Aeolian Sand Transport Research Articles

Consistency between the distributions for the two phases in aeolian sand transport: Impact of sand on wind

Consistency between the distributions for the two phases in aeolian sand transport: Impact of sand on wind

An equivalent analysis of unsteady sand transport in a wind tunnel

This study investigates the effect of unsteady wind on sand transport in a wind tunnel using sinusoidally varying wind velocity to represent the unsteady wind. By comparing sand transport fluxes under steady and unsteady winds, we demonstrate that cumulative sand transport flux varies sinusoidally with the sinusoidally varying wind velocity. In addition to the time-averaged wind velocity, the sand transport rate is also affected by the amplitude and period of wind velocity variations. We then propose an ‘equivalent’ steady aeolian sand transport process as a substitute for the actual unsteady transport process. In terms of wind velocity amplitude and period variations, an equivalent wind friction velocity factor u*′ is defined to capture differences caused by wind velocity variations with the same time-averaged wind velocity and friction velocity. Finally, a unified equation for aeolian sand transport is derived, demonstrating applicability for both steady and unsteady flow conditions. The results of this study contribute to a deeper understanding of aeolian sand transport under unsteady flow.

Schmidt number and turbulent structures in boundary layer aeolian sand transport

Schmidt number and turbulent structures in boundary layer aeolian sand transport

Identifying topographic changes at the beach using multiple years of permanent laser scanning

Sandy beach-dune systems make up a large part of coastal areas world wide. Their function as an eco-system as well as a protective barrier for human and natural habitat is under increased threat due to climate change. A thorough understanding of change processes at the sediment surface is essential to facilitate prediction of future development and management strategies to maintain their function. Especially slow and small scale processes happening over several days up to weeks at cm level, such as aeolian sand transport are difficult to identify and analyse. Permanent laser scanning (PLS) is a useful tool in the study and analysis of coastal processes as it captures a data representation of the evolution of the sediment surface over extended periods of time (up to several years) with high detail (at cm-dm level). The PLS data set considered for this study, consists of hourly acquired 3D point clouds representing the surface evolution of a section of the Dutch coast during three years. However, it is challenging to extract concrete information on specific change processes from the large and complex PLS data set. We use multiple hypothesis testing in order to reduce the PLS data set to a so-called inventory of trends, consisting of 12.8 million partial time series with associated rate of change and elevation. The inventory of trends proofs to be a suitable tool to identify natural processes such as storms and aeolian sand transport in our test area in the aeolian zone of a sandy beach-dune system on the Dutch coast. We identify these processes and provide a tool to derive summarising data from the complex PLS data set. We find that all partial time series identified as most likely representing aeolian sand transport, result in 1354 m3 of sand deposition in our study area over the course of three years. We also show a comparison with transects from JarKus data and find a correlation between anthropogenic activities and erosion in our test area with a correlation coefficient of 0.3.

Mineral abrasion experiments at Mars relevant temperatures

The aeolian transport of sand generates fine material through abrasion. On Mars this process occurs at lower temperatures than on Earth, however, there is minimal data on the effects of temperature on aeolian abrasion rates. Here, results are reported of laboratory experiments where a suite of single-phase, Mars relevant minerals (feldspar, olivine, pyroxene, quartz and opal) were exposed to conditions simulating aeolian abrasion at temperatures common to the Martian surface (193 to 293 K). Our results suggest that mineral specific differences in solid phase parameters result in non-similar changes in abrasion rates with temperature. We propose this will ultimately exert a control on the composition and reactivity of the Martian surface.

Experimental study of particle impact on cohesive granular packing.

We investigate experimentally the impact process of sand particles onto a cohesive granular packing made of similar particles. We use a sand-oil mixture with varying liquid content to tune the cohesive strength of the packing. The outcome of the impact is analyzed in terms of the production of ejected particles from the packing. We quantify this production as a function of the impact velocity of the particles for increasing cohesion strength. We identified three different regimes depending on the cohesion number Co, defined as the ratio of the interparticle cohesive force to the particle weight. For small cohesion (i.e., Co⪅1), the ejection process is not modified by the cohesion. For intermediate cohesion (i.e., 1⪅Co⪅20), the ejection process becomes less efficient: the number of ejected particles per impact for a given impact velocity is decreased but the critical impact velocity to trigger the ejection process remains unchanged. Finally, for strong cohesion (i.e., Co⪆20), we observed a progressive increase of the critical impact velocity. These experimental results confirm spectacularly the outcomes of recent numerical simulations on the collision process of a particle onto a cohesive packing and open avenues to model the aeolian transport of moist sand.

Aeolian saltation over plateau gobi: Field studies in the Zhongzaohuo gobi area, northeastern Tibetan plateau, China

Aeolian sand movement, affected by low air density, exhibits special characteristics. Our understanding of saltation, especially in plateau gobi regions, is limited. Here, we report results of aeolian saltation processes over gobi in the Zhongzaohuo area of the northeastern Tibetan Plateau using high-frequency saltation measurement. The key findings include: (1) In turbulent winds, aeolian sand saltation on plateau gobi occurs intermittently. As the aeolian sand transport activity parameter increases, the threshold wind speed Ut for sand movement decreases exponentially. Concurrently, the threshold wind speed required to move sand particles shows a significant increase, being 49%–68% higher than that in lower altitude gobi regions. (2) The average saltation height zq in plateau gobi ranges from 0.25 to 0.40 m, with a mean value of 0.36 ± 0.04 m, which is approximately 2.3–2.6 times higher than that in lower altitude gobi areas. (3) Sand transport rate in the plateau gobi is significantly reduced, being 57%–94% lower than that in lower altitude gobi areas. This research will deepen our knowledge of how turbulent winds affect aeolian sand movements in plateau gobi regions.

Characterizing sand transport with various micro‐ridge spacings and heights

AbstractIn arid regions, the use of ridge cover as a traditional agricultural strategy has been effective in mitigating soil wind erosion. However, few studies have focused on the sand transport characteristics of different micro‐ridge spacings and heights. This study aimed to identify the mechanism by which ridges change the near‐surface sand transport. In a controlled wind tunnel environment, the aeolian sand flux structure and sand transport flux (qz) at heights from 0 to 0.7 m were measured. The results showed that, compared with no ridges, ridge covering could significantly modify the structure of aeolian sand flux near the surface, yielding a substantial reduction in the proportion of sand transport to the total sand transport in the 0‐ to 0.1‐m height layer. In the absence of ridges, blown sand was mainly concentrated in the 0‐ to 0.1‐m height layer, and qz decreased exponentially as the height increased. With various micro‐ridge spacings and heights, when H was less than 0.1 m and L was more than 15H, the blown sand also remained concentrated in the 0‐ to 0.1‐m height layer, and qz decreased exponentially as the height increased. When H increased to 0.15 m from 0.1 m and L decreased from 15H to 5H, the height of the blown sand was concentrated in the 0.3‐ to 0.4‐m layer. The blown sand layer of most ridge structures was concentrated at a height of 0.1–0.4 m, and the structure of the aeolian sand flux resembled an ‘elephant nose effect’. The total sand transport rate of all ridge structures was significantly lower than that with no ridges, indicating that a reasonable ridge structure can effectively prevent wind erosion.

Spatial Characteristics of Aeolian Sand Transport Affected by Surface Vegetation along the Oshang Railway

Blown sand transport plays a pivotal role in determining the optimal placement of sand protection facilities along railways in sandy areas. Surface vegetation cover significantly influences blowing sand activities along the Oshang Railway (from Otog Front Banner to Shanghai-Temple Town). In this study, the spatial characteristics of aeolian sand transport along the railway were derived from field observations conducted at five different locations, each with varying fractional vegetation cover (FVC). The results indicate that sand-transport intensity does not fully correlate with the wind energy environment, primarily due to differences in surface vegetation cover among the observation sites. We utilize the dimensionless ratio Q·g·f/(0.136 × DP·ρa) to represent the sand transport rate (Q), the sand-moving wind frequency (f) and drift potential (DP), exhibiting a negative exponential trend with FVC. Sand transport is effectively restrained when FVC is greater than or equal to 20%. Conversely, when FVC is less than 20%, sand transport intensity exponentially increases with decreasing fractional vegetation cover. After careful analysis, we propose a simple empirical expression that incorporates the influence of both the wind field and fractional vegetation cover to assess sand transport on a flat surface. The study offers valuable insights for designing wind-blown sand protection measures along railways and evaluating wind-blown sand movement on a flat surface affected by vegetation.

Impact of temporal beach grain size variability on aeolian sediment transport and topographic evolution in a microtidal environment

While the impact of the spatio/temporal variability of grain size on morphological beach state is reasonably well understood, relatively little is known on its impact at a scale of days/months on aeolian sediment transport. This study focuses on five short intensive wind events during which aeolian sediment transport measurements, beach surface sampling and elevation change surveys were carried out for 1 to 3 days, over a 16-month period on a microtidal beach dominated by offshore winds. Monthly observations show a high temporal variability in beach grain size in relation to the decoupling between hydrodynamic and aeolian processes, from medium sand after a marine storm and inundation of the beach, to very coarse sand after several weeks of storm-force winds. During each wind event, topographic change on the beach ranged from zero, to 0.55 m. The time scale of coarsening depended on the initial beach grain size and could be very fast when the beach was composed of medium sand (e.g. 388 μm sand changed to coarse sand of 547 μm in 40 h). In contrast, it took one month to transition from a coarse median beach grain size of 883 μm to a very coarse one of 1323 μm. This variability in grain size results in dramatically different rates of sediment flux. For example, during average wind speeds of 10 to 14 m/s the sediment flux when the beach was composed of medium sized sand ranged between 21 and 154 kg/m/h compared to 0.4 to 50 kg/m/h when the beach was composed of coarse and very coarse sized grains. Overall, this study highlights the importance of beach grain size variability on aeolian sediment transport and shows that for similar incident wind and climatic conditions aeolian sediment transport rates vary dramatically. The study demonstrates the importance of taking care when using a constant median grain size in the calculation of long-term aeolian sand transport on beaches with heterogeneous spatial and temporal beach grain size variability.

Aeolian Sand Sorting and Soil Moisture in Arid Namibian Fairy Circles

We studied fairy circles 20 km west of Sesriem at one of the driest locations of fairy circles in Namibia, at the foot of the popular Sossusvlei dunes. These fairy circles lack the typical hexagonal order of the Namibian fairy circles. After years of drought, their pattern is more similar to that of vegetation rings, due to the sparse vegetation in the area between the circles. Cross-section measurements of the soil water content (SWC) show that the upper layer (12 cm) is very dry (~1%) and much below the wilting point of Stipagrostis ciliata grasses, whereas the deeper soil layer is wetter (4%). The grain size distribution of soil samples taken from inside and outside the fairy circles reveals considerable heterogeneity in the size fractions due to aeolian (wind-driven) sand sorting. The bare soil inside the fairy circles contains coarser grains, and the ground surface is covered by sand megaripples. There is a linear trend between the vertical soil moisture gradient and the median grain diameter. Fine particles trapped on the vegetated edges of the fairy circle result in small nebkhas that increase the soil water retention at the surface. The dry and loose coarser topsoil inside the fairy circles may prevent the recolonization of new seedlings with short root lengths inside the fairy circles. Our results highlight the role of aeolian sand transport and deposition in desert vegetation environments and seem to support the notion that fairy circle formation may be affected by the interplay between sand sorting and soil moisture gradients.

Statistically assessing vertical change on a sandy beach from permanent laser scanning time series

In the view of climate change, understanding and managing effects on coastal areas and adjacent cities is essential. Permanent Laser Scanning (PLS) is a successful technique to not only observe notably sandy coasts incidentally or once every year, but (nearly) continuously over extended periods of time. The collected point cloud observations form a 4D point cloud data set representing the evolution of the coast provide the opportunity to assess change processes at high level of detail. For an exemplary location in Noordwijk, The Netherlands, three years of hourly point clouds were acquired on a 1 km long section of a typical Dutch urban sandy beach. Often, the so-called level of detection is used to assess point cloud differences from two epochs. To explicitly incorporate the temporal dimension of the height estimates from the point cloud data set, we revisit statistical testing theory. We apply multiple hypothesis testing on elevation time series in order to identify different coastal processes, like aeolian sand transport or bulldozer works. We then estimate the minimal detectable bias for different alternative hypotheses, to quantify the minimal elevation change that can be estimated from the PLS observations over a certain period of time. Additionally, we analyse potential error sources and influences on the elevation estimations and provide orders of magnitudes and possible ways to deal with them. Finally we conclude that elevation time series from a long term PLS data set are a suitable input to identify aeolian sand transport with the help of multiple hypothesis testing. In our example case, slopes of 0.032 m/day and sudden changes of 0.031 m can be identified with statistical power of 80% and with 95% significance in 24-h time series on the upper beach. In the intertidal area the presented method allows to classify daily elevation time series over one month according to the dominating model (sudden change or linear trend) in either eroding or accreting behaviour.

Tracing provenance of eolian sands in the northern Ulan Buh Desert, China: Evidence from the REE geochemical characteristics

Desertification is a primary threat to crop farming, the livestock industry, and the eco-environment in northwestern China. Through field research and satellite imagery, it has been discovered that there are 4–5 sand belts in the shape of handclasps connecting major deserts in the Alxa Plateau region of northern China. These sand belts have been expanding for several decades. However, eolian sand transport between major deserts through these expanding sand belts is still underestimated. Identifying the source areas of eolian sands in deserts provide essential knowledge to prevent desertification. In this study, we collected samples from the northern Ulan Buh Desert (UBD), the Yamalik Desert (YD), and the Langshan Mountains and analyzed their REE contents in the <125 μm fraction. We compared REE geochemical characteristics of fine-grained sands in the UBD with other recently published REE data for potential source areas. Our findings indicate that the primary origins of fine-grained sands in the northern UBD are from the Qilian Mountains (QM) and Badain Jaran Desert (BJD), which are transported through the YD. Contributions from the Gobi-Altay Mountains to the north are relatively minor. The Langshan Mountains and Hawula Mountains are not considered as the primary sources of fine-grained sands in the UBD. In contrast, surface sediments in the southern UBD have a local mixture of the paleo-lacustrine deposits, the Gobi sediments, and minor external inputs from the BJD and QM. Our findings demonstrate that sand belts connecting these deserts play a crucial role in the eolian transport of fine-grained sands. Therefore, both the Chinese government and scientists should pay close attention to them in future efforts to combat desertification.

Sr-Nd isotopic compositions of aeolian sand in the northern Ulan Buh Desert and Yamalik Desert: Implications for provenance and transport processes

Tracing the provenance of aeolian sand in deserts is essential for comprehending the mechanisms underlying desert formation and sediment transportation processes. There are multiple sand belts that interconnect major deserts within the Alxa Plateau of northern China, as evidenced by satellite images and field research. However, the aeolian sand transport from the Badain Jaran Desert (BJD) to the Yamalik Desert (YD) and Ulan Buh Desert (UBD) through these expanding sand belts is still being underestimated. In this study, we analyzed trace metal contents and Sr-Nd isotopic compositions in the < 125 μm fraction of dune sands in the northern UBD and YD, along with bedrocks from the Langshan Mountains (LM). Based on bivariate plots of Zr vs. Hf, Zr/Hf vs. Hf, Cr/V vs. Y/Ni, and Th/Co vs. Th/Sc, the aeolian sands in the northern UBD and YD have similar sources but are different from those in the southern UBD. Comparison of Sr-Nd isotopes with potential source areas suggests that fine-grained aeolian sands in the YD and northern UBD are mainly sourced from the BJD and the southern Mongolian Gobi. Although the heterogeneity of bedrock samples impeded the precise interpretation of contributions from all potential sources, the εNd(0) signatures in the UBD suggest that up to 25% of the fine-grained sand is derived from the LM, which displays highly negative εNd(0) values. On a larger geographical scale, the northern UBD serves as a transfer intersection for long-distance transportation of sand and dust ultimately sourced from the Central Asian Orogen and northern Tibetan Plateau. This study also demonstrates the essential role of sand belts in facilitating aeolian transport within the Alxa Deserts.

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.

STRENGTHENING COASTAL DEFENCE WITH ARTIFICIAL DUNES

The Belgian coast is primarily sandy and is locally subject to erosion (Deronde, 2004). Traditionally, erosion was counteracted using hard engineering structures like groynes, seawalls and sea dikes. Nowadays, the Belgian government uses a different strategy by adopting a more soft and dynamic approach where possible. Many ‘soft’ managing activities at the Belgian coast are carried out in the form of regular and routine sand nourishments to cope for future flooding risks and coastal erosion (Houthuys, 2012). This method gives room to a new approach for coastal management where the natural elements and processes facilitate the development of new engineered dune areas at locations where traditional dike structures currently protect the hinterland. In the context of climate change and strict requirements for water safety, these hard to adapt engineering structures will not resist future flood events and more innovative solutions like dune in front of a dike principles to deal with rising sea level are receiving considerable attention. As a result, concepts are worked out in which traditional sea dikes are reinforced with newly created dune systems, offering a high level of protection of coastal infrastructure and at the same time offering a more natural appearance and higher ecological and socio economical values. In the framework of the SARCC project, one pilot site is realized in Raversijde. Besides the coastal defense function of the artificial dune, a second benefit is achieved: mitigating the nuisance, created by aeolian sand transport and preventing roads and tram tracks at the dike crest to be buried in sand.

BEACH BUILDINGS ON POLES AND THEIR IMPLICATIONS FOR DUNEWARD SEDIMENT TRANSPORT: A NUMERICAL STUDY

Sandy beaches are attractive worldwide. An increasing number of beach users induces a higher demand for buildings at the beach-dune interface. These buildings alter the local flow speed and direction which, in turn, changes the aeolian sand transport around buildings, and the sediment supply to the dunes. This study aims to understand how and to what extent beach buildings on poles affect duneward sediment transport.

EFFECTS OF SHELLS ON AEOLIAN SEDIMENT TRANSPORT ON A NATURAL BEACH

Studying aeolian sediment transport in coastal areas is challenging. Therefore, a location with a small number of supply-limiting elements (e.g., shells, moisture and vegetation) and a long fetch length is frequently chosen to allow for a better comparison of predicted and observed transport rates (Strypsteen et al., 2021). Many natural beaches, however, contain an abundance of shells and shell fragments/hash due to pounding and fracturing in the surf zone and literature on this topic is rather scarce. Direct field studies on the impact of shells on aeolian sediment transport rates have been limited. This study reports on a two-day measurement campaign on the upper beach of Koksijde, Belgium, where data on aeolian transport rates, mass flux profiles, surface moisture, wind conditions, and grain size distributions were collected. The goal of the experiment was to measure aeolian sand transport on the upper beach as input for dune growth and to find out how a shell pavement affected these transport rates during a strong, oblique onshore wind.

Turbulent Flow Structures Over a Gobi Surface and Their Impact on Aeolian Sand Transport

AbstractHere, we present the results of turbulent flow structures and their influence on aeolian sand transport over gobi surfaces based on quadrant analysis. The results show that the classified turbulent structures over gobi accounted for 43%–44% of the time frequency and contributed ∼97% to the Reynolds stress, yet the contribution rates to sand transport rate ranged from 45% to 52%. Sweeps (Quadrant 4) and ejections (Q2) were the two most frequent events and accounted for 70% of the total frequency for the classified turbulent structures. Sweeps made a major contribution of 51%–57% to sand transport rate over gobi. Outward interactions (Q1) were relatively rare, but they individually entrained as much sand as sweeps of comparable magnitude and duration. The turbulent structures over gobi show similar behaviors comparable to those of water flows over gravel riverbeds or rough seabeds yet different from those of air flow over flat sand surfaces.

The Application of CNN-Based Image Segmentation for Tracking Coastal Erosion and Post-Storm Recovery

Coastal erosion due to extreme events can cause significant damage to coastal communities and deplete beaches. Post-storm beach recovery is a crucial natural process that rebuilds coastal morphology and reintroduces eroded sediment to the subaerial beach. However, monitoring the beach recovery, which occurs at various spatiotemporal scales, presents a significant challenge. This is due to, firstly, the complex interplay between factors such as storm-induced erosion, sediment availability, local topography, and wave and wind-driven sand transport; secondly, the complex morphology of coastal areas, where water, sand, debris and vegetation co-exists dynamically; and, finally, the challenging weather conditions affecting the long-term small-scale data acquisition needed to monitor the recovery process. This complexity hinders our understanding and effective management of coastal vulnerability and resilience. In this study, we apply Convolutional Neural Networks (CNN)-based semantic segmentation to high-resolution complex beach imagery. This model efficiently distinguishes between various features indicative of coastal processes, including sand texture, water content, debris, and vegetation with a mean precision of 95.1% and mean Intersection of Union (IOU) of 86.7%. Furthermore, we propose a new method to quantify false positives and negatives that allows a reliable estimation of the model’s uncertainty in the absence of a ground truth to validate the model predictions. This method is particularly effective in scenarios where the boundaries between classes are not clearly defined. We also discuss how to identify blurry beach images in advance of semantic segmentation prediction, as our model is less effective at predicting this type of image. By examining how different beach regions evolve over time through time series analysis, we discovered that rare events of wind-driven (aeolian) sand transport seem to play a crucial role in promoting the vertical growth of beaches and thus driving the beach recovery process.