The effect of aggregation/disaggregation on suspended particulate matter transport in a tidal strait

Abstract

Predicting net suspended sediment fluxes in coastal regions is important for promoting good water quality, maritime transport and marine industries. Aggregation and disaggregation of suspended particulate matter responding to spatial and temporal variations in turbulence can control particle size and hence settling speed; little is currently known about how this influences the net suspended sediment flux. A tidal strait, in which the flux is confined to be in one of two directions, makes a good site for observing the effect of changes in particle size on the suspended sediment flux. The Menai Strait in North Wales is a narrow tidal strait in which the flow speeds increase to a maximum in the narrowest central region, the Swellies. Observations at sites on either side of the Swellies show that the median size of suspended particles varies with the speed of the tidal current, but also depends on the direction of the flow, with the smallest particles seen when the current is flowing out of the Swellies. The implication (supported by model results) is that the strong currents in the centre of the strait tear up flocs into small pieces which are then carried out towards both ends of the strait by the tide. Averaged over a tidal cycle, there is a net flux of larger particles into the Swellies and a net flux of small particles out. These results are relevant to the formation of isolated turbidity maxima around a region of strong turbulence.