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421.
The River Nile receives most of its sediment load from the Atbara and Blue Nile rivers, which carry eroded sediments north
from the Ethiopian mountains during the seasonal flood period between August and October.
Prior to the construction and operation of the High Aswan Dam, in 1964, 9–10 × 106 metric tons of suspended sediment were deposited annually in the flood plain of the Nile, while about 93% of the total average
annual suspended load of 124 × 106 metric tons was carried out into the Mediterranean Sea.
Since the full operation of the High Aswan Dam in 1968, the flood discharge of the Nile, below the dam, has been greatly modified
and more than 98% of the total suspended load has been retained within the reservoir.
Based on long-term records; estimated relationships between discharge and suspended load, and field measurements, the life
span of the dead storage capacity has been estimated at a minimum of about 360 years. Although this preliminary calculation
is less than the estimated design capacity of 450 years, it is expected that progressively more suspended solids will be released
in the outflow of the reservoir and that together with the use of flood diversion schemes the High Aswan Dam is likely to
approach its design life span. 相似文献
422.
423.
In shallow environments, under certain conditions of fetch, wind velocity, bathymetry and bottom characteristics, resuspension
can be generated by wind induced waves. In the tropical Ebrié lagoon, austral trade winds are dominant almost all year long,
and their velocity shows a marked diel pattern with maximum speed between noon and midnight. Only austral trade winds with
a speed >3 m s−1 allow particle resuspension which is effective for depths<1.5 m. In these areas, significantly higher values of chlorophyll
biomass and mineral seston are noted during the windy sequences. Granulometric and mineralogical analyses showed that only
the surficial sediment (0–3 cm) was involved in resuspension. This process induces several effects: 1) an increase of the
suspended matter concentration in the water and thus a light attenuation due to a higher turbidity, 2) a redistribution in
the whole water column of nutrients from the pore water and 3) a removal of the finer fractions from the superficial sediment.
On the contrary, for depths>1.5 m, particle sinking is permanent in depressions which are spontaneously transformed into anoxic
systems. At the lagoon scale, sedimentation is significantly modified by wind induced resuspension. According to the bathymetry
and the distance from a river, three sedimentary facies are recognized. Their grain size distributions are parabolic in areas
where resuspension occurs, logarithmic in areas where no resuspension is possible and hyperbolic in the hollows and the main
channels. Finally, a large part of the allochthonous inputs (from drainage and rivers) and autochthonous pelagic production
is trapped into the Ebrié lagoon and less than 10% of the particles entering the lagoon are exported toward the Atlantic Ocean. 相似文献
424.
N. Catsimpoolas E.M. Skrabut A.L. Griffith C.R. Valeri 《Journal of biochemical and biophysical methods》1980,2(3):147-153
A new method of sedimentation analysis of human blood leukocytes is described. Platelets, lymphocytes, monocytes, and polymorphonuclear cells isolated from normal human peripheral blood have been analyzed alone and in mixture by gravity sedimentation employing a computerized scanning instrument. All four classes could be clearly resolved from each other exhibiting sedimentation velocities of 0.6 ± 0.00, 1.04 ± 0.11, 1.27 ± 0.15 and 1.89 ± 0.21 · 10?4 cm/s, respectively, at 37°C in a 2.5–6.25% Ficoll gradient in Medium 199. Less than 106 cells can be used for analysis. Possible applications of the method are discussed. 相似文献
425.
The rates of sedimentation of fallout nuclides were determined by means of sediment traps during 28 months after the Chernobyl accident in southern Lake Päijänne, Finland. The spatial distribution of the radionuclide content of the lake bottom was studied on 35 sediment cores in winters 1987/88 and 1988/89. The results were compared with simultaneous observations of the radionuclide content of the water and seston. The role of different transfer mechanisms in the elimination of radionuclides from the water column is discussed.The values recorded for the flux to the lake sediments were on average of the same order of magnitude as the initial deposition on the lake surface (Cs-137 65 kBqm–2). The radionuclide flux to the sediments was rapid during the first months after the accident. After that the elimination of dissolved nuclides from the water mass became significantly slower. The highest flux rate was that of Ce-144 and the lowest that of Rh-106 (Ru-106). Of the radiocesium, about half of the initial inventory was transferred to the sediments after the first observation year.The content of radionuclides showed considerable spatial variation on the lake bottom (Cs-137 7–280 kBqm–2). Direct adsorption of radiocesium explains unexpectedly high concentrations on shallow erosion bottoms. There was a clear tendency for the concentrations to increase with depth, as a result of the focusing effect. Sediment resuspension had a significant impact on the total flux of radionuclides to deepwater sediments. Estimates were made of the resuspended flux of radionuclides. 相似文献
426.
Keiji Iwasaki 《Ecological Research》1995,10(3):307-320
On moderately wave-exposed rocky shores in middle Japan, the upper interidal mytilid,Septifer virgatus, and lower intertidal mytilid,Hormomya mutabilis, occur together, forming vertically contiguous mussed beds. Factors limiting the lower distribution limit ofSeptifer and the upper limit ofHormomya were investigated by collections of natural mussel clumps and single- and mixed-species transplantation experiments. Newly
settled juvenileSeptifer (<5 mm shell length) were significantly fewer in the natural and artificialHormomya clumps than in theSeptifer clumps. Both natural and artificialHormomya clumps accumulated a much greater amount of sediment than did theSeptifer clumps.Hormomya clumps inhibited the recruitment ofSeptifer, presumably through accumulation of sediment, which resulted in setting the lower limit of theSeptifer zone. Survivorship of small (5–10 mm) and large (>15 mm)Hormomya was much lower inHormomya clumps transplanted upwards into theSeptifer zone than it was inHormomya clumps in theHormomya zone. In mixed-species clumps in theSeptifer zone, however, survivorship of smallHormomya was not significantly different from that in mixed-species clumps in theHormomya zone.Septifer had a positive effect on the survival of smallHormomya and increased the upper limit ofHormomya, presumably by providing shelter and thus protecting them from desiccation. The upper limit ofHormomya zone was thus considered to be set by desiccation exceeding the physiological tolerance of the species. 相似文献