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361.
Energy dissipation, flow resistance and gas-liquid interfacial area in skimming flows on moderate-slope stepped spillways 总被引:2,自引:2,他引:0
With the re-evaluation and revision of a number of design floods, several embankment overtopping protection systems have been
developed and a common technique is the construction of a stepped spillway on the downstream slope. For such moderate slope
stepped channels, detailed air–water flow measurements were performed in a large facility with a focus on the rate of energy
dissipation, flow resistance, air–water interfacial areas and re-aeration rates. Past and present experimental results showed
a significant aeration of the flow. The median dimensionless residual head was about 3 × dc for the 21.8° sloping chute and smaller than that for flatter slopes (θ = 3.4° and 15.9°). The flow resistance results yielded an equivalent Darcy friction factor of about 0.25 implying a larger
flow resistance for the 21.8° slope angle than for smaller slope angles. The re-aeration rate was deduced from the integration
of the mass transfer equation using measured air–water interfacial areas and air–water flow velocities. The results suggested
an increasing re-aeration rate with increasing rate of energy dissipation. The stepped invert contributed to intense turbulence
production, free-surface aeration and large interfacial areas. The experimental data showed however some distinctive seesaw
pattern in the longitudinal distribution of air–water flow properties with a wave length of about two step cavities. While
these may be caused by the interactions between successive adjacent step cavities and their interference with the free-surface,
the existence of such “instabilities” implies that the traditional concept of normal flow might not exist in skimming flows
above moderate-slope stepped spillways. 相似文献
362.
Rajalakshmi M. Josiam Heinz G. Stefan 《Journal of the American Water Resources Association》1999,35(2):433-439
ABSTRACT: Theoretical equations that establish the relationship between sediment oxygen demand (SOD) in a lake and the flow velocity and dissolved oxygen concentration in the bulk water already exist. These theoretical equations for oxygen consumption in the sediment express biological consumption with Michaelis-Menten kinetics, and chemical consumption by a first order reaction. Data from laboratory experiments that were conducted to validate the theoretical equations also exist. These experiments were performed in a laboratory channel with well defined flow characteristics for three types of sediments. Herein, the theoretical equations are used to model the experimental data for the three types of sediments. The values used for the parameters in the theoretical equations are determined by iteration until a best fit is obtained for the relationship of SOD to flow velocity from both the theoretical model and experimental data. The goodness of fit is measured by the standard error of prediction and the regression coefficient. 相似文献
363.
Ohne Zusammenfassung
The online version of the original article can be found at 相似文献
364.
365.
Volker Berding Stefan Schwartz Michael Matthies 《Environmental science and pollution research international》1999,6(1):37-43
The interdependencies of parameters applied in the models of EUSES are visualised in a directed connectivity graph. The parameters
(inputs, defaults, state variables, outputs) are represented by boxes (nodes) and their relations by lines (edges). The visualisation,
on the one hand, clarifies the complexity of the models in EUSES and, on the other hand, creates an overview and transparency.
The parameters’ relations to each other can be recognised faster, and the models can be better understood. The complexity
was quantified by the number (variety), kind (substance parameter, physico-chemical parameter, concentration, other parameters),
and depth (dimension) of the parameter and the number of relations (connectivity). The variety of EUSES (without the modelsSimple Treat andSimple Box whose interior structure is not documented and without the effect and risk characterisation) amounts to 466, the connectivity
to 961, and the maximal dimension is 21. 相似文献
366.
Farkas J Peter H Christian P Gallego Urrea JA Hassellöv M Tuoriniemi J Gustafsson S Olsson E Hylland K Thomas KV 《Environment international》2011,37(6):1057-1062
The increasing number of nanomaterial based consumer products raises concerns about their possible impact on the environment. This study provides an assessment of the effluent from a commercially available silver nanowashing machine. The washing machine released silver in its effluent at an average concentration of 11μgL(-1), as determined by inductive coupled mass spectrometry (ICP-MS). The presence of silver nanoparticles (AgNPs) was confirmed by single particle ICP-MS as well as ion selective electrode measurements and filtration techniques. Size measurements showed particles to be in the defined nanosize range, with an average size of 10nm measured with transmission electron microscopy (TEM) and 60-100nm determined with nanoparticle tracking analysis (NTA). The effluent was shown to have negative effects on a natural bacterial community as its abundance was clearly reduced when exposed to the nanowash water. If washing machines capable of producing AgNPs become a common feature of households in the future, wastewater will contain significant loadings of AgNPs which might be released into the environment. 相似文献
367.
Harald G. Zechmeister Stefan Dullinger Gunda Koellensperger Siegrun Ertl Christian Lettner Karl Reiter 《Environmental science and pollution research international》2011,18(1):91-98
Purpose
This study aims at evaluating (a) whether concentrations of a suite of elements in mosses sampled in the arctic region around Zackenberg reflect background concentrations useful for estimating pollution levels in industrialized parts of the northern hemisphere as is attempted, e.g. in the framework of the UNECE ICP Vegetation monitoring programme, and (b) whether there are any influences from Zackenberg research station detectable in these concentrations. 相似文献368.
Modeling of Vertical Solute Dispersion in a Sediment Bed Enhanced by Wave‐Induced Interstitial Flow1
Qin Qian Vaughan R. Voller Heinz G. Stefan 《Journal of the American Water Resources Association》2009,45(2):343-354
Abstract: Mass (solute) transport in a stream or lake sediment bed has a significant effect on chemical mass balances and microbial activities in the sediment. A “1D vertical dispersion model” is a useful tool to analyze or model solute transfer between river or lake water and a sediment bed. Under a motionless water column, solute transfer into and within the sediment bed is by molecular diffusion. However, surface waves or bed forms create periodic pressure waves along the sediment/water interface, which in turn induce flows in the pores of the sediment bed. The enhancement of solute transport by these interstitial periodic flows in the pores has been incorporated in a 1D depth‐dependent “enhanced dispersion coefficient (DE).” Typically, DE diminishes exponentially with depth in the sediment bed. Relationships have been developed to estimate DE as a function of the characteristics of sediment (particle size, hydraulic conductivity, and porosity) and pressure waves (wave length and height). In this paper, we outline and illustrate the calculation of DE as well as the penetration depth (dp) of the flow effect. Sample applications to illustrate the computational procedure are provided for dissolved oxygen transfer into a stream gravel bed and release of phosphorus from a lake bed. The sensitivity of the results to input parameter values is illustrated, and compared with the errors obtained when interstitial flow is ignored. Maximum values of DE near the sediment surface can be on the order of 1 cm2/s in a stream gravel bed with standing waves, and 0.001 cm2/s in a fine sand lake bed under progressive surface waves, much larger than molecular diffusion coefficients. 相似文献
369.
This paper describes the behaviour of 137Cs in Lago Maggiore and other pre-alpine lakes as a consequence of atmospheric nuclear weapons testing fallout and the fallout from the nuclear accident in Chernobyl. It presents data on the 137Cs distribution in tributaries, lake water, bottom sediments and reveals the role of 137Cs as a marker of the sedimentation processes. The run-off of 137Cs from the watershed to the lake is described with a simple compartment model. Measurements of the activity concentration of 137Cs in sediments are compared with the output of a model (diffusion–convection type) which describes the input of 137Cs into and its vertical distribution within the sediment. Varying sedimentation rates (0.05–0.90 g (cm2 y)−1) in Lago Maggiore are compared with data of other authors. Sedimentation rates and total distribution coefficients (of about 105 L kg−1) in Lago Maggiore are discussed and compared with those of Lago di Lugano, Lake Constance, and Lake Vorsee. 相似文献
370.
Urban agriculture can have many different expressions, varying from backyard gardening to poultry and livestock farming. This
article focuses on crop production on larger open spaces in cities of sub-Saharan Africa (SSA) and investigates the sustainability
and dynamics of this type of land use, which is common on undeveloped plots particularly in lowlands, such as in inland valleys,
or along urban streams or drains. An adapted version of the Framework for Evaluating Sustainable Land Management (FESLM) developed
by the Food and Agriculture Organization of the United Nations (FAO) was used to assess the sustainability of urban agriculture.
As an example for dynamics, the spatio-temporal changes of open-space agriculture in Dar es Salaam, Tanzania, are analyzed
for the period from 1992 to 2005, and compared with data from other cities. Crop production on urban open spaces appears as
a market-driven, highly productive and profitable phenomenon. However, it is often constrained by tenure insecurity and non-agricultural
land demands. Also, the common use of polluted water limits the official support of irrigated urban farming. However, despite
these constraints, the phenomenon of urban farming appears persistent and resilient to its changing environment, although
individual farmers might have to shift to other sites when their plots are needed for construction. Open-space vegetable production
in urban areas is a dynamic, viable and largely sustainable livelihood strategy, especially for poor urban dwellers. Spatio-temporal
analysis shows that it is not a short-lived or transitional phenomenon—probably as long as it can maintain its comparative
market advantage. However, its informal nature and resulting lack of political recognition need to be addressed. 相似文献