Sensitivity of incipient particle motion to fluid flow penetration depth within a packed bed

A discrete element method is applied to a three‐dimensional analysis related to sediment entrainment on a micro‐scale. Sediment entrainment is the process by which a fluid medium accelerates particles from rest and advects them upward until they are either transported as bedload or suspended by the flow. Modelling of the entrainment process is a critically important aspect for studies of erosion, pollutant resuspension and transport, and formation of bedforms in environmental flows. Previous discrete element method studies of sediment entrainment have assumed the flow within the particle bed to be negligible and have only allowed for the motion of the topmost particles. At the same time, micro‐scale experimental studies indicate that there is a small slip of the fluid flow at the top of the bed, indicating the presence of non‐vanishing fluid velocity within the topmost bed layers. The current study demonstrates that the onset of particle incipient motion, which immediately precedes particle entrainment, is highly sensitive to this small fluid flow within the topmost bed layers. Using an exponential decay profile for the inner‐bed fluid flow, the discrete element method calculations are repeated with different fluid penetration depths within the bed for several small particle Reynolds numbers. For cases with slip velocity corresponding to that observed in previous experiments with natural sediment, the predicted particle velocity is found to be a few percent of the fluid velocity at the top of the viscous wall layer, which is a reasonable range of velocities for observation of incipient particle motion. This method for prescribing the fluid flow within the particle bed allows for the current discrete element method to be extended in future studies to the analysis of sediment entrainment under the influence of events such as turbulent bursting. Additionally, predictions for the slip velocities and fluid flow profile within the bed suggest the need for further experimental studies to provide the data necessary for additional improvement of the discrete element method models.     

The enlargement of local power in trans-scalar strategies of planning: recent tendencies in two European cases

The increasing ‘trans-scalar’ conditions of urbanity imply unique opportunities and threats for urban policies. When drawing up strategic urban policies, cities are confronted with the dominance of higher level policy programmes and even more so with the hegemonic power of globalizing markets. Could urban policies also undergo a similar change and actively pursue the enlargement of local power and energies by becoming actively involved in the higher level scaled networks? By building on useful concepts of urban regime theory and more recent rescaling theories, the authors argue that this element of ‘foreign policy’ should be integrated in the core analysis of urban policy approaches. The case is explored and illustrated in two European examples of strategic urban planning, namely Barcelona and Copenhagen. The cases provide evidence of the enlargement of local power via active trans-scalar policies. For civic groups, however, this strategy still appears to be a bridge too far in both cases.     

SOIL FINGERPRINTING IN THE PALOUSE BASIN, USA USING STABLE CARBON AND NITROGEN ISOTOPES

1 INTRODUCTION Watershed restoration efforts have been accelerated in recent years by mandates in the Clean Water Act, the Endangered Species Act, and increasing pressure from environmental groups (Wagner and Roberts, 1998). To address these mandates, water-quality management plans and Total Maximum Daily Loads (TMDLs) have been developed for surface waters, such as streams placed on the 303(d) list because of beneficial use impairment. Commonly, much of the degradation of surface…     

Time transformations and Cowell's method

The precise numerical integration of Cowell's equations of satellite motion is frequently performed with an independent variables defined by an equation of the form dt=cr ⁿ ds, wheret represents time,r the radial distance from the center of attraction,c is a constant, andn is a parameter. This has been primarily motivated by the uniformizing effects of such a transformation resulting in desirable analytic stepsize control for elliptical orbits. This report discusses the proper choice of the parametern defining the independent variables for various types of orbits and perturbation models, and develops a criterion for its selection.     

Genesis of post-hotspot,A-type rhyolite of the Eastern Snake River Plain volcanic field by extreme fractional crystallization of olivine tholeiite

Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene rhyolites of the Yellowstone–Snake River Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km³ of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical signatures including high whole-rock FeO_tot/(FeO_tot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine ± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛ_Nd = −4 to −6; ⁸⁷Sr/⁸⁶Sr_i = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the rhyolite lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO₂) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s of km³ each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves. Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and rhyolite parent magmas. Interestingly, models of rhyolite genesis that involve large degrees of melting of Archean crust or previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO₂), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the rhyolites and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing and fractional melting occur to limited degrees, but are not essential parts of the rhyolite formation process. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Estimating Tortuosity Coefficients Based on Hydraulic Conductivity

While the tortuosity coefficient is commonly estimated using an expression based on total porosity, this relationship is demonstrated to not be applicable (and thus is often misapplied) over a broad range of soil textures. The fundamental basis for a correlation between the apparent diffusion tortuosity coefficient and hydraulic conductivity is demonstrated, although such a relationship is not typically considered. An empirical regression for estimating the tortuosity coefficient based on hydraulic conductivity for saturated, unconsolidated soil is derived based on results from 14 previously reported diffusion experiments performed with a broad range of soil textures. Analyses of these experimental results confirm that total porosity is a poor predictor for the tortuosity coefficient over a large range of soil textures. The apparent diffusion tortuosity coefficient is more reliably estimated based on hydraulic conductivity.     

Trans‐dimensional Bayesian inversion of controlled‐source electromagnetic data in the German North Sea

This paper presents the first controlled‐source electromagnetic survey carried out in the German North Sea with a recently developed seafloor‐towed electrical dipole–dipole system, i.e., HYDRA II. Controlled‐source electromagnetic data are measured, processed, and inverted in the time domain to estimate an electrical resistivity model of the sub‐seafloor. The controlled‐source electromagnetic survey targeted a shallow, phase‐reversed, seismic reflector, which potentially indicates free gas. To compare the resistivity model to reflection seismic data and draw a combined interpretation, we apply a trans‐dimensional Bayesian inversion that estimates model parameters and uncertainties, and samples probabilistically over the number of layers of the resistivity model. The controlled‐source electromagnetic data errors show time‐varying correlations, and we therefore apply a non‐Toeplitz data covariance matrix in the inversion that is estimated from residual analysis. The geological interpretation drawn from controlled‐source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of ～1 Ωm at the seafloor, which are typical for fine‐grained marine deposits, whereas resistivities below ～20 mbsf increase to 2–4 Ωm and can be related to a transition from fine‐grained (Holocene age) to unsorted, coarse‐grained, and compacted glacial sediments (Pleistocene age). Interface depths from controlled‐source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments. Resistivities decrease again at greater depths to ～1 Ωm with a minimum resistivity at ～300 mbsf where a seismic reflector (that marks a major flooding surface of late Miocene age) correlates with an increased gamma‐ray count, indicating an increased amount of fine‐grained sediments. We suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine‐grained sediments. Concerning the phase‐reversed seismic reflector that was targeted by the survey, controlled‐source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector. We suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low‐velocity layer. Controlled‐source electromagnetic interface depths near the reflector are quite uncertain and variable. We conclude that the seismic interface cannot be resolved with the controlled‐source electromagnetic data, but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west.     

Occurrence and Distribution of Organic Contaminants in the Aquatic System in Berlin. Part I: Drug Residues and other Polar Contaminants in Berlin Surface and Groundwater

Several polar contaminants were found in screening analyses of 30 representative surface water samples collected from rivers, lakes, and canals in Berlin. Residues of pharmaceuticals and N-(phenylsulfonyl)-sarcosine originating from various sewage treatment plants effluents were found at concentrations up to the μg/L-level in the surface water, whereas the concentrations of polar pesticides such as dichlorprop and mecoprop were always below 0.1 μg/L. The pharmaceuticals most frequently detected in the surface water samples include clofibric acid, diclofenac, ibuprofen, propiphenazone, and two other drug metabolites. Additional investigations of groundwater wells of a drinking water plant have shown that polar contaminants such as drug residues or N-(phenylsulfonyl)-sarcosine easily leach through the subsoil into the groundwater aquifers when contaminated surface water is used for groundwater recharge in drinking water production.