Purging and Flooding of Pipeline Sewage Outfalls

The effluent discharge from most ocean outfalls is periodic while some others discharge only intermittently. When the rate of discharge falls below a critical level, seawater commences to flow into the deeper end of the diffuser forming a salt water wedge in the bottom of the diffuser section of the outfall. The paper examines two aspects of this phenomenon: the purging of seawater from a simple multiport pipeline outfall at start-up or after partial intrusion of seawater, and the flooding of the outfall following cessation of effluent discharge. Mathematical models are developed that describe each of these processes. It is shown that a certain minimum discharge is required before all ports on a sloping diffuser will be utilized. The flooding of a pipeline is described in terms of the volume fraction of the diffuser section that is flooded and a normalized time parameter. Two programs of hydraulic model testing provided verification for the mathematical models of the flooding and purging processes.