The internal hydraulics of the multiport diffuser section in long sea outfalls has been studied by laboratory experiments and numerical modelling. The study has run for five years as a coordinated effort at both the University of Liverpool and the University of Aalborg. The investigations have covered such phenomena as saline intrusion, wave influence and unsteady flow caused by pump operation. Results show that numerous flow regimes exist depending on the boundary conditions. Numerical models have been developed to simulate the flow field in the diffuser section and some of the most interesting phenomena, in particular the wave induced saline intrusion, can now be modelled satisfactorily. Some aspects of the behaviour relating to very low fresh water discharges, e.g. unsteady flow when saline wedges are present, are still not very well described.

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