A simple turbulent energy-based model of fine suspended sediments in the Irish Sea

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Energy is required to maintain a particle heavier than water in suspension in the sea. In equilibrium, energy must be supplied at a rate equal to the product of the weight of the particle and its sinking velocity. If the energy supply rate is known, it becomes possible to predict the total weight, and hence the concentration of suspended particles in the sea. This principle has been applied to a model of fine suspended sediments in the Irish Sea. The model is used to test the hypothesis that the supply of turbulent kinetic energy (TKE) alone controls both the spatial pattern and seasonal changes in concentration. TKE is supplied by the tide and wind, and a fixed fraction of this energy is used to keep sediment in suspension. There is assumed to be a readily available supply of fine sediments at the sea bed, which rises into suspension as soon as TKE becomes available, and sinks down again when it is not. The model incorporates 2-layer thermal stratification and vertical mixing between layers but there is no horizontal advection or diffusion. The model is driven by smoothly varying wind speeds and heating over the year and spatially varying tidal stirring. The particles have a constant settling velocity of 10 m day −1, and a density twice that of water. A fraction 0.00006 of the total (tide+wind) TKE energy input is used to stir the sediment. The results of the model are in agreement with both the observed spatial pattern and the seasonal variation of fine suspended sediments in the surface waters of the Irish Sea to within the limits of measurement accuracy and natural variability. This energy model performs at least as well as conventional stress-driven models in this regard. We therefore conclude that the general behaviour of fine suspended sediments in the Irish Sea can be explained by TKE supply alone. Other factors, such as changes in particle size and density over the year, can be used for fine tuning but are not necessary to give a broadly correct picture. The result is that we have a simple, physically coherent, method of predicting the turbidity (and hence the light regime) of a shelf sea from physical inputs alone.