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The 5m long Recirculating Flume at the School of Ocean and Earth Sciences (SOES), University of Southampton, Part II: Unidirectional flow characteristics
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Sediment resuspension events within the (microtidal) coastal waters of Thermaikos Gulf, northern Greece
No full textHigh-frequency flow, pressure and suspended sediment concentration (SSC) measurements are presented from the Paralia-Katerinis coastal area, in Thermaikos Gulf. The data were collected along a cross-shore transect, between the 6 and 12 m water depth contours. The relative importance of wave- and tidally-induced resuspension is examined. Resuspension events are shown to be dominated by wind-generated waves, especially under storm conditions. Some evidence is provided for tidal resuspension, but the overall impact of this process is minimal, compared to wave resuspension. Such resuspension, under storm conditions, increased the SSC levels in the waters of the nearshore zone to 35 mg/l; this is a >15-fold increase over the ambient levels (1–2 mg/l) of turbidity
The 5m long Recirculating Flume at the School of Ocean and Earth Sciences (SOES), University of Southampton, Part I: Descriptive manual
Full text linkShort-term dynamics and maintenance processes of headland-associated sandbanks: Shambles Bank, English Channel, UK
No full textThe short-term (over a spring tidal cycle) dynamics of a headland-associated sandbank (Shambles Bank, English Channel) are investigated, by means of field measurements (synchronous data, using an acoustic Doppler current profiler (ADCP) and repeated side-scan sonar imagery) and two-dimensional (2D) hydrodynamic and sediment transport models. The dynamics of the bank are described in terms of along- and cross-bank velocity components, sand transport pathways and bedform asymmetries.The results showed the occurrence of a net bedload convergent zone along the crest of the bank as a result of anti-clockwise veering of sediment transport towards the crest. This transport pattern is suggested to be the result of the dynamic interplay between two distinct tidally related processes acting over the sandbank, during each phase of the tidal cycle: (a) the formation and different stages of a transient tidal eddy that drives bedload movement during the flood phase of the tide and (b) the bottom-friction induced by the presence of the sandbank that governs the bedload transport dynamics during the ebb phase of the tidal cycl
The use of grain size trend analysis in macrotidal areas with breakwaters: Implications of settling velocity and spatial sampling density
No full textIn a macrotidal environment with offshore breakwaters (Elmer, West Sussex) a new approach for the identification of the sediment transport pathways with grain size trend analysis (GSTA) was undertaken using statistical parameters (mean, sorting and skewness) directly derived from settling velocities distributions. The same samples were analysed with sieving (quarter- and half-phi resolution) and GSTA was performed again in order to directly compare the resultant sediment transport directions derived with the two analytical techniques. Furthermore, both regular and irregular sampling distributions were used to recalculate GSTA. Hydrodynamic data were collected in different locations around the breakwaters and net sediment transport directions were calculated in order to assess the accuracy of the sediment transport pathway directions derived with the different analytical techniques.The accuracy of settling velocity in determining the statistical parameters of the grain size distribution is identified, especially for the fine-medium sand sediments. Settling velocities produced better results than the sieving; the quarter-phi resolution producing the poorer results in comparison with the coarser half-phi resolution. The results for the different spatial sampling strategies are found to depend upon the number of samples utilised; that shows that the accuracy of the GSTA is based upon the ability of representing, adequately, the spatial distribution of the sediment parameters.<br/
Wave period and flow asymmetry effects on the transition to turbulence in relation to sediment dynamics
Full text linkThe presence or absence of turbulence in near-bed flows is known to affect patterns of sediment transport. Therefore the accurate prediction of turbulence is important when considering the initiation of motion of medium to coarse sands as such grains may be eroded by conditions similar to those causing turbulence. An experimental study is described to investigate boundary layer stability under nonlinear (asymmetric) oscillatory flows. In addition, the results are combined with data collected elsewhere to better explain the dependence of initial transition to turbulence on bed roughness and oscillatory period. An oscillating trolley system simulating such oscillatory flows at prototype scale is described. The transition to turbulence under asymmetric (nonlinear) oscillatory flow is investigated utilizing visual techniques and the high-resolution measurement of near-bed velocity. These data are used in conjunction with the extensive data sets of Li [1954] and Manohar [1955] for linear oscillatory flows, collected using similar equipment and methods. Relationships are presented to describe the transition over smooth and fixed (granular) roughness beds under linear and nonlinear flows. The transition to turbulence is shown to have a strong positive linear correlation with both oscillatory period and grain roughness up to a maximum Reynolds number for linear flows. An additional positive nonlinear relationship with asymmetry is observed; this was proportionately greater with increasing bed roughness (from smooth beds to a uniform bed roughness of 550 ?m). These relationships suggest that transition is regulated by a balance between stabilizing and destabilizing forces or conditions, namely, rates of fluid acceleration, the time frame for development of turbulence, and sources of the initial flow perturbation. It is argued that (wave) period dependence observed in threshold of motion data may be explained by a combination of (a) period dependency in the transition to turbulence or of (b) translation of the proposed regulatory mechanisms for the transition to turbulence
Residual circulation and associated sediment dynamics in the eastern approaches to the Solent
No full textLaboratory investigations into the threshold of movement of natural sand-sized sediments under unidirectional, oscillatory and combined flows
No full textSeabed sediment transport pathway investigations: review of scientific approach and methodologies
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