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Jerry C. Ritchie Vernon L. Finney Kenneth J. Oster Carole A. Ritchie 《Geomorphology》2004,61(3-4):347-360
Over the past 150 years, major land use changes have occurred in the Stemple Creek Watershed in northern California that have caused erosion to move soils from the upland to the flood plain, stream channels, and the bay. The purpose of this study is to document the recent (1954 to present) sediment deposition patterns in the flood plain area adjacent to Stemple Creek using the 137Cesium technique. Sediment deposition ranged from 0.26 to 1.84 cm year−1 for the period from 1964 to 2002 with an average of 0.85±0.41 cm year−1. Sediment deposition rates were higher for the 1954 to 1964 period with a range of 0.31–3.50 cm year−1 and an average of 1.29±1.04 cm year−1. These data indicate that sediment deposition in the flood plain has decreased since the middle 1950s, probably related to reduction in row crop agriculture and an increase in pasturelands. This study shows that the flood plains in the Stemple Creek Watershed are a significant sink for the soils being eroded from the upland area. Given the significance of the flood plain for trapping eroded materials before they reach the stream channels or the bay, efforts need to be made to manage these flood plain areas to insure that they do not change and become a source rather than a sink for eroded materials as improved management practices on the upland areas reduce sediment input to the flood plain. 相似文献
3.
We have used a first-principle approach based on the calculation of generalised stacking faults (GSF) to study the influence
of pressure on the mechanical properties of forsterite. Six cases corresponding to [100] glide over (010), (021) and (001),
and [001] glide over (100), (010) and (110) have been considered. The relaxed energy barriers associated with plastic shear
have been calculated by constraining the Si atoms to move perpendicular to the fault plane and allowing Mg and O atoms to
move in every direction. These conditions, which preserve dilations as a relaxation process, introduce Si–O tetrahedral tilting
as an additional relaxation mechanism. Relaxed GSF show little plastic anisotropy of [100] glide over different planes and
confirms that [001] glide is intrinsically easier than [100] glide. The GSF are affected by the application of a 10 GPa confining
pressure with a different response for each slip system that cannot be explained by sole elastic effect. In particular, [100](010)
is found to harden significantly under pressure compared to [001](010). Our results give the first theoretical framework to
understand the pressure-induced change of dominant slip systems observed by Couvy et al. (in Eur J Mineral 16(6):877–889,
2004) and P. Raterron et al. (in GRL, submitted). It appears necessary to account for the influence of pressure on the mechanical
properties of silicates in the context of the deep Earth. 相似文献
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Mathilde Cordier 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2018,12(2):109-122
Structural sliding stability of gravity dams is generally quantified using deterministic factor of safety, FSdet. Large FSdet (e.g. 3 in normal condition), are used in existing guidelines to guard against material and load uncertainties. Some guidelines allow an arbitrary reduction in FSdet (e.g. 2) when the knowledge in strength parameters increases from material test data. Yet, those reduced FSdet are not based on a rational consideration of uncertainties. Propagation of uncertainties could be done using probabilistic analyses, such as Monte-Carlo simulations (MC) which are complex and challenging for practical use. There is thus a need to develop simplified reliability based safety assessment procedures that could rationalise the adjustment of FSdet. This paper presents a progressive analysis methodology using four safety evaluation formats of increasing complexity: (i) deterministic, (ii) semi-probabilistic (partial coefficient), (iii) reliability based Adjustable Factor of Safety (AFS), and (iv) probabilistic. Comprehensive comparisons are made for the sliding safety evaluation of a 80 m gravity dam. Results are presented in terms of sliding factors of safety, allowable water levels, and demand/capacity ratios. It is shown that the AFS formulation, using direct integration, is simple and practical to use in complement to existing dam safety guidelines before undertaking MC simulations. 相似文献
7.
Computational Geosciences - The modeling of thermodynamic equilibria leads to complex nonlinear chemical systems which are often solved with the Newton-Raphson method. But this resolution can lead... 相似文献
8.
Existing paradigms suggest that mangrove leaf litter is processed primarily via the detrital pathway in forests in the Caribbean biogeographic realm whereas herbivorous crabs are relatively more important litter processors in the Indo-West Pacific. To test this hypothesis, we used pitfall traps to collect intertidal crabs to characterize the crab fauna in a mangrove estuary in southwest Florida. We also tethered mangrove leaves to determine if herbivorous crabs are major leaf consumers there. We compared the results with previously published data collected in an analogous manner from forests in northeastern Australia. The crab fauna in Rookery Bay, Florida, is dominated by carnivorous xanthid and deposit-feeding ocypodid crabs whereas that of the Murray River in northeastern Australia is dominated by herbivorous grapsid crabs. No leaves tethered at five sites in the forests in Southwest Florida were taken by crabs. This contrasts greatly with reported values of leaf removal by crabs in Australian forests of 28–79% of the leaves reaching the forest floor. These differences in the faunal assemblages and in the fate of marked or tethered leaves provide preliminary support for the hypothesis that leaf litter is in fact processed in fundamentally different ways in the two biogeographic realms. 相似文献
9.
We describe the use of flume nets for passively, quantitatively, and nondestructively sampling fishes and macrocrustaceans on tidal marsh surfaces. We captured 3,765 organisms of 23 species in 118 samples using six such nets in a Virginia tidal freshwater marsh in 1984. Efficiency estimates for four common species of fishes range from 53 to 80%. Flume nets are most suited to the collection of long-term data and are particularly useful in elucidating seasonal trends in species composition and relative abundance. These nets are also useful in comparing different microhabitats within and between marshes. This method is most applicable to intertidal habitats with predictable lunar tides, including mud flats, mangrove swamps, and other wetlands. 相似文献
10.
Carole A. S. Mandryk 《Journal of Paleolimnology》1996,16(1):37-57
Stratigraphic pollen analysis done on sediment cores from two sites in the upper North Saskatchewan drainage basin of the eastern slopes foothills of the Rocky Mountains in west central Alberta, Canada combined with sedimentological data provide a local vegetational and environmental history. Radiocarbon AMS dates provide a chronology back to 17960 BP. Reconstruction and interpretation of the local pollen zones includes reevaluation of steppe and grassland as analogs for full- and late-glacial vegetation. Regional vegetation from c. 17960 to 16 100 BP is interpreted as an extremely cold semi-arid Artemisia steppe, the vegetation c. 16 100 to 11 900 BP as an Artemisia-Betula shrubland, and the vegetation c. 11 900–10 200 BP as a Picea woodland, in an environment characterized by consistently arid and windy conditions. This reconstruction emphasizes the significance of aridity, as opposed to simply low temperatures, as the critical factor in determining the late Quaternary vegetation of Alberta.This is the 18th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers. 相似文献