Studies of saltwater purging from a model sea outfall diffuser

Findings are reported of laboratory studies into the purging of salt water from a multi-riser manifold, representing the diffuser section of a long sea sewage outfall. Detailed measurements of instantaneous riser salinity and velocity are presented. The work complements previous investigations and, for the first time, offers quantitative data on the internal salt wedge front mixing and entrainment. For the soffit-connected riser system investigated, it is found that the established criterion for riser purging can be conservative and a revised formulation is proposed to more closely match the experimental observations. The paper provides an advance in qualitative understanding of the complexity of the flow processes which occur in outfall manifolds.