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31.
Mahmoud M. El Banna 《Environmental Geology》2008,54(6):1291-1299
Types, distribution, and origin of recent sand dunes between Rosetta and Idku, in the western sector of the Nile Delta, Egypt
were investigated. Sand samples from the dunes, beach, and seafloor were studied for grain size distribution and mineralogical
composition. It has been found that most of the dunes in the study area have been subjected to deterioration and removal due
to the construction of buildings and the International Coastal Highway. The remnant constitutes a damaged belt of foredunes
that extends from El Bouseily village to the west of Idku town. The dune’s origin is interpreted to be the result of coastal
drifting and the subsequent transport of sediments of the former Canopic Nile branch eastward by the predominant longshore
current and by aeolian processes. The blown sand grains accumulated to form a belt of coastal sand dunes of original longitudinal
and crescentic forms. Urbanization of the coast has severely altered the landscape. The study area is considered vulnerable
to the impacts of climate change and the expected rise in sea level. The outcome of potential sea level rise is serious; erosion
problems are expected to be exacerbated and vast areas from land and property would be lost. Thus, protection and preservation
the remaining dunes in the study area are vital requirements for shore protection. 相似文献
32.
M. Draoui J. Vias B. Andreo K. Targuisti J. Stitou El Messari 《Environmental Geology》2008,54(3):455-463
This paper presents the results of a comparative study relating to the application of four vulnerability mapping methods,
GOD, AVI, DRASTIC and SINTACS, in a pilot detritic aquifer situated in NW Morocco, known as the Martil–Alila aquifer. The
principal objective of this work is to determine the most suitable such methods for this aquifer type within a Mediterranean
context, and to show the effect of the rainfall variations that are characteristic of the Mediterranean climate on the degree
of vulnerability. The methods applied distinguish five classes of vulnerability, these being irregularly divided up in space,
with the division varying according to the method in question. The vulnerability maps obtained by the different methods strongly
suggest that the eastern half of the aquifer is more vulnerable to contamination than the western half, for all hydrological
situations. The effect of climatic conditions on the degree of vulnerability is well represented by the DRASTIC, according
to which the aquifer is moderately to strongly vulnerable during humid hydrological years and weakly to moderately vulnerable
during dry ones. For the other methods, this climatic effect is limited to the area occupied by the two predominant classes
(“High” and “Low” for GOD and “High” and “Moderate” for SINTACS) while it is null for AVI. In conclusion, DRASTIC appears
the most suitable for mapping the vulnerability to contamination of Mediterranean coastal detritic aquifers such as the Martil–Alila
aquifer. 相似文献
33.
34.
Numerical estimation of REV and permeability tensor for fractured rock masses by composite element method 总被引:2,自引:0,他引:2
The Monte Carlo method is used to generate parent stochastic discrete fracture network, from which a series of fractured rock samples of different sizes and orientations are extracted. The fracture network combined with a regular grid forms composite element mesh of the fractured rock sample, in which each composite element is composed of sub‐elements incised by fracture segments. The composite element method (CEM) for the seepage is implemented to obtain the nodal hydraulic potential as well as the seepage flow rates through the fractured rock samples. The application of CEM enables a large quantity of stochastic tests for the fractured rock samples because the pre‐process is facilitated greatly. By changing the sizes and orientations of the samples, the analysis of the seepage characteristics is realized to evaluate the variation of the permeability components, the existence of the permeability tensor and the representative element volume. The feasibility and effectiveness are illustrated in a numerical example. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
35.
Biaxial test simulations using a packing of polygonal particles 总被引:1,自引:0,他引:1
The mechanical response of cohesionless granular materials under monotonic loading is studied by performing molecular dynamic simulations. The diversity of shapes of soil grains is modelled by using randomly generated convex polygons as granular particles. Results of the biaxial test obtained for dense and loose media show that samples achieve the same void ratio at large strains independent of their initial density state. This limit state resembles the so‐called critical state of soil mechanics, except for some stress fluctuations, which remain for large deformations. These fluctuations are studied at the micro‐mechanical level, by following the evolution of the co‐ordination number, force chains and the fraction of the sliding contacts of the sample. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
36.
M. El Ghorfi F. Melcher T. Oberthür A. E. Boukhari L. Maacha A. Maddi M. Mhaili 《Mineralogy and Petrology》2008,92(1-2):59-80
Summary The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite.
Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr2O3) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios
[(Mg/(Mg + Fe2+)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in
sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing
laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel.
Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display
trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides,
arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition
of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel
in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated
through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides,
sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As
ores.
Author’s address: Dr. Frank Melcher, Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany 相似文献
37.
We test the hypothesis that flexural isostatic compensation of the mass removed by enhanced Quaternary erosion is responsible for uplift of the Western European Alps and their forelands. We use two well‐preserved and well‐dated (1.8 Ma) abandonment surfaces of foreland basin remnants in SE France (the Chambaran and Valensole plateaux) as passive benchmarks for tilting of the foreland. Estimating their initial slope from morphometric scaling relationships, we determine bulk post‐depositional tilting of 0.5–0.8% for these surfaces. The calculated isostatic response of the Alpine lithosphere to erosional unloading, using the method recently proposed by Champagnac et al. [Geology 35 (2007) 195–198] , yields a predicted tilting of 0.3–0.4% in the considered areas, explaining approximately half of the determined post‐depositional tilting. Such long‐term deformation being insensitive to cyclic loading/unloading because of glaciations, we suspect the other half to be related to as yet undetermined long‐wavelength and long‐lived tectonic process(es). 相似文献
38.
Nonlinear Analysis of Local Site Effects on Seismic Ground Response in the Bam Earthquake 总被引:1,自引:0,他引:1
M. H. T. Rayhani M. H. El Naggar S. H. Tabatabaei 《Geotechnical and Geological Engineering》2008,26(1):91-100
The influence of local geologic and soil conditions on the intensity of ground shaking is addressed in this study. The amplification
of the ground motion due to local site effects resulted in severe damage to dwellings in the Bam area during the 2003 Bam
Earthquake. A unique set of strong motion acceleration recordings was obtained at the Bam accelerograph station. Although
the highest peak ground acceleration recorded was the vertical component (nearly 1 g), the longitudinal component (fault-parallel
motion) clearly had the largest maximum velocity as well as maximum ground displacement. Subsurface geotechnical and geophysical
(down-hole) data in two different sites have been obtained and used to estimate the local site condition on earthquake ground
motion in the area. The ground response analyses have been conducted considering the nonlinear behavior of the soil deposits
using both equivalent linear and nonlinear approaches. The fully nonlinear method embodied in FLAC was used to evaluate the
nonlinear soil properties on earthquake wave propagation through the soil layer, and compare with the response from the equivalent
linear approach. It is shown that thick alluvium deposits amplified the ground motion and resulted in significant damage in
residential buildings in the earthquake stricken region. The comparison of results indicated similar response spectra of the
motions for both equivalent and nonlinear analyses, showing peaks in the period range of 0.3–1.5 s. However, the amplification
levels of nonlinear analysis were less than the equivalent linear method especially in long periods. The observed response
spectra are shown to be above the NEHRP building code design requirements, especially at high frequencies. 相似文献
39.
The interstitial water composition (
, alkalinity, Ca2+, Mg2+, Sr2+, Na+, K+) and the cation exchange capacity (CEC) were determined for the muddy sediments of the continental shelf off the Gironde Estuary (France), in the area where the sediment represents the deposit of the muddy suspension of the river. In comparison with seawater concentrations, the pore waters below 10 cm depth, show depletions of
and Ca2+ and below a 30 cm depth show depletions of Mg2+. Inversely, the upper 10 cm an enrichment of Ca2+ concentration, and an increase of K+ concentration to a 40 cm depth. High values of
are observed at the top 4 cm. Alkalinity enrichment is observed along the length of the core. Applying the alkalinity models for the sediment below a 10 cm depth demonstrates generally that calculated alkalinities are higher than the measured ones. Ca2+ dissolution occurs at the first 10 cm and authigenic carbonate precipitation starts beneath that level. Mg2+ depletion is accompanied by bicarbonate loss. This proves that Mg2+ depletion is due to a Mg-silicate reaction. The result of the CEC does not confirm the Mg2+ uptake by clay minerals in exchangeable site, under reducing conditions. Diffusion and bioturbation play an important role in the pore water concentration at the top of the core. 相似文献
40.
S. C. Gonalves J. C. Marques M. A. Pardal M. F. Bouslama M. El Gtari F. Charfi-Cheikhrouha 《Estuarine, Coastal and Shelf Science》2003,58(4):901-916
The biology, population dynamics, and production of Talorchestia brito were studied at two sandy beaches located on the Atlantic (Portugal) and on the Mediterranean (Tunisia) coasts, respectively. The seasonal variation in abundance and the overall densities were similar in both populations. Reproduction occurred from February to September in the Atlantic, and from March to early November in the Mediterranean. The sex ratio was male biased in the Atlantic, and female biased in the Mediterranean. Based on data from the Atlantic population, both abundance and the proportion of reproductive females were positively correlated with temperature, while the proportion of juveniles in the population was positively correlated with temperature and sediment moisture. On average, individuals from the Atlantic were larger than the ones from the Mediterranean. Life span was estimated at six to nine months in the Atlantic, and five to eight months in the Mediterranean. Talorchestia brito was shown to be a semiannual species, with iteroparous females producing two broods per year, and exhibited a bivoltine life cycle. The minimum age required for males' and females' sexual differentiation and for female sexual maturation was shorter in the Mediterranean. Growth production (P) was estimated at 0.19 g m−2 y−1 ash free dry weight (AFDW; 4.3 kJ m−2 y−1) in the Atlantic population, and 0.217 g m−2 y−1 AFDW (4.9 kJ m−2 y−1) in the Mediterranean one. Elimination production (E) was estimated at 0.35 g m−2 y−1 AFDW (7.9 kJ m−2 y−1) in the Atlantic, and 0.28 g m−2 y−1 AFDW (6.3 kJ m−2 y−1) in the Mediterranean. The average annual biomass (
) (standing stock) was estimated at 0.032 g m−2 in the Atlantic beach, and 0.029 g m−2 in the Mediterranean one, resulting, respectively, in
ratios of 5.9 and 7.5 and
ratios of 10.8 and 9.6. Like other talitrids, T. brito exhibited geographic variation in morphometrical characteristics, sex ratio, growth rates, life span, and reproduction period, with the Atlantic population presenting a slower life history. 相似文献