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排序方式: 共有674条查询结果,搜索用时 359 毫秒
91.
Olga Sarychikhina Ewa Glowacka Braulio Robles F. Alejandro Nava Miguel Guzmán 《Pure and Applied Geophysics》2015,172(11):3139-3162
92.
When fine particles are involved, cohesive properties of sediment can result in flocculation and significantly complicate sediment process studies. We combine data from field observations and state-of-the-art modeling to investigate and predict flocculation processes within a hypertidal estuary. The study site is the Welsh Channel located at the entrance of the Dee Estuary in Liverpool Bay. Field data consist of measurements from a fixed site deployment during 12–22 February 2008. Grain size, suspended sediment volume concentration, and current velocity were obtained hourly from moored instruments at 1.5 m above bed. Near-bottom water samples taken every hour from a research vessel are used to convert volume concentrations to mass concentrations for the moored measurements. We use the hydrodynamic model Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) coupled with the turbulence model General Ocean Turbulence Model (GOTM) and a sediment module to obtain three-dimensional distributions of suspended particulate matter (SPM). Flocculation is identified by changes in grain size. Small flocs were found during flood and ebb periods—and correlate with strong currents—due to breakup, while coarse flocs were present during slack waters because of aggregation. A fractal number of 2.4 is found for the study site. Turbulent stresses and particle settling velocities are estimated and are found to be related via an exponential function. The result is a simple semiempirical formulation for the fall velocity of the particles solely depending on turbulent stresses. The formula is implemented in the full three-dimensional model to represent changes in particle size due to flocculation processes. Predictions from the model are in agreement with observations for both settling velocity and SPM. The SPM fortnight variability was reproduced by the model and the concentration peaks are almost in phase with those from field data. 相似文献
93.
94.
Abstract Laboratory tests were conducted on compacted marine sediments to study the effect of salt concentration of permeating fluid on its permeability characteristics. Deep sea sediment samples were collected from water depths varying from 3700 to 4500 m off Mauritius coast. Liquid limit and plasticity index varied widely from 45 to 75 and 10 to 35, respectively. Permeability was found at different void ratios with distilled water and 0.2, 0.4, and 0.8 N NaCl solutions as permeating fluid. It was found that permeability increases with an increase in salt concentration for a given void ratio. This is explained by diffused double layer theory. Also, the rate of increase in permeability decreases with increase in salt concentration. The effect of salt concentration seemed to be less at higher void ratios. 相似文献
95.
Jennifer M. Brown Rodolfo Bola?os Michael J. Howarth Alejandro J. Souza 《Ocean Dynamics》2012,62(7):969-982
Sea level comprises a mean level, tidal elevation and a residual elevation. Knowledge of what causes maximum water levels is often key in coastal management. However, different methods to extract deviations in water level (residuals) from modelled and observed elevation can give different results. The Dee Estuary, northwest England is a macrotidal estuary that undergoes periodic stratification. It is used here to demonstrate methods to extract the residual water level in response to the following interactive processes: tidal, river-induced stratification and flow, meteorology and waves. Using modelling techniques, the interaction and contribution of different physical processes are investigated. Classical harmonic tidal analysis, model simulations and filtering techniques have been used to “de-tide” the total elevation for short-term (approximately month long) records. Each technique gives a different result highlighting the need to select the correct method for a required study. Analysis of the residual components demonstrates that all processes inducing residuals interact with the tide generating a semi-diurnal residual component. It is suggested that modelling methods enable the full effect of tidal interaction to remain in the residual, whilst harmonic tidal analysis (partly) modify and filtering methods (fully) remove this component of the residual. The analysis methods presented and their influences on the resultant residual are applicable to other study sites. However, when applied specifically to the mouth of the Dee Estuary, the external surge is found to be the main contributor to the total residual, whilst local wind and stratification effects are of secondary importance. 相似文献
96.
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98.
Inga Golbeck Xin Li Frank Janssen Thorger Brüning Jacob W. Nielsen Vibeke Huess Johan Söderkvist Bjarne Büchmann Simo-Matti Siiriä Olga Vähä-Piikkiö Bruce Hackett Nils M. Kristensen Harald Engedahl Ed Blockley Alistair Sellar Priidik Lagemaa Jose Ozer Sebastien Legrand Patrik Ljungemyr Lars Axell 《Ocean Dynamics》2015,65(12):1603-1631
99.
Yoana Del-Pilar-Ruso Jose Antonio de-la-Ossa-Carretero Francisca Giménez-Casalduero Jose Luis Sánchez-Lizaso 《Marine pollution bulletin》2010,60(11):1930-1938
The European Water Framework Directive (WFD) and the Urban Wastewater Treatment Directive (UWWTD) aim to improve water quality and reduce the environmental impact of wastewater, by requiring member states to introduce better treatment levels. This study was carried out in order to detect the most suitable treatment type or flow level for reducing benthic environmental impact. Therefore, soft-bottom polychaete assemblages affected by discharges of six treatment plants, with different treatment technologies and flow rates, were analysed over three consecutive years. Polychaete assemblages changed as a result of sewage discharge. These changes were related to treatment level and flow rates. Both disposal quality and quantity are important since changes in soft-bottom assemblages in stations affected by pre-treated sewage with medium and high flow rates are observed. However, the location with a low-flow pre-treated effluent has similar polychaete assemblages with regard to locations with biological treatment or controls. 相似文献
100.
In the paleogeographic reconstruction of Mexico and northern Central America, an ever-increasing amount of evidence shows
that the entire region is a collage of suspect terranes transported from abroad, whose timing and sense of motion are now
beginning to be understood. Among these, the Chortis block (nuclear Central America) and the Baja California Peninsula have
been proposed as pieces of continent separated from the Pacific coast of southwestern Mexico, that have moved either southeastward
by the Farallon plate or northwestward by the Kula plate. Previous studies mainly confined to the northern margin of the Chortis
block, confirmed a left-lateral displacement of 130 km in Neogene time. Further studies made northwestward along the Mexican
coast provided a better understanding of magmatic and metamorphic processes in the area, and suggested times of detachment
increased to 30 Ma, 40 Ma, and 66 Ma. The pre-detachment westernmost position of the block has changed, depending on the model
chosen, from Puerto Vallarta and beyond, to the current position. Here we show that the isotopic mineral ages from coastal
granites along the coast from Puerto Vallarta, Jalisco (80 Ma) to Puerto Angel, Oaxaca (11 Ma) record systematic decrease
of cooling ages from NW to SE. This pattern is interpreted to result from the progressive uplift of rocks exposed at the present-day
coast in that direction, such uplift occurred in response to the development of the Middle America Trench at the newly formed
continental margin when the Chortis block was sliding at an average rate of 1.5 cm/year in a sinistral sense to its present
position. Our results also constrain the position of the Kula-Farallon spreading axis north of Puerto Vallarta. These observations
led us to conclude that several indicators point to this time and region for the onset of strike-slip drifting of the Chortis
block toward its current position. Here, we also present several view points in terms of other possilble interpretations to
different tectonic, geologic and isotopic data sets published recently by different authors. 相似文献