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361.
Computer flow simulations using the HEC-2 step-backwater routine are used to demonstrate the effect of systematically varying river channel width, riffle spacing and channel roughness on the shear velocity, section-mean velocity and energy slope in fixed-bed pool-riffle sequences. Initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the River Severn. Subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored. The purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence ‘reversal’, such that pools scour at high flow whilst deposition occurs on riffles. It is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal. For prescribed conditions, the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width. Downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool-riffle topography. 相似文献
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Andrzej Hanyga 《Geophysical Journal International》1995,123(1):277-290
Uniformly asymptotic formulae for edge-and-vertex diffraction in the time-domain, involving elementary functions of time, traveltimes and GTD amplitudes, are derived. Explicit expressions for diffraction at a pyramid and a triangle are constructed. They can be applied to the numerical calculation of a field reflected and diffracted at 3-D objects with sharp edges and to reflection from triangulated surfaces. the computational cost is very low. 相似文献
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367.
Large-scale deformation associated with ridge subduction 总被引:1,自引:0,他引:1
Eric L. Geist Michael A. Fisher David W. Scholl 《Geophysical Journal International》1993,115(2):344-366
368.
Mapping the Geographical Diffusion of a Finnish Smallpox Epidemic from Historical Population Records
James L. Wilson 《The Professional geographer》1993,45(3):276-286
Smallpox mortality from an early 19th century epidemic in Finland is mapped at the parish village level. First, geographically referenced historical materials are used to construct a map showing the spread of smallpox mortality among villages. Next, the diffusion of smallpox morbidity is simulated by computer for the same set of villages. These two maps are then compared. Results indicate that at this scale maps of mortality diffusion can show general trends but have no direct spatial correspondence to the underlying pattern of morbidity diffusion. Mortality maps cannot be used as a surrogate measure of infectious contact behavior at micro scales of analysis. 相似文献
369.
370.
Robert A. Langel 《Journal of Earth System Science》1990,99(4):581-618
Among the first measurements made from near-Earth orbiting satellites were measurements of the magnetic field. The sources
of that field lie both within the Earth, in its core and crust, and in the surrounding ionosphere and magnetosphere. This
article summarizes some of the methodology and results for studies of the Earth’s mantle and crust. Mantle conductivity studies
can be made either by studying signals impressed on the Earth from outside, e.g., the ionosphere or magnetosphere, or by studying
signals originating in the core and transmitted through the mantle. Crustal field studies begin with a careful selection of
the data and subsequent removal of core and external fields by some sort of filtering. Average maps from different local times
sometimes differ, presumably due to the remaining presence of fields of external origin. Several techniques for further filtering
are discussed. Where large-area aeromagnetic maps are available, crustal maps derived from satellite data can be compared
with upward continued data. In general, the comparisons show agreement, with some differences, particularly in and near the
auroral belts. The satellite data are further reduced by various methods of inverse and forward modelling, sometimes including
reduction to the pole (RTP). These techniques are generally unstable at the equator. Common methods of stabilizing the inversions
include principle components analysis and ridge regression. Because of the presence of the core field, the entire crustal
contribution from the field is not known. Also, there is a basic nonuniqueness to the inverse solutions. Nevertheless, magnetizations
that are interpretable can be derived. 相似文献