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241.
A general thermo-hydro-mechanical framework for the modelling of internal erosion is proposed based on the theory of mixtures applied to two-phase porous media. The erodible soil is partitioned in two phases: one solid phase and one fluid phase. The solid phase is composed of nonerodible grains and erodible particles. The fluid phase is composed of water and fluidized particles. Within the fluid phase, species diffuse. Across phases, species transfer. The modelling of internal erosion is contributed directly by mass transfer from the solid phase towards the fluid phase. The constitutive relations governing the thermomechanical behaviour, generalised diffusion, and transfer are structured by the dissipation inequality. The particular case of soil suffusion is investigated with a focus on constitutive laws. A new constitutive law for suffusion is constructed based on thermodynamic conditions and experimental investigations. This erosion law is linearly related to the power of seepage flow and to the erosion resistance index. Owing to its simplicity, this law tackles the overall trend of the suffusion process and permits the formulation of an analytical solution. This new model is then applied to simulate laboratory experiments, by both analytical and numerical methods. The comparison shows that the newly developed model, which is theoretically consistent, can reproduce correctly the overall trend of the cumulated eroded mass when the permeability evolution is small. In addition, the results are provided for four different materials, two different specimen sizes, and various hydraulic loading paths to demonstrate the applicability of the new proposed law. 相似文献
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A new paleolimnological dataset from Lake Pacucha (13 °S, 3095 m elevation) in the Peruvian Andes provides evidence of changes in lake level over the past 24,700 yr. A late-glacial highstand in lake level gave way to an early-Holocene lowstand. This transition appears to have paralleled precessional changes that would have reduced insolation during the wet-season. The occurrence of benthic/salt-tolerant diatoms and CaCO3 deposition suggest that the lake had lost much of its volume by c. 10,000 cal yr BP. Pronounced Holocene oscillations in lake level included a second phase of low lake level and heightened volatility lasting from c. 8300 to 5000 cal yr BP. While a polymictic lake formed at c. 5000 cal yr BP. These relatively wet conditions were interrupted by a series of drier events, the most pronounced of which occurred at c. 750 cal yr BP. Paleolimnological changes in the Holocene were more rapid than those of either the last glacial maximum or the deglacial period. 相似文献
245.
Orozco-Borbón MV Rico-Mora R Weisberg SB Noble RT Dorsey JH Leecaster MK McGee CD 《Marine pollution bulletin》2006,52(10):1190-1196
This survey was part of a Binational Program (Mexico-United States) in microbiological water quality, with a goal to assess the shoreline bacteriological water quality from Tijuana to Ensenada, Mexico. Samples were collected at 29 sites (19 beaches and 10 outfalls), from the United States border to Punta Banda, Baja California, during summer (1998) and winter (1999). Total coliforms, fecal coliforms and enterococci were used as bacterial indicators. Standard methods were used for total and fecal coliforms, while the Enterolert quick method (IDEXX) was used for the enterococci. Compared with outfalls, the beaches exceeded water quality standards by a small percent, 25.3% in summer and 17% in winter. For outfalls, the percentage of shoreline that exceeded bacterial indicator thresholds had a minor value in summer (32.7%) than in winter (50%). Sites near wastewater discharges had the lowest quality and did not meet the microbiological water quality criteria for recreational use. 相似文献
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For almost two decades, politicians have been negotiating temperature limits to which anthropogenic global warming should be restricted, and 2 °C has emerged as benchmark for danger. However, there has been a lack of scientific research into the implications of such a change for African climate. This study aims to provide information for mitigation debates; through an examination of temperature and precipitation changes in Africa associated with 1 °C, 2 °C, 3 °C, and 4 °C of global warming. Data from Global Climate Models show little significant precipitation change at 1 °C, then larger anomalies at 2 °C which are strengthened and extended at 3 °C and 4 °C, including a wet signal in East Africa, and dry signals in Southern Africa, the Guinea Coast, and the west of the Sahel. Some of the models project changes with potential for severe societal implications. Despite the uncertainty attached to these projections, they highlight risks associated with 2 °C and beyond. Using these findings as a framework for impact assessment and evaluation, further research has the potential to uncover the implications of global warming for African regions. 相似文献
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Rachel Warren Jeff Price Andreas Fischlin Santiago de la Nava Santos Guy Midgley 《Climatic change》2011,106(2):141-177
In a meta-analysis we integrate peer-reviewed studies that provide quantified estimates of future projected ecosystem changes
related to quantified projected local or global climate changes. In an advance on previous analyses, we reference all studies
to a common pre-industrial base-line for temperature, employing up-scaling techniques where necessary, detailing how impacts
have been projected on every continent, in the oceans, and for the globe, for a wide range of ecosystem types and taxa. Dramatic
and substantive projected increases of climate change impacts upon ecosystems are revealed with increasing annual global mean
temperature rise above the pre-industrial mean (ΔTg). Substantial negative impacts are commonly projected as ΔTg reaches and exceeds 2°C, especially in biodiversity hotspots. Compliance with the ultimate objective of the United Nations
Framework Convention on Climate Change (Article 2) requires that greenhouse gas concentrations be stabilized within a time
frame “sufficient to allow ecosystems to adapt naturally to climate change”. Unless ΔTg is constrained to below 2°C at most, results here imply that it will be difficult to achieve compliance. This underscores
the need to limit greenhouse gas emissions by accelerating mitigation efforts and by protecting existing ecosystems from greenhouse-gas
producing land use change processes such as deforestation. 相似文献
250.
Rachel Stubbington Paul J. Wood Ian Reid 《Aquatic Sciences - Research Across Boundaries》2011,73(4):499-511
A key ecological role hypothesized for the hyporheic zone is as a refugium that promotes survival of benthic invertebrates
during adverse conditions in the surface stream. Many studies have investigated use of the hyporheic refugium during hydrological
extremes (spates and streambed drying), and recent research has linked an increase in the abundance of benthic invertebrates
within hyporheic sediments to increasing biotic interactions during flow recession in a temporary stream. This study examined
spatial variability in the refugial capacity of the hyporheic zone in two groundwater-dominated streams in which flow permanence
varied over small areas. Two non-insect taxa, Gammarus pulex and Polycelis spp. were common to both streams and were investigated in detail. Hydrological conditions in both streams comprised a four-month
period of flow recession and low flows, accompanied by reductions in water depth and wetted width. Consequent declines in
submerged benthic habitat availability were associated with increases in population densities of mobile benthic taxa, in particular
G. pulex. The reduction in the spatial extent of the hyporheic zone was minimal, and this habitat was therefore a potential refugium
from increasing biotic interactions in the benthic sediments. Concurrent increases in the hyporheic abundance and hyporheic
proportion of a taxon’s total (benthic + hyporheic) population were considered as evidence of active refugium use. Such evidence
was species-specific and site-specific, with refugium use being observed only for G. pulex and at sites dominated by downwelling water. A conceptual model of spatial variability in the refugial capacity of the hyporheic
zone during habitat contraction is presented, which highlights the potential importance of the direction of hydrologic exchange. 相似文献