Unidirectional Velocities Research Articles

Microplastic retention by marine vegetated canopies: Simulations with seagrass meadows in a hydraulic flume.

Marine canopies formed by seagrass and other coastal vegetated ecosystems could act as sinks of microplastics for being efficient particle traps. Here we investigated for the first time the occurrence of microplastic retention by marine canopies in a hydraulic flume under unidirectional flow velocities from 2 to 30cms-1. We used as model canopy-forming species the seagrass Zostera marina with four canopy shoot density (0, 50, 100, 200 shoots m-2), and we used as microplastic particles industrial pristine pellets with specific densities from 0.90 to 1.34gcm-3 (polypropylene PP; polystyrene PS; polyamide 6PA; and polyethylene terephthalate PET). Overall, microplastics particles transported with the flow were retained in the seagrass canopies but not in bare sand. While seagrass canopies retained floating microplastics (PP) only at low velocities (<12cms-1) due to a barrier created by the canopy touching the water surface, the retention of sinking particles (PS, PA, PET) occurred across a wider range of flow velocities. Our simulations revealed that less dense sinking particles (PS) might escape from the canopy at high velocities, while denser sinking particles can be trapped in scouring areas created by erosive processes around the eelgrass shoots. Our results show that marine canopies might act as potential barriers or sinks for microplastics at certain bio-physical conditions, with the probability of retention generally increasing with the seagrass shoot density and polymer specific density and decreasing with the flow velocity. We conclude that seagrass meadows, and other aquatic canopy-forming ecosystems, should be prioritized habitats in assessment of microplastic exposure and impact on coastal areas since they may accumulate high concentration of microplastic particles that could affect associated fauna.

SwathDop: Multibeam Pulse-Coherent Doppler Sonar for Scanning 2D Velocity Sections near the Sediment–Water Interface

AbstractThis paper considers the problem of accurately measuring the sediment transport over bedforms where flow evolves continuously both in time and space. For this purpose, we have developed a pulse-to-pulse coherent Doppler sonar system designed in bistatic geometry with two fan-beam transmitters symmetrically positioned on each side of a multielement receive array. The system resolves 2D velocity components within a ±20° (~0.5 m by ~0.5 m) swath. The software-defined radio data acquisition and control system limited us at present to eight independent receiver channels, and consequently the azimuthal resolution of the system is 4°. As a preliminary test of the sonar system, the system operation was simulated using a model developed to predict coherent sonar performance. The uncertainties with respect to the prescribed values and mean measurements in the model results were confined to 0.35 and 0.23 cm s−1, respectively, in the presence of strong shear (~150 s−1) and 50 cm s−1 horizontal flow. An important thing is that the model allowed us to test and develop the signal processing algorithms necessary to invert the multibeam sonar data. Using sand of 0.4-mm median diameter, the laboratory trials were carried out in active sediment transport conditions over dunes with 2-m wavelength and ~0.90 m s−1 unidirectional flow velocities. The results presented here focus mainly on 2D velocity field and indicate an average 4% deviation from the wake law and 8% from independent observations made with the wide-band multifrequency coherent Doppler profiler (MFDop) instrument under similar flow conditions.

Experimental Investigations of Combined Flow Sediment Transport

Abstract In shallow marine environments gravity-driven currents (e.g., hyperpycnal flows) often traverse surface wave fields, and the resulting complex flows are key mechanisms for offshore sediment transport. Our laboratory experiments illustrate how surface waves alter sediment transport in gravity-driven density currents. The addition of a wave field to a gravity-driven current resulted in a 7–8.5% increase in the downslope transport of the deposit volume. Additionally, oscillatory velocities recorded at downslope locations in surface-wave-altered turbidity currents were larger than wave-field velocities measured at the same location without a turbidity current. These observations indicate that surface waves alter turbidity currents in a longitudinally complex manner whereby the influence of oscillatory currents is transported downslope within the body of the turbidity current. These effects were observed in a conservative case: the maximum orbital velocities of the wave field were an order of magnitude less than the maximum unidirectional velocities of the current. We predict that if the velocities of the wave field and the current were sub-equal, a plausible scenario for hyperpycnal flows in near-shore, deltaic, and proximal shelf environments, these effects would be substantially more effective. This work has significant implications for modeling offshore sediment transport in shallow marine environments and for interpreting the deposits of such flows, most notably that the presence or absence of combined-flow ripples might not indicate whether the current was deposited above or beneath wave base.

Increased current flow enhances the risk of organic carbon loss from Zostera marina sediments: Insights from a flume experiment

AbstractHydrodynamic processes are important for carbon storage dynamics in seagrass meadows, where periods of increased hydrodynamic activity could result in erosion and the loss of buried carbon. To estimate hydrodynamic impacts on the resuspension of organic carbon (Corg) in seagrass‐vegetated sediments, we exposed patches (0.35 × 0.35 cm) of Zostera marina (with different biomass, shoot densities, and sediment properties) to gradually increased unidirectional (current) flow velocities ranging from low (5 cm s−1) to high (26 cm s−1) in a hydraulic flume with a standardized water column height of 0.12 m. We found that higher flow velocities substantially increased (by more than threefold) the proportion of Corg in the suspended sediment resulting in a loss of up to 5.5% ± 1.7% (mean ± SE) Corg from the surface sediment. This was presumably due to increased surface erosion of larger, carbon‐rich detritus particles. Resuspension of Corg in the seagrass plots correlated with sediment properties (i.e., bulk density, porosity, and sedimentary Corg) and seagrass plant structure (i.e., belowground biomass). However, shoot density had no influence on Corg resuspension (comparing unvegetated sediments with sparse, moderate, and dense seagrass bed types), which could be due to the relatively low shoot density in the experimental setup (with a maximum of 253 shoots m−2) reflecting natural conditions of the Swedish west coast. The projected increase in the frequency and intensity of hydrodynamic forces due to climate change could thus negatively affect the function of seagrass meadows as natural carbon sinks.

Acclimation of seagrass Zostera noltii to co-occurring hydrodynamic and light stresses

Seagrasses may frequently experience a combination of velocity and light stresses, as elevated hydrodynamics often enhances turbidity and the subsequent light reduction. The objective of this study was to investigate the effects that these stressors induce on morphometric and dynamic seagrass features depending on the initial biomass partitioning. For this purpose, a factorial meso- cosm experiment was conducted on plants of Zostera noltii subjected to combinations of 2 contrast- ing light levels (2.5 ± 0.6 and 15.6 ± 2.5 mol photons m -2 d -1 ) and 3 unidirectional flow velocities (0.35, 0.10 and 0.01 m s -1 ). No interactive effects between the 2 variables were recorded, except on plant survival and leaf length, and generally, light effects prevailed over hydrodynamic ones. Plants responded to light reduction regardless of the flow velocity treatments, showing low survival rates (which improved at high velocity), high aboveground/belowground biomass ratios (AG/BG) and a poorly developed root-rhizome system compared to plants under saturating light conditions. Plant morphometry only responded to hydrodynamic stress under saturating light: at high current velocity, plants preferentially allocated biomass into BG structures, bearing short leaves and displaying high internode and root appearance rates. Overall, light reduction promoted similar responses in plants with different AG/BG biomass ratios, but dissimilarities were recorded for current velocity. Thus, it can be concluded that, under simultaneous light and hydrodynamic stresses, light effects prevailed over hydrodynamic ones in Z. noltii, while acclimation to hydrodynamics only occurred under saturating light.

Spatial variations in nutrient supply to the red algae Eucheuma serra (J. Agardh) J. Agardh

SUMMARYThe nutrient supply rates within the canopy of the economically important red algal species, Eucheuma serra J. Agardh were determined experimentally in a recirculating flow‐chamber. A single individual was placed in the working section of the 2000 × 200 × 250 mm3 acrylic flow‐chamber and subjected to unidirectional water velocities from 1.0 to 9.3 cm s−1. Rates of nutrient supply were determined using 9.7 mm diameter CaSO4 (gypsum) spheres that were attached to the thallus inside and outside of the canopy. The supply rates within the canopy were 56% less than outside of the canopy. Increases in internal and external water velocity asymptotically increased the nutrient supply rates regardless of location. A model was developed to examine how changes in ammonium and nitrate supply compared with the physiologically maximum uptake rates of these nutrients. The results suggest that when the ammonium concentration in the water was 20 µmol L−1 uptake rates were limited by the supply rate especially at velocities below 5 cm s−1, whereas in the case of 20 µmol L−1 of nitrate, the supply of nitrate was more than adequate to maximize nutrient uptake.

Experiments on Oscillatory-Flow and Combined-Flow Bed Forms: Implications for Interpreting Parts of the Shallow-Marine Sedimentary Record

Abstract Analog laboratory experiments were conducted to investigate subaqueous bed forms generated under storm-like oscillatory and combined flow. Experiments were carried out in a large wave tunnel and used a range of sand sizes (fine and very fine), wave periods (10.5 and 8 s), oscillatory velocities (0 to 125 cm/s), and unidirectional velocities (0 to 25 cm/s). At low unidirectional velocities (≤10 cm/s), addition of an increasing collinear oscillatory flow caused the bed to evolve from small-scale (wavelength 100 cm), symmetric, orbital ripples, to plane bed. At higher unidirectional velocities (>10 cm/s), a similar trend was noted, but ripples were more asymmetric. Three phase diagrams are presented to summarize the observed relationships of bed configurations to flow conditions. These diagrams are intended to assist in the interpretation of shallow marine sedimentary environments where oscillatory and combined flow are thought to be omnipresent. Distinctive features of small-scale asymmetric combined-flow ripples generated are: a 3D planform, round crest, and convex-up sigmoidal profile with local pronounced scour at the toe of the stoss side giving the ripple profile a boxy appearance. Similarly, large-scale asymmetric combined-flow ripples had broad and round crests, convex-up stoss sides, and compressed due to scouring at the toe of the stoss side. Hummocky bed forms were generated under moderate to high oscillatory velocities and low unidirectional velocities. Hummocks were not observed as a distinct bed state but rather appeared to mark transitions in bed-form scale and symmetry. Hummocks were more prevalent at longer oscillatory periods and in finer-grained sediment. Stratification produced by synthetically aggrading hummocky bed profiles closely resembles hummocky cross-stratification. With the introduction of only a small unidirectional-flow component, hummocks evolve into downstream-migrating large-scale asymmetric ripples, and the resultant cross-stratification becomes similar to that produced by unidirectional-flow dunes. Accordingly, these experiments suggest that much of the hummocky cross-stratification observed in the stratigraphic record is produced by storm-generated long-period oscillatory-dominant combined flows. Inasmuch as long-period, high-energy waves require deep, wide basins to form, hummocky cross-stratification may therefore serve as a useful indicator of deposition in unrestricted, open-water conditions.

Sediment resuspension and boundary layer flow dramatically increase the growth rates of interface-feeding spionid polychaetes

Most spionid polychaetes switch from surface deposit feeding to suspension feeding as current velocity and the flux of suspended food particles increase in the benthic boundary layer. This hydrodynamically mediated feeding behavior has been well studied, but the impact of flow on the growth rates of juvenile spionids has not been tested extensively. I performed two experiments in counter-rotating annular flumes to measure the growth of Polydora cornuta and Streblospio benedicti in a range of flow conditions. Experiment 1 tested the effects of sediment resuspension. The body volumes of individual worms were measured before and after a 7-day exposure to steady, unidirectional flow velocities ranging from 0.5 to 18 cm s −1 ( U 5mm=velocity measured 5mm above bottom). Sediment collected from the worms' field site was added to each flume as the only food resource. The sediment's critical erosion velocity was ∼13 cm s −1 ( U *=0.9 cm s −1). When flumes were set to U 5mm≤12 cm s −1, the relative growth rate (RGR) of small Polydora (initial body length=1–4 mm) equalled 0.19 day −1 on average, and the RGR of a larger class of juvenile Polydora (6–9 mm) equalled 0.06 day −1. When U 5mm=15 or 18 cm s −1 and some sediment became resuspended, the RGR of small and large Polydora equalled 0.37 and 0.22 day −1, respectively. Streblospio grew more slowly than Polydora, but showed a similar relative increase in RGR when flow exceeded the erosion velocity. To better test the effects of non-erosive flows, a second experiment was performed with phytoplankton added to the flumes. Experiment 2 tested four velocities ( U 5mm=3, 6, 9, or 12 cm s −1), and each flow treatment was replicated four times. Increases in velocity and the flux of microalgae significantly increased the RGR of Streblospio and Polydora, with an average RGR of 0.53 day −1 for small Polydora exposed to U 5 mm=12 cm s −1. The growth rates of Polydora and Streblospio in the flume experiments are much greater than those reported for similar polychaetes in still water, demonstrating that studies of spionid nutrition and population dynamics must consider realistic flow regimes.

Transient production of F-region irregularities associated with TCV passage

Abstract. Transient production of F-region plasma irregularities due to traveling convection vortices (TCVs) was investigated using the Super Dual Auroral Radar Network (SuperDARN) combined with ground magnetometer networks and the POLAR ultraviolet imager. We selected two large-amplitude (100–200 nT) TCV events that occurred on 22 May 1996 and 24 July 1996. It is found that the TCV-associated HF backscatter arises in blobs with spatial scale of a few hundreds km. They traveled following tailward bulk motion of the TCV across the three fields-of-view of the SuperDARN HF radars in the prenoon sector. The spectra in the blobs showed unidirectional Doppler velocities of typically 400–600 m/s, with flow directions away from the radar. These unidirectional velocities correspond to the poleward and/or eastward convective flow near the leading edge of upward field-aligned current. The backscatter blobs overlapped the poleward and westward part of the TCV-related transient aurora. It is likely that the transient backscatter blobs are produced by the three-dimensional gradient drift instabilities in the three-dimensional current system of the TCV. In this case, nonlinear rapid evolution of irregularities would occur in the upward field-aligned current region. The spectral width of the backscatter blob is typically distributed between 50 and 300 m/s, but sometimes it is over 400 m/s. This suggests that the temporal broad spectra over 400 m/s are produced by Pc1–2 bursts, while the background spectral width of 50–300 m/s are produced by the velocity gradient structure of convection vortices themselves.Key words. Ionosphere (Electric fields and currents; Ionospheric irregularities; Plasma convection)

A dynamic two-phase flow model for a purification plant

A mathematical model for the flow through the siphon of a sewage purification machine is presented. The model is based on the average unidirectional flow velocities of the two phases (water and bubbles of air), the evaluation of hydrodynamic forces corresponding to the plant's geometry, and on estimates of the mechanical interaction of the two phases. Data from experiments with an industrial machine are used to identify the model parameters and a comparison of the model's trajectories with the experimental records is given.

Exploratory Flow-Duct Experiments on Combined-Flow Bed Configurations, and Some Implications for Interpreting Storm-Event Stratification

ABSTRACT Experiments were conducted to define equilibrium combined-flow bed configurations developed under a wide range of oscillatory (Uo) and unidirectional (Uu) velocity components, with a constant oscillation period of 8.5 sec and median grain size of 0.09 mm. In the range of flows studied (Uo, 0-0.80 m/sec; Uu, 0-0.25 m/sec) the bed phases are: no movement, small 2D ripples, small 3D ripples, large 3D ripples, and plane bed. Small 2D ripples are stable at low Uo and Uu; by increasing either velocity component the plan-view bed configuration becomes progressively less regular as small 3D ripples develop. Small 3D ripples are stable at low to moderate Uo and a wide r nge of Uu. Under purely oscillatory flow, as Uo increases beyond about 0.40 m/sec, large-scale, hummocky bed forms become stable. When a unidirectional component is applied, these bed forms quickly develop a downstream asymmetry and generally move in the direction of Uu. The dip angles of the lee flanks increase with increasing Uu, although they remain less than the angle of repose. At high oscillatory velocities (Uo, 0.70-0.80 m/sec), plane bed is the stable bed phase when Uu > 0.05 m/sec. Much hummocky cross-stratification shows a preferred dip direction of the coset laminae. Our experiments support the idea that such cross-stratification is produced by combined flow: the large 3D ripples produced at high oscillatory velocities and small to moderate unidirectional velocities would presumably generate what might be called anisotropic hummocky cross-stratification (if we had been able to aggrade the bed during the runs). This would be the case even in dominantly oscillatory flow; the experiments show that even a weak unidirectional current of several centimeters per second superimposed upon a strong oscillatory flow results in bed forms that move predominantly in a single direction and that leave sets of laminae that dip predominantly in that direction. In runs at large Uo, the purely oscillatory large-scale bed forms were quickly planed off as Uu was increased from zero, but a series of almost stationary gentle undulations on the bed surface persisted for indefinitely long times where the original topographic highs were situated, even at the highest unidirectional flow investigated, 0.25 m/sec. This suggests a possible origin for the gentle undulation observed in Cretaceous parallel-laminated shallow-marine sandstones described by Arnott (1987, 1988), with spacings up to a few meters and heights not much more than a centimeter.

On the threshold of transport of sand‐sized sediment under the combined influence of unidirectional and oscillatory flow

ABSTRACTWhile the transport of sediment by unidirectional currents or by oscillatory (wave‐induced) currents has been investigated, very little attention has been paid to the problem of the threshold of transport under the two mechanisms combined. Studies were carried out using cohesionless sand‐sized (mean diameters: 142, 363, 771 and 1134 μm) quartz grains in a unidirectional flume, within which an oscillatory carriage had been installed. The experiments were carried out under unidirectional velocities ranging between 0 and 27 cm s‐1, combined with simulated wave‐induced currents (at periods of 5 and 15 s) ranging from 0 to 35 cm s‐1. The threshold of transport was assessed, by visual observation, using the Yalin Criterion for unidirectional flow. This criterion permitted critical conditions to be estimated subjectively by observation of incipient transport from the flat beds. The results indicated the dependence of the critical threshold velocity combination on grain size and wave period. Thresholds tend to increase with increasing grain size or decreasing period. The grain‐size dependence is predictable from existing empirical relationships for the separate mechanisms; statistical fluctuations in near‐bed stress (bursting) however, are invoked to explain the wave period dependence. The latter effect acts in a reverse manner for wave and current combinations than for waves alone. Graphs are presented, relating grain size to critical threshold velocity combinations, to aid in the sedimentological interpretation of field data.

Carrier-mediated Transport of the Folic Acid Analogue, Methotrexate, in the L1210 Leukemia Cell

Abstract Transport of methotrexate (4-amino-10-methylpteroyl-glutamic acid), a structural analogue of folic acid, was studied in L1210 mouse leukemia cells grown in tissue culture. Intracellular methotrexate was present in two states. (a) There was an essentially nonexchangeable intracellular fraction probably bound to the enzyme dihydrofolate reductase. (b) The remaining intracellular methotrexate was rapidly exchangeable and likely present in the cell water in an osmotically active state. In addition, there was a small rapidly exchanging component of methotrexate associated with the cells which had the characteristics of a loose adsorption on or near to the cell membrane. Methotrexate was shown to be negligibly metabolized by this cell system. Because of the presence of tight intracellular binding sites for methotrexate, prior to saturation of these sites, free intracellular methotrexate does not appear and consequently efflux is negligible. This permits the accurate measurement of unidirectional uptake velocities. The major portion of the unidirectional influx process demonstrated Michaelis-Menten kinetics with a high degree of structural specificity, competitive inhibition by folic and folinic acids, and marked temperature dependence. Addition of these competing substances in excess to cells at the steady state with extracellular methotrexate leads to countertransport of intracellular methotrexate. Electrochemical potential gradients for free intracellular methotrexate were demonstrated at low levels of extracellular methotrexate and intracellular methotrexate concentrations at equilibrium exhibited Michaelis-Menten kinetics. Efflux of intracellular methotrexate was not first order and appeared to be partially saturated. The Michaelis constant and maximum velocity for the unidirectional efflux process were estimated, and when related to these parameters for the unidirectional influx, were compatible with an accumulating system. These studies indicate that methotrexate transport in the L1210 cell is carrier-mediated and uphill. Methotrexate exhibits several qualities which make it a valuable model substrate for the study of the transport characteristics of folic acid and its naturally occurring derivatives. The implication of these findings with respect to the development of more effective folic acid antagonists is considered.