Wastewaters generated in many coastal communities are discharged into the ocean through deepwater diffusers. There are several factors of interest in describing the performance of a deepwater diffuser, including: (a) the proportion of the time that the rising effluent does not reach the surface, but remains submerged; (b) the initial dilution of the effluent (that is, the dilution when all of the initial momentum and buoyancy of the effluent discharge has been used); (c) the level of fecal coliform or other public health indicator organisms in nearby bathing areas; and, (d) the settling of grit and sludge in the region of the outfall. It is a large design task to examine each of these factors, particularly because of the hourly, daily, and seasonal variations in effluent discharge rate, ocean currents, and vertical density profiles. Thus, for example, to develop an understanding of the position of the effluent field at different times of the year with a selected outfall design, it is necessary to carry out many simulations of the behaviour of the rising effluent field using different combinations of effluent discharge, vertical density profile, and longshore currents.