SALINITY INTRUSION INTO MULTI-PORT SEA OUTFALLS

Abstract

Sea water intrusion into the diffuser system of a long sea outfall is likely to reduce the efficiency of the outfall. Recent surveys have revealed outfalls with a significant proportion of their diffuser blocked, and others discharging through as few as 50% of their diffuser ports. It is suspected that intrusion is the cause of these malfunctionings. Intrusion may be encouraged by the design requirement of low efflux velocities to obtain optimum dilution, or by over-design to cater for future increases in discharge. Although intrusion can only commence at low discharge velocities, when a port Densiometric Froude number falls below about 1, once it has started sea water will continue to enter the diffuser system until a state of balance is reached. Subsequently a considerably greater flow will be required to purge the outfall of all sea water. Intrusion may also be prematurely triggered by wave action, currents, or by reduced flow through a damaged port. Hydraulic model tests at the University of Dundee are leading to an understanding of the intrusion mechanism as it affects the various diffuser systems in current use. A continuing research and site survey programme is aimed at design recommendations to eliminate intrusion or reduce its effects.