Drainage in heterogeneous sand columns with different geometric structures

This paper discusses multi-step drainage experiments in two heterogeneously packed sand columns (10 × 10 × 20 cm³). Different packing structures were generated using two different sand types. One purpose of the study was to test the influence of packing structures on the movement of water. The second purpose was to assess the quality of predictions for the outflow curves in both columns made with an upscaled model. The heterogeneous structures of the columns can be considered as two opposing extremes. The first column was packed with a random arrangement of two sand types that is not stochastically homogeneous and where a cluster running through the column exists for both materials. The second column was packed with a periodic pattern of coarse-sand inclusions in a fine-sand background and has a clearly defined unit cell. The depth-averaged (2D) spatial distribution of the water content in the columns was monitored during the whole multi-step outflow experiment using neutron radiography. The 3D water content was measured at the steady states by neutron tomography. The experimental results are compared with the model predictions of an upscaled model derived with the homogenization theory. The parameters for the upscaled model are calculated from the hydraulic parameters of the two sand types. These hydraulic parameters were first identified in independent measurements on samples of the two individual sand types, separately. Additionally, the hydraulic parameters of both sands were identified by fitting a numerical model to the measured outflow curves. The different column structures showed a significant effect on water retention and the effective retention function, as water was trapped in the coarse-sand inclusions of the periodic structure. We included this trapping effect in the effective retention function of the upscaled model with an apparent air entry pressure. Contrary to the retention, the different packing structures had no large effect on the dynamic behavior of the outflow. The effective conductivity of the columns is therefore not significantly influenced by the structure. The upscaled models predicted the movement of the averaged water content in the two columns well. This confirms the applicability of upscaled models even if the underlying requirements are not strictly met.     

Measuring the effect of structural connectivity on the water dynamics in heterogeneous porous media using speedy neutron tomography

The temporal and spatial distribution of water within a porous medium is affected by the medium’s structure, i.e., the spatial arrangement of its constituents. To analyze structural effects on the fluid dynamics, we measured the 3D water content distribution in a heterogeneous sand column during two drainage-wetting cycles using neutron transmission tomography. The sample with a volume of 105 cm³ contained 101 cubes of fine and 49 cubes of coarse sand with particles ranging from 0.01 to 0.05 and 0.03 to 0.09 cm, respectively. The pressure at the lower boundary was determined by the water reservoir positioned between 7 and 39 cm below the top of the column. The duration of one complete 3D scanning with a spatial resolution of 127 μm was 56 s. The signal to noise ratio of the measurements was low due to the short exposure time in the neutron beam, but it was possible to quantify the water content in the individual cubes and hence the effect of structure on macroscopic water distribution. Continuous structures of coarse sand drained faster than coarse sand without connection to the upper boundary. During the initial wetting phase, cubes of coarse sand material completely embedded in the fine material remained water unsaturated due to air entrapment. The effect of the coarse sand connectivity was analyzed in two-dimensional numerical simulations based on Richards equation. In contrast to the measurements, no effect of structure connectivity was found. The coarse sand cubes embedded within the fine matrix drain as quickly as the coarse sand cubes arranged in a continuous channel due to the model assumption of a continuous air phase.     

Characterization of Soil Particle Size Distribution with a Fractal Model in the Desertified Regions of Northern China

We constructed an aeolian soil database across arid, semi-arid, and dry sub-humid regions, China. Soil particle size distribution was measured with a laser diffraction technique, and fractal dimensions were calculated. The results showed that: (i) the predominant soil particle size distributed in fine and medium sand classifications, and fractal dimensions covered a wide range from 2.0810 to 2.6351; (ii) through logarithmic transformations, fractal dimensions were significantly positive correlated with clay and silt contents (R² = 0.81 and 0.59, P < 0.01), and significantly negative correlated with sand content (R² = 0.50, P < 0.01); (3) hierarchical cluster analysis divided the plots into three types which were similar to sand dune types indicating desertification degree. In a large spatial scale, fractal dimensions are still sensitive to wind-induced desertification. Therefore, we highly recommend that fractal dimension be used as a reliable and quantitative parameter to monitor soil environment changes in desertified regions. This improved information provides a firm basis for better understanding of desertification processes.     

Splash detachment of sand particles under varying contact stress field of wind-driven raindrop impact

The effects of wind-driven rain (WDR) on sand detachment were studied under various raindrop obliquities. Results suggested a significant reduction in compressive stress on sand surfaces for a two-dimensional experimental set-up in a wind tunnel. During experiments, sand particles in splash cups were exposed to both wind-free rain (WFR) and WDR driven by horizontal winds of 6.4, 8.9 and 12.8 m s⁻¹ and rainfall intensities of 50, 60, 75 and 90-mm h⁻¹ to assess the sand detachment rate (D, in g m⁻² s⁻¹). The effects of sand moisture state (dry and wet) on the detachment of different-sized particles (0.20–0.50 and 0.50–2.00 mm, respectively) were also tested. Factorial analysis of variance showed that shear and compressive stress components evaluated by horizontal and vertical kinetic energy flux terms (KE_x and KE_y, respectively, in J m⁻² s⁻¹) along with their vector sum (KE_r, in J m⁻² s⁻¹) explained the variation in D. Neither sand size nor sand moisture was statistically significant alone although binary interactions of KE_r, KE_x and KE_y with the sand size and three-way interaction of KE_x, sand size and moisture were statistically significant. These results can be explained by size-dependent variation in sand compressibility and surface friction related to the total stress field developed by a given partition of shear and compressive stresses of wind-driven oblique raindrops (KE_x/KE_y). Further analysis of the variation of the unit sand detachment rate (D_u = D/KE_r = g J⁻¹) with rain inclination (α, in degrees) better revealed the effect of WDR obliquity on D_u that further changed with sand size class and moisture state. Finally, the difference in the resulting stress field differentiable by the oblique raindrop trajectories of the experiment over sand surface significantly affected the non-cohesive particle detachment rates, to some extent interacted with size-dependent compressibility and interface shear strength of sand grains.     

Reynolds stress and sand transport over a foredune

Reynolds shear stress (RS = –u′ w′) and sand transport patterns over a vegetated foredune are explored using three‐dimensional velocity data from ultrasonic anemometers (at 0 · 2 and 1 · 2 m) and sand transport intensity from laser particle counters (at 0 · 014 m). A mid‐latitude cyclone on 3–4 May 2010 generated storm‐force winds (exceeding 20 m s^–1) that shifted from offshore to obliquely alongshore. Quadrant analysis was used to characterize the spatial variation of RS quadrant components (Q1 through Q4) and their relative contributions were parameterized using the flow exuberance relation, EX_FL = (Q1 + Q3)/(Q2 + Q4). The magnitudes of RS and sand transport varied somewhat independently over the dune as controlled by topographic forcing effects on flow dynamics. A ‘flow exuberance effect’ was evident such that Q2 (ejection‐like) and Q4 (sweep‐like) quadrants (that contribute positively to RS) dominated on the beach, dune toe, and lower stoss, whereas Q1 and Q3 (that contribute negatively to RS) dominated near the crest. This exuberance effect was not expressed, however, in sand transport patterns. Instead, Q1 and Q4, with above‐average streamwise velocity fluctuations (+u′), were most frequently associated with sand transport. Q4 activity corresponded with most sand transport at the beach, toe, and stoss locations (52, 60, 100%). At the crest, 25 to 86% of transport was associated with Q1 while Q4 corresponded with most of the remaining transport (13 to 59%). Thus, the relationship between sand transport and RS is not as straightforward as in traditional equations that relate flux to stress in increasing fashion. Generally, RS was poorly associated with sand transport partly because Q1 and Q4 contributions offset each other in RS calculations. Thus, large amounts of transport can occur with small RS. Turbulent kinetic energy or Reynolds normal stresses (u′², w′²) may provide stronger associations with sand transport over dunes, although challenges exist on how to normalize and compare these quantities. Copyright © 2013 John Wiley & Sons, Ltd.     

Regional grain size variations in aeolian sediments along the transition between Tibetan highlands and north‐western Chinese deserts – the influence of geomorphological settings on aeolian transport pathways

Sediment distribution is investigated applying grain size analysis to 279 surface samples from the transitional zone between high mountains (Qilian Shan) and their arid forelands (Hexi Corridor) in north‐western China. Six main sediment types were classified. Medium scale (10³ m) geomorphological setting is carefully considered as it may play an important role concerning sediment supply and availability. A tripartite distribution of sedimentological landscape units along the mountain to foreland transition is evident. Aeolian sediments (e.g. loess and dune sands) are widespread. They are used to identify aeolian transport pathways. The mU/fS‐ratio (5–11 µm/48–70 µm) among primary loess opposes the two grain size fractions being most sensitive to varying accumulation conditions. The first fraction is attributed to long‐distance transport in high suspension clouds whereas the latter represents local transport in saltation mode. The ratio shows strong correlation with elevation (R² = 0.77). Thus, it indicates a relatively higher far‐traveled dust supply in mountainous areas (>3000 m above sea level [a.s.l.]) compared to the foreland. The contribution of westerlies to high mountain loess deposits is considered likely. Hereby, the influence of the geomorphological setting on grain size composition of aeolian sediments becomes apparent: the contribution from distant dust sources is ubiquitous in the study area. However, the far‐distance contribution may be reduced by the availability of fine sand provided in low topography settings. Plain foreland areas support fine sand deflation from supplying river beds, allowing the formation of sandy loess in foreland areas and intramontane basins. In contrast, high mountain topography inhibits strong sand deflation into loess deposits. Eastern parts of the Hexi Corridor show higher aeolian sand occurrence. In contrast, the western parts are dominated by gravel gobi surfaces. This is attributed to higher sand supply in eastern parts provided by the Badain Jaran Desert and fluvial storages as sand sources. Copyright © 2014 John Wiley & Sons, Ltd.     

Implications of turbulent shear on clay floc break-up along the Atlantic estuary (Bouregreg), Morocco

Water depth,salinity,current,and suspended sediment concentration(SSC)were measured along with the grain size distribution of bed sediment along an estuarine longitudinal section.The floc size increased with increase in the percentage of clay and silt,while decreased with increase in the percentage of sand content of bed sediment.The turbulent shear,G,had a direct effect on floc size with its value increasing with increase in G up to a G value of 15 s^-1,while an inverse relation existed between floc size and G at higher G(G>15 s^-1).Further,higher turbulence enabled sand to get resuspended and cause additional shear leading to the break-up of flocs.An attempt was made to modify G to account for the combined effect of water turbulence(G)and shear imparted by sand(Ga)and the impact of the modification of G on the predictability of floc size was evaluated.A new model was developed which explains floc size in terms of sediment concentration(C),salinity gradient(S),and G for different scenarios based on the value of G.Sensitivity analysis was done for observed floc size(FS)and predicted floc size using four approaches:(I)FSαC^x;(II)FSαC^xS-y;(III)FSαC^xS^-yGz for G<15 s^-1and FSαC^xS^-yG-z for G>15 s^-1;and(IV)FSαC^xS^-yG_m^-zfor G>15 s^-1and G_m=G+Ga,where x,y,and z are determined by calibration.It was observed that the predictability of the floc size improved when the turbulence was modified to account for shear imparted by sand so that the coefficient of determination was increased from 0.78 for model III to 0.89 for model IV.Further,the settling velocity was expressed as a function of suspended sediment concentration,turbulent shear,and salinity gradient.The predictability of settling velocity was improved(R2 increased from 0.77 to 0.86)when the additional turbulence created by sand was incorporated in the non-dimensional empirical equation.The study highlights the influence of sand in causing the break-up of flocs and suggests that for turbulence shear values high enough to resuspend sand,and G has to be modified to account for the additional shear imparted by sand in mixed sediment estuarine environments.     

Particle dislodgement from a flat sand bed by wind

The rate, with respect to area and time, at which grains are dislodged from a sand bed for given wind conditions is an important factor in determining the grain transport rate and the intensity of grain activity in each of the transport modes. The literature of the subject contains little direct information about particle dislodgement. The paper describes a series of experiments in which dyed sand grains, spread on the surface of quartz dune sand in a wind-tunnel, were photographed at five second intervals while the sand was exposed to wind. The data on rate of loss of coloured grains was used, for two of three chosen size fractions, to deduce the dislodgement rate for each size fraction. The variation of this dislodgement rate with shear velocity is shown graphically for values of u_* between 24 cm s^?1 and 50 cm s^?1. Because of the artificial method of distribution of the coloured sand grains, the results should be applied with caution to natural conditions. The interpretation of the observations of dyed grain loss involved the numerical simulation of the process which comprises removal of coloured grains, slightly offset by replenishment as upwind coloured grains settle briefly in the observed zone. An estimation of grain excursion length has to be incorporated in the simulation. This estimation was made by trial, but general corroboration was found from earlier work. Comparisons are made between dislodgement rates obtained thus and rates estimated by Anderson (1986) and by Jensen and Sorensen (1986). Reasonable agreement with the latter is found in the u_* range 30 cm s^?1.     

Sand transport measurements in Chioggia inlet,Venice lagoon: Theory versus observations

This paper presents results of recent measurements of sand transport made in Chioggia inlet as part of an extensive monitoring programme in the Venetian inlets. Measurements were made in order: (1) to define a relationship between sand transport magnitude and tidal flow; (2) to derive the thresholds for sand transport; (3) to identify the dominant modes of transport; (4) to evaluate the concentration profiles of sand within the benthic boundary layer; (5) to compare bedload transport observations with model predictions using existent bedload formulae; and (6) to produce yearly estimates of bedload transport across the inlet. The vertical distribution of sand in the water column was sampled using modified Helley–Smith bedload samplers at three sites. Transport was found to vary according to the flow and bed grain size, with considerable temporal and spatial variability. A difference of up to three orders of magnitude in transport was observed through the inlet, with higher transport rates measured on the seaward part. The dominant mode of transport in the central inlet was suspension, while bedload was dominant in the mouths. The measured profiles of sand concentration varied with the tidal stage and seabed grain size according to the Rouse parameter (R). R was high at the inlet mouths (1<R<2), indicative of a well-developed bedload layer. The inverse movability number (W_s/U_*) was also higher at these sites and appeared to be grain size dependant. Formulae for bedload transport were tested against field data; stochastic methods such as Einstein–Brown, Engelund–Hansen and Van Rijn produce the best fits. The coupled model SHYFEM-Sedtrans05 appears to simulate well observed transport for most conditions of flow. Long-term bedload predictions indicate a dominant export of sand, with a yearly average of 4500 m³.     

Re-examination of the Source Mechanism of the 1998 Papua New Guinea Earthquake and Tsunami

— Simulation of tsunami propagation and runup of the 1998 Papua New Guinea (PNG) earthquake tsunami using the detailed bathymetry measured by JAMSTEC and adding bathymetric data at depths less than 60 m is carried out, reproducing the tsunami energy focus into Warapu and Arop along the Sissano Lagoon. However, the computed runup heights in the lagoon are still lower than those measured. Even if the error in estimating the fault parameters is taken into consideration, computational results are similar. Analysis by the wave ray method using several scenarios of the source size of the tsunami and location by the wave ray method suggests that a source characterized by small size in water 1,000-m deep approximately 25 km offshore the lagoon, best fits the arrival determined from the interviews with eyewitnesses. A two-layer numerical model simulating the interaction of the tsunami with a landslide is employed to study the behavior of a landslide-generated tsunami with different size sand depths of the initial slide just outside the lagoon. A landslide model with a volume of 4–8 × 10⁹ m³ is selected as the best in order to reproduce the distribution of the measured tsunami runup in the lagoon. The simulation of a tsunami generated in two stages, fault and landslide, could show good agreement with the runup heights and distribution of the arrival time, but a time gap of around 10 minutes remains, suggesting that a tsunami generated by the mainshock at 6:49 PM local time is too small for people to notice, and the following tsunami triggered by landslide or mass movement near the lagoon about ten minutes after the mainshock attacked the coast and caused the huge damage.     

Field observations of the vertical distribution of sand transport characteristics over fine,medium and coarse sand surfaces

The vertical distribution of sand transport characteristics is an important issue in aeolian research. Surface characteristics affect sand transport processes, but their effects are not yet fully understood. To provide more data on this subject, we observed sand transport in 16 field experiments above surfaces covered by fine, medium and coarse sand. The sand transport rate over relatively coarser‐grained medium and coarse surfaces could be expressed as a Gaussian peak function: q _z = a + b exp (?0.5[(|z – C _h|)/d ]^e), where q _z is the measured sediment transport at height z above the bed and a , b , C _h, d , and e are regression coefficients. The measured sand transport flux peak values (H _h) were linearly related to C _h, and both values were significantly related to the mean surface grain size. However, for the relatively finer‐grained medium and fine sand surfaces, the sediment transport could be expressed as an exponential function. The cumulative sand transport below 0.1 m was directly related to the mean surface grain size, and the relationship could be expressed as the following exponential function: C _z = f + g exp –M _z/i , where C _z is the cumulative sand transport at height z above the bed, M _z is the mean grain size and f , g , and i are regression coefficients. Above 0.1 m, there were no significant relationships between the cumulative sand transport and the mean surface grain size. The mean grain size decreased with increasing height below the peak height and then increased with increasing height. The surface grain size distribution and proportions of the particles in different grain size categories controlled the mean grain size as a function of height. The observed changes in the sand transport rate and grain size with height will provide support for sand disaster mitigation, numerical modelling and studies of dune formation. Copyright © 2016 John Wiley & Sons, Ltd.     

Wind tunnel tests of the dynamic processes that control wind erosion of a sand bed

Aeolian sand transport is a complicated process that is affected by many factors (e.g. wind velocity, sand particle size, surface microtopography). Under different experimental conditions, erosion processes will therefore produce different results. In this study, we conducted a series of wind tunnel experiments across a range of wind velocities capable of entraining sand particles (8.0, 10.0, 12.0, and 14.0 m s^-1) to study the dynamic changes of the shear velocity, aerodynamic roughness length, and sand transport. We found that the shear velocity and aerodynamic roughness length are not constant; rather, they change dynamically over time, and the rules that describe their changes depend on the free-stream air velocity. For wind tunnel experiments without feeding sand into the airflow, the sand bed elevation decreases with increasing erosion time, and this change significantly affected the values of shear velocity and aerodynamic roughness length. A Gaussian distribution function described the relationships between the sand transport rate (q_T) and the duration of wind erosion (T). It is therefore necessary for modelers to consider both deflation of the bed and the time scale used when calculating sand transport or erosion rates. © 2018 John Wiley & Sons, Ltd.     

Effects of FPOM size and quality on aquatic heterotrophic bacteria

Mechanical and biological processing in aquatic systems converts coarse particulate organic matter (CPOM) into fine particulate organic matter (FPOM). Other sources of particles with different size classes include flocculated dissolved matter, algae and soil particles. The relative magnitudes of these inputs are influenced by the degree of allochthony of a lake or stream. The size-reactivity hypothesis, formulated for dissolved organic matter, postulates that bacterial degradation rates are higher with high-molecular-weight fractions than with low-molecular-weight fractions. In this study, we investigated the effect of particle size on degradation of POM and on freshwater bacterial communities. We generated leaf-derived particle size classes of the same age (same diagenesis status) but differing in quality (maple and beech leaves). Contrary to our expectations, we found a strong effect of particle size and no significant effect of substrate quality on community respiration which decreased at smaller particle size, on C:N ratios which declined with particle size, and on δ¹⁵ N which showed a decreasing trend (though not significant) at smaller particle size in beech leaves. By contrast, bacterial community structure and ∂¹³C values responded mainly to particle quality. Bacterial biomass, estimated by qPCR, was affected by complex interactions between particle size and quality. These findings open an unanticipated perspective on the size-reactivity hypothesis for particulate organic matter.     

Differences in 10Be concentrations between river sand,gravel and pebbles along the western side of the central Andes

Cosmogenic nuclides in river sediment have been used to quantify catchment-mean erosion rates. Nevertheless, variable differences in ¹⁰Be concentrations according to grain size have been reported. We analyzed these differences in eleven catchments on the western side of the Andes, covering contrasting climates and slopes. The data include eight sand (0.5–1 mm) and gravel (1–3 cm) pairs and twelve sand (0.5–1 mm) and pebble (5–10 cm) pairs. The difference observed in three pairs can be explained by a difference in the provenance of the sand and coarser sediment. The other sand–pebble pairs show a lower ¹⁰Be concentration in the pebbles, except for one pair that shows similar concentrations. Two sand–gravel pairs show a lower ¹⁰Be concentration in the gravel and the other five pairs show a higher ¹⁰Be concentration in the gravel. Differences in climate do not reveal a particular influence on the ¹⁰Be concentration between pairs. The analysis supports a model where pebbles and gravel are mainly derived from catchment areas that are eroding at a faster rate. The five gravel samples with high ¹⁰Be concentrations probably contain gravel that were derived from the abrasion of cobbles exhumed at high elevations. In order to validate this model, further work should test if pebbles are preferentially exhumed from high erosion rate areas, and if the difference between pebbles with high ¹⁰Be concentrations and sand decreases when the erosion rate tends to be homogeneous within a catchment.     

The zoogeomorphology of case-building caddisfly: Quantifying sediment use

Caddisfly (Trichoptera) larvae are an abundant and widespread aquatic insect group characterized by the construction of silk structures, including nets and cases. Case-building caddisfly have the potential to modify the sorting and mobility of sand and fine gravel via: (1) case construction, resulting in altered sediment properties; (2) transporting sediment incorporated into cases over the river bed; and (3) changing the structure of river beds via burrowing activity. To investigate these mechanisms, it is necessary to understand the mass, size distribution and spatial variability of sediment use by case-building caddisfly larvae. We quantified the mineral sediment used by individuals and communities of case-building caddisfly in 27 samples, from three sites on a gravel-bed stream. The mass and size distribution of sediment in individual cases varied between taxa (mass = 0.001–0.83 g, D₅₀ = 0.17–4 mm). The mean mass of sediment used by the caddisfly community was 38 g m⁻² and varied locally. Sediment use was predominantly coarse sand (D₅₀ = 1 mm). 64% of sediment use was attributable to Agapetus fuscipes (Glossosomatidae). Due to within-species variability in case mass, the abundance of most taxa, including A. fuscipes, was only weakly associated with the mass of sediment used by this species, at the river scale. Whilst the caddisfly community used a small percentage of the total sediment available (average 2.99% of the 1–1.4 mm size fraction), A. fuscipes used more fine sediment in their cases at sites where it was more available. Despite variability in local habitat, all sites supported diverse case-building caddisfly communities utilizing mineral sediment. Consequently, geomorphological effects of case-building caddisfly are potentially widespread. The results provide novel insights into the specific grain sizes and quantities of fine sediment used by caddisfly larvae, which represents an important step towards understanding their zoogeomorphic activities. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Fine structure of microwave emission in the 25 August 1999 event and its association with the magnetic field fine structure

A study is made of the initial stage of the 25 August 1999, event using high time-resolution microwave observations, as well as the data from the Siberian Solar Radio Telescope and Yohkoh/SXT/HXT. Microwave emission revealed fine structures with a typical time scale of about 1 s with a frequency drift 1 GHz/s, and structures with a typical time scale of less than 0.1 s with a frequency drift 10 GHz/s. Emission bands with a slower frequency drift are generated by energetic electrons subjected to the cyclotron resonance condition. This slower frequency drift is a consequence of a change in plasma density caused by the coalescence of magnetic structures with a typical size of the order of 10⁸ cm, while the emission with a faster drift is generated by beams of energetic electrons generating Cherenkov resonance emission. The energetic electron beams are produced as a result of the interaction of magnetic structures 10⁶–10⁷ cm in size.     

Effect of particle density and inflow concentration of suspended sediment on bedload transport in rill flow

Laboratory flume experiments were carried out to evaluate the effect of particle density on bedload transport of sand‐sized particles and the effect of a suspended load of clay particles (kaolinite) on bedload transport of sand‐sized particles in rill flow conditions. Three materials in the range 400–600 µm were selected to simulate bedload transport of primary particles and aggregates: sand (2650 kg/m³), crushed brick (2450 kg/m³) and anthracite (1300–1700 kg/m³). In the two first experiments, two different methods were applied to determine bedload transport capacity of coarse particles for various conditions of flow discharge (from 2 to 15 L/min) and slope (2.2, 3 and 4%). In the third experiment, clear water was replaced with kaolinite–water mixture and bedload transport capacity of crushed brick particles was determined for a 4% slope and different concentrations of kaolinite (0, 7, 41 and 84 g/L). The results showed that bedload transport increased significantly with the decrease in particle density but the effect of particle density on transport rates was much less important than flow discharge. Velocity measurements of clear flow, flow mixed with coarse particles and coarse particles confirmed the existence of a differentiation between suspended load and bedload. In these experimental conditions, suspended load of kaolinite did not affect bedload rates of crushed brick particles. Three transport capacity formulae were tested against observed bedload rates. A calibration of the Foster formula revealed that the shear stress exponent should be greater than 1.5. The Low and the Govers unit stream power (USP) equations were then evaluated. The Low equation was preferred for the prediction of bedload rates of primary particles but it was not recommended in the case of aggregates of low density because of the limited experimental conditions applied to derive this equation. Copyright © 2008 John Wiley & Sons, Ltd.     

Influence of reed stem density on foredune development

Vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition, and therefore on profile development. In a long‐term monitoring field experiment, three plots were planted with regular grids of reed bundles in three different densities: 4, 2 and 1 bundles per m². This study reports on the differences in profile development under the range of vegetation densities. Topographic profiles were measured between May 1996 and April 1997. Results indicate important differences in profile development for the three reed bundle densities: in the highest density plot a distinct, steep dune developed, while in the lowest density a more gradual and smooth sand ramp was deposited. When the stems had been completely buried, differences in profile evolution vanished. After a second planting of reed stems in January 1997 the process was repeated. In May 1997, all plots had gained a sand volume ranging from 11·5 to 12·3 m³ m^?1, indicating that the sediment budget is relatively constant, regardless of the particular profile evolution. The field evidence is compared with simulations of profile development, generated by the foredune development model SAFE. The model successfully reproduces the overall profile development, but in general, the equations used for vegetation–transport interaction overestimate the effect of vegetation. This causes some deviations between field and model results. Several reasons for this are discussed. Based on the experiments reported here, recommendations are given for further research. Copyright © 2001 John Wiley & Sons, Ltd.