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111.
David R. Butler 《自然地理学》2013,34(1):72-87
Hazardous snow avalanches in Glacier National Park, Montana, are associated with a variety of meteorologic conditions: heavy snow; heavy snows followed by a rise in air temperature to above freezing; a rise in air temperature to above freezing, without precipitation; and rain in association with above-freezing air temperatures. Years of major, widespread avalanching may be recognized by examination of historical information and tree-ring data. Avalanche types include slab avalanches, wet snow avalanches, and dry loose snow avalanches. February is the peak avalanche month. Intraannual seasonalities of avalanche trigger mechanisms and type of avalanche are related. The presence of sun crusts in some cases provides unstable stratigraphic planes in the snowpack over which freshly deposited snow may glide. Destructive windblasts also occur in association with some avalanches. Insufficient data from east of the Continental Divide precluded a comparison of avalanche type and trigger mechanisms from the western and eastern portions of Glacier National Park. The general avalanche climate is more similar to that reported from the southern Canadian cordillera than to continental locations such as the mountains of Colorado. [Key words: Snow avalanches; avalanche trigger mechanisms; avalanche types; avalanche seasonality; Glacier National Park, Montana; northern Rocky Mountains; hazard planning.] 相似文献
112.
Soil denudation studies in urban environments are infrequent in the geomorphic literature. Here we describe the amount of soil erosion associated with pedestrian and bicycle pathways on an American university campus. The amount of soil denudation from thirty pedestrian/bicycle paths on the campus of the University of North Carolina was determined by morphometric measurements. Volumes denuded ranged from nearly 20 m3 to <0.2 m3 per site. Highest amounts were located at sites adjacent to a football stadium. Three types of paths were recognized, based on their utilization: paths running adjacent to existing sidewalks; paths cutting across lawns and through shrubbery from one sidewalk to another; and those formed by bicycles in order to avoid going up or down steps. 相似文献
113.
Jean-Pierre Lefebvre Sylvain Ouillon Vu Duy Vinh Robert Arfi Jean-Yves Panché Xavier Mari Chu Van Thuoc Jean-Pascal Torréton 《Geo-Marine Letters》2012,32(2):103-121
In the Bach Dang–Cam Estuary, northern Vietnam, mechanisms governing cohesive sediment aggregation were investigated in situ
in 2008–2009. As part of the Red River delta, this estuary exhibits a marked contrast in hydrological conditions between the
monsoon and dry seasons. The impact on flocculation processes was assessed by means of surveys of water discharge, suspended
particulate matter concentration and floc size distributions (FSDs) conducted during a tidal cycle at three selected sites
along the estuary. A method was developed for calculating the relative volume concentration for the modes of various size
classes from FSDs provided by the LISST 100X (Sequoia Scientific Inc.). It was found that all FSDs comprised four modes identified
as particles/flocculi, fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic
energy, their proportions varied but without significant modification of their median diameters. In particular, when the turbulence
level corresponded to a Kolmogorov microscale of less than ∼235 μm, a major breakup of flocs resulted in the formation of
particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater
discharge and by the tidal cycle. During the wet season, strong freshwater input induced a high turbulent energy level that
tended to generate sediment transfer from the coarser size classes (macroflocs, coarse microflocs) to finer ones (particles/flocculi
and fine microflocs), and to promote a transport of sediment seawards. During the dry season, the influence of tides predominated.
The turbulent energy level was then only episodically sufficiently high to generate transfer of sediment between floc size
classes. At low turbulent energy, modifications in the proportions of floc size classes were due to differential settling.
Tidal pumping produced a net upstream transport of sediment. Associated with the settling of sediment trapped in a near-bed
layer at low turbulent energy, this causes the silting up of the waterways leading to the harbour of Haiphong. 相似文献
114.
Michael Rowan-Robinson Tom Babbedge Seb Oliver Markos Trichas Stefano Berta Carol Lonsdale Gene Smith David Shupe Jason Surace Stephane Arnouts Olivier Ilbert Olivier Le Févre Alejandro Afonso-Luis Ismael Perez-Fournon Evanthia Hatziminaoglou Mari Polletta Duncan Farrah Mattia Vaccari 《Monthly notices of the Royal Astronomical Society》2008,386(2):697-714
115.
116.
Bootstrap Calibration and Uncertainty Estimation of Downhole NMR Hydraulic Conductivity Estimates in an Unconsolidated Aquifer
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A.D. Parsekian K. Dlubac E. Grunewald J.J. Butler R. Knight D.O. Walsh 《Ground water》2015,53(1):111-121
Characterization of hydraulic conductivity (K) in aquifers is critical for evaluation, management, and remediation of groundwater resources. While estimates of K have been traditionally obtained using hydraulic tests over discrete intervals in wells, geophysical measurements are emerging as an alternative way to estimate this parameter. Nuclear magnetic resonance (NMR) logging, a technology once largely applied to characterization of deep consolidated rock petroleum reservoirs, is beginning to see use in near‐surface unconsolidated aquifers. Using a well‐known rock physics relationship—the Schlumberger Doll Research (SDR) equation—K and porosity can be estimated from NMR water content and relaxation time. Calibration of SDR parameters is necessary for this transformation because NMR relaxation properties are, in part, a function of magnetic mineralization and pore space geometry, which are locally variable quantities. Here, we present a statistically based method for calibrating SDR parameters that establishes a range for the estimated parameters and simultaneously estimates the uncertainty of the resulting K values. We used co‐located logging NMR and direct K measurements in an unconsolidated fluvial aquifer in Lawrence, Kansas, USA to demonstrate that K can be estimated using logging NMR to a similar level of uncertainty as with traditional direct hydraulic measurements in unconsolidated sediments under field conditions. Results of this study provide a benchmark for future calibrations of NMR to obtain K in unconsolidated sediments and suggest a method for evaluating uncertainty in both K and SDR parameter values. 相似文献
117.
118.
Marijan Herak Ivo Allegretti Davorka Herak Krešimir Kuk Vlado Kuk Krešimir Marić Snježana Markušić Josip Stipčević 《Bulletin of Earthquake Engineering》2010,8(3):483-499
Horizontal-to-vertical spectral ratios (HVSR) of ambient vibrations measured in the ancient town of Ston (Croatia) on 99 locations, are shown to be well matched to the theoretical ones computed for body-waves as well as for the surface waves. This match is poorer for sites on the slopes of nearby hills. The ratios of measured peak horizontal ground acceleration during the damaging earthquake in 1996 (M L = 6.0) and the ones obtained using empirical attenuation laws is approximately equal to the mapped value of the dynamic amplification factor determined on the basis of observed HVSR in the vicinity of the accelerometric station. The HVSR of the accelerogram is very similar to the HVSR of the ambient noise. The damage to the building stock in the old town centre caused by the earthquake series of 1996 is closely related to the estimated soil amplification and its fundamental frequency. More measurements in buildings are needed to arrive at confident conclusions about possible soil-structure resonance. 相似文献
119.
A suite of management options for a prawn trawl fishery in Torres Strait, Australia was assessed for impacts on the benthic fauna using a dynamic management strategy evaluation approach. The specification of the management options was gained through consultation with stakeholders. Data for the model was drawn from several sources: the fleet data from fishery logbooks and satellite vessel monitoring systems, benthic depletion rates from trawl-down experiments, benthic recovery rates from post-experiment recovery monitoring studies, and benthic distribution from large-scale benthic surveys. Although there were large uncertainties in the resulting indicators, robust measures relevant to management were obtained by taking ratios relative to the status quo. The management control with the biggest effect was total effort; reducing trawl effort always led to increases in benthic faunal density of up to 10%. Spatial closures had a smaller benefit of up to 2%. The effect of closing a set of buffer zones around reefs to trawling was indistinguishable from the status quo option. Closing a larger area, however, was largely beneficial especially for sea cucumbers. When the spatial distributions of fauna prior to fishing were accounted for, fauna with distributions positively correlated with effort improved relative to those negatively correlated. The reduction in prawn catch under effort reduction scenarios could be ameliorated by introducing temporal closures over the full-moon period. 相似文献
120.
Alejandro Carrillo Pablo Huq Mari C. Pérez José M. Redondo 《Estuarine, Coastal and Shelf Science》2008
Spatial and temporal variations in the distribution of the marine picoplanktic cyanobacteria population and mixing conditions were found in the Ebro River estuary outflow to the Mediterranean Sea in Spain. Six sampling surveys were undertaken between July 1999 and February 2000 for distances up to 15 km from the river mouth. Measurements were taken of flow velocity, salinity, temperature, depth and picocyanobacteria (PCB) abundances. Gradient Richardson (Rig) and Reynolds (Re) numbers were determined to evaluate hydrodynamics. In summer, large values of Rig arise from the small flow rates, and small values of velocity shear between the surface fresh water layer and the bottom saline layer; conversely, in winter the large flow rates and attendant large velocity shears between the layers give rise to small values of Rig. Flow conditions in the fall are an intermediate case between the summer and winter cases. Vertical abundance distributions were resolved through the river water, interfacial region, and the bottom salt wedge; longitudinal gradients of PCB abundances were also resolved. Seasonal differences in the PCB abundance values were observed. Analysis of cell numbers (C) showed that the variable dC/dS, the dependence of cell number upon salinity gradient (dS) was critical. A non-dimensional number; the cellular gradient number (Cg) is introduced. Cg has useful biological interpretations that can potentially be included in ecological modeling. For example, Cg = 1 pertains to perfect adaptability of the organism to adjust to changing environmental conditions, whereas Cg = 0 describes total mortality. For a system with strong advection there is insufficient time for cells to adapt to the changing environment, and so those cell counts are unchanged. This is the case for the Ebro estuary in winter as advection of salinity (and hence PCB abundance) dominates the other loss processes for large flow rates. 相似文献