Line Plume and Ocean Outfall Dispersion

The three-dimensional flowfield created by a line plume of finite length in a steady unstratified current was investigated experimentally and theoretically. The results are applicable to ocean sewer outfall design. Even though the diffuser length may be much greater than the water depth, the flowfield cannot be considered to be two dimensional. The most important dynamic parameter is a Froude number, F, given by the ratio of the current speed cubed to the buoyancy flux discharged per unit diffuser length. Different mixing regimes can occur depending on the value of F and the plume can be attached to the lower boundary. Dilution depends on current speed and direction only for F>0.1. For F>0.1, dilution increases with current speed and diffusers perpendicular to the current will result in greater dilution than if parallel. The surface wastefield spreads rapidly in a parallel current, showing that Y or similarly shaped diffusers are not essential to produce a widely dispersed wastefield. The buoyant spreading of the surface wastefield is a complex phenomenon, having growth laws that change with distance downstream.