from published studies investigating the impact of the near-surface wind on anomalously highly-directional backscattering of radio waves based on the results of radar remote sensing researches of Aeolian sand and dust transport processes in desert regions that, in turn, could have been used later to determine the parameters of such transport process. This article presents the results of analysis of the data obtained from long-term studies of desert regions of El-Djuf, Akshar and Trarza in Mauritania by means of space-borne SAR Envisat-1. The purpose of the analysis was actually to identify the specifics of the effect that the near-surface wind has on the anomalously highly-directional backscattering of radio waves which is identified by radar based researches of Aeolian processes of sand and dust transport in desert regions, so can be used for remote determination of such transportation parameters.

Using Video Monitoring to Test a Fetch-Based Aeolian Sand Transport Model

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 (&lt;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 (&gt;weeks).

Experimentally Derived Thresholds for Windblown Sand on Mars

AbstractAeolian sand transport frequently occurs on Mars despite wind speeds rarely exceeding predicted thresholds for motion. This dissonance is problematic for understanding aeolian processes and the resulting geomorphologic responses in contemporary and ancient atmospheres. To address the apparent discrepancy between speeds required to initiate sand motion and transport observed from orbital and in situ observations, we conducted a series of wind tunnel experiments at Martian atmospheric pressures and used buoyancy as the similitude parameter to simulate the force required to initiate particle motion on Mars. Shear velocities were derived from velocity profiles corresponding to the onset of sporadic transport, which can induce downwind cascading motion. Here, we find that threshold shear velocities are slower than previously thought by a factor of 1.6 to 2.5. Measured wind speeds on Mars exceed our observed thresholds, thus offering one mechanism behind the dissonance between observations of transport below previous thresholds of motion.

Craters as sand traps: Dynamics, history, and morphology of modern sand transport in an active martian dune field

Aeolian transport of sand is abundant on modern-day Mars, as revealed by remote sensing measurements of the motion of dunes, and of the meter-scale ripples that mantle them. We study a large-scale natural sand trap within the Meroe Patera dune field: a 1.8-km diameter crater which features a dune-free “shadow” in its lee. We compare the volume of sand trapped within this crater to the sand volume that would be expected to cover the area of the crater and its dune-free shadow behind it if the crater were not present. We find that the crater holds less sand than this “missing” volume would predict, implying that sand escapes from the crater over time. Modern day imagery shows an apparent lack of sand escaping from the Meroe crater, however, suggesting that changes in the wind regime at the site may have allowed sand to escape in the past. The persistence of an altered dune morphology all the way to the far downwind edge of the dune field suggests consistent wind conditions over the time of the crater-dune field interaction.

Studying the spatial and temporal changes in aeolian sand transport in a wind tunnel using 3D terrestrial laser scanning

AbstractThe spatial and temporal changes in aeolian sand transport are important in modelling wind erosion and parameterizing sand dunes and ripples. In this study, we explored the wind erosion processes that occur in sand beds using a Trimble 3D laser scanner. We found that the erosion intensity varied with distance along the sand bed and that the sand bed surface could be divided into three zones. In the pre‐bowl zone, the sand bed surface showed small and uniform variation, indicating weak erosion intensity in this zone. Downwind of the pre‐bowl zone, the bowl zone developed where the sand surface was eroded seriously and a kind of morphology analogous to blowouts occurred. The post‐bowl zone, which followed the bowl zone, was characterized by alternating accumulation and erosion. Wind velocity affected the spatial development of this bed morphology, whereas the erosion duration only affected the erosion depth. We developed an equation of the form QL = Qsat/(1 + e(−10(L‐s)/k)) to describe the relationship between sediment flux (QL) and bed length (L) in the pre‐bowl and bowl zones and found the fitting coefficients R2 were very high (1.0, 0.98, 0.97 at wind velocities 10.0, 11.0 and 12.0 m s−1, respectively). Using a fine spatial resolution for the measurements revealed features of the sand flux profile that were missed at coarser resolution. However, it will be necessary to use a longer sand bed to study the full length of the post‐bowl zone.Highlights The erosion intensity at different sites of the sand bed was studied. The roughness of the bed surface increased with increasing erosion duration. A sigmoid curve could describe the changes of erosion along the fetch length.

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.

Artificial simulation of complex unsteady wind in an ABL wind tunnel with dual axial fans

The wind tunnel test plays an important role in studying the mechanism of aeolian sand transport. An atmospheric boundary layer (ABL) wind tunnel with dual axial flow fans to simulate unsteady wind is described in this paper. Air flow is generated using two fans arranged at each end of the wind tunnel. During the experiment, the rotation rates of the fans are changed by varying the frequency of the converters, which can be controlled by PLC. The speed of each fan varies according to a sinusoidal function. As a result of changing the parameters of the sinusoidal function, the two fans produce different forms of sinusoidal wind, and finally compose the controllable unsteady wind, which manifests as typical unsteady wind characteristics in the test section of the wind tunnel. The experimental results show that the wind speed profile of unsteady wind generated in the wind tunnel conforms to the logarithmic profile of the ABL. The proposed method is very useful for experimental research on the unsteady sand transport mechanism.

Characteristics of Aeolian Dune, Wind Regime and Sand Transport in Hobq Desert, China

A systematic study of the wind regime characteristics in a region can not only accurately grasp the dynamic factors of the development of aeolian geomorphology, but also provide a scientific basis for the prevention and treatment of regional sand disasters. Taking the Hobq Desert as the study area, the basic characteristics of dune are analyzed by using remote sensing images. Based on the annual meteorological data of six meteorological stations from 2009 to 2018, the spatial and temporal distribution characteristics of wind speed were obtained. With the daily wind data of three stations from 2009 to 2018, we have figured out the wind regime and sand transport characteristics of the Hobq Desert. The results show that the sand dune height of the Hobq Desert ranges large, the highest height is 5010 m and the lowest is 10 m. It decreases gradually from the west to the east. The height of dune mainly distributed below 1500 m, followed by 1500–2000 m. Migratory sand dunes in Hobq Desert accounts for 51.8% and is mainly distributed in the west of the desert. The distribution area of fixation sand dunes in Hobq Desert is the least, accounting for 8.3%. The migratory dune pattern is trellis dune, semimigrated dune and semifixed dune patterns include honeycomb dune, parabolic duneand brush dune, and fixation dune pattern is grass dune. Annual wind speed was greatest in the southeast and decreased moving to the northwest. The dominant wind direction was W and SW from 2009 to 2018 in the Hobq Desert, the average wind speed of the prevailing winds mainly distributed at 4–8 m/s. The frequency of wind speed exceeding 10 m/s is very low, with a maximum value of 10% or below. There is a low energy wind environment in the Hobq Desert, with intermediate annual directional variability and obtuse or acute bimodal wind regime. The resultant drift direction (RDD)at Dongsheng station was relatively constant from 2009 to 2018, it was about 350°. RDD differed significantly at Baotou and Linhestations were 181 ± 169° and 231 ± 121°, respectively.The relationship between drift potential (DP) and the average and maximum wind speed was expressed as a power function. DP was strongly correlated with them. There is no significant correlated between the temporal changes in DPandprecipitation and temperature from 2009 to 2018 in the Hobq Desert.

A fully predictive model for aeolian sand transport

This paper presents the development, calibration and verification of a fully predictive model for wind-blown dry sand. The model is based on the modification of the well-known Bagnold equation by including the threshold shear velocity, which is also slightly modified to ensure a smoother transition from the non-transport regime to the transport regime. The sand transport rate is related to the static and dynamic grain roughness by using a semi-empirical roughness predictor, which is calibrated based on sand transport data from wind tunnel experiments with a flat mobile sand surface. The model input for dry sand is the wind velocity at one height above the surface and the sand characteristics (d50, d90). The effects of moisture, vegetation and shells on the sand transport process are included by fairly simple expressions which are acting on the shear velocity, the threshold value and the transport rate. In all, 105 high-quality data sets have been used for verification of the proposed predictive model. About 72% of the predicted values are within a factor of 2 of measured values of the transport rate.

The Scale Characteristics and Formation Mechanism of Aeolian Sand Streamers Based on Large Eddy Simulation

AbstractAeolian sand transport near the surface in the field often occurs in the form of sand streamers, which is very important to understand in‐depth the physics of aeolian transport and accurately predict the sand transport rate. In this paper, sand streamers were successfully reproduced by combining the Lagrangian trajectory simulation of particles with large eddy simulation of turbulence. The simulated sand streamers are hill‐like. Their characteristic scales are extracted by the two‐point correlation of sand concentration. It is found that sand streamers corresponding to the correlation threshold of are 2.0–4.5 m long and 0.3–0.4 m wide, while 30–100 m long and 1.2–2.4 m wide for , which are substantially consistent with the existing field observation results and increases with particle size. Conditional averaging method provided an ensemble‐mean visualization of flow structures that are responsible for the formation of streamers. It is found that sand streamers on a flat and erodible surface exist in the near‐surface wake area of large/very large scale turbulent motions (LSMs/VLSMs), indicating that sand streamers are the footprint of LSMs/VLSMs. Moreover, the characteristic scales of sand streamers will be affected by the splashing process of sand saltation. On a nonerodible surface, sand streamers trailing from upstream sandy surface become narrow though it can propagate downstream for a long distance.

Evaluating the SandFlow, an acoustic sediment transport sensor

The SandFlow is an acoustic device for detecting and measuring aeolian sand transport. It is based on the FlowCapt sensor, an instrument developed to measure aeolian snow transport. This study investigates the performance of the SandFlow in relation to the Saltiphone and the Modified Wilson and Cooke sampler, two devices frequently used in aeolian sand transport. The performance of the SandFlow was tested during three wind erosion events on the Dutch barrier island Terschelling and also in laboratory tests. Results show that the SandFlow adequately registers the periods of aeolian sand transport provided sand transport is sufficiently intense. For low sand transport the SandFlow is less accurate, although the periods with sand transport may still be detected. The sand transport fluxes measured by the SandFlow were lower than those measured by the MWAC although they remained within the same order of magnitude. Laboratory measurements showed that the efficiency of the SandFlow to register sand transport is close to 100% for sand coarser than 300 µm. For finer sand, efficiency decreases rapidly although the instrument remains usable for particles coarser than 150 µm provided appropriate corrections for efficiency are made. This study shows that the SandFlow can be used for aeolian sand transport provided the erosion event is strong enough and the particles are not too fine.