Showing papers on "Water flow published in 1995"

HEC-RAS River Analysis System. Hydraulic Reference Manual. Version 1.0.

Abstract: : Welcome to the Hydrologic Engineering Center's River Analysis System (HEC-RAS). This software allows you to perform one-dimensional steady flow, unsteady flow, and sediment transport calculations. The current version of HEC-RAS only supports one-dimensional, steady flow, water surface profile calculations. This manual specifically documents the hydraulic capabilities of the Steady flow portion of HEC-RAS. Documentation for unsteady flow and sediment transport calculations will be made available as these features are added to the HEC-RAS. This chapter discusses the general philosophy of HEC-RAS and gives you a brief overview of the hydraulic capabilities of the modeling system. Documentation for HEC-RAS is discussed, as well as an overview of this manual.

A coupled atmosphere‐ocean model for transient climate change studies

Abstract: A new coupled atmosphere‐ocean model has been developed for climate predictions at decade to century scales. The atmospheric model is similar to that of Hansen et al. (1983) except that the atmospheric dynamic equations for mass and momentum are solved using Arakawa and Lamb's (1977) C grid scheme and the advection of potential enthalpy and water vapour uses the linear upstream scheme (Russell and Lerner, 1981). The new global ocean model conserves mass, allows for divergent flow, has a free surface and uses the linear upstream scheme for the advection of potential enthalpy and salt. Both models run at 4° × 5° resolution, with 9 vertical layers for the atmosphere and 13 layers for the ocean. Twelve straits are included, allowing for subgrid‐scale water flow. Runoff from land is routed into appropriate ocean basins. Atmospheric and oceanic surface fluxes are of opposite sign and are applied synchronously. Flux adjustments are not used. Except for partial strength alternating binomial filters (Shap...

Nature's Pulsing Paradigm

Abstract: While the steady state is often seen as the final result of development in nature, a more realistic concept may be that nature pulses regularly to make a pulsing steady stata—a new paradigm gaining acceptance in ecology and many other fields. In this paper we compare tidal salt marshes, tidal freshwater marshes, and seasonally flooded fresh-water wetlands as examples of pulsed ecosystems. Despite marked differences in species composition, biodiversity, and community structure, these wetland types are functionally similar because of the common denominator of water flow pulses. Often a period of high production alternates with a period of rapid consumption in these fluctuating water-level systems, a biotic pulsing to which many life histories, such as that of the wood stork, are adapted. Pulsing of medium frequency and amplitude often provides an energy subsidy for the community thus enhancing its productivity. The energy of large-scale pulses such as storms are usually dissipated in natural ecosystems with little harm to the biotic network; however, when seawalls, dikes, or stabilized sand dunes are constructed to confront these strong pulses, the whole ecosystem (and associated human structures) may be severly damaged when the barriers fail because too much of the storm energy is concentrated on them. The relationship between biologically mediated internal pulsing, such as plant-herbivore or predator-prey cycles, and physical external pulsing is discussed not only in wetlands but in other ecosystem types as well. An intriguing hypothesis is that ecosystem performance and species survival are enhanced when external and internal pulses are coupled. We suggest that if pulsing is general, then what is sustainable in ecosystems, is a repeating oscillation that is often poised on the edge of chaos.

Tracer Characteristics of Brilliant Blue FCF

Abstract: Dyes are widely used to stain the travel paths of water and solutes in soils. Brilliant Blue FCF (C.I. Food Blue 2) is a valuable dye tracer for visualizing water flow patterns. Because this dye is nontoxic, it is particularly suited for field use. Depending on pH, the dye is either neutral or dissociates to a mono- or bivalent anion. The water solubility is 200 kg m -3 . In this study, properties of the dye that characterize the adsorbing behavior in soil were determined. Batch adsorption experiments were carried out in the laboratory, and a field experiment was conducted to compare the mobility of Brilliant Blue FCF with that of I - and Br - . The batch studies gave distribution coefficients K d of 0.19, 3.00, and 5.78 dm 3 kg -1 for three different soils. The results of the batch studies suggest that the dye forms ion pairs with Ca 2+ . The field experiment showed that the dye is retarded compared with the conservative tracer Br - . A comparison between Brilliant Blue FCF and I - analyzed with the iodide-starch method resulted in a relative retardation of 1.2 Brilliant Blue FCF compared with I -

Effects of Mercuric Chloride on the Hydraulic Conductivity of Tomato Root Systems (Evidence for a Channel-Mediated Water Pathway)

Abstract: A pressure-flux approach was used to evaluate the effects of HgCl2 on water transport in tomato (Lycopersicon esculentum) roots. Addition of HgCl2 to a root-bathing solution caused a large and rapid reduction in pressure-induced root water flux; the inhibition was largely reversible upon addition of [beta]-mercaptoethanol. Root system hydraulic conductivity was reduced by 57%. There was no difference between treatments in the K+ concentration in xylem exudate. The results are consistent with the presence of a protein-mediated path for transmembrane water flow in tomato roots.

Computer simulation of a silica gel-water adsorption refrigeration cycle - the influence of operating conditions on cooling output and COP

Abstract: This work deals with the use of adsorption refrigerant cycles driven by waste heat of near ambient temperature. A parametric study was conducted by computer simulation to determine the effects of operating conditions on cooling output and coefficient of performance (COP). A simulation program verified the influence of operating temperatures (hot and cooling water), water flow rates, and adsorption-desorption cycle times. The most influential parameter is the operating temperatures, followed by water flow rates. Cycle time is less influential in quantitative terms but qualitatively is very important.

Design and placement of a multi-species riparian buffer strip system

Abstract: A multi-species riparian buffer strip (MSRBS) system was designed and placed along a Central Iowa stream in 1990. Bear Creek, is typical of many streams in Central Iowa where the primary land use along the stream's length is row crop (corn and soybeans) production agriculture or intensive riparian zone grazing. The Bear Creek watershed is long (∼ 35 km), narrow (3–6 km), and drains 7,661 ha of farmland. The MSRBS system is a 20 m wide filter strip consisting of four or five rows of fast-growing trees planted closest to the stream, then two shrub rows, and finally a 7 m wide strip of switchgrass established next to the agricultural fields. The 1.0 km long system, is located on an operational farm and is laid out in a split block design on both sides of Bear Creek. An integral part of this system is a streambank stabilization soil bioengineering component and a constructed wetland to intercept NPS pollutants in field drainage tile water flow. It is hypothesized that this system will function effectively as a nutrient, pesticide, and sediment sink for NPS pollutants coming from the upslope agricultural fields. Prior to establishment of the MSRBS system, the riparian zone along Bear Creek was grazed and row cropped to the stream edge. Since 1990 there has been dramatic alteration in the appearance and functioning of this riparian zone. After four growing seasons, the fast-growing tree species (cottonwood, silver maple, willow, and green ash) range in height from 2.4 m to over 5.5 m. Mean (four-year) biomass production of silver maple was 8.4 dry Mg ha−1, more than twice to seven times the yield from other silver maple research plots in Central Iowa. The shrub species, selected because of desired wildlife benefits, have done well in terms of survival and growth with ninebark, Nannyberry viburnum and Nanking cherry doing the best. The switchgrass grass has developed into a dense stand that effectively stops concentrated flow from the agriculture fields and allows for infiltration rates well above the field rate. Early root biomass data indicate significantly more roots below the MSRBS than agricultural fields. This suggests better soil stabilization, absorption of infiltrated water, and soil-root-microbe-NPS pollutant interaction characteristics within the MSRBS system than the cropped fields. Nitrate-nitrogen concentrations in the MSRBS never exceed 2 mg l−1 whereas the levels in the adjacent agricultural fields exceed 12 mg l−1. The water quality data collected suggest that the MSRBS is effective in reducing NPS pollutants in the vadose and saturated zone below the system. The soil bioengineering revetments have stabilized the streambank and minimized bank collapse. Initial results (from 4 months of operation) from the constructed wetland (built in summer 1994) indicate nitrate-nitrogen concentrations of the tile inflow water >15 mg l−1 whereas, the outflow water had a nitrate-nitrogen concentration of <3 mg l−1. Over time this wetland should become more effective in removing excess nitrogen moving with the tile flow from the agricultural fields because of the accumulation of organic matter from the cattails. Overall the MSRBS system seems to be functioning as expected. This MSRBS system offers farmers a way to intercept eroding soil, trap and transform NPS pollution, stabilize streambanks, provide wildlife habitat, produce biomass for on-farm use, produce high-quality hardwood in the future, and enhance the aesthetics of the agroecosystem. As a streamside best management practice (BMP), the MSRBS system complements upland BMPs and provides many valuable private and public market and non-market benefits.

Assessing temporal variations in soil water composition with time domain reflectometry

Abstract: Time domain reflectometry (TDR) can be used to study temporal variations in volumetric soil water content (θ) and bulk soil electrical conductivity (σ a ). The variations in σ a are associated with changes in θ and the soil water composition. Laboratory and field experiments were conducted to verify if TDR can be used to monitor the temporal variation in the soil water composition between solution sampling occasions. Effects of cable length and temperature on the σ a measurement were evaluated. Including the series resistance of the cable and connectors in the analysis improves measurements at high electrical conductivity levels. The temperature factor of the bulk soil appears to be similar to the temperature factor of soil extracts. Laboratory experiments showed that the theoretical model giving σ a as function of θ and the electrical conductivity of the soil solution (σ w ) combined with the water retention function was capable of describing σ w measured on soil solution extracted with ceramic cup solution samplers under static water flow conditions. After optimization of a single parameter, the model was able to describe σ w values of the soil solution obtained in the laboratory, whereas literature values were sufficient for field data. Concentrations of a number of solutes in a field data set spanning 3 yr were positively correlated with σ w . Site-specific regressions between solute concentration and σ w combined with automated TDR measurements of σ a and θ enable a more meaningful interpretation of the temporal variation of the concentration of major solutes present in the soil solution between sampling occasions.

Ground‐penetrating radar and coring used to study the large‐scale structure of point‐bar deposits in three dimensions

Abstract: Detailed three-dimensional (3-D) observations of sandy point-bar deposits from the River South Esk in Scotland were made using very closely spaced (metres) vibracores and ground-penetrating radar (GPR) profiles. In order to explain the origin of the observed patterns of deposition, use was made of previous studies of channel geometry, flow and sediment transport. In addition, the mode and nature of channel migration and point-bar accretion were determined using published maps, aerial photographs and detailed topographic surveys. Point-bar deposits accumulated in response to channel-bend expansion and downstream migration, resulting in preservation of sequences that fine upwards and downstream. Lower-bar deposits are mainly very-coarse to coarse sands with medium-scale trough cross-strata overlying basal gravels: associated radar facies are generally low-amplitude, relatively discontinuous inclined reflectors. Upper-bar deposits are mainly fine to medium sands with medium- and small-scale cross-strata and vegetation-rich layers: associated radar facies are generally moderate- to high-amplitude, laterally continuous, inclined reflectors. Large-scale inclined stratasets seen in GPR profiles resulted from episodic point-bar accretion. Abrupt lateral changes in inclination of these stratasets, and preservation of distinct unit bars (bar heads, scroll bars), lower-bar platforms and inner-bank swale fills, record discrete episodes of erosion and deposition associated with floods with recurrence intervals of decades to centuries. Such detailed 3-D description and interpretation of these large-scale features of point-bar deposits was only possible through the use of GPR profiles tied closely to cores, and through the availability of much previously collected information on channel geometry, water flow, sediment transport, erosion and deposition.

Near-Saturated Hydraulic Conductivity in Soils of Contrasting Texture Measured by Tension Infiltrometers

Abstract: Soil structure is known to strongly affect water flow and solute transport, yet little information is available concerning soil hydraulic properties close to saturation. This study reports detailed measurements of near-saturated hydraulic conductivity in undisturbed field soils. Steady-state infiltration rates from tension infiltrometers were measured at the surface of six tilled soils of contrasting texture ranging from loamy sand to silty clay. Hydraulic conductivities (K) at supply pressure heads (h) in the range from -100 mm to zero were obtained using Wooding's solution For infiltration from a circular source. The paired K,h data were summarized using the technique of functional normalization with three alternative models of K(h): a single exponential function (Gardner's model), a two-line exponential model, and the Mualem-van Genuchten model. The commonly used single exponential model was found to be inappropriate in all cases. Both the two-line exponential and Mualem-van Genuchten models satisfactorily described the data, with the former performing marginally better for five of the six soils. In the supply pressure head range -100 mm to zero, K increased by three to four orders of magnitude in the finer textured soils and by about two orders of magnitude in two sandy soils. Estimates of field-saturated hydraulic conductivity (K fs ) were largest in the fine-textured soils, presumably because of the influence of continuous surface-vented macropores. Spatial variability in measured K(h) was small to moderate for all soils. Predicting unsaturated K from soil water release data using K fs as a matching point was shown to result in serious overestimations

Model for Instantaneous Residential Water Demands

Abstract: Residential water use is visualized as a customer-server interaction often encountered in queueing theory. Individual customers are assumed to arrive according to a nonhomogeneous Poisson process, then engage water servers fro random lengths of time. Busy servers are assumed to draw water at steady but random rates from the distribution system. These conditions give rise to a time-dependent Markovian queueing system having servers that deliver random rectangular pulses of water. Expressions are derived for the mean, variance, and probability distribution of the flow rate and the corresponding pipe Reynolds number at points along a dead-end trunk line. Comparison against computer-simulated results shows that the queueing model provides an excellent description of the temporal and spatial variations of the flow regime through a dead-end trunk line supplying water to a block of heterogeneous homes. The behavior of the local flow field given by the queueing model can be coupled with water-quality models that require ultra-fine temporal and spatial resolutions to predict the fate of contaminants moving through municipal distribution systems.

Soil temperature and water content beneath a surface fire

Abstract: We report here the results of laboratory and computer simulations designed to supply information on soil temperatures under forest and range fires. Measurements of temperature and water content in a soil column that was heated strongly at the surface showed a consistent pattern of warming and drying. In initially wet soil, temperature rose to around 95°C and remained there until the water content of the soil at that depth dropped below about 0.02 m 3 m -3 . When the soil was initially dry, the temperature increased more rapidly, but even the moisture present in air-dry soil was sufficient to slow the rate of temperature rise when temperatures reached 90°C. A linked-transport model, which simultaneously computes changes in temperature and water content, simulated the main features of heat and water flow in a soil column heated to high temperature. There were no consistent deviations of measured from modeled temperatures, but the water content simulations consistently showed a greater buildup of moisture ahead of the heating front than did the measurements, and less drying of the soil in the heated layers when the initial soil water content was low. Soils from sand to clay, and with differing mineralogies, water contents, and bulk densities were used to compare measurements and simulations. The model performed well in all cases. Since the temperature simulations are reasonable, the model appears suitable for predicting fire effects in the field.

Odor Plumes and Animal Navigation in Turbulent Water Flow: A Field Study.

Abstract: Turbulence causes chemical stimuli to be highly variable in time and space; hence the study of animal orientation in odor plumes presents a formidable challenge. Through combined chemical and physical measurements, we characterized the transport of attractant released by clam prey in a turbulent aquatic environment. Concurrently, we quantified the locomotory responses of predatory crabs successfully searching for sources of clam attractant. Our results demonstrate that both rheotaxis and chemotaxis are necessary for successful orientation. Perception of chemical cues causes crabs to move in the upstream direction, but feedback from attractant distributions directly regulates movement across-stream in the plume. Orientation mechanisms used by crabs difler from those employed by flying insects, the only other system in which navigation relative to odor plumes has been coupled with fluid dynamics. Insects respond to odors by moving upstream, but they do not use chemical distributions to determine across-stream direction, whereas crabs do. Turbulent eddy diffusivities in crab habitats are 100 to 1000 times lower than those of terrestrial grasslands and forests occupied by insects. Insects must respond to plumes consisting of highly dispersed, tiny filaments or parcels of odor. Crabs rely more heavily on spatial aspects of chemical stimulus distributions because their fluid dynamic environment creates a more stable plume structure, thus permitting chemotaxis.

Waste-Water Minimization with Flow-Rate Constraints

Abstract: This paper addresses water and wastewater minimization in the process industries through re-use in situations where there are fixed flowrate constraints. Water losses from operations and situations with multiple sources of freshwater with different qualities are also considered. Previously published targeting methods have been substantially improved to take these constraints into account. The insights gained through targeting have allowed new design rules to be developed which allow flowrate constraints to be met. Local recycling or splitting of water-using operations can be used as alternative ways to meet flowrate constraints. The design method allows novel water flow schemes to be developed based on local recycling and splitting of operations.

Water Recharge and Solute Transport Through the Vadose Zone of Fractured Chalk Under Desert Conditions

Abstract: This study focuses on water flow and solute migration through unsaturated fractured chalk in an arid area. The chalk underlies a major industrial complex in the northern Negev desert, where groundwater contamination has been observed. Four dry-drilling holes were bored through the vadose zone. Core and auger samples, collected at 30- to 50-cm intervals, were used for chemical and isotopic analyses, enabling the construction of the following profiles: (1) a tritium profile, to estimate the rate of water flow through the unsaturated zone; (2) oxygen 18 and deuterium profiles, to assess the evaporation of water at land surface before percolation, and in the upper part of the vadose zone after infiltration; and (3) chloride and bromide profiles, as tracers for inert solutes and pollutants. The tritium and bromide profiles showed the rate of infiltration through the unsaturated matrix to be very slow (1.6–11 cm/yr). The chemical and isotopic data from the core holes suggested that the pore water changes characteristics with depth. Close to land surface, the pore water is strongly evaporated (δ18O = +5.94‰) and highly concentrated (∼29 meq Cl/100 g rock), but changes gradually with depth to amore dilute concentration (∼4 meq Cl/100 g rock) and isotopically depleted composition (δ18O = −4.4‰), closer to the isotopic composition of precipitation and groundwater. Nearby monitoring wells have shown anthropogenic contribution of heavy metals, organic compounds, and tritium (Nativ and Nissim, 1992). A conceptual model is proposed in which a small portion of the rainwater percolates downward through the matrix, while a larger percentage of the percolating water moves through preferential pathways in fractures. The water flowing through the fractures penetrates the matrix across the fracture walls, where it increases the tritium concentrations, depletes the stable isotopic composition, and dilutes the salt concentrations. The observed rapid downward migration of tritium and heavy metals through the profuse fractures makes the chalk inefficient as a hydrologic barrier.

Estimating volume flow rates through xylem conduits

Abstract: We discuss the errors in common approximations of the volume flow rate for laminar flow through conduits with noncircular transverse sections Before calculating flow rates, ideal geometric shapes are chosen to represent the noncircular transverse sections The Hagen-Poiseuille equation used with hydraulic diameter underestimates the volume flow rate for laminar flow through conduits even with such ideal shapes Correction factors that have been proposed for the Hagen-Poiseuille equation also lead to underestimates of the volume flow rate for those shapes The exact solutions are sometimes difficult to attain, but rates calculated using the exact solutions for the ideal shapes may be as much as five times higher than the approximated rates for common transversely elongated shapes Either the exact solutions or the approximations may be used to calculate volume flow rates through the xylem of plants Both of these methods actually approximate flow through the original conduits because the shapes used are approximations of the conduits' transverse sections We recommend using the exact solutions whenever possible; they should be closer to the real solution than other approximations We give tables of correction factors for use in the cases where calculating volume flow rate from the approximate solution, the Hagen-Poiseuille equation, is more feasible Obtaining theoretical volume flow rates that are larger than previously thought highlights the need to clarify the causes of differences between the theoretical rates and the smaller measured volume flow rates in plant xylem As part of an ongoing project to understand the dynamics of water flow through the xylem we discuss exact equations to calculate water flow through noncircular ideal capillaries and investigate the errors in frequently used approximations In determining theoretical volume flow rates through the xylem of plants, the Hagen-Poiseuille equation for laminar flow through conduits is often used The Hagen-Poiseuille equation provides the exact solution for laminar flow in individual ideal capillaries with circular transverse sections, but the application of this equation to conduits with noncircular transverse sections using the hydraulic diameter results in errors in the calculated volume flow rates Exact solutions for the determination of volume flow rate through capillaries with some common noncircular transverse sections have recently been published in English (White, 1991) In general, errors in calculated flow rates may result from anatomical deviations of the xylem conduits from ideal capillaries, or from methods used to calculate the volume flow rate This paper investigates the latter more completely than previous botanical literature (eg, Leyton, 1975; Nonweiler, 1975; Petty, 1978; Pickard, 1981; Zimmermann, 1983; Calkin, Gibson, and Nobel, 1986; Lewis, 1992), discusses some exact solutions for volume flow rates through some common noncircular conduits, and recommends methods for the calculation of theoretical volume flow rates based on desired accuracy, speed, and ease of calculation

Reversible closing of water channels in Chara internodes provides evidence for a composite transport model of the plasma membrane

Abstract: Treatment of internodal cells of Chara corallina with the water channel blocker HgCl 2 caused a decrease of the hydraulic conductivity of the membrane (Lp) by a factor of three to four. In the presence of (practically) non-permeating solutes such as sugars or salts, the osmotic responses were similar to those found in controls, i.e. reflection coefficients ( s ) were close to unity. However, when treating the internodes with osmotic solutions of rapidly permeating lipophilic substances such as low molecular weight alcohols or acetone, the pirture of biphasic pressure relaxations due to the exchange of both water and solutes changed considerably. In the presence of HgCl 2 , reflection coefficients were substantially reduced and even became negative for some solutes (anomalous osmosis). Different from reflection coefficients, permeability coefficients (P s ) remained constant upon treatment. When using heavy water (HDO) as a small hydrophilic solute which should cross the membrane largely via water channels, results were different: the reflection coefficient of HDO increased and permeability decreased. Treating the cells with 5 mM 2-mercaptoethanol to remove the mercury from transport proteins (water channels) reversed changes in Lp and s . The results disagree with conventional membrane models, which imply a homogeneous membrane. They are readily explained by a composite model of the membrane in which proteinaceous arrays with specific water channels are arranged in parallel with lipid arrays. The latter account for most of the permeability of the small organic test solutes used and are not affected by the channel blocker. From the data, detailed information is obtained about transport properties of the arrays (water channels).

Effect of root systems on preferential flow in swelling soil

Abstract: Permeability problems on irrigated soils may be alleviated by root systems that increase water flow by creating macropores. Infiltration rates have been shown to increase where plant roots decay and serve as preferential flow paths. For low‐organic‐matter swelling soil, there is a question whether macropores are able to resist the lateral swelling forces of the soil. The objective of this study was to observe preferential water flow paths in a swelling soil under two cropping systems. A Holtville silty clay (clayey‐over‐loamy, montmorillonitic Typic Torrifluvent) was observed in situ. Two crops, alfalfa (Medicago sativa, L.) and wheat (Triticum turgidum, L.) provided sharply contrasting root systems, with wheat possessing fine, fibrous roots; alfalfa on the other hand, has a taproot system. Macropores were observed after applying soil‐adsorbing methylene blue dye to irrigation water. Shrinkage cracks failed to conduct dye after 10 minutes into a flood irrigation. Earthworm (Lubricus terrestris) c...

Alluvial Channel Geometry: Theory and Applications

Abstract: The downstream hydraulic geometry of alluvial channels, in terms of bank-full width, average flow depth, mean flow velocity, and friction slope, is examined from a three-dimensional stability analysis of noncohesive particles under two-dimensional flows. Four governing equations (flow rate, resistance to flow, secondary flow, and particle mobility) are solved to analytically define the downstream hydraulic geometry of noncohesive alluvial channels as a function of water discharge, sediment size, Shields number, and streamline deviation angle. The exponents of hydraulic geometry relationships change with relative submergence. Four exponent diagrams illustrate the good agreement with several empirical regime equations found in the literature. The analytical formulations were tested with a comprehensive data set consisting of 835 field channels and 45 laboratory channels. The data set covers a wide range of flow conditions from meandering to braided, sand-bed and gravel-bed rivers with flow depths and channe...

Dispersal dynamics in a wind-driven benthic system

Abstract: Bedload and water column traps were used with simultaneous wind and water velocity measurements to study postlarval macrofaunal dispersal dynamics in Manukau Harbour, New Zealand. A 12-fold range in mean wind condition resulted in large differences in water flow (12-fold), sediment flux (285-fold), and trap collection of total number of individuals (95-fold), number of the dominant infaunal organism (84-fold for the bivalve Macomona liliana), and number of species (4-fold). There were very strong, positive relationships among wind condition, water velocity, sediment flux, and postlarval dispersal, especially in the bedload. Local density in the ambient sediment was not a good predictor of dispersal. Results indicate that postlarval dispersal may influence benthic abundance patterns over a range of spatial scales.

Ion exchange membrane fuel cell power plant with water management pressure differentials

Abstract: A proton exchange membrane fuel cell device with an internal water management and transfer system includes a plurality of adjacently arranged proton exchange membrane assemblies including a proton exchange membrane component; a pair of porous anode and cathode catalyst layers situated on either side of the proton exchange membrane; and porous plate assemblies interposed between and in contact with each of the adjacent proton exchange membrane assemblies. Oxidant gas is supplied to oxidant gas supply channels, and fuel gas to fuel gas supply channels formed in the porous plate assemblies for distribution to the cathode and anode catalyst layers, respectively. A water coolant circulating system is formed in each of the porous plate assemblies and causes each of the porous plate assemblies to become saturated with coolant water. The reactant flow fields are pressurized to a pressure which exceeds the coolant water circulating pressure by a selected ΔP so as to ensure that product water formed on the cathode side of each membrane assembly will be pumped through the porous plates into the coolant water flow field and become entrained in the circulating coolant water stream.

Experimental investigation of solute transport in large, homogeneous and heterogeneous, saturated soil columns

Abstract: Laboratory tracer experiments were conducted to investigate solute transport in 12.5-m long, horizontally placed soil columns during steady saturated water flow . Two columns having cross-sectional areas of 10 x IOcm' were used: a uniformly packed homogeneous sandy column and a heterogeneous column containing layered, mixed, and lenticular formations of various shapes and sizes. The heterogeneous soil column gradually changed, on average, from coarse-textured at one end to fine-textured at the other end . NaCl breakthrough curves (BTC's) in the columns were measured with electrical conductivity probes inserted a t 50- or 100-cm intervals. Observed BTC's in the homogeneous sandy column were relatively smooth and sigmoidal (S-shaped), while those in the heterogeneous column were very irregular, nonsigmoidal, and exhibited extensive tailing. Effective average pore-water velocities (tr,rr) and dispersion coefficients (&r) were estimated simultaneously by tilting an analytical solution of the convection-dispersionequation to the observe d BTC's. Velocity variations in the heterogeneousmedium were found to be much larger than those in the homogeneous sand. Values of the dispersivity, (Y = Derr/ecrrr for the homogeneous sandy column ranged from 0.1 to 5.0cm, while those for the heterogeneous column were as high as 200cm. The dispcrsivity for transport in both columns increased with travel distance or travel time, thus exhibiting scale- dependency. The heterogeneous soil column also showed the effects of preferential flow. i.e., some locations in the column showed earlier solute breakthrough than several locations closer to the inlet boundary. Spatial fluctuations in the dispersivity could bc explained qualitatively by the particular makeup of the heterogeneities in the column.

Fractal geometry characterization of geothermal reservoir fracture networks

Abstract: As a new procedure for modeling geothermal energy extraction systems, a two-dimensional modeling technique for subsurface fracture networks on the basis of `fractal geometry` is presented. Models of fracture networks are generated by distributing fractures randomly in space and by using the fractal relation between fracture length r and the number of fractures N expressed with a fractal dimension D as N = Cr(exp -D), where C is a constant that signifies the fracture density within the rock mass. This procedure makes it possible to characterize geothermal reservoirs by parameters measured from field data, such as from core sampling. In this characterization the fracture density parameter C of a geothermal reservoir is used as a parameter to model the subsurface fracture network. Using this fracture network model, the connectivities of the water flow paths between wells are calculated by means of a Monte Carlo simulation, and the result is then compared with that derived from a percolation model. We show that many fewer fractures are required to connect two wells for the fracture network model than for the percolation model. The transmissivities between wells for the fracture network model are also obtained as a function of the fracture density parametermore » C. The results show that the transmissivities in geothermal reservoirs are significantly dependent upon the fracture density of rock mass, and they can be predicted from the fracture density parameter C of the reservoirs.« less

Three-dimensional numerical modelling of water flow in a river with large bed roughness

Abstract: A numerical model for three-dimensional simulation of water flow in rivers with large roughness elements is developed. The bed roughness elements are large stones and rocks in the rivers. The model uses a finite volume method to solve the Navier-Stokes equations for three dimensions on a general non-orthogonal grid. The k-e turbulence model is used to solve the Reynolds-stress term. A porosity model is developed to model the bed roughness elements. The porosity model is used in combination with the solution of the Navier-Stokes equations to calculate interactions between porous and non-porous areas. To test the model, a reach of the Norwegian river Sokna is modelled. Velocity measurements from the river are taken at a number of locations and at several discharges. The measured velocities compare well with the results from the numerical model.

Flux of nitrogen and sediment in a fringe mangrove forest in terminos lagoon, Mexico

Abstract: Fluxes of dissolved inorganic and organic nitrogen, particulate nitrogen, and total suspended sediments were measured in a fringe mangrove forest using the flume technique during a 15-month period in Terminos Lagoon, Mexico. The 12-m flume extended through a fringe forest from a tidal creek to a basin forest. There was a net import of dissolved inorganic nitrogen (NH+4 and NO−2+NO−3) from the creek and basin forest, while particulate (PN) and dissolved organic nitrogen (DON) were exported to the creek and basin forest. The tidal creek was the principal source of NH+4 (0·53 g m−2 year−1) and NO−2+NO−3 (0·08 g m−2 year−1) to the fringe forest, while the basin forest was the main source of total suspended sediments (TSS; 210 g m−2 year−1). Net export of PN occurred from the fringe forest to the tidal creek (0·52 g m−2 year−1) while less PN was exported to the basin forest (0·06 g m−2 year−1). The decrease in salinity during the rainy season indicated that nutrient concentrations in the tidal creek may have been influenced by inputs from rainfall and river discharge to the lagoon. There was a net import of TSS to the fringe forest from both the creek and basin forests, but the net input was 3·5 times higher at the fringe/basin interface. Particulate material exported from the forest during ebb tides generally had a higher C/N ratio than particulate matter imported into the forest on the flooding tide. This suggested that there was a greater nitrogen demand during ebb tide caused by the export of nitrogen-deficient detritus from fringe and basin mangroves. The exchange of nutrients among the tidal creek, the fringe, and basin forests in Estero Pargo is strongly influenced by seasonal weather forcing, such as winter storms, that can influence the magnitude and direction of water flow. The net annual import of inorganic nitrogen and the export of DON and PN suggest, in contrast to other mangrove systems, that the fringe mangrove forest in Estero Pargo acts as a sink of inorganic nitrogen and as a source of dissolved and particulate nitrogen.

Movement of pesticides to surface waters from a heavy clay soil

Abstract: A field experiment at Cockle Park, Northumberland on a clay loam soil (Dunkeswick series) cropped with winter wheat investigated the effects of drainage and season of application on pesticide movement. Isoproturon, mecoprop, fonofos and trifluralin were applied in two consecutive seasons at normal agricultural rates of three hydrologically isolated plots each of 0.25 ha. Two of the plots were mole-drained and the third was an undrained control. Surface-layer flow and drainflow from each plot were monitored at 10-min intervals. Samples of flow were analysed for pesticides to evaluate transport of applied chemicals from the site. Despite widely differing properties (K oc 20-8000 ml g -1 , t 1/2 10-60 days), all four pesticides were found in surface-layer flow and mole drainflow from the site. Maximum concentrations of pesticides in flow ranged from 0.1 to 121 μg litre -1 (aqueous phase) and <0.2 to 48 μg litre -1 (particulate phase). Over two contrasting seasons, total losses of pesticides in flow followed total amounts of flow and were approximately four and five times larger, respectively, in 1990/91 than in 1989-90. The maximum loss occurred from the undrained plot and was 2.8 g isoproturon (0.45% of that applied). Total losses of autumn-applied pesticides from an undrained plot were up to four times greater than losses from a mole-drained plot. Mole drainage decreased movement of pesticides from this slowly permeable soil by reducing the amount of surface-layer flow. Maximum concentrations of mecoprop and isoproturon in drainflow were 10-20 times larger following spring application than after application in autumn. Bypass flow drown soil cracks was an important process by which pesticide was lost from the site, with transport to the drainage system via mole channels (55 cm depth) after less than 0.5 and 6.7 mm net drainage in the two winters