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11.
Hopper dredging operations release suspended sediment into the environment by agitation of the bed and by discharge of overflow slurries. Monitoring of turbidity and suspended sediment concentrations in central Chesapeake Bay revealed two plumes: (1) an upper plume produced by overflow discharge and (2) a near-bottom plume produced by draghead agitation and rapid settling from the upper plume. The upper plume dispersed over 5.7 km2 extending 5,200 meters form the discharge point. Redeposited sediment accumulated on channel flanks covering an area of 6.4 km2 and reached a thickness of 19 cm. Altogether dredging redistributed into the environment an estimated 100,000 tons of sediment or 12 percent of the total material removed.Near-field concentrations of suspended sediment, less than 300 m from the dredge, reach 840 to 7,200 mg/L or 50 to 400 times the normal background level. Far-field concentrations (>300 m) are enriched 5 to 8 times background concentrations and persist 34 to 50 percent of the time during a dredging cycle (1.5 to 2.0 h). The overflow discharge plume evolves through three dispersion phases: (1) convective descent, (2) dynamic collapse, and (3) long-term passive diffusion (Clark and others 1971). The bulk of the material descends rapidly to the bottom during the convective descent phase, whereas the cloud that remains in suspension is dispersed partly by internal waves. Although suspended sediment concentrations in the water column exceed certain water quality standards, benthic communities survived the perturbation with little effect. 相似文献
12.
Observations are presented of the transverse and vertical structure of residual water, salt and sediment transport in the upper reaches of a partly mixed estuary. Measurements were made over spring and neap tidal cycles at three sections. The residual transport for each variable exhibited a characteristic transverse structure. This structure is interpreted in terms of fundamental physical processes. The results are used to estimate the relative importance of transverse shear, vertical shear and tidal pumping to the axial dispersion of salt and sediment. 相似文献
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Calcite was synthesized by four methods, and the luminescence decay-time was measured for nine samples before and after heating
hydrothermally in the temperature range 200–400°C. Decay-time data were collected between room temperature and approximately
15 K. The decay time at room temperature is approximately 50 ms, with little difference between a given calcite before and
after hydrothermal treatment. The decay time at 15 K is always greater than at room temperature as the effect of thermal quenching
diminishes. Differences in decay time before and after heating are more apparent at low temperature owing to this reduction
in thermal quenching. The decay time decreased significantly in two samples, and an increase in decay time was observed in
the remaining seven samples following heating. Among the latter group, the change in decay time was insignificant in three
samples. The results are compared with previous data in which it was shown that the effect of heating is to increase the intensity
of luminescence. 相似文献
16.
Thermogravimetric Fourier transform infrared spectroscopy (TG-FTIR) analyses were carried out on two sets of isolated kerogens covering a wide maturity range from low mature (0.46% Ro) through the end of oil and gas generation (maximum Ro = 5.32%). Data onweight percent and Tmax for evolution of methane, volatile tars, ethylene, SO2, NH3, CO2, and CO are reported. The Tmax of methane shows the most consistent response to increasing maturation in both sets of samples. Results are comparable to those of whole rocks from an Alaskan North Slope well analyzed previously. The collective data for both whole rocks and isolated kerogens shows a generally linear correlation between %Ro and Tmax of methane, with the exception of Ro of about 2.0% where a dip in the curve occurs. The slope of the correlation line was steeper for the predominantly terrigenous Wilcox kerogen than for more marine Colorado kerogen or for the Alaskan North Slope whole rock samples, probably reflecting differences in the chemical nature of various kerogen sets, which is also reflected by differences in the shapes of the pyrolysis curves of SO2, CO2, CO, H2O, and ethylene. These preliminary data indicate that Tmax of methane is a good maturation indicator for whole rocks and isolated kerogens up to an Ro of about 4%, which includes all of the wet gas and a considerable portion of the dry gas generation zones. This correlation was also observed for samples containing migrated bitumen, where it was not possible to obtain a reliable Tmax for the volatile tar (S2) peak. The more terrigenous Wilcox kerogens also showed a good correlation of the Tmax of ethylene with %Ro. Tmax of ammonia evolution did not correlate with maturity and occurred 100–200°C lower than previously found for whole rocks, consistent with a whole-rock source of pyrolytic ammonia for Alaskan whole rock samples. HI and OI indices were calculated in several ways and plotted to reflect kerogen type as well as both the residual oil and gas generation potential. The ratio of pyrolyzable to combustible sulfur (evolved as SO2) was independent of maturity and showed a clear difference between the more terrigenous Wilcox kerogens and the more marine Colorado kerogens. 相似文献
17.
K. Weston T. D. Jickells L. Fernand E. R. Parker 《Estuarine, Coastal and Shelf Science》2004,59(4):559-573
Nitrate and ammonium uptake rates were measured during a series of cruises in the well-mixed region of the southern North Sea from February to September. Water column-integrated uptake rates ranged between 0.01 and 8.7 mmol N m−2 d−1 and 0.01 and 12.2 mmol N m−2 d−1 for nitrate and ammonium, respectively, with ammonium uptake dominating after the phytoplankton spring bloom in May. A moored buoy continuously measuring nitrate and chlorophyll a and seabed current meters were also deployed in the central southern North Sea in the region of the East Anglian plume—a permanent physical feature which transports nutrients towards continental Europe. This enabled the flux of water and hence of nutrients across the southern North Sea to be determined and an assessment of the contribution of freshwater nutrients to the flux to be made. A simple box model is developed to relate the phytoplankton uptake of nitrate and ammonium to the transport of nitrate, ammonium and particulate organic matter (POM) across the southern North Sea. This showed the importance of the plume region of the North Sea in the processing of nitrogen, with nitrate dominating total nitrogen transport prior to the spring bloom (10 340×103 kg N inflow to the plume in March) and transport of nitrogen as ammonium, nitrate and POM in approximately equivalent amounts during summer (2560, 2960 and 2151×103 kg N inflow to the plume, respectively, in July). The box model also demonstrates more generally the need to assess nitrogen transport as nitrate, ammonium and POM if an improved understanding of the impact of nutrient input in shelf seas is to be achieved. 相似文献
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19.
Kirstin Dow 《Natural Hazards》2010,54(2):497-518
Regional newspaper coverage from 1998 to 2007 is examined to expand our understanding of the complexity of drought impacts
and vulnerability in the Carolinas. Coverage at the height of two droughts reports drought effects extending beyond first-order
impacts on broadly recognized sectors, such as agriculture, livestock, and water supplies to recreation and tourism and impacts
on businesses, manufacturing, and households. Impacts were accompanied by social controversies with near-term and long-term
planning and development implications. Key concepts in vulnerability analysis were used to structure the review of vulnerability
reporting. The coverage of differential vulnerability, although limited, identified individual, household, local, and cross-scale
factors that influenced the severity of impacts. Articles also highlighted additional interacting stresses in some sectors
contributing to the severity of impacts experienced. An elaborated understanding of drought impacts and vulnerabilities is
a necessary, early step in advancing efforts to developing a risk-based drought management approach. 相似文献
20.
Cusp geometry in MHD simulations 总被引:2,自引:0,他引:2
George Siscoe Nancy Crooker Keith Siebert Nelson Maynard Daniel Weimer Willard White 《Surveys in Geophysics》2005,26(1-3):387-407
The MHD simulations described here show that the latitude of the high-altitude cusp decreases as the IMF swings from North to South, that there is a pronounced dawn–dusk asymmetry at high-altitude associated with a dawn–dusk component of the IMF, and that at the same time there is also a pronounced dawn–dusk asymmetry at low-altitude. The simulations generate a feature that represents what has been called the cleft. It appears as a tail (when the IMF has a By component) attached to the cusp, extending either toward the dawn flank or the dusk flank depending on the dawn–dusk orientation of the IMF. This one-sided cleft connects the cusp to the magnetospheric sash. We compare cusp geometry predicted by MHD simulations against published observations based on Hawkeye and DMSP data. Regarding the high-altitude predictions, the comparisons are not definitive, mainly because the observations are incomplete or mutually inconsistent. Regarding the low-altitude prediction of a strong dawn–dusk asymmetry, the observations are unambiguous and are in good qualitative agreement with the prediction. 相似文献