Effect of source width and tidal elevation changes on aeolian transport on an estuarine beach

Data from a moderate energy, meso-tidal beach on the east side of Delaware Bay, New Jersey, USA, revealed the significance of both beach width as a source for aeolian transport and the effect of tidal rise on source width. Wind speeds averaged over 17·1 min, recorded 6 m above the crest of a 0·5 m high dune, ranged from 11·6 to 12·7 m s^?1 during the experiment. The highest observed rate of transport on the beach was 0·0085 kg m^?1 s^?1, monitored at rising low tide when the average wind speed was 11·6 m s^?1 across 0·35 mm diameter surface sediments. The wind direction was oblique to the shoreline, creating a source width of 34 m. The reduction in the width of the beach as a source for aeolian transport during rising tide was approximately arithmetic, whereas the reduction in volume of sediment trapped was exponential. Aeolian transport effectively ceased when source width was less than 8 m. Wind conditions, moisture content of the surface sediments and presence of binding salts did not appear to vary dramatically, and no coarse grained lag deposit formed on the surface of the beach. The decrease in rate of sediment trapped through time in the tidal cycle is attributed to differences in source width. Sediment deposited in the litter behind the active beach by strong winds during the rising tide was eroded during the high water period by the high waves and storm surge generated by these winds, and net losses of sediment were observed despite initial aeolian accretion.     

Estuary-specific variation in the air-water gas exchange coefficient for oxygen

Oxygen air-water gas exchange was measured using floating chambers in two shallow tidal estuaries of differing bathymetry and local terrain, near Waquoit Bay, Massachusetts (United States). The specific chamber design permitted measurements of gas flux in 15 min, allowing analysis of the relationship with wind speed and tidal stage. Exchange coefficients ranged from 0.5 to 2.5 g O₂·m^?2 h^?1 atm^?1 (equivalent to piston velocities of 1.5 to 7 cm h^?1) for wind speeds of 0.3 to 9 m s^?1 at 10 m elevation. While the relationships for each estuary appear linear (significant linear regressions with wind speed were shown for each estuary, and the slopes were different at the 99.5% confidence level), the range of speeds differed at the two sites and an exponential function of wind speed was consistent with the combined data from both estuaries. A power function of wind speed was not an acceptable model. The exchange coefficients for our estuaries are from 57% to as low as 9% of that predicted by previously published generic equations. Because the atmospheric correction can be significant in shallow, metabolically active coastal waters, we suggest that empirically determined relationships for gas exchange versus wind for a specific estuary are preferable to the predictions of the general equations. While the floating chamber method should be used cautiously, at low winds speeds (below 8 m s^?1) and in slowly flowing waters, it provides a convenient approach for quantifying these site-specific differences. The differences, especially those between shallow sheltered systems and the open waters best fit by some published relationships, are ecologically important and do not appear yet to be measurable by other methods.     

Characteristics of particle size for creeping and saltating sand grains in aeolian transport

Creep and saltation are the primary modes of surface transport involved in the fluid‐like movement of aeolian sands. Although numerous studies have focused on saltation, few studies have focused on creep, primarily because of the experimental difficulty and the limited amount of theoretical information available on this process. Grain size and its distribution characteristics are key controls on the modes of sand movement and their transport masses. Based on a series of wind tunnel experiments, this paper presents new data regarding the saltation flux, obtained using a flat sampler, and on the creeping mass, obtained using a specifically designed bed trap, associated with four friction velocities (0·41, 0·47, 0·55 and 0·61 m sec^?1). These data yielded information regarding creeping and saltating sand grains and their particle size characteristics at various heights, which led to the following conclusions: (i) the creeping masses increased as a power function (q = ?1·02 + 14·19u_*³) of friction wind velocities, with a correlation (R²) of 0·95; (ii) the flux of aeolian sand flow decreases exponentially with increasing height (q = a exp(–z/b)) and increases as a power function (q = ?26·30 + 428·40 u_*³) of the friction wind velocity; (iii) the particle size of creeping sand grains is ca 1·15 times of the mean diameter of salting sand grains at a height of 0 to 2 cm, which is 1·14 times of the mean diameter of sand grains in a bed; and (iv) the mean diameter of saltating sand grains decreases rapidly with increasing height whereas, while at a given height, the mean diameter of saltating sand grains is positively correlated with the friction wind velocity. Although these results require additional experimental validation, they provide new information for modelling of aeolian sand transport processes.     

The morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae

Experimental studies of subcritical, unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves. Flow depth was 0·11 m, mean flow velocities were 0·86–1·0 m s^?1, shear velocities were 0·058–0·71 m s^?1 and dimensionless shear stresses were 0·56–0·87. Bedwaves are asymmetrical in profile and range from 0·75 to 11 mm in height (mainly 2–6 mm), from 0·7 to 1·3 m in wavelength and have mean celerities of 10 mm s^?1. Flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs. High resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds. Lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point; individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced. Laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave.     

Inter‐annual variability of southerly winds in a coastal area of the Atacama Desert: implications for the export of aeolian sediments to the adjacent marine environment

The analysis of the aeolian content of marine cores collected off the coast of the Atacama Desert (Mejillones Bay, Chile) suggests that marine sediments can record inter‐annual to inter‐decadal variations in the regional southerly winds responsible for particle entrainment at the surface of the nearby desert. However, the establishment of a simple and direct correlation between the sediment and wind records is complicated by the difference of time scales between the erosion and accumulation processes. The aim of this work is to: (i) assess the inter‐annual variability of the surface winds responsible for the sand movements; and (ii) determine whether the integration over periods of several months completely smoothes the rapid changes in characteristics of the transported and deposited aeolian material. To accomplish this aim, 14 years of 10 m hourly wind speed, measured at the Cerro Moreno (Antofagasta) Airport between 1991 and 2003 and at the Orica Station between 2000 and 2004, were analyzed. For each year, the wind speed statistical distribution can be represented by a combination of two to three Weibull functions. Winds of the lowest Weibull mode are too weak to move the sand grains at the surface of the pampa; this is not the case for the intermediate mode and especially for the highest speed mode which are able to erode the arid surface and transport particles to the bay. In each individual year of the period of study, the highest speed mode only accounted for a limited number of strong erosion events. Quantitative analysis of the distribution of the friction velocities and of their impact on erosion using a saltation model suggests that, although all wind speeds above threshold produce erosion events, values around 0·45 m sec^?1 contribute less to the erosion flux. This gap allows separation of the erosion events into low and high saltation modes. The correlation (r = 0·997) between the importance of the third Weibull mode and the extent of higher rate saltation indicates that the inter‐annual variability of the erosion at the surface of the pampa, as well as the transport of coarse particles (>100 μm), are directly related to inter‐annual variations in the prevalence of the strongest winds. Finally, a transport and deposition model is used to assess the possible impact of the wind inter‐annual variability on the deposition flux of mineral particles in the bay. The results suggest that inter‐annual differences in the wind speed distributions have a quantifiable effect on the intensity and size‐distribution of this deposition flux. This observation suggests that a detailed analysis of the sediment cores collected from the bay could be used for reconstructing the inter‐annual variability of past winds.     

The effect of surface moisture on the entrainment of dune sand by wind: an evaluation of selected models

Abstract Reliable predictions of wind erosion depend on the accuracy of determining whether erosion occurs or not. Among the several factors that govern the initiation of soil movement by wind, surface moisture is one of the most significant. Some widely used models that predict the threshold shear velocity for particle detachment of wet soils by wind were critically reviewed and evaluated. Wind‐tunnel experiments were conducted on pre‐wetted dune sand with moisture contents ranging from 0·00 to 0·04 kg kg^?1. Sand samples were exposed to different wind speeds for 2 min. Moisture content was determined gravimetrically before and after each experiment, and the saltation of sand particles was recorded electronically with a saltiphone. Shear velocities were deduced from the wind speed profiles. For each moisture content, the experiments were repeated at different shear velocities, with the threshold shear velocity being determined by least‐squares analysis of the relationships between particle number rates and shear velocity. Within the 2‐min test runs, temporal changes in particle number rates and moisture contents were detected. A steep increase in the threshold shear velocity with moisture content was observed. When comparing the models, large differences between the predicted results became apparent. At a moisture content of 0·007 kg kg^?1, which is half the moisture content retained to the soil matrix at a water tension (or matric potential) of ?1·5 MPa, the increase in ‘wet’ threshold shear velocity predicted with the different models relative to the dry threshold shear velocity ranged from 117% to 171%. The highest care should therefore be taken when using current models to predict the threshold shear velocity of wet sediment. Nevertheless, the models of Chepil (1956; Proc. Soil Sci. Soc. Am., 20, 288–292) and Saleh & Fryrear (1995; Soil Sci., 160, 304–309) are the best alternatives available.     

The influence of wind speed on surface layer stability and turbulent fluxes over southern Indian peninsula station

Surface to atmosphere exchange has received much attention in numerical weather prediction models. This exchange is defined by turbulent parameters such as frictional velocity, drag coefficient and heat fluxes, which have to be derived experimentally from high-frequency observations. High-frequency measurements of wind speed, air temperature and water vapour mixing ratio (eddy covariance measurements), were made during the Integrated Ground Observation Campaign (IGOC) of Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) at Mahabubnagar, India (16^°44^′N, 77^°59^′E) in the south-west monsoon season. Using these observations, an attempt was made to investigate the behaviour of the turbulent parameters, mentioned above, with respect to wind speed. We found that the surface layer stability derived from the Monin–Obukhov length scale, is well depicted by the magnitude of wind speed, i.e., the atmospheric boundary layer was under unstable regime for wind speeds >4 m s^?1; under stable regime for wind speeds <2 m s^?1 and under neutral regime for wind speeds in the range of 2–3 m s^?1. All the three stability regimes were mixed for wind speeds 3–4 m s^?1. The drag coefficient shows scatter variation with wind speed in stable as well as unstable conditions.     

Vertical sediment fluxes and wave-induced sediment resuspension in a shallow-water coastal lagoon

The present study describes variations in the vertical fluxes measured concurrently with sediment traps at both a shallow water (4 m) and a deeper water (7.5 m) position in a coastal lagoon in April 1995. A tripod equipped with five sediment traps (trap openings at 0.35 m, 0.75 m, 1.05 m, 1.40 m, and 1.80 m above the seabed) was placed at the shallow water position. This tripod was deployed three times during the study period and deployment periods varied between 2 d and 5 d. The second sediment trap, placed at the deep water position in the central part of the lagoon, measured vertical flux for intervals of 12 h at 1.4 m above the seabed. The horizontal distance between the sediment traps was 8 km. The average maximum vertical flux at the shallow water position reached 27.9 g m⁻² d⁻¹ during a period of high, westerly wind speeds, and a maximum vertical flux of 16.9 g m⁻² d⁻¹ was reached at the deep water position during a period of high, easterly wind speeds. Both strong resuspension events were closely related to increased wave shear stress derived from surface waves. Maximum wave-induced resuspension rate was 10 times higher at the shallow water position and 3.8 times higher at the deep water position compared with the net sedimentation rate in the lagoon. Small resuspension events occurred at the shallow water position during periods of increased current shear stress, Estimations of conditions for transport of sediment between shallow water and deep water showed that particles must be resuspended to a height between 3 m and 4 m and that current speeds must be higher than about 0.1 m s⁻¹. An average sedimentation rate of 3.8 g m⁻² d⁻¹ was obtained at the shallow water position during a period without wave shear stress and low current shear stress. This rate measured by sediment traps is similar to a net sedimentation rate in the lagoon of 4.4 g m⁻² d⁻¹, which was determined by radiocarbon dating of a sediment core (Kristensen et al. 1995).     

Aeolian sediment transport and deposition in a modern high‐latitude glacial marine environment

Aeolian sand and dust in polar regions are transported offshore over sea ice and released to the ocean during summer melt. This process has long been considered an important contributor to polar sea floor sedimentation and as a source of bioavailable iron that triggers vast phytoplankton blooms. Reported here are aeolian sediment dispersal patterns and accumulation rates varying between 0·2 g m^?2 yr^?1 and 55 g m^?2 yr^?1 over 3000 km² of sea ice in McMurdo Sound, south‐west Ross Sea, adjacent to the largest ice free area in Antarctica. Sediment distribution and the abundance of southern McMurdo Volcanic Group‐derived glass, show that most sediment originates from the McMurdo Ice Shelf and nearby coastal outcrops. Almost no sediment is derived from the extensive ice free areas of the McMurdo Dry Valleys due to winnowed surficial layers shielding sand‐sized and silt‐sized material from wind erosion and because of the imposing topographic barrier of the north‐south aligned piedmont glaciers. Southerly winds of intermediate strength (ca 20 m sec^?1) are primarily responsible for transporting sediment northwards and offshore. The results presented here indicate that sand‐sized sediment does not travel more than ca 5 km offshore, but very‐fine sand and silt grains can travel >100 km from source. For sites >10 km from the coast, the mass accumulation rate is relatively uniform (1·14 ± 0·57 g m^?2 yr^?1), three orders of magnitude above estimated global atmospheric dust values for the region. This uniformity represents a sea floor sedimentation rate of only 0·2 cm kyr^?1, well below the rates of >9 cm kyr^?1 reported for biogenic‐dominated sedimentation measured over much of the Ross Sea. These results show that, even for this region of high‐windblown sediment flux, aeolian processes are only a minor contributor to sea floor sedimentation, excepting areas proximal to coastal sources.     

Estimating palaeowind strength from beach deposits

Abstract The geological record of past wind conditions is well expressed in the coarse gravel, cobble and boulder beach deposits of Quaternary palaeolakes in the Great Basin of the western USA and elsewhere. This paper describes a technique, using the particle‐size distribution of beach deposits, to reconstruct palaeowind conditions when the lakes were present. The beach particle technique (BPT) is first developed using coarse beach deposits from the 1986–87 highstand of the Great Salt Lake in Utah, combined with instrumental wind records from the same time period. Next, the BPT is used to test the hypothesis that wind conditions were more severe than at present during the last highstand of Lake Lahontan (≈ 13 ka), which only lasted a decade or two at most. The largest 50 beach clasts were measured at nine beach sites located along the north, west and south sides of Antelope Island in the Great Salt Lake, all of which formed in 1986–87. At these sites, the largest clast sizes range from 10 to 28 cm (b‐axis), and fetch lengths range from 25 to 55 km. Nearshore wave height was calculated by assuming that the critical threshold velocity required to move the largest clasts represents a minimum estimate of the breaking wave velocity, which is controlled by wave height. Shoaling transformations are undertaken to estimate deep‐water wave heights and, ultimately, wind velocity. Wind estimates for the nine sites, using the BPT, range from 6·5 to 17·4 m s^?1, which is in reasonable agreement with the instrumental record from Salt Lake City Airport. The same technique was applied to eight late Pleistocene beaches surrounding the Carson Sink sub‐basin of Lake Lahontan, Nevada. Using the BPT, estimated winds for the eight sites range from 9·7 to 27·1 m s^?1. The strongest winds were calculated for a cobble/boulder beach with a fetch of 25 km. Instrumental wind records for the 1992–99 period indicate that wind events of 9–12 m s^?1 are common and that the strongest significant wind event (≥ 9 m s^?1 for ≥ 3 h) reached an average velocity of 15·5 m s^?1. Based on this preliminary comparison, it appears that the late Pleistocene western Great Basin was a windier place than at present, at least for a brief time.     

Theoretical and measured aeolian sand transport on a barrier island, Louisiana, USA

Over the past 100 years, the Isles Dernieres, a low lying barrier island chain along the coast of central Louisiana, Usa , has undergone more than 1 km of northward beach face retreat with the loss of 70% of its surface area. The erosion results from a long term relative sea level rise coupled with day to day wind and wave action that ultimately favours erosion over deposition. At a site in the central Isles Dernieres, 8 days of wind and beach profile measurements during the passage of one winter cold front documented aeolian erosion and deposition patterns under both onshore and offshore winds. For offshore winds, the theoretical erosion rate, based on wind shear velocity, closely matched the measured erosion rate; for onshore winds, the theoretical rate matched the measured rate only after being corrected by a factor that accounted for beach face morphology. In late February 1989, a strong cold front moved into coastal Louisiana. That cold front stalled over the Gulf of Mexico, resulting in 4 days of strong northerly winds at a study site on the Isles Dernieres. During those 4 days, the wind moved sand from the backshore to the upper beach face. When the cold front finally moved out of the area, the wind shifted to the south and decreased in strength. The onshore wind then restored some of the upper beach face sand to the backshore while increased wave activity moved the rest into the nearshore. The theoretical estimate of 1·28 m³ m^?1 for the rate of sand transport by the northerly wind compares well with the measured backshore erosion rate of 1·26 m³ m^?1, which was determined by comparing beach profiles from the start and end of the period of northerly winds. The theoretical estimate of 0·04 m³ m^?1 for the rate of sand transport by the southerly wind, however, is notably less than the measured rate of 0·45 m³ m^?1. The large discrepancy between the two rates can be explained by a difference in the shear velocity of the wind between the beach face, where the erosion occurred, and the backshore, where the wind stress was measured. Using an empirical relationship for the wind shear drag coefficient as a function of coastal environment, the theoretical estimate for the rate of sand transport by the southerly wind becomes 0·44 m³ m^?1     

Amino acid composition of suspended particles,sediment-trap material,and benthic sediment in the Potomac Estuary

Sediment trap deployments in estuaries provide a method for estimating the amount of organic material transported to the sediments from the euphotic zone. The amino acid composition of suspended particles, benthic sediment, and sediment-trap material collected at 2.4 m, 5.8 m, and 7.9 m depths in the Potomac Estuary was determined in stratified summer waters, and in well-mixed oxygenated waters (DO) in late fall. The total vertical flow, or flux, of material into the top traps ranged from 3 g m^?2 d^?1 in August to 4.9 g m^?2 d^?1 in October. The carbon and nitrogen fluxes increased in the deepest traps relative to the surface traps during both sampling periods, along with that of the total material flux (up to 47.3 g m^?2 d^?1 in the deepest trap), although the actual weight percent of organic carbon and organic nitrogen decreased with depth. Amino acid concentrations ranged from 129 mg g^?1 in surface water particulate material to 22 mg g^?1 in particulate material in 9-m-deep waters and in the benthic sediment. Amino acid concentrations from 2.4-mg-depth sediment traps averaged 104±29 mg g^?1 in stratified waters and 164±81 mg g^?1 in well-mixed waters. The deep trap samples averaed, 77.3±4.8 mg g^?1 amino acids in summer waters and 37±16 mg g^?1 in oxygenated fall waters. Amino acids comprised 13% to 39% of the organic carbon and 12% to 89% of the orgnaic nitrogen in these samples. Analysis of the flux results suggest that resuspension combined with lateral advection from adjacent slopes can account for up to 27% of the material in the deep traps when the estuary was well-mixed and unstratified. When the estuary was stratified in late summer, the amino acid carbon produced by primary productivity in the euphotic zone decreased by 85% (86% for total organic carbon) at the pycnocline at 6 m depth, leaving up to 15% of the vertical organic flux available for benthic sediment deposition.     

Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic

Abstract A study of the seafloor of the Gulf of Cadiz west of the Strait of Gibraltar, using an integrated geophysical and sedimentological data set, gives new insights into sediment deposition from downslope thermohaline bottom currents. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components. Changes in bedform morphology across the study area indicate a decrease in the peak velocity of the MO from >1 m s^?1 to <0·5 m s^?1. The associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. A series of downslope‐oriented channels, formed by the MO, are found where the MO starts to descend the continental slope at a water depth of ≈700 m. These channels are up to 40 km long, have gradients of <0·5°, a fairly constant width of ≈2 km and a depth of ≈75 m. Sand waves move down the channels that have mud wave‐covered levees similar to those seen in turbidite channel–levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. Cores from the lobes recovered up to 8·5 m of massive, well‐sorted, fine sand, with occasional mud clasts. This work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel–levee systems. The distribution of sand within these systems is of particular interest, with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings.     

Sedimentology of the Minette oolitic ironstones of Luxembourg and Lorraine: a Jurassic subtidal sandwave complex

The oolitic ironstones of the Minette were deposited during Toarcian/Aalenian times in a nearshore environment of the Paris Basin. The sedimentary sequence comprises up to 13 coarsening upward depositional cycles. The development of the cycles and different facies types are described. The iron ooids accumulated in a subtidal environment under the effects of tidal currents. Sand waves, which form an important part of the Minette sediments, were deposited under both time-velocity symmetrical and time-velocity asymmetrical tides. From the study of their internal structures the time-velocity patterns of the currents, current speeds (up to 0±9m s^?1) and net transport rates have been estimated (29 g m^?1 s^?1 in the case of large-scale sand waves controlled by time-velocity symmetrical tides). Sand wave heights and calculated near-bed current speeds suggest a mesotidal regime. Sedimentary facies include large-scale and waves, subtidal shoals with channels and an offshore muddy shelf. Conclusions as to bathymetry are drawn from ichnofaunal associations and from the study of wave ripple marks. Finally a depositional model of the Minette iron formation is proposed which may be applicable to other oolitic ironstones.     

Flow parameters of turbidity currents in a low-sinuosity giant deep-sea channel

Flow parameters (velocity and density) for turbidity currents in the Northwest Atlantic Mid-Ocean Channel (NAMOC) have been determined based on two different approaches, channel geometry and grain-size distributions of turbidites. Channel geometry has been obtained by a quantitative morphological analysis of the NAMOC which shows three genetically different segments in the upper 2000 km: (1) an upper 350 km-long ‘equilibrium channel’, (2) a middle 700 km-long ‘modified equilibrium channel’and (3) a lower ‘basement-controlled channel’which is more than 1000 km-long. In contrast to other meandering submarine channels the NAMOC has very low sinuosities and gradients. A consistently higher right-hand levee limits mean flow velocities to 3ms^?1 and channel geometry indicates mean flow velocities of 0·86 m s^?1 that decrease within the equilibrium channel to 0·05 m s^?1. Grain-size distributions on the levees and in the channel suggest strong vertical velocity and density gradients for bank-full flows with velocities of up to 8 m s^?1 and excess densities up to 87 kg m^?3 at the base, and 0·45 m s^?1 and 4 kg m^?3 at the top. The internal shear produced by these strong vertical gradients results in a decoupling of the current head and body. Channel geometry appears to be mainly the result of the slowly moving dilute body of the current.     

Deep‐water dunes on drowned isolated carbonate terraces (Mozambique Channel,south‐west Indian Ocean)

Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec^?1 and maximum speeds of 57 cm sec^?1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology.     

Rate and budget of blown sand movement along the western bank of Lake Nasser,southern Egypt

Three sets of Landsat? satellite images for the years 1993, 1998, and 2003 show that the sand dunes at the southwestern Desert of Egypt are generally moving towards southeast direction with a mean annual creeping speed over ground attaining 15 m/year. The manual-stickled field measurements show that the net annual extension of the longitudinal dunes in the coastal area is between 4 and 5 m/year, while the inland longitudinal dunes showed a net movement ranging between 5 and 6 m/year. Seasonal variations of drift potential and sand movement refer to a strongly high energy wind desert environment in the spring season, high energy wind desert environment in the summer season, and relatively high to intermediate in the autumn and winter seasons, respectively. The total annual estimated volume of transported sand which falls down into Lake Nasser basin attains 16,225,808 m³ as calculated by Bagnold's equation and quantities of sand collected from the sand traps. Comparing this value with the total volume of Lake Nasser Basin, which attains 120?×?10⁹ m³, we can conclude that the sand sheets or sand accumulations may represent serious natural hazards to Lake Nasser in some locations. However, the sand drifting towards the lake may be obstructed by high contour topography hindrance, and the mean grain size of the sand sheets is bigger than 0.25 mm, which needs high wind velocity more than 4 m/s. In addition, the direction of the prevailing wind is N-NNW to S-SSE, and this direction sometimes is parallel to Lake Nasser in some places according to the meandering of the lake. The total lengths of hazardous areas along the western bank of Lake Nasser, which receive the most amounts of the drifted sands, attain 43.6 km only.     

An empirical model for the development and equilibrium morphology of current ripples in fine sand

A flume study on the development and equilibrium morphology of current ripples in fine sand (D₅₀ = 0·238 mm) was performed to extend an empirical model for current ripple stability in 0·095 mm sand to larger grain sizes. The results of the flume experiments agree with the very fine sand model that current ripple development from a flat bed is largely independent of flow velocity. At all flow velocities, ripples evolve from incipient, through straight, sinuous and non-equilibrium linguoid, to equilibrium linguoid plan morphology. The time needed to achieve an equilibrium linguoid plan form is related to an inverse power of flow velocity and ranges from several minutes to more than hundreds of hours. Average equilibrium height and length are 17·0 mm and 141·1 mm respectively. These values are about 20% larger than in very fine sand. Equilibrium ripple height and length are proportional to flow velocity near the stability field of dunes. In the same velocity range, a characteristic grouping of ripples with smaller ripples migrating on the upstream face of larger ripples was observed. Bed-form development shows a conspicuous two-phase behaviour at flow velocities < 0·49 m s^?1. In the first phase of development, ripple height and length increase along an exponential path, similar to that at higher flow velocities, thus reaching intermediate equilibrium values of 14·8 mm and 124·5 mm respectively. After some time, however, a second phase commences, that involves a rapid increase in bed-form size to the typical equilibrium values for 0·238 mm sand. A comparison with literature data shows that the results obtained for 0·238 mm sand agree reasonably well with other flume studies at similar grain size. Yet considerable variability in the relationships between ripple dimensions and flow strength ensues from, among others, underestimation of equilibrium time, shallow flow depths and differences in sediment texture.     

Sediment waves in a modern high-energy glacilacustrine environment

A 4·7 km² field of sediment waves occurs in front of the Slims River delta in Kluane Lake, the largest lake in the Yukon Territory. Slims River heads in the Kaskawulsh Glacier, part of the St Elias Ice Field and discharges up to 400 m³ s^?1 of water with suspended sediment concentrations of up to 7 g l^?1. The 19 km long sandur of Slims River was created in the past 400 years since Kaskawulsh Glacier advanced and dammed the lake and the sandur has advanced into Kluane Lake at an average rate of 48 m a^?1. However, this rate is decreasing as flow is diverted from Slims River because of the retreat of the Kaskawulsh Glacier. The sandur and a road constructed on the delta remove coarse‐grained sediment, so the river delivers dominantly mud to the lake. Inflow during summer generates quasi‐continuous turbidity currents with velocities up to 0·6 m s^?1. The front of the delta consists of a plane surface sloping lakeward at 0·0188 (1·08°). A field of sediment waves averaging 130 m in length and 2·3 m in amplitude has developed on this surface. Slopes on the waves vary from ?0·067 (?3·83°, i.e. sloping in the opposite direction to the regional slope) to 0·135 (7·69°). The internal structure of the sediment waves, as documented by seismic profiling, shows that sedimentation on the stoss portion of the wave averages 2·7 times that on the lee portion. Rates of sediment accumulation in the wave field are about 0·3 m a^?1, so these lacustrine waves have formed in a much shorter period of time (less than 200 years) and are advancing upslope towards the delta much more quickly (1 to 2 m a^?1) than typical marine sediment waves. These waves formed on the flat surface of the lake floor, apparently in the absence of pre‐existing forms, and they are altered and destroyed as the wave field advances and the characteristics of the turbidity currents change.     

Particle size characteristics of aeolian ripple crests and troughs

Ripples are prevalent in aeolian landscapes. Many researchers have focused on the shape and formation of sand ripples, but few have studied the differences in the particle size of sand on crests and in troughs along bed, especially the variations caused by changes in friction velocity and the wind‐blowing duration. A particle size of 158 μm (d ) was used to create aeolian ripples in a wind tunnel under four friction velocities (u *) with different wind duration times (t ). Samples were collected from the surfaces of ripple crests and troughs, respectively, at seven sites, and particle sizes were measured using a Malvern Mastersizer 2000. The main results were: (i) The particle size distributions of sand in troughs are unimodal with slight variations of particle size parameters, including mean particle size, standard deviation, skewness and kurtosis, etc., under different conditions, while these particle size parameters of sand on crests change with friction velocity and deflation time. Moreover, some of the particle distribution curves for the sand on crests do not follow typical unimodal curves. (ii) With increasing friction velocity or deflation duration, the sand on the crests shows a coarsening process relative to those on the bed surface. The particle size of sand on crests at a 1 m bed increases linearly with friction velocity (d  =  344·27 + 34·54 u *) at a given wind‐blowing duration. The particle sizes of sand on crests at 1 m, 2 m and 4 m beds increase with a power‐law relationship (d  = a + t ^b, where a and b are fitting parameters) with deflation time at a given friction velocity. (iii) The probability cumulative curves of sand showed a three‐section pattern in troughs and on most of the crests but a four‐section pattern at crest locations due to increased influence by friction velocity and deflation time. The proportions of the sediment moved by suspension, saltation and creep in the three‐section pattern were within the ranges of 0·2% to 2·0%, 97·0% to 98·9%, and 0·8% to 3·0%, respectively. For the four‐section pattern, suspension accounted for 0·3% and 3·0%, and the proportion of creep increased with friction velocity and deflation time, while saltation decreased accordingly. Although these results require additional validation, they help to advance current understanding of the grain‐size characteristics of aeolian ripples.