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.     

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.     

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.     

Sand grain threshold, in relation to bed 'stress history': an experimental study

Besides particle size, density and shape, the erodibility of a sediment bed depends also upon the exposure to prethreshold velocities in the overlying flow. Such flow effectively rearranges the grains (at and below the bed surface), causing them to become more resistant to subsequent erosion. The effects of the ‘stress history’, leading up to the critical condition for sediment movement, are investigated for unidirectional flows generated in a recirculating laboratory flume. The sediment beds investigated consisted of cohesionless quartz sand grains, with mean grain diameters of 0·194 mm (fine sand), 0·387 mm (medium sand) and 0·774 mm (coarse sand), with narrow particle-size distributions. The critical (threshold) shear velocity (target value) for the three beds was established, within 2·5 min of increasing the flow from zero velocity. The subsequent experiments were performed under prethreshold velocities at 70% (for 5, 10, 20, 40 and 80 min exposure duration), 80% (for 5, 10, 20, 40 and 80 min exposure duration), 90 and 95% (for 5, 10, 20, 40, 80 and 120 min exposure duration) of the target value. Following exposure to these different prethreshold conditions, the flow was increased then to reach actual critical conditions, within a period of 2·5 min. The critical condition for the initiation of sediment movement was established using visual observation (supplemented by video recordings), according to the Yalin criterion. The results show that if the exposure duration to prethreshold velocities remains constant, then the critical shear velocity increases with increasing prethreshold velocity. Likewise, if the prethreshold velocity remains constant, then the critical shear velocity increases with increasing exposure duration. In some circumstances, the critical shear velocity was found to increase by as much as 27%. An empirical formula is proposed to account for the exposure correction to be applied to the critical shear velocities of sand-sized sediment beds; this is prior to their inclusion into bedload transport formulae, for an improved prediction of the magnitude and nature of transport.     

Field measurements of the flux and speed of wind-blown sand

A field experiment was conducted to measure the flux and speed of wind-blown sand under known conditions in a natural setting. The experiment, run at Pismo Beach, California, involved a tract 100 m long (parallel with the wind) by 20 m wide. The site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary-layer, temperature probes to determine atmospheric stability and wind vanes to determine wind direction. From these measurements, wind friction speeds were derived for each experimental run. In order to measure sand saltation flux, a trench 3 m long by 10 m wide (transverse to the wind direction) by 0·5 m deep was placed at the downwind end of the tract and lined with 168 collector bins, forming an ‘egg-box’ pattern. The mass of particles collected in each bin was determined for four experimental runs. In order to assess various sand-trap systems used in previous experiments, 12 Leatherman traps, one Fryberger trap and one array of Ames traps were deployed to collect particles concurrently with the trench collection. Particle velocities were determined from analysis of high-speed (3000 and 5000 frames per second) motion pictures and from a particle velocimeter. Sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined. Fluxes for each run were calculated using various previously published expressions, and then compared with the flux derived from the trench collection. Results show that Bagnold's (1941) model and White's (1979) equation most closely agree with values derived from the trench. Comparison of the various collector systems shows that the Leatherman and Ames traps most closely agree with the flux derived from the trench, although these systems tended to under-collect particles. Particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories. Results show that particle velocities from the velocimeter are in the range 0·5–7·0 m s^?1, compared to a wind friction velocity of 0·32–0·43 m s^?1 and a wind velocity of 2·7–3·9 m s^?1 at the height of the particle measurements. Descending particles tended to exceed the speeds of ascending particles by ～ 0·5 m s^?1.     

Sediment flux distribution in the Southern Brazil Basin during the late Quaternary: the role of deep-sea currents

Detailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm^?2 10^?3 yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm^?2 10^?3 yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine-grained silty-clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm^?2 10^?3 yr) are higher than on the middle continental rise, as a consequence of fine-grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth-related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep-sea currents which dominate the sedimentation in the abyssal domains.     

Hydraulic characteristics of bioclastic deposits: new possibilities for environmental interpretation using settling velocity fractions

This paper examines the hydraulic behaviour of heterogeneous bioclastic sediments using settling velocity fractions. Bioclastic deposits are divided into fractions by splitting hydraulically sorted samples. Compositional analysis of fractions shows that grains exhibit marked variation in composition, shape, size and density which control the hydraulic behaviour of bioclastic deposits. Despite the heterogeneous grain properties of fractions (analogous to sieve fractions) sedimentation analysis shows that they possess a narrow range of settling velocity values. Flume experiments also show that the settling velocity fractions possess a narrow range of threshold velocity values. A threshold relationship is identified in which the velocity required to entrain mixed settling fractions increases from 20.5 to 65-0cm s ^?1 for settling fractions with mean settling values of 5.5 chi (2-2 cm s ^?1) to 1.4 chi (36.0 cm s ^?1), respectively. Heterogeneous settling fractions, due to intergrain effects, were found to possess larger threshold velocities than individual components showing that the composition of bioclastic deposits controls hydraulic behaviour. Thus, caution must be exercised in using the threshold relationship for homogeneous sediments or deposits of markedly different composition. As settling velocity fractions reflect narrow hydraulic (settling and threshold) properties of sediments the mean settling velocity of fractions is considered a good indicator with which to interpret transport and depositional processes. Comparison of settling and sieve-size distributions shows that size distributions do not reflect the hydraulic behaviour of bioclastic deposits and should not be used to interpret environmental processes. The study indicates that examination of settling velocity fractions (for which hydraulic settling and threshold properties are known) coupled with compositional analysis of these fractions will allow much greater environmental interpretation of deposit-forming and energy processes in reef environments.     

The stability of sublittoral, fine-grained sediments in a subarctic estuary

The erodibility of natural estuarine sediments was measured in sit along a longitudinal transect of Manitounuk Sound, Hudson Bay, using the benthic flume Sea Carousel. Sedimentation processes along the transect varied from continuous, rapid, post-glacial sedimentation in the inner Sound, to glacial outcrops and seabed reworking of the outer Sound. The grain size and physical bulk properties reflect changes in depositional environment and correlate with sediment erosion threshold stress (τ_c), erosion rate (E), erosion type and still-water mass settling rate. There was a steady increase in τ_c (0·8–2·0 Pa) with distance down the Sound in parallel with the decreasing sedimentation rate (0·003–0·001 m yr^?1) and increasing sediment bulk density (1650–2010 kg m^?3). The near-surface friction coefficient varied up to 68° in proportion to the clay content of post-glacial material. Glacial sediments were characterized by variable results and generally higher friction coefficients. Seabed erosion in Sea Carousel began with surface creep of loose aggregates, pellets and organic debris. This was followed by Type I bed erosion at rates that varied between 0·0002 and 0·0032 kg m^?2 s^?1 (mean 0·0015). Type I peak erosion rate was inversely related to applied bed shear stress (τ_o). Type II erosion succeeded Type I, often after a broad transitional period. Simulations of suspended sediment concentration in Sea Carousel were made using four commonly used erosion (E) algorithms. The best results were obtained using Krone's dimensionless ratio relationship: E=M(τ_o/τ_c-1). Simulations were highly sensitive to the definition of erosion threshold with sediment depth [τ_c(z)]. Small errors in definition of τ_c(z) caused large errors in the prediction of suspended sediment concentration which far exceeded differences between the methods tested.     

The stability of a remediated bed in Hamilton Harbour,Lake Ontario,Canada

ABSTRACT In situ measurements of lakebed sediment erodibility were made on three sites in Hamilton Harbour, Lake Ontario, using the benthic flume Sea Carousel. Three methods of estimating the surface erosion threshold (τ_c(0)) from a Carousel time series were evaluated: the first method fits measures of bed strength to eroded depth (the failure envelope) and evaluates threshold as the surface intercept; the second method regresses mean erosion rate (E_m) with bed shear stress and solves for the floc erosion rate (E_f) to derive the threshold for E_m = E_f = 1 × 10^?5 kg m^?2 s^?1; the third method extrapolates a regression of suspended sediment concentration (S) and fluid transmitted bed shear stress (τ₀) to ambient concentrations. The first field site was undisturbed (C) and acted as a control; the second (W) was disturbed through ploughing and water injection as part of lakebed treatment, whereas the third site (OIP) was disturbed and injected with an oxidant used for remediation of contaminated sediment. The main objectives of this study were: (1) to evaluate the three different methods of deriving erosion threshold; (2) to compare the physical behaviour of lacustrine sediments with their marine estuarine counterparts; and (3) to examine the effects of ploughing and chemical treatment of contaminated sediment on bed stability. Five deployments of Sea Carousel were carried out at the control site. Mean erosion thresholds for the three methods were: τ_c(0) = 0·5 (±0·06), 0·27 (±0·01) and 0·34 (±0·03) Pa respectively. Method 1 overpredicted bed strength as it was insensitive to effects in the surface 1–2 mm, and the fit of the failure envelope was also highly subjective. Method 2 exhibited a wide scatter in the data (low correlation coefficients), and definition of the baseline erosion rate (E_f) is largely arbitrary in the literature. Method 3 yielded stable (high correlation coefficients), reproducible and objective results and is thus recommended for evaluation of the erosion threshold. The results of this method correlated well with sediment bulk density and followed the same trend as marine counterparts from widely varying sites. Mass settling rates, expressed as a decay constant, k, of S(t), were strongly related to the maximum turbidity at the onset of settling (S_max) and were also in continuity with marine counterparts. Thus, it appears that differences in salinity had little effect on mass settling rates in the examples presented, and that biological activity dominated any effects normally attributable to changes in salinity. Bedload transport of eroded aggregates (2–4 mm in diameter) took place by rolling below a mean tangential flow velocity (U_y) of 0·32 ms^?1 and by saltation at higher velocities. Mass transport as bedload was a maximum at U_y = 0·4 ms^?1, although bedload never exceeded 1% of the suspended load. The proportion of material moving as bedload was greatest at the onset of erosion but decreased as flow competence increased. Given the low bulk density and strength of the lakebed sediment, the presence of a bedload component is notable. Bedload transport over eroding cohesive substrates should be greater in estuaries, where both sediment density and strength are usually higher. Significant differences between the ploughed and control sites were apparent in both the erosion rate and the friction coefficient (φ), and suggest that bed recovery after disruption is rapid (< 24 h). τ_c(0) increased linearly with time after ploughing and recovered to the control mean value within 3 days. The friction coefficient was reduced to zero by ploughing (diagnostic of fluidization), but increased linearly with time, regaining control values within 6 days. No long‐term reduction in bed strength due to remediation was apparent.     

Lead-210 sediment geochronology in a changing coastal environment

Sediment accumulation rate studies utilizing excess ²¹⁰Pb and ¹³⁷Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm^?2 · yr^?1 or approximately 8.4 to 11.8 cm · yr^?1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the ²¹⁰Pb profile in order to establish a geochronology. Grain size ²¹⁰Pb distribution measurements at three depths reveal that the specific activity (dpm · g^?1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess ²¹⁰Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events.Sixteen ¹³⁷Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm^?2 · yr^?1.     

Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf,South Australia

Spencer Gulf is a large (ca 22 000 km²), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m^?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m^?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m^?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec^?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.     

Molybdenum in marine deposits

The distribution and rates of accumulation of Mo in marine deposits have been determined and compared with the same parameters for U and Mn. High concentrations of Mo are associated both with oxidizing environments represented by the presence of ferro-manganese oxide-rich sediments (where Mo/U ～- 3) and with reducing environments (where Mo/U is about unity). The supply of Mo by streams is more than adequate to balance the measured removal rate in normal deep-sea deposits and no submarine volcanic ‘emanations’ need be involved. On an ocean-wide basis, 4 · 3 μg Mo/cm²/1000 yr is supplied in solution by streams. Of this, 2·0 μg Mo/cm²/1000 yr is removed in deep-sea sediments and manganese nodules. The remaining 2·3 μg Mo/cm²/1000 yr is probably removed in primarily (but not exclusively) near-shore reducing sediments. The average Mo accumulation rate in these environments is about 1000 μg Mo/cm²/1000 yr; thus only 0·23 per cent of the world ocean area need be such reducing sites.     

Engineering Properties, Microstructure and Strength Development of Lightweight Concrete Containing Pumice Aggregates

Lightweight aggregate concretes are widely incorporated in construction and development. This study presented an experimental investigation on the engineering properties of volcanic pumice lightweight aggregates concretes. Three groups of lightweight concretes: 1—coarse pumice aggregates (2–6 mm), 2—course pumice aggregates (2–6 mm)/sand size pumice aggregates and 3—course pumice aggregates (2–6 mm)/sand fraction were built and the physical/mechanical aspects of them were studied. The results of the compressive strength, density, water absorption, pH and shrinkage showed that these lightweight concretes were affected by the type of aggregates, the cement paste and the interfacial zone between cement and aggregates. One grain size pumice (2–6 mm) showed best compressive strength (65 kg/cm²), density of 0.60 g/cm³, linear shrinkage 0.4 % and high water absorption 29.73 %. Strength minerals represented by calcium-silicate-hydrate (CSH) and calcium-aluminate-hydrate (CAH) leading the concrete strength.     

Asynchronous alkenone and foraminifera records from the Benguela Upwelling System

Radiocarbon stratigraphy is an essential tool for high resolution paleoceanographic studies. Age models based on radiocarbon ages of foraminifera are commonly applied to a wide range of geochemical studies, including the investigation of temporal leads and lags. The critical assumption is that temporal coupling between foraminifera and other sediment constituents, including specific molecular organic compounds (biomarkers) of marine phytoplankton, e.g. alkenones, is maintained in the sediments.To test this critical assumption in the Benguela upwelling area, we have determined radiocarbon ages of total C₃₇-C₃₉ alkenones in 20 samples from two gravity cores and three multicorer cores. The cores were retrieved from the continental shelf and slope off Namibia, and samples were taken from Holocene, deglacial and Last Glacial Maximum core sections. The alkenone radiocarbon ages were compared to those of planktic foraminifera, total organic carbon, fatty acids and fine grained carbonates from the same samples. Interestingly, the ages of alkenones were 1000 to 4500 yr older than those of foraminifera in all samples.Such age differences may be the result of different processes: Bioturbation associated with grain size effects, lateral advection of (recycled) material and redeposition of sediment on upper continental slopes due to currents or tidal movement are examples for such processes.Based on the results of this study, the age offsets between foraminifera and alkenones in sediments from the upper continental slope off Namibia most probably do not result from particle-selective bioturbation processes. Resuspension of organic particles in response to tidal movement of bottom waters with velocities up to 25 cm/s recorded near the core sites is the more likely explanation.Our results imply that age control established using radiocarbon measurements of foraminifera may be inadequate for the interpretation of alkenone-based proxy data. Observed temporal leads and lags between foraminifera based data and data derived from alkenone measurements may therefore be secondary signals, i.e. the result of processes associated with particle settling and biological activity.     

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.     

An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits

The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass‐ballotini sand of particle size 35·5 μm < d < 250 μm and volume fractions, φ_s, up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 μm and volume fractions, φ_m, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at φ_s ～ 0·2 and entirely inhibited at φ_s ≥ 0·6. In noncohesive and cohesive mixtures with low sand concentrations (φ_s < 0·2), particle segregation was initially suppressed at φ_m ～ 0·07 and entirely suppressed at φ_m ≥ 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear‐flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The ‘abrupt’ change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud‐poor, graded sandy deposits or mud‐rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean ‘turbidite’ or muddy ‘debrite’ sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal ‘linked’ debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ≤ φ_s ≤ 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near‐bed sediment concentrations.     

A recent turbidity current event, Hueneme Fan, California: reconstruction of flow properties

A recent (100 yr old) turbidite is described from Hueneme Fan, California Continental Borderland. Dense sampling over the fan surface has allowed excellent delineation of the characteristics of this deposit. It exhibits Bouna DE sequences and has a distinctly bimodal, sandy silt grain size distribution. Through the use of generalized fluid dynamics equations, it is possible to reconstruct original flow properties of the current which deposited this material. The calculated velocities ranged from 10–90 cm s^-1 and excess density (above ambient seawater) from 0·001–0·005 g cm^-3 in the lower midfan and upper fan channel regions, respectively. Height of the current ranged from 5–15 m, on slopes from 1·5 to 0·15°. A total of 10⁷ m³ of sediment was deposited during 10 days. The turbidity current is conjectured to have originated from direct river input during the floods of 1884. An older event is also described, which has distinctly different properties and origins. The grain sizes of this older deposit are much coarser, and sedimentary structures suggest higher flow regimes. This turbidite is conjectured to have been deposited from a higher density, faster current thought to have been generated by slumping. The need for a better understanding of the controls on the characteristics of turbidity currents and their effect on fan morphology is emphasized.     

Texture and sedimentation rates in Lake Geneva

Thirty two cores were collected from Lake Geneva sediments along one longitudinal and eight transverse profiles. Rates of sedimentation determined by¹³⁷Cs vary from 0.01 to 1.86 g cm⁻² y⁻¹. The average deposition rates in coastal and slope areas amounts to 0.37 g cm⁻² y⁻¹ in the Upper Lake (Grand Lac) and 0.12 g cm⁻² y⁻¹ in the Lower Lake (Petit Lac). In the deep basins, average rates of 0.13 and 0.05 g cm⁻² y⁻¹ were found for the Grand Lac and Petit Lac, respectively. The estimated mass of sediment deposited yearly outside of the principal deltas and turbidity current depositional areas is about 1.0 million tons (about 13% of the estimated total river load). One turbidite is clearly identified in the deepest, central lake area. There is little variation of surface sediment texture (mean grain size about 8–9μm) with the exception of delta areas. Since the beginning of the twentieth century, both carbonate and organic matter have increased as a result of lake eutrophication.     

Threshold of motion of bivalve and gastropod shells under oscillatory flow in flume experiments

The threshold of motion of non-fragmented mollusc shells was studied for the first time under oscillatory flow. In this regard, flume experiments were used to investigate the threshold of motion of three bivalve and three gastropod species, two typical mollusc classes of coastal coquina deposits. The sieve diameters ranged from 2·0 to 15·9 mm. These experiments were performed on a flat-bottom setup under regular non-breaking waves (swell) produced by a flap-type wave generator. The critical Shields values for each species of mollusc were plotted against the sieve and nominal diameter. Moreover, the dimensionless Corey shape factor of the shells was evaluated in order to investigate the effect of mollusc shell shapes on the threshold of motion. According to their critical Shields parameter, the mollusc threshold data under oscillatory flow present smaller values than the siliciclastic sediments when considering their sieve diameter. In addition, the mollusc datasets are below the empirical curves built from siliciclastic grain data under current and waves. When considering the nominal diameter, the critical Shields parameter increases and the mollusc data are closer to siliciclastic sediments. Bivalves, which have a flat-concave shape (form factor: 0·27 to 0·37), have a higher critical Shields parameter for smaller particles and more uniform datasets than the gastropod scattered data, which have a rounded shape (form factor: 0·58 to 0·62) and have varied morphologies (ellipsoidal, conical and cubic). The comparison between previous current-driven threshold data of bioclastic sediment motion and the data of mollusc whole shells under oscillatory flow shows a fair correlation on the Shields diagram, in which all datasets are below the mean empirical curves for siliciclastic sediments. These findings indicate that the shape effect on the transport initiation is predominant for smaller shells. The use of the nominal diameter is satisfactory to improve the bioclastic and siliciclastic data correlation.     

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.