At two contrasting locations variations in particle size have been observed; temporally at a high energy site in the Irish Sea, and spatially at a seasonally stratified area of the Celtic Sea. The response of the particle size to changes in the turbulent regime, and vertical structure of the water column has been studied.
In an area of strong tidal energy and on the fringe of a turbidity maximum in the Irish Sea, the variation in sediment particle size and concentration has been measured over a tidal cycle. Particle size data were collected and subjected to harmonic analysis. The results show a classical 'twin-peaked' signal consisting of a quarter-diurnal cycle superimposed on a semi-diurnal one. Analysis of the quarter-diurnal signal for different particle sizes reveals three processes acting on particles at the site: resuspension and aggregation of fine material and disaggregation of larger flocs, in response to strong shear at maximum flood and ebb.
Investigation of the advection signal also reveals the presence of two suspended
sediment concentration gradients, a gradient of small diameter particles increasing towards the turbidity maximum and a concentration gradient of larger diameter particles increasing away from the turbidity maximum. A hypothesis is proposed that due to high turbulent shear at the turbidity maximum, the particles are predominantly fine, while the sediment in the surrounding water is made up of larger flocs. Diffusion of small particles out of the turbidity maximum balanced by the diffusion of aggregated material towards it provides a mechanism for the maintenance of the maximum.
Spatial particle size data collected on the either side of the Celtic Sea front reveals the presence of strong vertical and lateral gradients in response to water column structure. Clear surface waters, a highly productive thermocline and large median particle sizes in the bottom mixed layer are all regular features of stratified conditions. In contrast, on the mixed side of the front, conditions are far more uniform. A model is proposed for the growth of flocs in shelf seas and is found to fit observations at both the Celtic Sea front and at Irish Sea site.