A model of equilibrium bed topography for meander bends with erodible banks     

Stephen E. Darby  Ivana Delbono 《地球表面变化过程与地形》2002,27(10):1057-1085

Channel curvature produces secondary currents and a transverse sloping channel bed, along which the depth increases towards the outer bank. As a result deep pools tend to form adjacent to the outer bank, promoting bank collapse. The interaction of sediment grains with the primary and secondary flow and the transverse sloping bed also causes meanders to move different grain sizes in different proportions and directions, resulting in a consistent sorting pattern. Several models have been developed to describe this process, but they all have the potential to over‐predict pool depth because they cannot account for the influence of erodible banks. In reality, bank collapse might lead to the development of a wider, shallower cross‐section and any resulting flow depth discrepancy can bias associated predictions of flow, sediment transport, and grain‐size sorting. While bed topography, sediment transport and grain sorting in bends will partly be controlled by the sedimentary characteristics of the bank materials, the magnitude of this effect has not previously been explored. This paper reports the development of a model of flow, sediment transport, grain‐size sorting, and bed topography for river bends with erodible banks. The model is tested via intercomparison of predicted and observed bed topography in one low‐energy (5·3 W m^?2 specific stream power) and one high‐energy (43·4 W m^?2) study reach, namely the River South Esk in Scotland and Goodwin Creek in Mississippi, respectively. Model predictions of bed topography are found to be satisfactory, at least close to the apices of bends. Finally, the model is used in sensitivity analyses that provide insight into the influence of bank erodibility on equilibrium meander morphology and associated patterns of grain‐size sorting. The sensitivity of meander response to bank cohesion is found to increase as a function of the available stream power within the two study bends. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Predicting average annual groundwater levels from climatic variables: an empirical model   总被引：8，自引：0，他引：8  

Zhuoheng Chen  Stephen E. Grasby  Kirk G. Osadetz 《Journal of Hydrology》2002,260(1-4):102-117

On the basis of one-dimensional theoretical water flow model, we demonstrate that the groundwater level variation follows a pattern similar to recharge fluctuation, with a time delay that depends on the characteristics of aquifer, recharge pattern as well as the distance between the recharge and observation locations. On the basis of a water budget model and the groundwater flow model, we propose an empirical model that links climatic variables to groundwater level. The empirical model is tested using a partial data set from historical records of water levels from more than 80 wells in a monitoring network for the carbonate rock aquifer, southern Manitoba, Canada. The testing results show that the predicted groundwater levels are very close to the observed ones in most cases. The overall average correlation coefficient between the predicted and observed water levels is 0.92. This proposed empirical statistical model could be used to predict variations in groundwater level in response to different climate scenarios in a climate change impact assessment.  相似文献   

Modeling of Wide-Angle Seismic Attributes Using the Gaussian Beam Method     

Stacy  Stephen M.  Nowack  Robert L. 《Studia Geophysica et Geodaetica》2002,46(4):667-690

In this study, observed seismic attributes from shot gather 11 of the SAREX experiment are used to derive a preliminary velocity and attenuation model for the northern end of the profile in southern Alberta. Shot gather 11 was selected because of its prominent Pn arrivals and good signal to noise ratio. The 2-D Gaussian beam method was used to perform the modeling of the seismic attributes including travel times, peak envelope amplitudes and pulse instantaneous frequencies for selected phases. The preliminary model was obtained from the seismic attributes from shot gather 11 starting from prior tomographic results. The amplitudes and instantaneous frequencies were used to constrain the velocity and attenuation structure, with the amplitudes being more sensitive to the velocity gradients and the instantaneous frequencies more sensitive to the attenuation structure. The resulting velocity model has a velocity discontinuity between the upper and lower crust, and lower velocity gradients in the upper and lower crust compared to earlier studies. The attenuation model has Q _p ^-1 values between 0.011 and 0.004 in the upper crust, 0.0019 in the lower crust and a laterally variable Q _p ^-1 in the upper mantle. The Q _p ^-1 values are similar to those found in Archean terranes from other studies. Although the results from a single gather are non-unique, the initial model derived here provides a self-consistent starting point for a more complete seismic attribute inversion for the velocity and attenuation structure.  相似文献   

Geopolitics, water and development in South Asia: cooperative development in the Ganges–Brahmaputra delta     

Stephen Brichieri-Colombi  Robert W Bradnock† 《The Geographical journal》2003,169(1):43-64

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Petrogenesis of the Greenhills Complex,Southland, New Zealand: magmatic differentiation and cumulate formation at the roots of a Permian island-arc volcano     

Carl J. Spandler  Richard J. Arculus  Stephen M. Eggins  John A. Mavrogenes  Richard C. Price  Anthony J. Reay 《Contributions to Mineralogy and Petrology》2003,144(6):703-721

A Permian (~265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes. Cumulates, which include dunite, olivine clinopyroxenite, olivine gabbro, and hornblende gabbro-norite, are related products of parent-magma fractionation. Both primary (magmatic) and secondary platinum-group minerals occur within dunite at one locality. Using the composition of cumulus minerals, mafic dykes and melt inclusions, we have determined that the parent magmas of the complex were hydrous, low-K island-arc tholeiites of ankaramitic affinities. Progressive magmatic differentiation of this parent magma generated fractionated melt of high-alumina basalt composition which is now preserved only as dykes which cut the Complex. Field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate-rock formation. Recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface. Similar processes are inferred to account for volcanic-rock compositions in other parts of this arc terrane and in modern island-arc systems.  相似文献   

Chlorine-36 in groundwater of the United States: empirical data     

Stanley N. Davis  Stephen Moysey  DeWayne L. Cecil  Marek Zreda 《Hydrogeology Journal》2003,11(2):217-227

Natural production of the radionuclide chlorine-36 (³⁶Cl) has provided a valuable tracer for groundwater studies. The nuclear industry, especially the testing of thermonuclear weapons, has also produced large amounts of ³⁶Cl that can be detected in many samples of groundwater. In order to be most useful in hydrologic studies, the natural production prior to 1952 should be distinguished from more recent artificial sources. The object of this study was to reconstruct the probable preanthropogenic levels of ³⁶Cl in groundwater in the United States. Although significant local variations exist, they are superimposed on a broad regional pattern of ³⁶Cl/Cl ratios in the United States. Owing to the influence of atmospherically transported ocean salt, natural ratios of ³⁶Cl/total Cl are lowest near the coast and increase to a maximum in the central Rocky Mountains of the United States. Electronic Publication  相似文献   

Laser Ablation ICP-MS Analysis of Geological Materials Prepared as Lithium Borate Glasses   总被引：9，自引：0，他引：9  

Stephen M. Eggins 《Geostandards and Geoanalytical Research》2003,27(2):147-162

A simple, rapid and precise method is described for determining trace elements by laser ablation (LA)-ICP-MS analysis in bulk geological materials that have been prepared as lithium borate glasses following standard procedures for XRF analysis. This approach reliably achieves complete sample digestion and provides for complementary XRF and LA-ICP-MS analysis of a full suite of major and trace elements from a single sample preparation. Highly precise analysis is enabled by rastering an ArF excimer laser (λ= 193nm) across fused samples to deliver a constant sample yield to the mass spectrometer without inter-element fractionation effects during each analysis. Capabilities of the method are demonstrated by determination of twenty five trace elements (Sc, Ti, V, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf, Ta, Pb, Th and U) in a diverse range of geological reference materials that includes peridotites, basalts, granites, metamorphic rocks and sediments. More than 90% of determinations are indistinguishable from published reference values at the 95% confidence level. Systematic bias greater than 5% is observed for only a handful of elements (Zr, Nb and U) and may be attributed in part to inaccurate calibration values used for the NIST SRM 612 glass in the case of Zr and Nb. Detection limits for several elements, most notably La, are compromised at ultra-trace levels by impurities in the lithium borate flux but can be corrected for by subtracting appropriate procedural blanks. Reliable Pb analysis has proved problematic due to variable degrees of contamination introduced during sample polishing prior to analysis and from Pt-crucibles previously used to fuse Pb-rich samples. Scope exists for extending the method to include internal standard element/isotope spiking, particularly where integrated XRF analysis is not available to characterise major and trace elements in the fused lithium borate glasses prior to LA-ICP-MS analysis.  相似文献   

The effects of pH, ionic strength, and iron-fulvic acid interactions on the kinetics of non-photochemical iron transformations. II. The kinetics of thermal reduction     

Michael J Pullin  Stephen E Cabaniss 《Geochimica et cosmochimica acta》2003,67(21):4079-4089

A combination of flow-injection analysis and kinetic analysis was used to examine the speciation of iron(II) and iron(III) in fulvic acid solutions as a function of pH, ionic strength, and time. This methodology was used to follow a shift in iron speciation from faster to slower reacting species over a timescale of several days. This speciation data shows that both iron(II) and iron(III)-fulvic acid complexes are important iron species in humic-containing natural waters and that their amounts and their rates of transformation to colloidal iron are controlled primarily by the kinetics of thermal (dark) reduction and iron(II) oxidation. The kinetic analysis methodology also yielded the rate constants for the thermal reduction of iron by the fulvic acid. These rate constants decrease with increasing pH and are independent of ionic strength. While thermal reduction was found to be too slow to produce large amounts of steady state iron(II) at circumneutral pH, it does provide a mechanism for iron redox cycling in the absence of photochemical or biochemical processes.  相似文献   

Naturally precipitating vaterite (μ-CaCO₃) spheres: unusual carbonates formed in an extreme environment     

Stephen E Grasby 《Geochimica et cosmochimica acta》2003,67(9):1659-1666

Vaterite, a rare hexagonal CaCO₃ polymorph, was identified in precipitates forming at a supraglacial sulfur spring, in Borup Fiord Pass, northern Ellesmere Island, Canadian High Arctic. Vaterite occurs in a precipitate mound along with calcite, gypsum, and native sulfur. The unusual conditions of the site, including an extremely cold climate, supersaturated alkaline waters, and the presence of gypsum, mimics conditions used to grow vaterite in laboratory experiments. Stable isotope data suggest that vaterite may preferentially form during colder periods of the year. Vaterite found at the site is characterized by 2- to 10-μm rounded to spherical shaped particles (comprising smaller 0.5- to 2-μm spheres) as both individuals and in chainlike structures. The spherical habit of vaterite resembles carbonate structures that have been interpreted to be organic; however, δ¹³C values are indicative of an inorganic origin. The thick permafrost, and the extreme cold and dry environment make this site a good terrestrial analog for carbonate precipitates that might be expected at potential deposits associated with water discharge on other planetary bodies.  相似文献   

Analysis of Re, Au, Pd, Pt and Rh in NIST Glass Certified Reference Materials and Natural Basalt Glasses by Laser Ablation ICP-MS   总被引：1，自引：0，他引：1  

Paul J. Sylvester  Stephen M. Eggins 《Geostandards and Geoanalytical Research》1997,21(2):215-229

A new technique for the in situ analysis of Re, Au, Pd, Pt and Rh in natural basalt glass by laser ablation (LA)-ICP-MS is described. The method involves external calibration against NIST SRM 612/613 or 614/615 glass certified reference materials, internal standardisation using Ca, and ablation with a 200 μm wide beam spot and a pulsed laser repetition rate of 50 Hz. Under these conditions, sensitivities for Re, Au, Pd, Pt and Rh analyte ions are ˜ 5000 to 100,000 cps/μg g^-1. This is sufficient to make measurements precise to ˜ 10% at the 2-10 μg g^-1 level, which is well within the range of concentrations expected in many basalts. For LA-ICP-MS calibration and a demonstration of the accuracy of the technique, concentrations of Re, Au, Pd, Pt and Rh in the NIST SRM 610/611 (˜ 1 to 50 μg g^-1), 612/613 (˜ 1 to 7 μg g^-1), 614/615 (˜ 0.2 to 2 μg g^-1) and 616/617 (˜ 0.004 to 2 μg g^-1) glasses were determined by solution-nebulisation (SN)-ICP-MS. Using the 612/613 or 614/615 glasses as calibration standards, LA-ICP-MS measurements of these elements in the other NIST glasses fell within ˜ 15% of those determined by SN-ICP-MS. Replicate LA-ICP-MS analyses of the 612/613 and 614/615 glasses indicate that, apart from certain anomalous domains, the glasses are homogeneous for Re, Au, Pd, Pt and Rh to better than 3.5%. Two LA-ICP-MS analyses of natural, island-arc basalt glasses exhibit large fractionations of Re, Au and Pd relative to Pt and Rh, compared to the relative abundances in the primitive mantle.  相似文献