A study of the temporal variability in particle size in a high-energy regime

The variation in sediment particle size and concentration has been measured over a tidal cycle in an area of strong tidal energy and on the fringe of a turbidity maximum in the Irish Sea. Laser In-Situ Scattering and Transmissometry (LISST 100) particle size data were collected and subjected to harmonic analysis. The results show a classical ‘twin-peaked’ signal consisting of a quarterdiurnal cycle superimposed on a semi-diurnal one. In previous studies this has been interpreted as a combination of resuspension and settling (producing the quarter-diurnal cycle) and advection of a horizontal gradient (producing the semi-diurnal one). However, analysis of the quarter-diurnal signal for different particle sizes shows that it is in phase with a likely resuspension signal for small particles, but out of phase for the larger particles. An explanation of this, proposed here, is that the larger particles become disaggregated by the strong shear at maximum flow speeds, producing a maximum in the concentration of fine particles and a minimum in the large particle concentration. The reverse is seen to occur at slack water. We estimate that this aggregation/ disaggregation process accounts for approximately 1/4 of the quarter-diurnal signal. The conclusion is that previous studies may have considerably over-estimated the resuspension of suspended sediments by tidal streams.