The present study investigates the distribution of trace metals (Zn, Hg, Cd, Cu, and Pb), as indicators of pollution, in the
surficial offshore shelf sediments along the northern coast of Heraklion Prefecture (Crete, Mediterranean Sea). The concentrations
and the spatial distribution of the different trace metals, in relation to the sedimentological characteristics and the water
circulation pattern of the entire continental shelf, are associated with human inshore sources of pollutants located along
the coastline of the study area. Although the trace metal concentrations measured are higher than the background values, they
are not considered to be dangerous to human health, as they are lower than the standard values given by the World Health Organisation,
with only a few localised exceptions. Furthermore, results reveal the important role of local hydrodynamism that moves fine-grained
material and associated trace metals offshore (seawards to wave breaking zone) and then transports them eastwards by entrapping
them in the prevailing offshore shelf-water circulation. 相似文献
The present investigation concerns the formation and evolution of the dune field of the central part of the Kyparissiakos Gulf (western coast of Peloponnesos, Greece). This dune field is associated with the Kaiafas lagoon and consists of four dune lines that lie at distances of 600, 200, 100 and 70 m from the coastline. The dune field has developed on top of a barrier beach that formed subsequently to the completion of the last phase of rapid sea level rise, i.e. after 6,000 BP, consisting mostly of medium sand with good sorting, due to its aeolian formation. Assuming a steady wind regime and adequate sediment availability during the late Holocene, a period of approximately 1,350 years has been estimated to be the minimum time required for the formation of the dune field; this formation period may also include intervals when the development processes were more or less intensive. On the basis of radio-carbon dating, secondary fluctuations of air temperature and published information, it is proposed that the 4th (oldest) dune line started forming between the years 400 AD and 1,000 AD, whilst the 1st (youngest) dune line started forming after 1,520 AD. The dune field, especially its youngest line seems to be in equilibrium with its adjacent beach zone and the nearshore hydrodynamics, being beyond the reach of wave run-up. On the other hand, the dune field, over the past decades, has been subjected to intense human intervention (agriculture, construction, forest fires, etc.) that has locally destroyed and/or destabilised part of the dunes. Finally, the expected sea level rise, due to global warming, is undoubtedly a threat to the existence of the dune field. 相似文献
The transboundary Evros River discharges into the Alexandroupolis Gulf, located in the inner shelf of the northeastern Aegean
Sea, where it has formed an extended delta. Grain-size and mineralogical analyses of five sediment cores, collected in the
subaqueous delta, provide the following information about recent sedimentation processes in the northeastern part of the Aegean
shelf: (a) river mouth deposits, consisting of coarse-grained sediments, are mainly deposited in front of the active mouth,
whilst some sandy material is expected to be transported alongshore by nearshore currents; (b) delta front deposits are characterised
by fine-grained sediments that include evidence of human activities which have taken place, in a more intense way, since the
1950s; and (c) prodelta deposits are represented by almost uniform riverine mud that cover the pre-existed relict sands of
the shelf, indicating also the limit (some 15 km to the SW) of the influence of riverine sedimentation on the seabed of the
inner shelf of the Alexandroupolis Gulf. 相似文献
In this paper, a numerical procedure based on the finite element method is outlined to investigate pile behaviour in sloping ground, which involves two main steps. First a free-field ground response analysis is carried out using an effective stress based stress path model to obtain the ground displacements, and the degraded soil stiffness and strength over the depth of the soil deposit. Next a dynamic analysis is carried out for the pile. The interaction coefficients and ultimate lateral pressure of soil at the pile–soil interface are calculated using degraded soil stiffness and strength due to build-up of pore pressures, and the soil in the far field is represented by the displacements calculated from the free-field ground response analysis. Pore pressure generation and liquefaction strength of the soil predicted by the stress path model used in the free-field ground response analysis are compared with a series of simple shear tests performed on loose sand with and without an initial static shear stress simulating sloping and level ground conditions, respectively. Also the numerical procedure utilised for the analysis of pile behaviour has been verified using centrifuge data, where soil liquefaction has been observed in laterally spreading sloping ground. It is demonstrated that the new method gives good estimate of pile behaviour, despite its relative simplicity. 相似文献
An apparent shear flow instability occurred in the stably stratified night-time boundary layer on 6 October 1999 over the Cooperative Atmosphere–Surface Exchange Study (CASES-99) site in southeast Kansas. This instability promoted a train of billows which appeared to be in different stages of evolution. Data were collected by sonic anemometers and a high-frequency thermocouple array distributed on a 60 m tower at the site, and a high resolution Doppler lidar (HRDL), situated close to the tower. Data from these instruments were used to analyze the characteristics of the instability and the billow event. The instability occurred in a layer characterized by a minimum Richardson number Ri0.13, and where an inflection in the background wind profile was also documented. The billows, which translated over the site for approximately 30 min, were approximately L320 m in length and, after billow evolution they were contained in a layer depth H30 m. Their maximum amplitude, determined by HRDL data, occurred at a height of 56 m. Billow overturns, responsible for mixing of heat and momentum, and high-frequency intermittent turbulence produce kurtosis values above the Gaussian value of 3, particularly in the lower part of the active layer. 相似文献
A numerical method, based on a simplified elastic continuum boundary element method, is presented for the settlement analysis of axially loaded vertical piles and pile groups. The soil flexibility coefficients are evaluated using the analytical solutions for a layered elastic half space. Results are presented and compared with existing published solutions for the following cases: (i) piles in homogeneous soil, (ii) piles in finite soil layer, (iii) piles end-bearing on stiffer layer, (iv) piles socketted into stiffer bearing layer, and (v) piles in Gibson soil. Reasonably good agreement is obtained between the present solutions and existing published solutions. 相似文献
Summary Numerical simulations of increasing complexity are conducted to investigate topographic controls and ambient wind effects upon drainage flows along a portion of the Colorado Front Range in the central Rocky Mountains. A series of two-dimensional simulations show the effects upon the drainage flow of changing slope gradient at the mountain-plain interface. For a given mountain slope, a decrease in the slope of the plain decelerates the mountain drainage jet as it approaches the plain and causes the jet to elevate. The integrated effects of slope and valley drainage are presented with particle trajectories for a particular drainage basin along the Front Range. A nested grid simulation of drainage flow from multiple basins along the Front Range shows that basin area is an important factor in the strength of the drainage flow and that canyon topography variations greatly affect the behavior of the drainage jet as it flows through the canyon mouth onto the plain. Strong drainage winds developed on each of four case night simulations due to the presence of only weak ambient wind below mountaintop. The weak winds represent a decoupling of the near-surface from stronger winds above mountaintop. The canyon drainage exhibited substantial temporal variability in wind speed with the inclusion of ambient winds, due to interactions between ambient and drainage winds.With 11 Figures 相似文献
Τhis paper presents an analytical method for calculating the steady-state impedance factors of pile groups of arbitrary configuration subjected to harmonic vertical loads. The derived solution allows considering the effect of the actual pile geometry on the contribution of pile-soil-pile interaction to the response of the group, via the introduction of a new dynamic interaction factor, defined on the basis of soil resistance instead of pile displacements. The solution is first validated against a published solution for single piles that accounts for the effect of pile geometry on the generated ground vibrations. Accordingly, we show that the derived soil attenuation factor agrees well with existing solutions for pile groups in the high frequency range, but considerable differences are observed in both the stiffness and damping components of the computed impedance when the relative spacing between piles decreases. Numerical results obtained for typical problem parameters suggest that ignoring pile geometry effects while estimating the contribution of pile-soil-pile interaction in the response may lead to inaccurate results, even for relative large pile group spacings.