The use of variable orifice “duckbill” valves hydraulically optimizes marine effluent diffusers by generating higher jet velocity at low flows, reduced headloss at peak flow, and a more uniform port discharge distribution. Recent laboratory experiments showed that near field mixing and dilution were substantially improved with duckbill valves, and the purging of salt water from an outfall occurs at much lower flow than the theoretical flow required to purge a conventional outfall. An inherent characteristic of the duckbill valve is that it is a check valve that prevents the ingress of salt water, sediment, and marine organisms into the outfall during periods of low flow and hydraulic transients.

This paper presents the hydraulic advantages of variable orifice duckbills verses fixed orifices with emphasis on enhancements in initial dilution and minimization of headloss at peak flow. The process of salt water purging will also be presented along with a brief discussion on failure mechanisms of conventional diffusers.

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