Aspects of modelling sediment transport and night conditions in Lake Marken

In lake Marken (Markermeer), a shallow lake in the Netherlands, sediment transport and suspended solids concentration are dominated by wind induced resuspension of sediment. The suspended solids concentration in the lake determines the attenuation of light. A 2-dimensional sediment transport model, STRESS-2d, was used to estimate the special distribution of the silt content for periods with different wind conditions. The model was calibrated using measured values of the fall velocity distributions of sediment, freshly deposited material and suspended solids. The specific light attenuation coefficient of the various sediment classes was also measured and used in the light attenuation model, CLEAR, together with simulation results from the STRESS-2d model, to simulate the light attenuation variations in time and space. By comparison of the simulation results for different scenarios, the influence of regional planning features on the sediment balance and the light attenuation can quantified. A provisional example of this method is presented.     

Modelled influences of non-exchanging trichomes on leaf boundary layers and gas exchange

The two main resistances in the exchange of gases between plants and the atmosphere are stomatal and boundary layer resistances. We modeled boundary layer dynamics over glabrous and pubescent leaves (assuming non-exchanging trichomes) with leaf lengths varying from 0.01 to 0.2 m, and windspeeds of 0.1-5.0 m x s(-1). Results from theoretical and semi-empirical formulae were compared. As expected, boundary layer thickness decreased with decreasing leaf length and increasing windspeed. The presence of trichomes increased leaf surface roughness, resulting in lowered Reynolds numbers at which the boundary layer became turbulent. This effect is especially important at low windspeeds and over small leaves, where the Reynolds number over glabrous surfaces would be low. We derived a new simple dimensionless number, the trip factor, to distinguish field conditions that would lead to a turbulent boundary layer based on the influence of trichomes. Because modeled rates of CO2 and H2O(v) exchange over turbulent boundary layers are one or more orders of magnitude faster than over laminar boundary layers, a turbulent boundary layer may lead to increased carbon uptake by plants. The biological trade-off is potentially increased transpirational water loss. However, in understory habitats characterized by low windspeeds, even a few trichomes may increase turbulence in the boundary layer, thus facilitating photosynthetic gas exchange. Preliminary field data show that critical trip factors are exceeded for several plant species, both in understory and open habitats.     

Benthic Diatoms as Indicators of Stream Sediment Concentrationin Hong Kong

Diatoms are photosynthetic unicellular, eukaryotic, microorganisms (algae) that are distinguished by their silicified (SiO₂‐nH₂O) cell walls. They have often been employed to assess salinity, pH, and nutrient conditions. Our data suggest that, they may also be used to assess suspended solid levels in streams. The ratio of motile to attached benthic diatoms from five different stream sites in Hong Kong was correlated with the level of suspended solids for each of these sites. Sites with high concentrations of suspended solids had a high percentage of motile diatoms on their rocks while sites with low concentrations of suspended solids had a high percentage of attached (non‐motile) diatoms on their rocks. When water carried by a stream has a high concentration of suspended solids, benthic diatoms in the stream are often covered in a layer of silt. Those diatoms that are motile are able to get on top of this silt layer while attached diatoms cannot. If the silt layer is not washed away, the attached diatoms perish and the percentage of motile diatoms increases. Thus, streams with high concentrations of suspended solids display a higher proportion of motile diatoms than do clear water streams with low concentrations of suspended solids. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A model of constant-flow ventilation in a dog lung

A semiempirical model of constant-flow ventilation (CFV) is developed to test the hypothesis that a three-zone serial model with the following characteristics can explain the adequate CO2 transport observed during CFV: 1) a zone of jet recirculation immediately downstream of the catheter in which convection dominates; 2) a zone influenced by turbulence but with little or no bulk flow; and 3) a peripheral zone, free of turbulence, in which transport is governed by molecular and augmented diffusion. Interactions between turbulent eddies and cardiogenic oscillations are included using a modification of Taylor dispersion theory according to the formulation of Kamm et al. Predicted values for arterial PCO2 are reasonably similar to experimental results for He-O2, air, and SF6-O2 mixtures for catheter flow rates from 0.2 to 1.6 l/s. Specific impedance to gas exchange was found to be largest immediately proximal to the end of turbulent mixing zone, where transport is governed by low-level eddy mixing and molecular diffusion. Simulations suggest that, during CFV, cardiogenic oscillations augment gas exchange primarily by promoting turbulent eddy dispersion in the distal airways and by extending the length of the turbulent mixing zone. Even small displacements of the catheter are shown to have a dramatic effect on gas exchange.     

A semi-analytical model for dissolved oxygen mass transfer coefficient at the sediment–water interface

Relying on the fundamental principles of mass transport in a turbulent flow, we have developed a semi-analytical model for a mass transfer coefficient in a sediment water interface over a smooth bed. The governing equations with boundary conditions reflect a mechanism of shear dispersion in a turbulent flow. The model is formulated in terms of the Sherwood-Reynolds-Schmidt functional dependence. Unlike previous regression-type models, the mass transfer coefficient at the sediment-water interface is parameterized by the friction coefficient. Flow conditions over a smooth bed ranging in Reynolds number from 1800 to 7000 were used to verify the model. The predicted mass transfer coefficients are in very good agreement with the experimental data.     

Daily Fish and Zooplankton Abundances in the Littoral Zone of Lake Texoma,Oklahoma-Texas,in Relation to Abiotic Variables

Many studies have shown the effects of yearly or monthly environmental conditions on the structure of fish and zooplankton communities. Environmental conditions can also vary greatly on much shorter time scales. We tested the effects of abiotic conditions on the daily abundance of fish and zooplankton in the littoral zone of Lake Texoma, Oklahoma-Texas. After date was removed statistically from the analysis, no environmental variables were particularly important in determining the daily abundance of zooplankton, whereas, numbers of fish in the littoral zone were related to changes in wind velocity and wave height. Regressions of daily fish abundance against wave height showed that the response differed among species and among life-history intervals within species. Numbers of juvenile Dorosoma petenense, Notropis atherinoides, and Morone chrysops were positively correlated with wave height, whereas juvenile Menidia beryllina were negatively correlated with wave height. We suggest that changes in the abundance of particular species may be associated with (1) avoidance of inshore areas to escape possible physical damage by wave induced turbulence, (2) attraction to inshore areas to feed on prey organisms suspended in the water column by wave induced turbulence, and (3) avoidance of inshore areas to escape high predator abundance and increased possibility of being eaten due to turbulence.     

Wave and sediment dynamics along a shallow subtidal sandy beach inhabited by modern stromatolites

To help define the habitat of modern marine stromatolites, wave-dominated flow and sediment transport were studied in the shallow subtidal region (1-2 m depth) along the slightly concave, windward face of Highborne Cay, Exuma, Bahamas - the only face of the cay that includes a population of stromatolites concentrated near the region of highest curvature of the beach. Wave energy impacting this island's most exposed beach was driven by local wind forcing which increases largely in response to the passage of atmospheric disturbances that typically affect the region for periods of a few days. Although some wave energy is almost always noted (maximum horizontal orbital speeds at the bottom are rarely <10 cm s(-1)), wave conditions remain comparatively calm until local winds increase above speeds of approximately 3-4 m s(-1) at which point maximum wave speeds rapidly increase to 50-80 cm s(-1). Stromatolites, which are largely restricted to the shoreward side of a shallow platform reef, are sheltered by the reef beyond which wave speeds are one to four times higher (depending on tidal stage). Moreover, stromatolite populations are predominantly found along a region of this wave-exposed beach that experiences comparatively reduced wave energy because of the curved morphology of the island's face. Maximum wave speeds are 1.4 to 2 times higher along more northern sections of the beach just beyond the locus of stromatolite populations. A quantitative model of sediment transport was developed that accurately predicted accumulation of suspended sediment in sediment traps deployed in the shallow subtidal zone along this beach. This model, coupled with in situ wave records, indicates that gross rates of suspended sediment deposition should be two to three times higher northward of the main stromatolite populations. Regions of the beach containing stromatolites nevertheless should experience significant rates of gross suspended sediment deposition averaging 7-10 g cm(-2) day(-1) ( approximately 4-6 cm day(-1)). Results suggest that one axis of the habitat of modern marine stromatolites may be defined by a comparatively narrow range of flow energy and sediment transport conditions.     

Modelling sediment transport in shallow lakes — interactions between sediment transport and sediment composition

In shallow, wind exposed lakes, the light conditions, the cycling of nutrients, heavy metals and organic micro-pollutants and changes in the local composition of the sediment top layer can be dominated by resuspension/erosion of bottom sediment and sedimentation of suspended solids. A 2 dimensional model for Sediment Transport, Resuspension and Sedimentation in Shallow lakes (STRESS-2d), based on an existing transport model, is discussed. In the model, mass balance equations for the water compartment and the bottom sediment are solved numerically. Up to 7 sediment fractions can be taken into account, each having a specific set of resuspension/erosion and sedimentation parameter values. Several options for modelling the changes in the bottom sediment composition are available.A simulation experiment for Lake Veluwe (The Netherlands), in which model options with and without the distinction of sediment fractions were used, showed that using sediment fractions to account for the variability in the sediment composition leads to an improvement of the model results, particularly the simulated phosphorus sediment-water exchange fluxes. For Lake Ketel (The Netherlands) two options for modelling changes in the bottom sediment composition are compared. It is shown that an option in which a thin water-sediment layer on top of the more consolidated bottom sediment is simulated provides an improvement in the simulation of the suspended solids concentration.     

The nature of the fluid boundary layer and the selection of parameters for benthic ecology

1. The nature of flow structure close to a river bed is reviewed and suggestions made as to appropriate equations to apply to given hydrodynamic regimes. 2. It is emphasized that in natural rivers the laminar sublayer observed immediately above the bed in some experimental studies is completely disrupted for flows characterized by high turbulence levels. 3. Instead of resorting to direct topographical measurement of bed undulations, the roughness of the river bed also can be quantified using hydraulic data obtained from velocity profiles. 4. Ambuhl's experimental findings of 1959 underpin modern ecological research into the nature of the benthic boundary layer. Common misconceptions concerning Ambuhl's contribution are corrected and it is shown that his results only apply to certain prescribed hydrodynamic conditions. 5. The adoption of a consistent approach to describing the benthic boundary layer is to be preferred, so that diverse studies can be usefully compared.     

Kinetics of methanogenic degradation of phenol by activated-carbon-supported and granular biomass

A continuous-feed recycle bioreactor was used to study the kinetics of methanogenic degradation of phenol at 35 degrees C by bacteria supported on a bed of granular activated carbon (GAC). At dilution rates well above the growth rate of the culture, the cells not only populated the GAC, but also formed a layer of granular biomass. This layer was stabilized by the presence of the GAC, and accounted for over half of the phenol-degrading activity in the bioreactor. The specific phenol degradation rates for GAC-attached biomass, suspended biomass, and granular biomass were all in the range 0.15 to 0.22 mg phenol/mg volatile solids per day as measured under pseudo-steady-state conditions. (c) 1992 John Wiley & Sons, Inc.     

Effects of hydrodynamics on phosphorus concentrations in water of Lake Taihu,a large,shallow, eutrophic lake of China

To understand the effect of hydrodynamical process on water phosphorus concentration, wind, wave, and several water quality indices were observed in Meiliang Bay, a shallow and eutrophic bay locates in north of Lake Taihu. During the 7 day observation period, wind speed and significant wave height were recorded more than 3 h per day, and water samples were collected in five water-depth layers once a day. Hydrodynamical disturbance had no significant correlationship with the water quality at the top layer when the significant wave height was smaller than 30 cm, but it significantly increased suspended solids (SS) concentration of the bottom water layer. Concentrations of nutrients showed no positive correlationship with SS concentration in the water body. Intensive sediment resuspension may not have occurred when the hydrodynamic stress on sediment was only a little higher than the critical stress for sediment resuspension. A new method for confirming the critical stress for intensive sediment resuspension and nutrient release still needs to be developed. The range of the water quality indices was quite high during the seven days of observation. High variation seems to be a common character of large shallow lakes like Taihu.     

Sediment transport in an inland river in North Queensland

Rivers in northern Queensland are ephemeral and carry water mainly as a direct response to heavy rainfall. Sediment is transported downstream with the runoff and sediment deposition may be a major problem in many proposed reservoirs. Hence information about sediment transport, particularly under high flow conditions, is required for planning and design of water storage reservoirs. In this region, bed material samples can be obtained during low flow periods and suspended sediment sampling during floods is possible but only with difficulty. Little reliable data is available.This paper outlines a possible approach to predicting sediment loads in such rivers. Suspended sediment samples have been analysed to give both particulate concentrations and their grain size distributions. The latter have been compared with bed material size distributions, and the concentrations of suspended bed material and wash load components have been estimated.After investigations of a number of methods for predicting bed material transport, those which treat bed load and suspended load independently have been selected. Field data have been used to determine the wash load and the suspended bed material load. The bed load was then computed so that the total sediment load could be determined.This approach has been applied to the Flinders River at Glendower, based on field data obtained by the Queensland Water Resources Commission in 1982/83.     

Interpretation of sedimentation data measured in a former tidal channel Lake Volkerak

In Lake Volkerak, situated in the southwest of the Netherlands, downward settling fluxes are related to external inputs of suspended solids and wind action. The settling fluxes, measured using sediment traps, were 55 g (dw) m ^–2 d ^–1 on average. The ratio of metal concentration to scandium concentration was used to discriminate between external (polluted) suspended solids and internal (relatively clean) suspended solids. Generally, the contribution of the river suspended solids was small compared to that of resuspended material; the river-transported material was mainly deposited in the centre and to the east of the lake. The amount of material trapped increased substantially with increasing wind velocity.A simple model was used to interpretate the data. This model does not have a predictive capacity, but can be used to interpret and assess the significance of material retained in the sediment traps. Erosion was related to the wind velocity, using an empirical relationship between the orbital velocity of the wind-generated waves at the bottom and the wind velocity. The critical wind velocity for erosion to occur was estimated to be 5.5 m s^–1. The extremely high amounts retained in the sediment traps in shallow areas during storms emphasised the importance of these wind conditions for the transport of fine sediments.     

The effect of a turbulent jet on gas transport during oscillatory flow

Axial mass transport due to the combined effects of flow oscillation and a turbulent jet was studied both experimentally and with a simple theoretical model. The experiments show that the distance over which turbulence enhances transport is greatly increased by flow oscillation, and is particularly sensitive to tidal volume. The jet flow rate and jet configuration are relatively less important. To analyze the results, the region influenced by the jet is divided into two zones: a near field in which the time-mean flow velocities are larger than the turbulent fluctuations, and a far field where the time-mean flow is essentially zero. In the far field, axial mass transport is increased due to the turbulence which decays in strength away from the jet. When oscillatory flow is superimposed upon the steady jet flow, the turbulence in the far field interacts with the flow oscillations to augment the transport of turbulence energy and of mass. This transport enhancement is modeled by introducing an effective axial diffusivity analogous to that used in laminar oscillatory flow.     

Effects of near‐bed turbulence on the suspension and settlement of freshwater dreissenid mussel larvae

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A simulation of the process of sedimentation of suspended solids in the Yoshii River estuary

A vertical plane 2D model has been applied to the Yoshii River estuary, Japan, as a means of simulating the process of sedimentation. The current velocity and isohaline distributions were well reproduced in the estuary and the model simulated the concentration distribution of suspended solids for 8 particle size classes by using a transport equation based only on physical processes. The changes of the particle size distribution in surface water were well reproduced by the model. The vertical velocity component plays an important role in the behaviour of suspended solids.     

A simple theory of algal deposition

SUMMARY. A model describing the deposition of algae in a turbulent environment is outlined, it being assumed that all algae falling on the bed are trapped there forever. Its main features are that, above a certain threshold of turbulence the rate of deposition is independent of the turbulent intensity, and the rate of algal loss by sedimentation, in such conditions, is described by a first order kinetic equation. The hydraulic conditions under which algae settle at their intrinsic, still-water rate and those for which the model is applicable, are examined. Some ecological implications of the model are discussed.     

The turbidity maximum in the Tamar estuary

Many estuaries of medium to high tidal range exhibit an accumulation of fine cohesive material in their upper reaches in the region of the limit of saline intrusion. Much, or all, of this material is suspended each tidal cycle and the entire region undergoes a seasonal variation which appears to depend on fluvial input. Two factors which are throught to influence the formation and maintenance of turbidity maxima are the differing magnitudes of the bed shear stress (τ₀) on flood and ebb tides and the large vertical density gradient which developes on the ebb tide. Crucial to the importance of the first factor is that τ₀ exceeds a critical value, at which erosion occurs, for a greater period on the flood than on the ebb. The effect of the density gradient is that upward propagation of bed generated eddies is inhibited and the sediment is not transported into the upper part of the flow where it will be most effectively transported. It is not clear which, if either, of these mechanisms is dominant. Data consisting of vertical profiles of velocity, salinity and suspended solids were collected at four stations in the Tamar estuary during a high range tidal cycle. One station, at which the depth mean salinity (S _d ) varied from 0.0 to ∼ 12.0‰, was occupied permanently. The other stations were occupied such that data were collected asS _d varied in the range 0.0 to ∼ 4.0‰. In this way each station was occupied for a period of time on the ebb and flood tide. Observations show that during the early ebb, when the flow is relatively deep and slow, stratification persist untilS _d ∼ 0.0‰ and that no significant transport occurs while the flow is saline but that there is a rapid increase in suspended solids concentrations after this time. During the later ebb the shallower faster flow allowed the density gradient to be erode and significant transport was observed atS _d ∼ 5.0‰. On the flood tide the flow in the low salinity region is well-mixed troughout. Computation of the fluxes and total transport per unit breath of estuary show that on the ebb tide the quantity of solid material being transported by the low salinity (0–3‰) region remains nearly constant as this region of the flow is advebted seaward. On the flood tide, however, as the same region is advected landward the quantity of material being transported increases. It is concluded that in the Tamar estuary the early ebb tide stratification contributed to the formation and maintenance of a turbidity maximum which is strongly associated with the low salinity region of the flow. It is also speculated that the differences in the ebb and flood tide transport are caused by differences in the availability of mobile material on the bed at different stages of the tidal cycle.