Experimental observations of salt purging in a model sea outfall diffuser with eight soffit connected risers

Improved rehabilitation design for the long-term hydraulic function of marine outfalls with soffit connected risers is the main aim of the research programme reported here. As a step towards this goal a better understanding of the processes of saline intrusion and saltwater purging has been developed from detailed laboratory studies. Unsteady flow cycles have been investigated and observed riser purging sequences have been compared against theoretical criteria. From the monitoring of riser flow salinity, quantitative measures of the salt wedge front entrainment process have been obtained together with indicative figures for salt wedge expulsion rate. This interim outcome is intended to alert the designer and operator of outfalls of the potential problems with soffit connected riser systems.