Dye Studies of Initial Dilution and the Applicability of the Stagnant Water Design
In the preliminary design of an ocean outfall to discharge buoyant effluent a standard estimate of the initial dilution achieved at the water surface above the diffuser is useful to compare options for diffuser depth and diffuser design. The standard estimate or prediction of initial dilution that is often used is that pertaining to a worst case for surface contamination occurring during stagnant (zero current)-uniform (zero stratification) sea conditions. This paper describes a series of measurements of the initial dilution achieved by an operating outfall and by a pilot full-scale experimental set-up. The measurements, conducted near Cape Town, South Africa, using Rhodamine-B dye as tracer were performed on days when sea conditions very close to stagnant-uniform pertained. A consistent trend for the lowest measured dilution to exceed the predicted stagnant-uniform dilution by a factor of 2 to 3 times uas evident from four separate field experiments. The applicability of the stagnant-uniform design approach to a prototype, operating outfall is consequently discussed in the paper.