Effect of Shallowness on Dilution of Unidirectional Diffusers

The effect of receiving water depth on the dilution of a unidirectional diffuser discharging dense effluent in quiescent conditions is analyzed. Dilution measurements from previous studies are reviewed to determine the effect of shallowness on the mixing of submerged single port outfalls. For a unidirectional multiport diffuser, the width of the effluent plume is observed to contract in shallow water. A model is proposed relating the dilution of a unidirectional diffuser to the dilution of a single jet using the contracted width. Measurements of the contracted width in shallow water are made to support the proposed model, which is verified by additional measurements of dilution. Three flow regimes, namely, deep, shallow, and vertically mixed, are identified based on the value of D0F0/H. Here, D0 is the diameter of the jet, F0 is the densimetric Froude number of the jet, and H is the depth of receiving water. It is shown that shallow water depth affects the dilution in two ways - the dilution of individual jets is reduced, and the contraction in plume width leads to further reduction in dilution. © 2019 Elsevier B.V., All rights reserved.