A model for the flow dynamics and mixing in the near-field of a multiple port outfall diffuser is postulated. It employs the equations for gross conservation of mass, momentum, and buoyancy of the fluid in the discharge jet or plume. These equations are obtained by assuming flow similarity and then integrating over the cross-sectional area of the plume. Two distinct interactions of the discharge jets or plumes are included in the model. These are the interaction of the individual round jets from the diffuser ports and the merging of the plume from either side of the diffuser, over the top of the diffuser. The resulting equations are closed by the “entrainment assumption” and solved numerically. Results provide the velocity, width, and dilution of the jet or plume. Calculations were made for a number of cases where experimental results were available. The model gives reasonable agreement with the experiments over a wide range of discharge conditions and over the complete range of flow patterns. In most cases it slightly underestimates the mixing or dilution. Therefore the model should be useful in determining the minimum dilution that can be expected from any marine outfall.

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