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141.
The effects of frictional forces acting on the sidewalls of buried box culverts are presented as determined with finite element method (FEM) and detailed soil modelling. The possibility of reducing earth pressure on deeply buried concrete box culverts by the imperfect trench installation (ITI) method has been contemplated during the last several decades. There have been limited research results published primarily regarding the qualitative aspect of load reduction in ITIs. It was found during the course of this study that significant frictional forces develop along the sidewalls of box culverts and adjacent sidefills in ITIs. Current American Association of State Highway and Transportation Officials provisions do not consider these frictional forces, but they cannot be neglected in ITIs, as their effect is dominant. An optimum geometry for the soft zone in ITIs is presented to maximize earth load reductions. The soil–structure interaction at the box culvert–soil interface was found to have a significant effect on total earth pressure acting on the bottom slab. Predictor equations for earth load reduction rates were formulated for ITIs incorporating the optimum soft zone geometry based on the FEM. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
142.
We describe a powerful signal processing method, the continuous wavelet transform, and use it to analyze radial structure in Cassini ISS images of Saturn's rings. Wavelet analysis locally separates signal components in frequency space, causing many structures to become evident that are difficult to observe with the naked eye. Density waves, generated at resonances with saturnian satellites orbiting outside (or within) the rings, are particularly amenable to such analysis. We identify a number of previously unobserved weak waves, and demonstrate the wavelet transform's ability to isolate multiple waves superimposed on top of one another. We also present two wave-like structures that we are unable to conclusively identify. In a multi-step semi-automated process, we recover four parameters from clearly observed weak spiral density waves: the local ring surface density, the local ring viscosity, the precise resonance location (useful for pointing images, and potentially for refining saturnian astrometry), and the wave amplitude (potentially providing new constraints upon the masses of the perturbing moons). Our derived surface densities have less scatter than previous measurements that were derived from stronger non-linear waves, and suggest a gentle linear increase in surface density from the inner to the mid-A Ring. We show that ring viscosity consistently increases from the Cassini Division outward to the Encke Gap. Meaningful upper limits on ring thickness can be placed on the Cassini Division (3.0 m at r∼118,800 km, 4.5 m at r∼120,700 km) and the inner A Ring (10-15 m for r<127,000 km). 相似文献
143.
Matthew S. Lachniet 《Quaternary Science Reviews》2009,28(5-6):412-432
Variations in speleothem oxygen-isotope values (δ18O) result from a complicated interplay of environmental controls and processes in the ocean, atmosphere, soil zone, epikarst, and cave system. As such, the controls on speleothem δ18O values are extremely complex. An understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem δ18O as paleoclimate and paleoenvironmental proxies, and is best complemented by study of site-specific cave processes such as infiltration, flow routing, drip seasonality and saturation state, and cave microclimate, among others. This review is a process-based summary of the multiple controls on δ18O in the atmosphere, soil, epikarst, and speleothem calcite, illustrated with case studies. Primary controls of δ18O in the atmosphere include temperature and relative humidity through their role in the multiple isotope “effects”. Variability and modifications of water δ18O values in the soil and epikarst zones are dominated by evaporation, mixing, and infiltration of source waters. The isotopically effective recharge into a cave system consists of those waters that participate in precipitation of CaCO3, resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state. Recent modeling, experimental, and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid CaCO3, and have implications for the ‘Hendy’ test. To assist interpretation of speleothem δ18O time series, quantitative and semi-quantitative δ18O-climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records. Finally, several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of δ18O to reconstruct past climate changes in regions that have been typically poorly represented in paleoclimate records, such as tropical and subtropical terrestrial locations. The new approach to speleothem paleoclimatology emphasizes climate teleconnections between regions and attribution of forcing mechanisms. Such investigations allow paleoclimatologists to infer regional to global-scale climate dynamics. 相似文献
144.
Matthew S. Lachniet Leah Johnson Yemane Asmerom Stephen J. Burns Victor Polyak William P. Patterson Lindsay Burt April Azouz 《Quaternary Science Reviews》2009,28(27-28):3348-3360
We present a high-resolution terrestrial archive of Central American rainfall over the period 100–24 and 8.1–6.5 ka, based on δ18O time series from U-series dated stalagmites collected from a cave on the Pacific Coast of Costa Rica. Our results indicate substantial δ18O variability on millennial to orbital time scales that is interpreted to reflect rainfall variations over the cave site. Correlations with other paleoclimate proxy records suggest that the rainfall variations are forced by sea surface temperatures (SST) in the Atlantic and Pacific Oceans in a fashion analogous to the modern climate cycle. Higher rainfall is associated with periods of a warm tropical North Atlantic Ocean and large SST gradients between the Atlantic and Pacific Oceans. Rainfall variability is likely linked to the intensity and/or latitudinal position of the intertropical convergence zone (ITCZ). Periods of higher rainfall in Costa Rica are also associated with an enhanced sea surface salinity gradient on either side of the isthmus, suggesting greater freshwater export from the Atlantic Basin when the ITCZ is stronger and/or in a more northerly position. Further, wet periods in Central America coincide with high deuterium excess values in Greenland ice, suggesting a direct link between low latitude SSTs, tropical rainfall, and moisture delivery to Greenland. Our results indicate that a stronger tropical hydrological cycle during warm periods and large inter-ocean SST gradients enhanced the delivery of low latitude moisture to Greenland. 相似文献
145.
Matthew J.R. Simpson Glenn A. Milne Philippe Huybrechts Antony J. Long 《Quaternary Science Reviews》2009,28(17-18):1631-1657
We constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. Our new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. Our results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat. 相似文献
146.
Russell Cannon Michael Drinkwater Alastair Edge Daniel Eisenstein Robert Nichol Phillip Outram Kevin Pimbblet Roberto De Propris Isaac Roseboom David Wake Paul Allen Joss Bland-Hawthorn Terry Bridges Daniel Carson Kuenley Chiu Matthew Colless Warrick Couch Scott Croom Simon Driver Stephen Fine Paul Hewett Jon Loveday Nicholas Ross Elaine M. Sadler Tom Shanks Robert Sharp J. Allyn Smith Chris Stoughton Peter Weilbacher Robert J. Brunner Avery Meiksin Donald P. Schneider 《Monthly notices of the Royal Astronomical Society》2006,372(1):425-442
147.
Two populations of minor bodies in the outer Solar System remain particularly elusive: Scattered Disk Objects and Sedna-like objects. These populations are important dynamical tracers, and understanding the details of their spatial- and size-distributions will enhance our understanding of the formation and on-going evolution of the Solar System. By using newly-derived limits on the maximum heliocentric distances that recent pencil-beam surveys for trans-neptunian objects were sensitive to, we determine new upper limits on the total numbers of distant SDOs and Sedna-like objects. While generally consistent with populations estimated from wide-area surveys, we show that for magnitude-distribution slopes of α ? 0.7-1.0, these pencil-beam surveys provide stronger upper limits than current estimates in literature. 相似文献
148.
An important aspect of shellfish restoration projects is to evaluate whether potential sites are likely to be recolonised naturally once disturbances are removed and habitats are restored. Similarly, if active translocation or reseeding of juveniles or adults is undertaken, it is important to understand whether these introduced populations will be self‐maintaining in future, and whether any seed they produce are likely to be retained within the restored area. We used a combined hydrodynamic and particle tracking model to predict larval dispersion patterns for the common cockle Austrovenus stutchburyi under different hydrodynamic scenarios for seven release locations in Whangarei Harbour, New Zealand. Our results implied that sites varied substantially in their potential for self‐seeding and for exporting seed to other locations. For sites with more restricted dispersal, the model predicted that most larvae originating at these sites would settle inside the release region (68–94% for passive particle simulations), whereas relatively few larvae originating from the other release sites settled at these sites. In contrast, model larvae released from sites exhibiting high connectivity dispersed to all sub‐regions in the harbour, and export outside of the model region was high. Forthcoming field validation of these model predictions will result in better integration of hydrodynamic connectivity in whole estuary restoration programs. 相似文献
149.
Macroinvertebrate community composition was compared across streams draining catchments dominated by either native bush, agricultural or urban land uses within the Water of Leith stream catchment near Dunedin, New Zealand. Land use was associated with differences in taxon richness and faunal composition of communities present in each stream. The mean abundance levels of Ephemeroptera, Plecoptera, and Trichoptera were highest in native bush catchments, and lowest in urban catchments. In contrast, the mean abundance of Oligochaeta exhibited the opposite pattern. Increasing dominance of the urban and agricultural streams by pollution tolerant taxa was reflected in the Macroinvertebrate Community Index and Quantitative Macroinvertebrate Community Index scores. 相似文献
150.
The WFD has introduced an international commitment to assess the ecological status of transitional waters (TWs), within which fish communities are a key biological monitoring component. The Transitional Fish Classification Index (TFCI) outlined in this paper uses 10 ecological measures to analyse fish populations caught from various ecological niches using a variety of gear types within the Thames estuary. These reach and method-specific communities are then compared to a reference population created from a 'healthy' population from TWs of a similar type. The results indicate a progressive downstream increase the quality of fish communities, consistent with previous work; variation between methods can be accounted for by gear selectivity. Overall, the TFCI is an effective communication tool for converting ecological information into an easily understood format for managers, policy makers and the general public. 相似文献