Crust strength: a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfaces

A wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength. The sediments consisted of varying proportions of large sand‐sized particles with a fine particle cement. The energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface. It is important to consider distributions of parameters rather than mean values only, since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities. At the impact velocities used in this study (mean velocity 4·4 m s^?1, with standard deviation of 0·51), surfaces containing less than 12 per cent fine material were easily eroded, but insignificant erosion occurred when the fine particle content exceeded 60 per cent. Small amounts of cementing material were easily ruptured, allowing the large sand grains to be moved (largely in creep) by the bombarding particles. A significant amount of energy was lost to the bed. As the percentage of fine material increased, the surface became more difficult to break up and less energy was lost to the bed. The probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles. Copyright © 2001 John Wiley & Sons, Ltd.     

Modeling the effect of saltation on surface layer turbulence

Particle–turbulence interaction has been a research focus in the field of pneumatic transport, especially in aeolian environments. However, knowledge regarding the effect of saltating particles on the turbulence characteristics is very limited. In this article, a process of sand-laden flow from forming sand streamers to stability is investigated via a coupled mathematical model of wind-blown sand that includes the spatiotemporal development. The variations in the turbulence characteristics, such as the mean velocity and turbulence intensity in clean air or sand-laden flow field, are analyzed. The results show that the splash process of sand grains near the wall decrease the wind speed in the saltation layer and destroy the low-speed streaks. Moreover, the profiles of streamwise turbulence intensity exhibit a transition from ‘decreasing’ to ‘increasing’ and approximately intersect at an ‘intensity focus’, which is presented for the first time. Furthermore, it was found that saltating particles could enhance the Reynolds stress. Meanwhile, it was also noticed that the shear stress at the wall surface is greater than the impact threshold and that there is a tendency towards the impact threshold. Therefore, saltation makes the particle Reynolds number of sand-laden flow higher than that under non-saltation conditions, thus changing the particles’ effect on the turbulence intensity. Gravity-dominated saltation is probably the most essential difference between wind-blown sand and other traditional two-phase flows. © 2020 John Wiley & Sons, Ltd.     

Experimental investigation of the velocity of a sand cloud blowing over a sandy surface

The velocity of a wind‐blown sand cloud is important for studying its kinetic energy, related erosion, and control measures. PDA (particle dynamics analyser) measurement technology is used in a wind tunnel to study the probability distribution of particle velocity, variations with height of the mean velocity and particle turbulence in a sand cloud blowing over a sandy surface. The results suggest that the probability distribution of the particle velocity in a blowing sand cloud is stochastic. The probability distribution of the downwind velocity complies with a Gaussian function, while that of the vertical velocity is greatly complicated by grain impact with the bed and particle–particle collisions in the air. The probability distribution of the vertical velocity of ?ne particles (0·1–0·3 mm sands) can be expressed as a Lorentzian function while that of coarse particles (0·3–0·6 mm sands) cannot be expressed by a simple distribution function. The mean downwind velocity is generally one or two orders greater than the mean vertical velocity, but the particle turbulence in the vertical direction is at least two orders greater than that in the downwind direction. In general, the mean downwind velocity increases with height and free‐stream wind velocity, but decreases with grain size. The variation with height of the mean downwind velocity can be expressed by a power function. The particle turbulence of a blowing sand cloud in the downwind direction decreases with height. The variations with height of the mean velocity and particle turbulence in the vertical direction are very complex. It can be concluded that the velocity of a sand cloud blowing over a sandy surface is mainly in?uenced by wind velocity, grain impact with the bed and particle–particle collisions in the air. Wind velocity is the primary factor in?uencing the downwind velocity of a blowing sand cloud, while the grain impact with the bed and particle–particle collisions in the air are the primary factors responsible for the vertical velocity. Copyright © 2004 John Wiley & Sons, Ltd.     

Changes in the saltation flux following a step‐change in macro‐roughness

The effect of a step change in macro‐roughness on the saltation process under sediment supply limited conditions was examined in the atmospheric boundary layer. For an array of roughness elements of roughness density λ = 0.045 (λ = total element frontal area/total surface area of the array) the horizontal saltation flux was reduced by 90% (±7%) at a distance of ≈150 roughness element heights into the array. This matches the value predicted using an empirical design model and provides confidence that it can be effectively used to engineer roughness arrays to meet sand flux reduction targets. Measurements of the saltation flux characteristics in the vertical dimension, including: saltation layer decay (e‐folding) height and particle size, revealed that with increasing distance into the array, the rate of mass flux change with increasing height decreased notably, and (geometric) mean particle diameter decreased. The distribution of the saltation mass flux in the vertical remains exponential in form with increasing distance into the roughness array, and the e‐folding height increases as well as increasing at a greater rate as particle diameter diminishes. The increase in e‐folding height suggests the height of saltating particles is increasing along with their mean speed. This apparent increase in mean speed is likely due to the preferential removal, or sequestration, of the slower moving particles across the size spectrum, as they travel through the roughness array. Copyright © 2018 John Wiley & Sons, Ltd.     

A wind tunnel investigation of particle segregation,ripple formation and armouring within sand beds of systematically varied texture

This paper reports on a wind tunnel investigation of particle segregation, ripple formation and surface armouring within sand beds of systematically varied particle size distribution, from coarsely skewed to bimodal. By design, the system was closed with no external inputs of mass from an external particle feed. Particles too coarse to travel in saltation for the given range in wind speed were dyed red in order to distinguish them in optical images from finer sand particles, which could be entrained into the unidirectional airflow. A 3D laser scanner measured the changing bed topography at regular time intervals during 18 experiments involving varied combinations of wind speed and bed texture. Image classification techniques were used to investigate the coincident self‐organization of the two populations of particles, as distinguished by their colour. As soon as saltation commenced, some of the red particles segregated into thin discontinuous patches. Particle trapping and sheltering on these rough patches was strongly favoured, causing them to grow preferentially. During the earliest stages of formation, bedform growth coincided with: (i) rapid coarsening of the surface texture; and (ii) the merging of proto‐ripple ‘crests’ to generate larger rhythmic bedforms of lower frequency. Consistent with previous work, ripple size was observed to increase under stronger winds when not exceeding the threshold for entrainment of the coarse‐mode or red particles from the crest. With declining rates of mass transport and particle segregation as the bed surface armoured, and the consequent deceleration of ripple propagation through to the end of each experiment, all surfaces eventually attained a steady‐state morphometry. At saturation, the largest ripples developed on beds having the lowest initial concentration of red particles. Copyright © 2016 John Wiley & Sons, Ltd.     

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

In 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 & Sons, Ltd.     

Spatial variations in surface sediment structure in riffle–pool sequences: a preliminary test of the Differential Sediment Entrainment Hypothesis (DSEH)

Riffle–pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows; however the Differential Sediment Entrainment Hypothesis (DSEH) postulates that differential entrainment can instead result from spatial sedimentological contrasts. Here we use a novel suite of in situ grain‐scale field measurements from a riffle–pool sequence to parameterize a physically‐based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution digital elevation models (DEMs) acquired using terrestrial laser scanning, from which particle exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. This is because pool grains have looser packing, hence greater exposure and lower pivoting angles. Conversely, riffle and pool exit grains have denser packing, lower exposure and higher pivoting angles. A cohesive matrix further stabilizes pool exit grains. The resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a two‐dimensional computational fluid dynamics (CFD) model of the reach. The CFD model predicts that, under bankfull conditions, pools experience lower shear stresses than riffles and pool exits. However, the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles, resulting in differential entrainment under bankfull flows. Significantly, this differential entrainment does not require a reversal in flow velocities or shear stress, suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle–pool sequences. This finding has implications for the prediction of sediment transport and the morphological evolution of gravel‐bed rivers. Copyright © 2012 John Wiley & Sons, Ltd.     

The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach

This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta‐T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two‐dimensional sonic anemometer. Transport intensity was measured at a height of 2–4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10–20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1–2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance. Copyright © 2007 John Wiley & Sons, Ltd.     

Particle travel distances and bed and sediment compositions associated with rain‐impacted flows

Laboratory experiments were performed with rain of uniform drop size (2·7 mm, 5·1 mm) impacting flows over non‐cohesive beds of uniform sized sand (0·11–0·9 mm) and coal (0·2–0·9 mm) particles with flow velocities (20 mm s^?1, 40 mm s^?1) that were insufficient for the flow to entrain the particles without the aid of raindrop impact. Measurement of particle travel distance under rain made up of 2·7 mm drops confirmed a theoretical relationship between settling velocity and the distance particles travel after being disturbed by drop impact. Although, in theory, a relationship between settling velocity and particle travel distance exists, settling velocity by itself was unable to account for the effect of changes in both particle size and density on sediment discharge from beds of uniform non‐cohesive material. Particle density was also a factor. Further study of how particle characteristics influence sediment discharge will aid modelling of the impact of the soil in process‐based models of erosion by rain‐impacted flow. Copyright © 2001 John Wiley & Sons, Ltd.     

The response of saltation to wind speed fluctuations

The response time of saltation to spatial or temporal wind speed fluctuations constitutes an important control parameter for aeolian sediment transport and deposition. In this paper, we present direct measurements of the response time obtained from several field experiments. The sand transport was studied using six small microphones arranged in a vertical profile and collocated with a sonic anemometer, a webcam and a cup anemometer tower. The webcam was coupled with the sonic anemometer via a personal computer and provides information on creeping and saltating grains with a sampling rate of 10 Hz. Sediment transport measurements were obtained over four periods. The Wiener filter, a signal processing technique, is used to obtain a discrete transfer function that relates the horizontal wind speed and the non‐intermittent sand transport. The transfer function can be established using an exponential function with a time constant or characteristic response time τ without time shift. The response time fluctuated between zero and 1·5 seconds depending on the turbulence intensity, the saltation activity, the measuring height and sampling rates. The Wiener filter coefficients suggest that the response of saltation to wind speed alterations is determined by more than one process. Copyright © 2012 John Wiley & Sons, Ltd.     

Development of the saltation system under controlled environmental conditions

The transport of sand by the wind occurs predominantly by the process of saltation. Following the entrainment of sand by an above threshold wind, the saltation system is regulated by the mutual interaction of the atmospheric boundary‐layer, the sand cloud and the sand bed. Despite existing data on the spatial and temporal development of the sand transport system, very little is known about the development of the saltation system towards equilibrium. Results are presented from wind‐tunnel experiments that were designed to address the simultaneous spatial and temporal development of the saltation system, with and without artificial sand feed. The development of the saltation system was monitored over a streamwise length of 8 m during a period of 3600 s. Mass flux data were measured simultaneously at 1 m intervals by the downwind deployment of seven Aarhus sand traps. Wind velocity data were collected throughout the experiments. The downwind spatial development of the saltation system is manifested by an overshoot in mass flux and friction velocity prior to declining towards a quasi‐equilibrium. Mass flux overshoots at approximately 4 m downwind, in remarkable agreement with existing data of a comparable scale. Friction velocity overshoots at approximately 6 m downwind, a result not previously witnessed in saltation studies. The overshoot of mass flux prior to the overshoot in friction velocity is a spatial manifestation of the time lag between the entrainment of grains and the deceleration of the wind by the grains in transport. Temporally, the development of the saltation system is controlled by the availability of entrainable grains from the sand bed. Through time the saltation system develops from a transport‐limited to a supply‐limited system. The depletion of the sand bed through time limits the appropriateness of the assumption of ‘equilibrium’ for the universal prediction of mass flux. Copyright © 2002 John Wiley & Sons, Ltd.     

Effect of saltating sediment on flow resistance and bed roughness in overland flow

The acceleration of saltating grains by overland flow causes momentum to be transferred from the flow to the grains, thereby increasing flow resistance and bed roughness. To assess the impact of saltating sediment on overland flow hydraulics, velocity profiles in transitional and turbulent flows on a fixed sand-covered bed were measured using hot-film anemometry. Five discharges were studied. At each discharge, three flows were measured: one free of sediment, one with a relatively low sediment load, and one with a relatively high sediment load. In these flows from 83 to 90 per cent of the sediment was travelling by saltation. As a result, in the sediment-laden flows the near-bed velocities were smaller and the velocity profiles steeper than those in the equivalent sediment-free flows. Sediment loads ranged up to 87·0 per cent of transport capacity and accounted for as much as 20·8 per cent of flow resistance (measured by the friction factor) and 89·7 per cent of bed roughness (measured by the ratio of the roughness length to median grain diameter). It is concluded that saltating sediment has a considerable impact on overland flow hydraulics, at least on fixed granular beds. Saltation is likely to have a relatively smaller effect on overland flow on natural hillslopes and agricultural fields where form and wave resistance dominate. Still, saltation is generally of greater significance in overland flow than in river flow, and for this reason its effect on overland flow hydraulics is deserving of further study. © 1998 John Wiley & Sons, Ltd.     

Stabilization of fine gravels by net‐spinning caddisfly larvae

We examined the impact of Hydropsychidae caddisfly larvae on the incipient motion of two sizes of narrowly graded fine‐gravel (4–6 and 6–8 mm). This impact was assessed relative to the collective impact of other abiotic and biotic processes that are potentially important conditioning agents of fine‐gravels. Trays of gravel were placed in the River Soar, Leicestershire, UK, where they were colonized to natural densities by caddisfly larvae. Identical trays that were surrounded by a 250 µm mesh were also deployed, preventing colonization but allowing field conditioning of sediments, including minor reworking of grains and biofilm development. After 21 days in the river, trays were removed to a laboratory flume where grain entrainment stresses were established. In addition to the colonized and conditioned treatments, critical shear stresses were measured for identical sediments that were not placed in the river (laboratory gravels). Gravels that were colonized by Hydropsychidae required significantly greater shear stresses for entrainment than conditioned trays (p ≤ 0·002), however, there was no significant difference between conditioned and laboratory gravels. This implies that the presence of caddisfly can be a more important influence on fine‐gravel stability than some conditioning processes. Shields parameter was compared across treatments and across the two gravel size‐fractions using two‐way ANOVA. No significant differences or interactions were observed, indicating that 4–6 mm gravel was stabilized to a similar degree as 6–8 mm gravel by conditioning and colonization processes. Our results extend earlier studies in two important ways: (1) entrainment stresses were established for fine gravels that were colonized at natural densities, under natural stream conditions; and (2) the caddisfly effect was measured relative to both field‐conditioned and unconditioned laboratory controls. The temporal and spatial distribution of silk‐spinning caddisfly larvae suggests that they have the potential to influence fine‐sediment mobility in many rivers, worldwide. Copyright © 2008 John Wiley & Sons, Ltd.     

Recent ventifact development on the central Oregon coast,western USA

The unusual location of ventifacts, on a boulder‐built jetty at the mouth of the Siuslaw River, Oregon coast, western USA, allows ventifact age and wind abrasion rates to be estimated with some precision. The jetty was built mainly between 1892–1901 and extended throughout the twentieth century. Consideration of historical shoreline position and the history of jetty construction and repair suggests the ventifacts have formed since about 1930. Morphologically the ventifacts are aligned south‐to‐north reflecting winter winds and sediment transport from the adjacent beach. Wind‐parallel grooves and ridges with sharp, sinuous crests are developed on inclined boulder surfaces on top of the jetty and reflect suspended sand transport in wind vortices. Deeply pitted surfaces on steep boulder surfaces nearest the beach reflect impact by saltating sand grains. Based on present wind regimes (1992–2000) from three regional weather stations, southerly winds above the sand transport threshold occur for 21·9–29·6 per cent of the time. Based on estimated depth of loss from boulder surfaces, wind abrasion rates are calculated to be on the order of 0·24–1·63 mm a^?1. This is the first well‐constrained field estimate of ventifact age and ventifaction rate from a modern coastal environment. Copyright © 2003 John Wiley & Sons, Ltd.     

A framework for predicting abrasion rupture of crusts in wind erosion

Crusts play a crucial role in the reduction or control of wind erosion. In this regard, the resilience and durability of crusts are of prime importance. Crusts have high resilience and durability against wind flow shear stresses; however, they are prone to abrasion induced by saltating particles. Therefore, estimating crust durability in abrasion rupture has practical importance. In this study, a cyanocrust and a biocemented sand crust were subjected to a controlled flux of saltating particles for different sandblasting periods to provide a framework for predicting crust rupture. The velocity and pre- and post-collision energy of the saltating particles were measured using high-speed photography. The changes in the strength of the crusts after different periods of sandblasting were determined using a scratch test. The results suggested that the average strength of the cyanocrust and biocemented sand crust became 0.25 and 0.7 of their corresponding initial values after 30 min of sandblasting. Also, the average stiffness of the cyanocrust and biocemented sand crust decreased to 0.5 and 0.9 of their initial values, respectively. Furthermore, the amount of impact energy absorbed by the crusts increased by the deterioration of the crusts. Compiling the results of the wind tunnel experiment and scratch tests yielded an exponential equation which can be used to estimate crust durability in a given condition of saltation. Based on this equation, the cyanocrust and biocemented sand crust will break down entirely after 23 and 449 min, respectively, at a wind velocity of 6.8 m/s and a saltation flux of 1 g/s/m.     

Effects of hydrophobicity on splash erosion of model soil particles by a single water drop impact

Raindrop impact can be a major contributor to particle mobilization for soils and other granular materials. In previous work, water repellent soils, comprised of hydrophobic particles, have been shown to exhibit greater splash erosion losses under multiple drop impact. However, the underlying principle differences in splash behavior between hydrophobic and hydrophilic granular surfaces have not been studied to date. In this study the effects of particle hydrophobicity on splash behaviour by a single water drop impact were examined using high‐speed videography. Water drops (4 mm in diameter) were dropped on beds of hydrophilic and hydrophobic glass beads (sieved range: 350–400 µm), serving as model soil particles. The drop velocity on impact was 2.67 m s^‐1, which corresponds to ~30% of the terminal velocity of a raindrop of similar size. The resulting impact behaviour was measured in terms of the trajectories of particles ejected from the beds and their final resting positions. The response to the impacting water drop was significantly different between hydrophilic and hydrophobic particles in terms of the distance distribution, the median distance travelled by the particles and number of ejected particles. The greater ejection distances of hydrophobic particles were mainly the result of the higher initial velocities rather than differences in ejecting angles. The higher and longer ejection trajectories for hydrophobic particles, compared with hydrophilic particles, indicate that particle hydrophobicity affects splash erosion from the initial stage of rainfall erosion before a water layer may be formed by accumulating drops. The ~10% increase in average splash distance for hydrophobic particles compared with hydrophilic particles suggests that particle hydrophobicity can result in greater net erosion rate, which would be amplified on sloping surfaces, for example, by ridges in ploughed agricultural soils or hillslopes following vegetation loss by clearing or wildfire. Copyright © 2012 John Wiley & Sons, Ltd.     

Large roughness element effects on sand transport,Oceano Dunes,California

The effect of large roughness elements on sand transport efficiency was evaluated on a coastal sand sheet by measuring sand flux with two types of sand traps [Big Spring Number Eight (BSNE) and the Cox Sand Catcher (CSC)] at 30 positions through a 100 m‐long × 50 m‐wide roughness array comprised of 210 elements each with the dimensions 1·17 m long × 0·4 m high × 0·6 m wide. The 210 elements were used to create a roughness density (λ) of 0·022 (λ = n bh/S, where n is the number of elements, b the element breadth, h the element height, and S is the area of the surface that contains all the elements) in an area of 5000 m². The mean normalized saltation flux (NSF) values (NSF = outgoing sand flux/incoming sand flux) at the furthest downwind distance for the two trap types were 0·44 and 0·41, respectively. This is in excellent agreement with an empirical model prediction of 0·5. The reduction in saltation flux is similar to an earlier separate study for an equivalent λ composed of elements of similar height (0·36 m), even though the roughness element forms were different (rectangular in this study as opposed to circular) as were the horizontal porosity of the arrays (49% versus 16%). This corroborates earlier results that roughness element height is a critical parameter that enhances reduction in sand transport by wind for similar λ configurations. The available data suggest the form of the relationship between transport reduction efficiency and height is likely a power relationship with two limiting conditions: (1) for elements ≤ 0·1 m high the effect is minimized, and (2) as element height matches and then exceeds the maximum height of the saltation layer (≥ 1 m), the effect will stabilize near a maximum of NSF ≈ 0·32. Copyright © 2012 John Wiley & Sons, Ltd.     

Evaluation of the saltation process of bed materials by video imaging under altered bed roughness

The purpose of this paper is to examine the nature of particle saltation and movement over the beds of fixed roughness from flume experiments. A series of experiments are carried out to study the saltation of individual sand particles of different sizes over rough beds under different flow conditions. A 3‐D acoustic Doppler velocimeter is used to record the fluid velocity components; subsequently, under different flow conditions, the images of released sand particles are recorded using high‐speed video imaging technique. Systematic analysis is made with regard to the forces acting on the grains and the variation of their magnitudes along the saltation trajectories of the grains. Relations between the saltation parameters, flow intensity and bed roughness are developed. The distributions of the angle of orientations during a single saltation follows almost a Gaussian distribution. The shape of the Gaussian distribution depends on the particle size and bed roughness. Particle collisions with rough beds and the resulting coefficients of restitution are also discussed. A theoretical framework is developed to compute the mean particle velocity considering the spin in the energy balance equation. Results of the detailed analysis using the imaging technique are much better than in previously reported studies. Copyright © 2013 John Wiley & Sons, Ltd.     

The factors influencing the abrasion efficiency of saltating grains on a clay‐crusted playa

The entrainment and subsequent transport of PM₁₀ (particulate matter <10 µm) has become an important and challenging focus of research for both scientific and practical applications. Arid and semi‐arid environments are important sources for the atmospheric loading of PM₁₀, although the emission of this material is often limited by surface crusts. It has been suggested that the primary mechanisms through which PM₁₀ is released from a crusted surface are abrasion by saltating grains or disturbance by agricultural and recreational activities. To examine the importance of saltation abrasion in the emission of PM₁₀, a series of field wind tunnel tests were conducted on a clay‐crusted surface near Desert Wells, Arizona. In a previous part of this study it was found that the emission rate varies linearly with the saltation transport rate, although there can be considerable variation in this relationship. This paper more closely examines the source of the variability in the abrasion efficiency, the amount of PM₁₀ emitted by a given quantity of saltating grains. The abrasion efficiency was found to vary with the susceptibility of the surface to abrasion, the ability of the sand to abrade that surface and the availability of material with a caliper size <10 µm within the crust. Specifically, the results of the study show that the abrasion efficiency is related to the crust strength, the amount of surface disturbance and the velocity of the saltating grains. It is concluded that the spatial and temporal variability of these controls on the abrasion efficiency imposes severe contextual limitations on experimentally derived models, and can make theoretical models too complex and impractical to be of use. Copyright­© 2001 John Wiley & Sons, Ltd.     

Particle velocity and concentration profiles in bedload experiments on a steep slope

Depth profiles of particle streamwise velocity, concentration and bedload sediment transport rate were measured in a turbulent and supercritical water flow. One‐size 6 mm diameter spherical glass beads were transported at equilibrium in a two‐dimensional 10% steep channel with a mobile bed. Flows were filmed from the side by a high‐speed camera. Particle tracking algorithms made it possible to determine the position, velocity and trajectory of a very large number of particles. Approximately half of the sediment transport rate was composed by rolling grains, and the other half by saltation. This revealed a complex structure, with several concentration and flux peaks due to rolling, and one peak due to saltation. With an increase of the sediment transport rate, the depth structure remained the same at the water/granular interface, with peak value increases but with no shift in elevations. The saltation region expanded towards higher elevations with an increase of the particle velocity commensurate to the water velocity. The proportion of the sediment transport rate in saltation did not vary significantly. The particle streamwise velocity profiles exhibited three segments: an exponential decay in the bed, a linear increase where rolling and saltation co‐existed, and above this, a logarithmic‐like shape due to saltating particles. These results are comparable to profiles measured and modelled in dry granular free surface flows and in more intense bedload such as sheet flows. Copyright © 2014 John Wiley & Sons, Ltd.