Search

The quantification of saltation intermittency--the proportion of time when sand transport occurs--provides valuable information about the nature of aeolian systems, including insights concerning threshold conditions for sand transport. Intermittency has been measured in numerous studies, but a lack of measurement standardization often makes the comparison of results difficult. Four methodological factors influence estimates of intermittency: sample frequency, sample duration, sample area, and sensor elevation. For a given transport rate, the value of intermittency may decrease with sensor elevation, increased sample frequency, or decreased sensor area; it may increase or decrease as a consequence of sample duration. We therefore simulate the influences of these factors and suggest a common protocol for measuring and reporting intermittency based on measurements at 1 Hz with too m[m.sup.2] sensor area at 50 mm elevation above the surface, or equivalent values. Keywords: activity parameter (AP), aeolian processes, sand transport., Saltation is the principle mode of sand transport by wind, and an important agency of landform modification on Earth and extraterrestrial surfaces such as those of Mars, Venus, and Titan. [...]

Transport parameters obtained from laboratory tracer experiments were used to evaluate the stochastic form of the equilibrium convection--dispersion equation (CDE) in describing the transition of scale, i.e. from the column or local scale to a larger field scale. Local-scale solute breakthrough curves (BTCs) were measured in 1-m-long and 0.3-m- diameter undisturbed soil columns by means of time-domain reflectometry at six depths for a 79-h input pulse of chloride. The local-scale data were analysed in terms of the equilibrium CDE and the mobile immobile non-equilibrium transport model (MIM). At the local scale, the MIM transport model better described the observed early breakthrough and the tailing of the BTC than did the CDE. A linear regression analysis indicated that the relationship between the hydrodynamic dispersion D and pore-water velocity v was of the form D=31[v.sup.1.92] (correlation [ρ.sub.v,D] = 0.74). Averaging of the local-scale BTCs across the field produced a large-scale or field-scale mean BTC; at the greatest observation depth (0.8 m) the field-scale dispersivity / = λ equals 0.656 m. The results further showed that for large values of the mean dispersion coefficient, , local-scale dispersion is an important mechanism for field-scale solute spreading, whereas the standard deviation, [σ.sub.D], and the correlation between v and D, [ρ.sub.vD], have negligible effects on field-scale transport. Stochastic stream tube models supplemented with statistical properties of local-scale transport parameters provide a practical and computationally efficient tool to describe heterogeneous solute transport at large spatial scales. Additional keywords: convection-dispersion model, mobile--immobile model, stochastic stream tube model, time-domain reflectometry, parameter optimisation, contaminant transport., Introduction Protection of shallow groundwater systems will greatly benefit from our ability to accurately predict the field-scale transport of pollutants from the soil surface to the groundwater table. Remediation of [...]

Downstream changes in channel-bed and bank particle size that occur in the Pascagoula River system, a humid sub-tropical, sand-bed stream which flows across the Gulf Coastal Plain are summarized in [...]

Turbidity current is produced when a particle-laden fluid flows under lighter ambient fresh fluid. The streaming of particle-laden fluid is called a turbidity current and this kind of current is an important mechanism for sediment transportation in lakes and oceans. In the present research, the main concentration is on the effect of obstacle with an isosceles right triangular cross section on the behavior of turbidity current. A series of laboratory experiments were carried out with various obstacle heights and different inlet densimetric Froude numbers. In each experiment, velocity profiles upstream and downstream of the obstacle were measured, using an acoustic Doppler velocimeter. Kaolin was used as the suspended material. Experiments showed that the density current with lower inlet Froude number, reacts to the presence of the obstacle more rapidly compared to the currents with higher values of inlet Froude number. At the upstream section of the obstacle, an increase in the height of the obstacle resulted in a reduction in the current inertia and the maximum velocity. Also the local Froude numbers had an abrupt increase when the dense layer passed the obstacle. Key words: turbidity current, obstacle, Froude number, experiment, velocity profiles, ADV. Un courant de turbidite est produit lorsqu'un fluide charge de particules s'ecoule sous un fluide frais ambiant plus leger. L'ecoulement d'un fluide charge de particules est appele un courant de turbidite et ce type de courant est un mecanisme important de transport de sediments dans les lacs et les oceans. La presente recherche porte principalement sur l'effet de l'obstacle ayant une section transversale triangulaire isocele a angle droit sur le comportement du courant de turbidite. Une serie d'experiences en laboratoire a ete realisee en utilisant des obstacles de hauteurs variees et de differents nombres densimetriques de Froude a l'entree. Dans chaque experience, les profils bathymetriques ont ete mesures en amont et en aval de l'obstacle au moyen d'un profileur de courant acoustique Doppler. Du kaolin a ete utilise comme materiau en suspension. Les experiences ont montre que le courant de densite avec un nombre de Froude plus faible a l'entree reagit plus rapidement a la presence de l'obstacle que les courants ayant des valeurs plus elevees du nombre de Froude a l'entree. Dans la section amont de l'obstacle, une augmentation de la hauteur de l'obstacle a engendre une reduction de l'inertie de courant et de la vitesse maximale. De plus, les nombres de Froude locaux augmentent de maniere abrupte lorsque la couche dense a passe l'obstacle. [Traduit par la Redaction] Mots-cles: courant de turbidite, obstacle, nombre de Froude, experience, profils bathymetriques, profileur de courant acoustique Doppler., 1. Introduction Density currents are formed when gravity acts upon a density difference between two different fluids, and the driving force is the buoyancy force. Since density currents are formed [...]

Culvert design must account for a large range of flow rates but most of the time culverts convey flows that are smaller than the design flow. Repeated high flows passing through the culverts lead to a gradual increase of the sediment deposits in the culvert vicinity, eventually resulting in reduced flow and sediment capacity. Sedimentation at culverts is quite an involved process due to the combined effect of flow non-uniformity and unsteadiness, as well as of the flow-sediment interaction. This paper reports results from a series of laboratory experiments aimed at understanding the hydro and morpho-dynamic changes occurring in the vicinity of a three-box culvert. For this purpose, velocity and turbulence characteristics upstream of the culvert were mapped globally and locally during the transition from an original flatbed to equilibrium sediment transport condition using contemporary acoustics, light detection and ranging, and image-based technologies. The experimental results reveal flow complexities not currently accounted for in the design of the multi-barrel culverts, but which can be detrimental for their long-term operation. Considerations on practical approaches to efficiently model the complexities were formulated. Key words: culvert, non-uniform flow, LSPIV, LiDAR, acoustic instrument. La conception des ponceaux doit tenir compte d'une grande plage de debits mais, la plupart du temps, les ponceaux font passer des debits qui sont inferieurs au debit prevu. Les debits eleves passant a repetition dans les ponceaux engendrent une augmentation graduelle du depot de sediments aux environs du ponceau, ce qui conduit eventuellement une capacite reduite de debit et de transport solide. La sedimentation aux ponceaux est un processus complexe en raison de l'effet combine de la non-uniformite et de l'instabilite du debit ainsi que de l'interaction debit-sediments. Le present article presente les resultats d'une serie d'experiences en laboratoire visant a comprendre les changements hydrodynamiques et morpho-dynamiques survenant aux environs d'un ponceau tricorps. A cette fin, les caracteristiques de vitesse et de turbulence a l'amont du ponceau ont ete cartographiees globalement et localement durant la transition d'un lit plat initial a une condition de transport a l'equilibre de sediments en utilisant l'acoustique contemporain, le LiDAR et les technologies basees sur les images. Les resultats experimentaux revelent des complexites de debits dont on ne tient pas presentement compte dans la conception de ponceaux multitubes, mais qui pourraient nuire a leur fonctionnement a long terme. Nous avons formule des recommandations concernant les approches pratiques afin de modeliser efficacement ces complexites. [Traduit par la Redaction] Mots-cles: ponceau, debit non uniforme, LSPIV, LiDAR, instrument acoustique., 1. Introduction Culverts are common hydraulic structures for secondary roads crossing small streams in the rural US Midwestern area. In this region, the sedimentation at culverts is a chronic problem [...]

Runoff volume, sediment yield and infiltration on hillslope in the hilly region of Loess Plateau were studied using a rainfall simulator. Two land cover (natural grassland, NG, and bare hillslope, BS) and three different rainfall intensities (2.0, 1.5, 0.75 mm [min.sup.-1]) were implemented. The runoff time of the [NG.sub.2.0] treatment was 4.8 min earlier, the average infiltration rate was 5.60% lower, the runoff rate was 1.12 times higher and the sediment production rate was 3.28 times lower than those in the [BS.sub.2.0] treatment. The runoff time for the [NG.sub.1.5] and the [NG.sub.0.75] treatments were 6 and 27 min slower, respectively, the average infiltration rate were 10.78 and 23.52% higher, respectively, the runoff rate were 1.67 and 4.25 times lower, respectively, and the sediment production rate were 5.21 and 16.75 times lower, respectively, than those for the [BS.sub.1.5] and [BS.sub.0.75] treatments. The effects of slope on sediment reduction were significantly greater than the effects of its runoff reduction, and there occurred more significant sediment-reducing effects with the decrease of rainfall intensity. Efficient use of rainfall runoff in the natural grassland at high intensity should be taken into considerations especially under current conditions of the co-existing drought-caused water shortage and soil erosion in the loess hilly regions. Additional keywords: rainfall intensity, natural grassland, runoff, sediment yield, infiltration, rainfall simulation., Introduction The hilly regions of the Loess Plateau in northern Shaanxi province of China situated in the middle reaches of the Yellow River (36° 59'-38° 84'N and 109° 48'-110° 49'E) [...]

Branching river networks are one of the most widespread and recognizable features of Earth's landscapes and have also been discovered elsewhere in the Solar System (1,2). But the mechanisms that create these patterns and control their spatial scales are poorly understood. Theories based on probability (3-5) or optimality (3,6-8) have proven useful (9), but do not explain how river networks develop over time through erosion and sediment transport. Here we show that branching at the uppermost reaches of river networks is rooted in two coupled instabilities: first, valleys widen at the expense of their smaller neighbours, and second, side slopes of the widening valleys become susceptible to channel incision. Each instability occurs at a critical ratio of the characteristic timescales for soil transport and channel incision. Measurements from two field sites demonstrate that our theory correctly predicts the size of the smallest valleys with tributaries. We also show that the dominant control on the scale of landscape dissection in these sites is the strength of channel incision, which correlates with aridity and rock weakness, rather than the strength of soil transport. These results imply that the fine-scale structure of branching river networks is an organized signature of erosional mechanics, not a consequence of random topology., Observers have long recognized that branching river networks, in which tributaries merge in the downstream direction, are a fundamental outcome of erosion by flowing water. Early field observations led to [...]

Oxygen consumption in marine sediments is often coupled to the oxidation of sulphide generated by degradation of organic matter in deeper, oxygen-free layers. Geochemical observations have shown that this coupling can be mediated by electric currents carried by unidentified electron transporters across centimetre-wide zones. Here we present evidence that the native conductors are long, filamentous bacteria. They abounded in sediment zones with electric currents and along their length they contained strings with distinct properties in accordance with a function as electron transporters. Living, electrical cables add a new dimension to the understanding of interactions in nature and may find use in technology development., Marine sediments become anoxic because oxygen is consumed by microbial processes at the surface. Without available oxygen the microorganisms living below the surface are supposed to depend on energetically less [...]

Recent marine sedimentary deposits and river discharge in two subarctic fjords in Nunatsiavut (Northern Labrador, Canada) have been studied to elucidate patterns and mechanisms of fluvial sediment transfer and accumulation in the fjords, to further our understanding of the longer-term sedimentary record. Multibeam and sub-bottom acoustic data and sediment cores were collected in Nachvak and Saglek fjords, within Canada's Torngat Mountains National Park, as part of the most extensive study of the park's marine resources to date. Cores were subsampled for X-radiography, grain size, and [sup.210]Pb/[sup.137]Cs geochronology. Muddy basin sediments within each fjord are bioturbated, indicating circulation of oxygenated bottom water. Depositional fluxes and inventories of [sup.210]Pb indicate efficient marine scavenging of [sup.210]Pb by fine suspended sediments. In Nachvak Fjord, with small rivers and steep, presently glaciated catchments, postglacial and recent sediment accumulation rates are similar, implying relatively constant sedimentation over time. In Saglek Fjord, fed by larger rivers with more extensive catchments that lack glaciers, recent sediment accumulation is more rapid than that averaged over postglacial time. Present mass accumulation rates for the Nachvak Fjord basin are on average 39 000 t x [year.sup.-1] for the entire basin, and for Saglek 43 000 t x [year.sup.-1] for the entire basin, with sediment-gravity flows being one likely mechanism for sediment delivery to deep basins. Results collectively suggest that both marine basins are excellent natural sediment traps. Comparison of accumulation rates from [sup.137]Cs and [sup.210]Pb suggest that sediment fluxes to Nachvak Fjord may have decreased slightly over the past ~130 years. Des depots recents de sediments marins et de decharge de rivieres dans deux fjords subarctiques au Nunatsiavut (Nord du Labrador, Canada) ont ete etudies afin d'interpreter les patrons et les mecanismes de transfert et d'accumulation de sediments dans les fjords et de mieux comprendre les donnees sedimentaires d'une longue periode de tem[ρ.s]. Les donnees acoustiques multifaisceaux incluant celles du fond du sous-sol ainsi que des carottes de sediments ont ete recueillies dans les fjords Nachvak et Saglek, dans le parc national des monts Torngat, au Canada, dans le cadre de l'etude la plus approfondie a ce jour des ressources du parc marin. Les carottes ont ete sous- echantillonnees pour des analyses aux rayons-X, la granulometrie et la geochronologie [sup.210]Pb/[sup.137]Cs. Dans les deux fjords, les sediments du bassin boueux ont ete perturbes par bioturbation, indiquant une circulation oxygenee de l'eau de fond. Les flux de deposition et les inventaires de [sup.210]Pb indiquent une elimination marine efficace du [sup.210]Pb par de fins sediments en suspension. Dans le fjord de Nachvak, ou l'on retrouve de petites rivieres et des bassins de captage abrupts, actuellement recouverts de glace, les taux post-glaciaux et recents d' accumulation des sediments sont similaires, impliquant une sedimentation relativement constante dans le temps. Dans le fjord de Saglek, alimente par de plus grosses rivieres et ayant de plus grands bassins de captage libres de glaciers, l'accumulation recente de sediments est plus rapide que la moyenne d'accumulation des temps postglaciaires. Les taux actuels d' accumulation de masse du fjord Nachvak sont en moyenne de 39 000 tonnes/annee pour tout le bassin et de 43 000 tonnes/ annee pour tout le bassin de Saglek; un mecanisme probable de transport de sediments vers les bassins profonds est l'ecoulement gravitaire des sediments. L'ensemble des resultats suggere que les deux bassins marins sont d'excellents pieges naturels de sediments. Selon une comparaison des taux d'accumulation, determines par [sup.137]Cs et [sup.210]Pb, les flux de sediments dans le fjord Nackvak auraient diminue legerement au cours des 130 dernieres annees environ. [Traduit par la Redaction], Introduction The goal of this study is to evaluate recent marine records of fluvial sediment supply to two subarctic fjords in northern Labrador, to better understand the development of these [...]

The flux of organic particles below the mixed layer is one major pathway of carbon from the surface into the deep ocean. The magnitude of this export flux depends on two major processes--remineralization rates and sinking velocities. Here, we present an efficient method to measure sinking velocities of particles in the size range from approximately 3-400 µm by means of video microscopy (FlowCAM®). The method allows rapid measurement and automated analysis of mixed samples and was tested with polystyrene beads, different phytoplankton species, and sediment trap material. Sinking velocities of polystyrene beads were close to theoretical values calculated from Stokes' Law. Sinking velocities of the investigated phytoplankton species were in reasonable agreement with published literature values and sinking velocities of material collected in sediment trap increased with particle size. Temperature had a strong effect on sinking velocities due to its influence on seawater viscosity and density. An increase in 9°C led to a measured increase in sinking velocities of ~40%. According to this temperature effect, an average temperature increase in 2°C as projected for the sea surface by the end of this century could increase sinking velocities by about 6% which might have feedbacks on carbon export into the deep ocean., Introduction Sinking velocities of marine particles are generally determined by two major properties--size and excess density over seawater. Most organic particles within the oceans are relatively small ( It has [...]

Contaminant transport through a single hole in a geomembrane (GM) that forms part of composite liner with a geosynthetic clay liner (GCL) for applied heads (0.3 and 1 m) and an applied pressure of about 100 kPa is modeled and compared with experimental results. The system was first permeated with distilled water until a steady state was reached. The permeant was then switched to an NaCl solution. The inferred interface transmissivity at steady state (after 280 days) is about 2.3 x [10.sup.-11] [m.sup.2]/s for both heads examined. After 800 days permeation with an 0.14 mol/L NaCl solution, the interface transmissivity between the geomembrane and GCL was inferred to have decreased from 2.3 x [10.sup.-11] to 1.1 x [10.sup.-11] [m.sup.2]/s for experiments with heads of both 0.3 and 1 m. Modeling of the flow and changes in the GCL hydraulic conductivity arising from interaction between the bentonite and the 0.14 mol/L NaCl permeant is shown to give excellent agreement with the observed leakage. The differences between modeling a uniform and nonuniform interface transmissivity is found to be small--with the leakage being dominated by the interface transmissivity 0.1 to 0.15 m from the hole in the GM. Typical published values for the diffusion coefficient for both the GCL and sandy soil are shown to give good predictions of effluent concentration with time and chloride distribution in the silty sand in contact with the composite liners at the end of the test. Key words: numerical modeling, leakage, composite liner, geosynthetic clay liner, advection, transport, landfill. Cet article presente la modelisation du transport de contaminants a travers un trou unique dans de la geomembrane (GM) faisant partie d'un revetement composite avec argile et geosynthetique (GCL) pour des tetes appliquees (0,3 et 1 m) et des pressions appliquees d'environ 100 kPa. Les resultats de la modelisation sont ensuite compares a des resultats experimentaux. Le systeme est d'abord infiltre avec de l'eau distillee jusqu'a ce que le regime permanent soit atteint. Ensuite, le liquide permeant est change par une solution de NaCl. La transmissivite inferee a l'interface en regime permanent (apres 280 jours) est d'environ 2,3 x [10.sup.-11] [m.sup.2]/s pour les deux tetes testees. Apres 800 jours de permeation avec une solution de 0,14 mol/L NaCl, la transmissivite de l'interface entre la geomembrane et le GCL est supposee avoir diminue de 2,3 x [10.sup.-11] [m.sup.2]/s a 1,1 x [10.sup.-11] [m.sup.2]/s pour les essais avec des tetes de 0,3 et de 1 m. La modelisation de l'ecoulement et des variations de conductivite hydraulique du GCL survenant en reponse aux interactions entre la bentonite et la solution de 0,14 mol/L NaCl demontre des correspondances excellentes avec les fuites observees. Les differences entre la modelisation d'une transmissivite a l'interface uniforme ou non uniforme s'averent etre faibles, les fuites etant dominees par la transmissivite a 0,1 m et 0,15 m du trou dans la GM. Des valeurs typiques publiees du coefficient de diffusion autant pour le GCL que pour le sol sablonneux permettent d'obtenir des bonnes predictions de la concentration a l'effluent avec le temps et de la distribution du chlorure dans le sable silteux en contact avec les revetements composites a la fin de l' essai. Mots-cles : modelisation numerique, fuite, revetement composite, revetement d'argile et geosynthetique, advection, transport, site d' enfouissement. [Traduit par la Redaction], Introduction Composite liners used at the base of modern landfills often involve a high-density polyethylene (HDPE) geomembrane (GM) over a geosynthetic clay liner (GCL). The primary role of the GCL [...]

Browsing and trampling by nonnative feral pigs (Sus scrofa) negatively impact native flora and fauna in forested ecosystems and cause soil compaction. However, their impact on runoff and erosion is largely unknown. This study addressed this knowledge gap by investigating effects of feral pigs on runoff volume and total suspended solids (TSS) in runoff from the upper forested area of a Hawaiian watershed. Correlations between TSS, runoff, and other environmental variables were also examined. Runoff was collected monthly after 11 individual storm events from June 2008 to April 2009 at seven sites in the Manoa watershed on the island of O'ahu. Each site consisted of paired runoff plots (5.04 [m.sup.2]) with one plot located inside a fenced pig exclosure (exclosures 1 yr old at study initiation) and the other located in an adjacent area open to feral pigs. Forest composition and structure (stem density, stand basal area, and seedling/ sapling counts) were quantified at each site. Soil moisture, throughfall, runoff volume, and TSS in runoff were sampled for each storm event. The seven sites varied considerably in terms of forest structure, with stem densities ranging from 1,500 to 9,000 stems ha-1 and basal areas ranging from 20 to 132 [m.sup.2] ha-1. Vegetation at all sites was dominated by nonnative species. Runoff volumes from fenced and unfenced plots were highly variable, ranging from 128 liters. TSS levels in runoff ranged from, Little is known about the impact of nonnative ungulates on soil system function, but the few studies conducted to date indicate that feral pigs (Sus scrofa), in particular, can have [...]

SUMMARY A new surficial geology map of the St. Lawrence River Estuary illustrates sediment distribution and variability in a relatively confined environment. This map is based on recently collected geophysical [...]