Resuspension and transport of sediment in the Eastern Irish Sea.
The large surface area of cohesive sediments enables them to act as sinks for marine
contaminants. Thus the dispersal of such contaminants is partly controlled by the
resuspension and transport of fine sediments. The Cohesive Sediment Dynamics Study
(COSEDS) group was established to gain a better understanding of the erosion,
resuspension,t ransport and deposition of fine particulate matter, particularly during storm
conditions. As part of this study, measurementso f wave heights and orbital velocities, near
bed currents and suspended sediment concentrations were monitored using instruments
housed on free-standing frames deployed in Morecambe Bay and on the Sellafield mud
patch in the Eastern Irish Sea.
Acoustic and Miniature Optical Backscatter Sensors (ABS and MOBS) were calibrated in
a resuspension tank which allowed conversion of the raw backscattered field data into
information on the size and concentration of the material in suspension. The contrast in
particle size sensitivity of the ABS and the MOBS results in a big difference in the response
of the ABS and the MOBS to the particle size distribution of the field site sediments. Thus,
in an environment of mixed particle sizes, the ABS and the 'MOBS can be used in
conjunction with one another to provide information about the suspended sediments.
Hydrodynamic data from four sites on the Sellafield mud patch enabled shear velocity
estimates to be made using the Inertial Dissipation Method. Time-series of drag. coefficient
and roughness length measurementss howed both temporal and spatial variation in the
observations. The spatial difference in the observations has been attributed to a change in
the size distribution of the bed sediments between sites. The cause of the temporal
variation in the observed roughness is thought to have been due to a changing flow regime,
a combination of wave-current interaction and the presence of bed forms. Simple boundary
layer models enabled the reproduction of the shear velocity values and the drag coefficient
in both rough-turbulent, smooth-turbulent and transitional flow regimes.
The direct relationship between the suspended sediment and the tidal currents indicated
local erosion and, pickup from the seabed. The optical backscatter data revealed the
resuspensiono f sedimentb y waves at times of high orbital velocities and by tidal currents
at other times. Estimateso f suspendeds edimentf lux were computed from the product of
the vertical integration of velocity profiles and concentration profiles, which were predicted
assuming a Rouse-type profile. Flux estimates at each of the sites indicated a net
movement of sediment northwards throughout the duration of the two week deployment.
For a more detailed picture of transport patterns, and hence contaminant paths, it has been
shown that nearb ed hydrodynamicm easurementsa re required for a longer time period