Turbulence control of the properties and flux of suspended matter in a tide-stirred shelf sea

Turbulence and sediment interactions have been studied widely over recent years, this work being mainly carried out in estuarine environments. Due to the development of instrumentation and processing techniques it is now possible to obtain good quality measurements of turbulence and sediment properties on the same temporal and spatial scales over reasonably long durations. Therefore, this study was designed to investigate the turbulence and sediment interactions over numerous tidal cycles at a shelf sea site. To this end the variation in suspended particulate matter (SPM) volume concentration, mass concentration and size, in conjunction with turbulent kinetic energy data, was investigated at a high energy tide-stirred site in the Irish Sea. The study site was located off the north-west coast of Anglesey and was notable for the presence of a turbid patch. Initial conclusions drawn from harmonic analysis, entropy analysis, and graphs of particle numbers indicated that possible sediment dynamics mechanisms controlling SPM magnitude and variation at the site were: resuspension, aggregation, disaggregation and advection of the turbid patch. It was found that the range of sediment sizes present at the site could be described by 2 characteristic sediment size populations: one fine (-50µm diameter) and one coarse (150µm). Two models were developed within the study to test these findings upon the 2 characteristic sediment size populations. The first, an advection model, which included no vertical mixing, was able to reproduce the underlying signal present within the observations (both in terms of magnitude and variability). This model was then incorporated into the second model which included turbulent vertical mixing, settling, erosion (resuspension) due to tidally generated shear stresses and turbulence controlled aggregation and disaggregation. 11 The full sediment dynamics model reproduced the variability and magnitude of the observations reasonably well. Small scale variability was also replicated by the model. Sensitivity analysis was then performed on the model to quantify the relative importance of each of the sediment dynamics processes at the study site. Results showed that for both size populations the 2 mechanisms controlling the magnitude of the SPM mass concentrations are erosion (resuspension) and aggregation/disaggregation, erosion being dominant for the coarse population and aggregation/disaggregation being dominant for the fine population. In terms of the variability, advection is by far the dominant mechanism for controlling the coarse population. Aggregation/disaggregation and advection control most of the variability in the fine population at this site