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1.
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 (qT) 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.  相似文献   

2.
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.  相似文献   

3.
One‐dimensional simulations of the unsteady saltation process show that the transport rate's response depends on the amplitude and frequency of the wind fluctuations. At frequencies higher than f ≈ 0·5 Hz the transport rate was found not to respond to the wind changes. The initial overshoot reported by previous investigators was found not to appear for simulation heights smaller than 50 to 60 cm. This is due to the fast propagation of the grains' influence upward in the flow and the immediate deceleration of the wind. Confirmation of these findings comes from reports of experiments conducted in wind tunnels of different sizes. Further test calculations show that the discretization time step size Δt has an influence on the model's temporal behaviour. The reason for this is the better coupling of the wind–sand system when a smaller Δt is used. The implications of bed area choice on the statistical accuracy of predicted transport rate is also demonstrated. In the one‐dimensional case the grain cloud's total forward momentum equals transport rate, which is independent of model geometry. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

4.
Aeolian sand transport is a widespread physical phenomenon on the surface of Earth, as well as on Mars and Titan. Accurate measurements of the components of the transport system are necessary if we are to understand the nature of the physical processes. Sand traps are typically used to measure sediment transport rates, and issues associated with the sampling efficiency of traps and the development of reliable traps have received considerable attention in recent decades. In this study, we measured aeolian transport rate at five distances from a wind tunnel sidewall using a vertically‐segmented sand trap. Total transport rates were determined by weighing the bed sediment before and after each experiment, and with and without a trap installed. The following results were obtained: (1) sand transport increased linearly with the distance away from the sidewall, and the appropriate location to measure maximum transport is within the central 20% of the wind tunnel; (2) current methods overestimate the sampling efficiency of sand traps when comparing trap data to transport rate data obtained by weighing sand moved through the entire tunnel because the effects of the sidewalls in decreasing total transport are neglected; (3) the efficiency of the vertically‐segmented trap that we tested ranged from 11.57% to 31.68% using our revised methods, whereas standard methods caused efficiency to be overestimated by 32–72% of the efficiency; (4) using either method, the efficiency of the trap increased exponentially with shear velocity for the range we used. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

5.
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.  相似文献   

6.
Aeolian sand transport was studied at the Lanphere Dunes, a coastal dune complex in northern California, by comparing slipface advance rates with transport predicted based on local wind data. The slipfaces of a 2·5 m high transverse ridge and 10 m high parabolic dune were monitored over a period of three months to estimate sand discharge. The study was performed during the dry season, which has the maximum sand‐driving potential. Over the three month study period, average sand discharge was 12·5 m3 per m width per year at the transverse ridge and 8·8 m3 per m width per year at the parabolic dune. A method was developed for modelling slipfaces that are sinuous and where sediment transport rates are not constant across the width of the slipface. Field measurements were used to generate three‐dimensional representations of dune slipfaces. Periodic measurements over the course of three months were used to compute the volume of displaced sediment. Theoretical sand transport was computed from local wind data using the Bagnold model and compared with the observed transport rates. Predicted rates were substantially lower than observed rates. Wind velocities rarely exceeded the threshold velocity. Discrepancies between the observed and predicted values appear to be caused by a combination of wind data recording procedures and differences between wind velocities at the anemometer location and the site where sand transport was measured. Wind data collected by weather bureaux have been utilized in numerous studies for modelling sediment transport. Such data typically have sample intervals of one hour or greater and are often averaged prior to reporting. The effect of averaging was investigated by comparing sand transport estimates based on daily average wind velocities with those based on the original hourly observations. The daily average data were depleted of high velocity winds and sand transport estimates were accordingly much lower than those based on the hourly data. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

7.
A simple low‐cost sand catcher has been developed for use in wind‐tunnel experiments. It is 500 mm high with a 300 mm by 20 mm inlet opening, and is designed to measure total mass transport directly. The length is 280 mm and its walls slowly expand at an angle of 5·6°. The top view is similar to that of the widely used BSNE sampler. Each sidewall is vented with three ventilation screens. Two sets of experiments were conducted on dune sand and at different wind velocities to investigate to what degree the catcher samples isokinetically, ef?ciently and non‐selectively. The results indicate that the catcher measures suf?ciently isokinetically (inlet wind velocity over outside wind velocity >0·9). Catch ef?ciency was around 0·75 and the degree of non‐selectivity was >0·80 for 95 per cent of the particle sizes of the test material. It was also observed that the in?uence of wind velocity on the tested sampler characteristics was of minor importance so that no speci?c corrections need to be made for different wind velocities. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

8.
Transport and retention of Escherichia coli through the mixture of quartz, Al‐coated and Fe‐coated sands was examined using column experiments to investigate the effect of geochemical heterogeneity on bacteria transport. The first set of the experiments was performed in quartz, Al‐coated and Fe‐coated sand mixtures (coated sand: 0, 5, 10, 25, 50, 100%) to examine the influence of positively‐charged sand grains on bacteria transport. The second experiments were carried out to observe the impact of pH (range 6·74–8·21) on bacteria transport in the mixture of quartz 50% and Fe‐coated sand 50%. The third experiments were conducted to analyse the effect of ionic strength (0, 50, 100, 200 mM) on bacteria transport in the mixture of quartz 50% and Al‐coated sand 50%. The first experiments show that bacterial mass recoveries were in the range of 3·6–43·4%, decreasing nonlinearly as the content of Al‐ and Fe‐coated sands increased. In the second experiments, the bacterial mass recoveries were in the range of 35·5–79·2%, increasing linearly as the solution pH increased. In the third experiments, the mass recovery was 3·4% at 0 mM. As the ionic strength increased to 50mM, the mass recovery decreased to 0%. When the ionic strength increased further to 100 and 200 mM, no bacterial mass was recovered as in the case of 50 mM. It indicates that in the mixed medium of quartz 50% and Al‐coated sand 50% both positive (increment of bacterial adhesion) and negative (decrement) effects of ionic strength may be counterbalanced, minimizing the impact of ionic strength on the bacterial adhesion. This study helps to understand the role of metal oxides and solution chemistry in the transport of bacteria in geochemically heterogeneous media Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

9.
Wind erosion measurements were carried out in Nellis Dunes Recreation Area, southern Nevada, USA. Gross erosion (the total mass of sediment effectively blown away from a surface), gross deposition (the total mass of sediment effectively depositing on a surface) and net erosion (the difference in sediment mass before and after an event) were measured for 1 year, on 17 different types of surfaces developed on loose dune sand, compacted sand, loose silt, compacted and/or aggregated silt, rock‐covered sands and silts, mixtures of sand, silt and clay, exposed petrocalcic horizons, gravelly substrata and bedrock. Results showed that net erosion, which is the type of erosion measured in field and laboratory experiments, strongly differs from gross erosion. Activity on a surface is much higher than classic net erosion measurements suggest. Future studies on wind erosion should better acknowledge the distinction between the two types of process. Also, a grain diameter of maximum susceptibility to wind erosion (‘optimum deflation diameter’) near 70 µm as proposed by the aeolian literature only exists for net wind erosion. No such optimum diameter was found for gross wind erosion within the particle range 0–100 µm delineating the transport modes of suspension and modified saltation. In addition, desert surfaces predominantly composed of sand did not show an optimum deflation diameter (for net erosion) around 70 µm. Instead, there was a preferential grain size around 15 µm at which particles were most vulnerable to net emission. Desert surfaces poor in sand showed the classic value of 70 µm. This suggests that interactions exist between the type of surface and the susceptibility of particles to wind erosion. This study is solely based on field data. Although results are supported by two previous wind tunnel studies, more wind tunnel experiments documenting the interactions between gross erosion and gross deposition are necessary. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

10.
The study concerns sand deposition within a regular array of vertical cylinders placed in the path of a sand-laden wind. Twelve wind tunnel experiments using three preselected shear velocities (28·78, 32·86 and 45·1 cm s−1), with associated rates of sand feed (0·3, 2·0 and 3·8 g cm−1 s−1), and four roughness element concentrations (λ = 0·046, 0·092, 0·184 and 0·369) were carried out to evaluate the factors that affect sand deposition and sand flux in the presence of immobile rough elements. The measurements showed that as the concentration of non-erodible elements increased, the percentage reduction in the initial sand flux increased and a particularly sharp reduction occurred when λ ≥ 0·18. The pattern of reduction was found to be qred = qeq (d/H) [Δy/(Δy−d)](0·68 −3·5λ) when λ ≤ 0·18, and qred = qeq(d/H)[Δy/(Δy−d)](0·025) when λ > 0·18, where qeq is the equilibrium rate of sand transport arriving at the best bed, d is the diameter of the cylinder, H is the height of the cylinder, and Δy is the width of unit area associated with a cylinder. The experimenal results also showed that the sand flux downstream of the array started to increase immediately upon the commencement of burial of the array's cylinders. Thus the sand deposition and sand flux along an array consisting of regularly distributed, non-erodible elements were shown to be neither uniform nor steady. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

11.
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.  相似文献   

12.
A one-day field investigation on an unvegetated backbeach documents the importance of surface sediment drying to aeolian transport. Surface sediments were well sorted fine sand. Moisture content of samples taken in the moist areas on the backbeach varied from 2·9 to 9·2 per cent. Lack of dry sediment inhibited transport prior to 08:50. By 09:10 conspicuous streamers of dry sand moved across the moist surface. Barchan-shaped bedforms, 30 to 40 mm high and composed of dry sand (moisture content <0·10 per cent), formed where sand streamers converged. The surface composed of dry sand increased from 5 per cent of the area of the backbeach at 09:50 to 90 per cent by 12:50 Mean wind speeds were beetween 5·6 and 8·6 m s−1 at 6 m above the backbeach. Corresponding shear velocities were always above the entrainment threshold for dry sand and below the threshold for the moist sand on the backbeach. Measured rates of sand trapped (by vertical cylindrical traps) increased during the day relative to calculated rates. The measured rate of sand trapped on the moist foreshore was higher than the rate trapped on the backbeach during the same interval, indicating that the moist foreshore (moisture content 18 per cent) was an efficient transport surface for sediment delivered from the dry portion of the beach upwind. Measured rates of sand trapped show no clear relationship to shear velocities unless time-dependent surface moisture content is considered. Results document conditions that describe transport across moist surfaces in terms of four stages including: (1) entrainment of moist sediment from a moist surface; (2) in situ drying of surface grains from a moist surface followed by transport across the surface; (3) entrainment and transport of dry sediment from bedforms that have accumulated on the moist surface; and (4) entrainment of sand from a dry upwind source and transport across a moist downwind surface. © 1997 John Wiley & Sons, Ltd.  相似文献   

13.
Vertical profiles of the streamwise mass flux of blown sand in the near-bed (< 17 mm) region are analysed from high-resolution measurements made using an optical sensor in a wind tunnel. This analysis is complemented by detailed measurements of mass flux and mean velocity profiles throughout the boundary layer depth (0·17 m) using passive, chambered sand traps of small dimensions and armoured thermal anemometers, respectively. The data permit a preliminary analysis of the relations between the observed forms of the profiles of near-bed fluid stress and horizontal mass flux within a carefully conditioned boundary layer. Profiles of mass flux density are found to be characterized by three regions of differing gradient with transitions at about 2 mm and 19 mm above the bed. The exponential decay of mass flux with height is confirmed for elevations above 19 mm, and when plotted as a function of u*2/g (a parameter of mean vertical trajectory height in saltation), the gradient of mass flux in this region scales with the wake-corrected friction velocity (u), where u > 0·30 m s−1. A separate near-bed region of more intense transport below 19 mm is identified which carries 80 per cent of the total mass flux. This region is evident in some previous field and wind tunnel data but not in profiles simulated by numerical models. Ventilated passive sand traps underestimate mass flux in this region by 37 per cent. At slow or moderate wind speeds a third significant region below 2 mm is observed. These regions are likely to be related to grain populations in successive saltation, low-energy ejections and intermittent bed contact, respectively. Optical measurements reveal locally high grain concentrations at some elevations below 5 mm; these heights scale with transport rate, mass flux gradient and wind speed. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

14.
Recent studies of soil loss by the integrated action of raindrop impact and wind transport have demonstrated the significance of this mechanism. This paper presents data obtained during wind‐tunnel experiments examining the ‘Raindrop Detachment and Wind‐driven Transport’ (RD‐WDT) process to investigate average sand particle trajectory and the spatial extent at which the process operates. In the experimental design, at the same time as the horizontal wind velocities of 6·4, 10, and 12 m s–1 passed through the tunnel, rainfall was simulated falling on very well sorted dune sand. The aspect and slope of the sand bed was varied to reproduce both windward (Ww) and leeward (Lw) slopes of 4º and 9º with respect to the prevailing wind direction. The average sand particle trajectories by the RD‐WDT process ( ) were estimated by a mass‐distribution function, which was integrated over a 7‐m uniform slope segment. The results showed that depended statistically upon the wind shear velocity (u*), and the effect of the slope gradient (θ) was insignificant on . This was different from that of the windless rain process ( ), ‘Raindrop Detachment and Splash‐driven Transport’ (RD‐ST), the spatial range of which relies strongly on θ. Additionally, was approximately 2·27 ± 2·2 times greater than the average path of a typical saltating sand particle of the rainless wind ( ), ‘Wind Erosion Saltation Transport’ (WE‐ST). Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

15.
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.  相似文献   

16.
This paper deals with the effect of rainfall on the process of wind erosion of beach sands and presents results from both field and wind tunnel experiments. Although sediment transport by splash is of secondary importance on coastal dunes, splash–saltation processes can move sediments in conditions where no motion is predicted by aeolian processes. The effect of raindrop impact on the movement of soil particles by wind was measured on a sand beach plain using an acoustic sediment sampler. In general, an increase of particle movement by wind at the sensor heights was observed during rainfall. Rainfall also affected the wind erosion process during and after rain by changing the cohesive conditions of the surface. The influence of the surface moisture content on the initiation of wind erosion and on the vertical distribution of transported sand particles was studied in a wind tunnel. Moisture significantly increased threshold wind velocities for the initiation of sediment transport and modified vertical sediment profiles.  相似文献   

17.
Experiments were conducted on Magilligan Strand, Northern Ireland, to assess the influence of the fetch effect on aeolian sediment transport. During each experiment surface sediments were uniformly dry and unhindered by vegetation or debris. The leading edge of erodible material was well defined, with the limit of wave up‐rush demarcating the wet–dry boundary; the work was conducted during low tides. A number of electronic and integrating traps were utilised, with two ultrasonic anemometers used to measure wind direction and velocity at 1 Hz. The combination of 1o direction data and trap locations resulted in a range of fetch distances, from 2 to 26 m. Data integrated over 15‐minute intervals (corresponding to the integrating trap data) revealed a distinct trend for all the experiments. An initial rapid increase in the transport rate occurred over a short distance (4–9 m). This maximum transport rate was maintained for a further 5–6 m before a steady decay in the flux followed, as fetch distance increased. A measured reduction in wind speed (6–8%) across the beach suggests a negative feedback mechanism may be responsible for the diminishing transport rate: the saltating grains induce energy dissipation, thus reducing the capability of the wind to maintain transport. For one experiment, the presence of compact sediment patches may also have contributed to the reduction of the transport rate. The decay trend calls into question the utility of the fetch effect as an important parameter in aeolian studies that seek to understand sediment budgets of the foredune‐beach zone. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

18.
Comparison of eolian transport during five high-velocity wind events over a 29 day period on a narrow estuarine beach in Delaware Bay, New Jersey, USA, reveals the temporal variability of transport, due to changes in direction of wind approach. Mean wind speed measured 6 m above the dune crest for the five events ranged from 8·5 to 15·9 ms?1. Mean wind direction was oblique to the shoreline (63° from shore-normal) during one event but was within 14° of shore-normal during the other events. Eolian transport is greatest during low tide and rising tide, when the beach source area is widest and when drying of surface sediments occurs. The quantity of sediment caught in a vertical trap for the five events varied from a total of 0·07 to 113·73 kgm?1. Differences in temperature, relative humidity and moisture and salt content of surficial sediments were slight. Mean grain sizes ranged from 0·33 to 0·58 mm, causing slight differences in threshold shear velocity, but shear velocities exceeded the threshold required for transport during all events. Beach width, measured normal to the shoreline, varied from 15·5 to 18·0 m; beach slope differed by 0·5°. The oblique wind during one event created a source width nearly double the width during other days. Beach slope, measured in the direction of the wind, was less than half as steep as the slope measured normal to the shoreline. The amount of sand trapped during the oblique wind was over 20 times greater than any other event, even those with higher shear velocities. The ability of the beach surface to supply grains to the air stream is limited on narrow beaches, but increased source width, due to oblique wind approach, can partially overcome limitations of surface conditions on the beach.  相似文献   

19.
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.  相似文献   

20.
Stream–subsurface exchange strongly influences the transport of contaminants, fine particles, and other ecologically relevant substances in streams. We used a recirculating laboratory flume (220 cm long and 20 cm wide) to study the effects of particle size, overlying velocity, and biofilm formation on stream–subsurface exchange of particles. Sodium chloride was used as a non‐reactive dissolved tracer and 1‐ and 5‐µm fluorescent microspheres were used as particulate tracers. Surface–subsurface exchange was observed with a clean sand bed and a bed colonized by an autotrophic–heterotrophic biofilm under two different overlying velocities, 0·9 and 5 cm s?1. Hydrodynamic interactions between the overlying flow and sand bed resulted in a reduction of solute and particle concentrations in the water column, and a corresponding accumulation of particles in both the sediments and in the biofilm. Increasing overlying velocity and particle size resulted in faster removal from the overlying water due to enhanced mass transfer to the bed. The presence of the biofilm did not affect solute exchange under any flow condition tested. The presence of the biofilm significantly increased the deposition of particles under an overlying velocity of 5 cm s?1, and produced a small but statistically insignificant increase at 0·9 cm?1. The particles preferentially deposited within the biofilm matrix relative to the underlying sand. These results demonstrate that hydrodynamic transport conditions, particle size, and biofilm formation play a key role in the transport of suspended particles, such as inorganic sediments, particulate organic matter, and pathogenic microorganisms in freshwater ecosystems, and should be taken into consideration when predicting the fate and transport of particles and contaminants in the environment. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

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