Plume Dilution for Diffusers With Multiport Risers

The near field plume dilutions from ocean outfall line diffusers consisting of evenly spaced risers with clusters of two to eight ports per riser have been measured in a two‐dimensional sectional hydraulic model. Tests were run under both idealized and measured ocean current and stratification conditions. A mathematical model was successfully correlated with tests not involving ambient currents, thus providing a useful design tool. The laboratory results show that, for the cases studied, the initial dilution is at most a weak function of whether the discharge ports are clustered or evenly distributed along the diffuses and that the minimum dilution increases with decreasing port size (limited, ultimately, to that of a line source). Therefore, the most cost effective design for riser equipped ocean wastewater diffusers may involve the smallest port diameters permitted by other design considerations, and clustering many ports per riser to decrease the number of risers needed.