Aeolian Transport Models Research Articles

The importance of grain‐related shear velocity in predicting multi‐monthly dune growth

AbstractThis study assesses the accuracy of the aeolian transport model proposed by van Rijn and Strypsteen in 2020 in predicting sediment transport rates for dune growth, with a focus on the importance of grain‐related shear velocity. A 9‐month dataset of local and regional wind characteristics and dune growth from a new artificial dune with planted marram grass in an environment with minor supply limitations was analysed. The study site is located in Oosteroever, Belgium. The results revealed that utilizing local measured overall shear velocities resulted in significant overprediction of dune volume changes. Incorporating sub‐models for grain‐related bed roughness and shear velocity improved the predictions of dune growth drastically with high statistical results (r2 = 0.98 and RMSE = 0.64 m3/m). The primary driver contributing to dune growth at the study site was due to oblique onshore moderate winds of 10 m/s, as revealed from a frequency‐magnitude analysis. The model could be used with either local wind measurements or regional data transformed to local beach conditions. The study highlights the importance of considering grain‐related shear velocity in future studies to improve the prediction of sand transport rates and enhance our understanding of dune evolution.

Modeling Multi‐Fraction Coastal Aeolian Sediment Transport With Horizontal and Vertical Grain‐Size Variability

AbstractGrain size affects the rates of aeolian sediment transport on beaches. Sediment in coastal environments typically consists of multiple grain‐size fractions and exhibits spatiotemporal variations. Still, conceptual and numerical aeolian transport models are simplified and often only include a single fraction that is constant over the model domain. It is unclear to what extent this simplification is valid and if the inclusion of multi‐fraction transport and spatial grain‐size variations affects aeolian sediment transport simulations and predictions of coastal dune development. This study applies the numerical aeolian sediment transport model AeoLiS to compare single‐fraction to multi‐fraction approaches for a range of grain‐size distributions and spatial grain‐size scenarios. The results show that on timescales of days to years, single‐fraction simulations with the median grain size, D50, often give similar results to multi‐fraction simulations, provided the wind is able to mobilize all fractions within that time frame. On these timescales, vertical variability in grain size has a limited effect on total transport rates, but it does influence the simulation results on minute timescales. Horizontal grain‐size variability influences both the total transport rates and the downwind bed grain‐size composition. The results provide new insights into the influence of beach sediment composition and spatial variability on total transport rates toward the dunes. The findings of this study can guide the implementation of grain‐size variability in numerical aeolian sediment transport models.

The fetch effect on sand transport above a farmland surface and conditions for saturation

The existing aeolian transport models have been developed for saturated sand flow. For unsaturated sand flow, these models produce larger errors. To mitigate this problem, this paper attempts to clarify the fetch effect on the sand transport rate above a farmland surface so as to improve sand transport model. We first defined the C value in the 1978 Lettau and Lettau model as a sand transport coefficient. For saturated sand flow, the C value equals the saturated sand transport coefficient (i.e., C=Csat=6.7); but for unsaturated flow, the C values would be less than 6.7. We found that C increased with increasing fetch length (L) and shear velocity (u*) using data from a field investigation above a farmland surface. The measured C was far less than Csat even at the maximum fetch length (100 m) and the largest shear velocity (0.809 m s−1) during the sand collection in field observations. Based on the relationships among C, L, and u*, we developed an aeolian transport model that accounted for the shear velocity and fetch length. We confirmed that the essential condition for the sand flow to reach saturation was wind speed is strong enough to transport most of the surface soil particles. Thus, the critical shear velocity at which the sand flow can reach saturation (u*c-sat) is close to the threshold shear velocity for entrainment of the largest soil particles. When u* >u*c-sat, the sand transport saturation fetch length decreased rapidly with increasing wind speed. However, due to the high wind speed requirement, it is difficult for the sand flow to reach saturation, which leads to the farmland surface in an environment dominated by wind erosion under normal wind conditions.

Characterizing the spatial variations of wind velocity and turbulence intensity around a single Tamarix tree

Characterizing airflow and turbulence patterns around real vegetation is essential for predicting aeolian erosion. However, few studies have explored the airflow field around live plants. In this field study, airflow field and turbulence patterns were investigated around a single Tamarix tree. Wind speeds were monitored at 34 locations around the tree at the heights of z/h = 0.13 h, 0.5 h, 1.1 h, and 1.8 h (where h is the height of the tree) in a wind speed of 12.5 m s−1. The results showed that a half-ellipse deceleration zone formed downwind and full-ellipse acceleration zones on the sides of the tree. The largest turbulence intensity was observed at lower heights and in the leeside of the tree where wind speed was highest, and the smallest was found on the sides where wind speed accelerated. On the sides of the tree the structure of wind speed and turbulence profiles was similar and no significant differences (P < 0.001) in wind speeds were found between identical locations. The mean wind velocities downwind of the tree at 0.5 h–7 h and DR1-DR4 (at the right side of the tree edge) were significantly different to the approach flow (wind speeds at −7 h) but no statistical differences were observed between wind speeds in other locations and the approach flow. The values of Rc∆x, z (wind velocity reduction coefficient) were higher in the wake of the tree than upwind and on the sides of the tree indicating that the sheltering effect of the tree extended to the distance of seven times the height tree. This study represents an important investigation of the plant effect on airflow and the results can be used to inform Tamarix management and support parameterization of drag partition schemes to improve the accuracy of aeolian transport models in landscapes with Tamarix.

Wind regime, sediment transport, and landscape dynamics at a Mars analogue site in the Andes Mountains of Northwestern Argentina

The Puna Plateau of northwestern Argentina is a promising region for analogue investigations of Martian aeolian processes owing to its high altitude, low atmospheric pressure, aridity, geology, and widespread granular and bedrock aeolian landforms. Despite a growing body of research about the region's enigmatic aeolian landforms, little is known about the contemporary aeolian environment, processes, and landscape dynamics. To address this gap, we used a combination of modelled weather data, satellite imagery, and field measurements to characterize the wind regime, aeolian sediment transport, and allied surface changes. The study focused on the Campo de Piedra Pómez (CPP) – a volcanic landscape with a variety of Mars analogue landforms: gravel-mantled megaripples, yardangs, ventifacts, periodic bedrock ridges, and aeolian rat-tails. Modelled weather data from the European Center for Medium-Range Forecasting's ERA-Interim reanalysis dataset confirm the contemporary dominance of northwesterly winds in this region, consistent with the primary orientation of yardangs. These data also indicate the presence of less frequent southeasterly winds, which may explain why re-entrant profiles are observed on the southeast flanks of a small portion of the yardang population, and why some aeolian rat-tails point to the northwest. Observations from multi-temporal satellite imagery indicate that deposition and displacement of small sediment patches overlying ignimbrite bedrock is relatively common across the region. Samples retrieved from sediment traps deployed over one year in the region suggest a broad range of particle densities are mobilized by wind, which implies a wide range of wind speed for fluid entrainment. Conventional aeolian transport models do not consider the effects of particle density variations on transport processes. However, in areas with multiple density sediment present, our results point towards a need for aeolian models to consider density when quantifying and qualifying sediment transport.

A prediction method for the size distribution of saltating particles above a mixed‐size bed

AbstractIn aeolian saltation, the sand bed is a mixture of sand particle with a wide range of particle sizes. Generally, the particle size distribution (PSD) of saltating particles is ignored by previous aeolian transport models, which will result in differences between predictions and observations. To better understand the saltation process, a prediction method of the PSD of saltating particles was proposed in this article. The probability of contact between incident sand and bed sand was introduced into the particle‐bed collision process. An artificial PSD of the incident saltating particles was set as the initial condition. A stochastic particle‐bed collision model considering contact probability was then used in each iteration step to calculate a new PSD of saltating particles. Finally, the PSD of saltating particles can be determined when aeolian saltation reaches a steady state (saltation is in a steady state when its primary characteristics, such as horizontal mass flux and the concentration of saltating particles, remain approximately constant over time and distance). Meanwhile, according to the experimental results, a calculation formula for the contact parameter n is given, which characterizes the shielding effect of particles on each other. That is, if soil PSD and friction velocity were given, the PSD of saltating particles can be determined. Our results do not depend on the initial conditions, and the predicted results are consistent with the experimental results. It indicated that our method can be used to determine the PSD of saltating particles. © 2020 John Wiley &amp; Sons, Ltd.

A note on the use of drag partition in aeolian transport models

Sediment transport equations used in wind erosion and dust emission models generally incorporate a threshold for particle motion (u*t) with a correction function to account for roughness-induced momentum reduction and aerodynamic sheltering. The prevailing approach is to adjust u*t by the drag partition R, estimated as the ratio of the bare soil threshold (u*ts) to that of the surface in the presence of roughness elements (u*tr). Here, we show that application of R to adjust only the entrainment threshold (u*t = u*ts/R) is physically inconsistent with the effect of roughness on the momentum partition as represented in models and produces overestimates of the sediment flux density (Q). Equations for Q typically include a friction velocity scaling term (u*n). As Q scales with friction velocity at the soil surface (us*), rather than total friction velocity (u*) acting over the roughness layer, u*n must be also adjusted for roughness effects. Modelling aeolian transport as a function of us* represents a different way of thinking about the application of some drag partition schemes but is consistent with understanding of aeolian transport physics. We further note that the practice of reducing Q by the vegetation cover fraction to account for the physically-protected surface area constitutes double accounting of the surface protection when R is represented through the basal-to-frontal area ratio of roughness elements (σ) and roughness density (λ). If the drag partition is implemented fully, additional adjustment for surface protection is unnecessary to produce more accurate aeolian transport estimates. These findings apply equally to models of the vertical dust flux.

Size‐Independent Susceptibility to Transport in Aeolian Saltation

AbstractNatural wind‐eroded soils contain a mixture of particle sizes. However, models for aeolian saltation are typically derived for sediment bed surfaces containing only a single particle size. To nonetheless treat natural mixed beds, models for saltation and associated dust aerosol emission have typically simplified aeolian transport either as a series of noninteracting single particle size beds or as a bed containing only the median or mean particle size. Here we test these common assumptions underpinning aeolian transport models using measurements of size‐resolved saltation fluxes at three natural field sites. We find that a wide range of sand size classes experience “equal susceptibility” to saltation at a single common threshold wind shear stress, contrary to the “selective susceptibility” expected for treatment of a mixed bed as multiple single particle size beds. Our observation of equal susceptibility refutes the common simplification of saltation as a series of noninteracting single particle sizes. Sand transport and dust emission models that use this incorrect assumption can be both simplified and improved by instead using a single particle size representative of the mixed bed.

Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements

AbstractWind‐blown sand and dust models depend sensitively on the threshold wind stress. However, laboratory and numerical experiments suggest the coexistence of distinct fluid and impact thresholds for the initiation and cessation of aeolian saltation, respectively. Because aeolian transport models typically use only a single threshold, existence of separate higher fluid and lower impact thresholds complicates the prediction of wind‐driven transport. Here we extend the statistical Time Frequency Equivalence Method to derive the first field‐based estimates of distinct fluid and impact thresholds from high‐frequency wind and saltation measurements at three field sites. Our measurements show that when saltation is mostly inactive, its instantaneous occurrence is governed primarily by wind exceedance of the fluid threshold. As saltation activity increases, so too does the relative importance of the impact threshold, until it dominates under near‐continuous transport conditions. Although both thresholds are thus important for high‐frequency saltation prediction, our results suggest that the time‐averaged saltation flux is primarily governed by the impact threshold.

High-frequency measurements of aeolian saltation flux: Field-based methodology and applications

Aeolian transport of sand and dust is driven by turbulent winds that fluctuate over a broad range of temporal and spatial scales. However, commonly used aeolian transport models do not explicitly account for such fluctuations, likely contributing to substantial discrepancies between models and measurements. Underlying this problem is the absence of accurate sand flux measurements at the short time scales at which wind speed fluctuates. Here, we draw on extensive field measurements of aeolian saltation to develop a methodology for generating high-frequency (up to 25 Hz) time series of total (vertically-integrated) saltation flux, namely by calibrating high-frequency (HF) particle counts to low-frequency (LF) flux measurements. The methodology follows four steps: (1) fit exponential curves to vertical profiles of saltation flux from LF saltation traps, (2) determine empirical calibration factors through comparison of LF exponential fits to HF number counts over concurrent time intervals, (3) apply these calibration factors to subsamples of the saltation count time series to obtain HF height-specific saltation fluxes, and (4) aggregate the calibrated HF height-specific saltation fluxes into estimates of total saltation fluxes. When coupled to high-frequency measurements of wind velocity, this methodology offers new opportunities for understanding how aeolian saltation dynamics respond to variability in driving winds over time scales from tens of milliseconds to days.

Grain size and hydrodynamic sorting controls on the composition of basaltic sediments: Implications for interpreting martian soils

Impact shattering of bedrock and wind-driven sorting have been identified as primary processes modifying the size, shape, and roundness of basalt-derived sediments at the Spirit rover landing site in Gusev Crater, Mars. We experimentally explore how physical sorting can cause significant compositional modification of sediment relative to bedrock in the absence of chemical weathering. Two basalt samples, chosen to provide a range of source rock compositions and textures, were crushed and sieved. We quantify variations in sediment mineral proportions as a function of grain size, as well as accompanying changes in major- and trace-element geochemistry. Redistribution of phenocrysts imposes the primary compositional control on basalt-derived sediments, although minor mineral fractionations affect trace-element concentrations. Olivine accumulation pairs with increasing MgO, Fe2O3(T), Ni, and Co, and Al2O3 depletion. The observed chemical changes in physically sorted sediments in our experiments mimic those measured in martian soils. Aeolian transport models demonstrate that sorting of mineral components of basalts under martian conditions is likely. Given the potential for sorting by winds on Mars we argue that sorting must be considered, in addition to chemical weathering, when interpreting the geochemical changes in martian sediments.

Evaluation of a new model of aeolian transport in the presence of vegetation

Aeolian transport is an important characteristic of many arid and semiarid regions worldwide that affects dust emission and ecosystem processes. The purpose of this paper is to evaluate a recent model of aeolian transport in the presence of vegetation. This approach differs from previous models by accounting for how vegetation affects the distribution of shear velocity on the surface rather than merely calculating the average effect of vegetation on surface shear velocity or simply using empirical relationships. Vegetation, soil, and meteorological data at 65 field sites with measurements of horizontal aeolian flux were collected from the Western United States. Measured fluxes were tested against modeled values to evaluate model performance, to obtain a set of optimum model parameters, and to estimate the uncertainty in these parameters. The same field data were used to model horizontal aeolian flux using three other schemes. Our results show that the model can predict horizontal aeolian flux with an approximate relative error of 2.1 and that further empirical corrections can reduce the approximate relative error to 1.0. The level of error is within what would be expected given uncertainties in threshold shear velocity and wind speed at our sites. The model outperforms the alternative schemes both in terms of approximate relative error and the number of sites at which threshold shear velocity was exceeded. These results lend support to an understanding of the physics of aeolian transport in which (1) vegetation's impact on transport is dependent upon the distribution of vegetation rather than merely its average lateral cover and (2) vegetation impacts surface shear stress locally by depressing it in the immediate lee of plants rather than by changing the bulk surface's threshold shear velocity. Our results also suggest that threshold shear velocity is exceeded more than might be estimated by single measurements of threshold shear stress and roughness length commonly associated with vegetated surfaces, highlighting the variation of threshold shear velocity with space and time in real landscapes.

PROCESS-BASED MODELING OF COASTAL DUNE DEVELOPMENT

In this paper, the aeolian transport model DUNE (Sauermann et al., 2001, Kroy et al., 2002) that describes important features and dynamics of typical desert dunes, is extended such that it can be applied in sandy coastal areas. Initial tests explore the limitations of the model in coastal areas after which adaptations are proposed and implemented. The final model version is applied to a coastal profile near Vlugtenburg (Dutch Holland coast).

An aeolian transport model for flat shifting sand fields under dynamic-limiting conditions

Estimating aeolian transport rates in the field is a difficult task even though various transport equations have been proposed. In this study, we used 5 years of field data on aeolian transport from an experimental plot for aeolian research located in the southeastern Tengger Desert to develop a practical aeolian transport equation capable of estimating transport rates in flat shifting sand fields. The resulting equation relates transport to the average wind velocity during a given period through a modified power function. The transport rate increases linearly with the cube of the average wind velocity. The equation can be generalized rate to a form that contains a proportionality coefficient and a threshold average wind velocity. The proposed model has an advantage over previous models in that it is based on a much larger sample size and corrected to account for variability in wind direction, sampling efficiency, and the height of the wind measurements.

Representation of land-surface processes in aeolian transport models

Mathematical modelling provides an essential tool for understanding aeolian transport of dust and sand, its spatial and temporal dynamics, and its role in the earth system. As much of the crucial action takes place at or just above land surfaces, adequate representation of land-surface processes is essential. In this paper, we review progress in aeolian transport modelling during last two decades, with emphasis on the representation of several key land-surface processes: (1) saltation; (2) dust uplift; (3) the threshold for particle motion, including the effects of surface sheltering and cohesion; (4) turbulent diffusion near the ground; and (5) deposition of particles to the surface. These process models are then placed in the context of integrated models of aeolian transport, in which the conservation equation for atmospheric dust is solved over a large region or the globe. We identify and assess four challenges in large-scale aeolian transport modelling: (1) the fidelity of process representations, (2) upscaling point-scale process models in the presence of unresolved heterogeneity in space and time (where a new approach is suggested), (3) availability of spatial data on model inputs and boundary conditions, and (4) large-scale parameter estimation.

Predicting Aeolian Sand Transport: Revisiting the White Model

The derivation and history of the frequently cited aeolian transport model of White are considered in light of the continued replication of an error in the original expression. The error may have escaped notice because the expression is still dimensionally correct and it yields predictions that appear reasonable in comparison with both the predictions of other models with field data. The incorrect expression has come to be identified as a distinct model. However, the correct formulation of the ‘White model’ is, in fact, a rearrangement of the Kawamura model with a slightly smaller (c.6%) empirical coefficient. © 1997 John Wiley & Sons, Ltd.

Predicting Aeolian Sand Transport: Revisiting the White Model

The derivation and history of the frequently cited aeolian transport model of White are considered in light of the continued replication of an error in the original expression. The error may have escaped notice because the expression is still dimensionally correct and it yields predictions that appear reasonable in comparison with both the predictions of other models with field data. The incorrect expression has come to be identified as a distinct model. However, the correct formulation of the ‘White model’ is, in fact, a rearrangement of the Kawamura model with a slightly smaller (c.6%) empirical coefficient. © 1997 John Wiley & Sons, Ltd.