Three-dimensional modelling of intermittent effluent discharges using highly accurate schemes

Water quality and cost are the two most important factors to be considered in designing a sea outfall. One way in achieving the optimum of these two factors is by changing the outfall operating procedure such that the effluent is initially stored in a holding tank and then discharged into the sea in a controlled way. The main objective of this study is to investigate: (i) if there is a significant difference in containment dilution between the steady and intermittent cases? and (ii) if so, can the unsteady discharge of pollutants be optimised to give maximum dilution in the receiving waters? For this purpose a highly accurate 3-D water quality model has been used, which is able to predict intermittent discharges and unsteady decay coefficients. The model has been applied to predict bacteria concentrations in the bathing waters along the beach of Swansea Bay, UK. Initial results of the numerical predictions are presented.