Effects of sedimentation on the storage capacity of the High Aswan Dam reservoir

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.     

Sedimentation modified by wind induced resuspension in a shallow tropical lagoon (Cote d'Ivoire)

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.     

Gravity sedimentation analysis of human blood leukocytes

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.     

Sedimentation and distribution of gamma-emitting radionuclides in bottom sediments of southern Lake Päijänne,Finland, after the Chernobyl accident

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.     

Factors delimiting the boundary between vertically contiguous mussel beds ofSeptifer virgatus (Wiegmann) andHormomya mutabilis (Gould)

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.