Dispersion of Buoyant Waste Water Discharged from Outfall Diffusers of Finite Length

Abstract

The three-dimensional flow field created by a simple line plume of finite length in a steady current of uniform density was investigated in a laboratory basin. The results can be used to aid in the prediction of dispersion of buoyant waste water released from line diffusers, particularly sewage discharges into the ocean. The experimental results for minimum surface dilution, S_m, were found to be independent of L/H, in the range 3.7 < L/H < 30 where L is the diffuser length and H the water depth, and independent of Reynolds number, Re = 4uH/ν, in the range 1190 < Re < 12,900 where u is the current velocity. The results are expressed graphically in the form: (S_(m)q)/uH = f(F,θ), where q is the volume flux per unit length, and θ the orientation of the line diffuser to the current. F is a type of Froude number defined by F = u^(3)/b, where b is the buoyancy flux per unit length. The initial momentum flux is assumed to be small. For a current perpendicular to the diffuser, and F > 0.2, the effluent mixes over the receiving water depth due to self-induced turbulence. When the diffuser is of finite length, the diluted effluent separates from the bottom at some point downstream and forms a two-layer flow. However, currents parallel to the diffuser do not produce mixing over the depth, and the flow forms a two-layer system immediately, even for Froude numbers as high as 100. For F < 0.1, dilution is independent of current speed and direction. For F > 0.1, dilutions when the current is perpendicular to the diffuser are proportional to the current speed. For 0.1 < F < 100 this dilution is about 60% of that predicted assuming uniform mixing of the effluent over the receiving water depth. This is due to the development of a vertically stable density profile. For F > 0.1, a diffuser placed perpendicular to the current will result in greater dilutions than if parallel. The ratio of minimum surface dilution when the current is perpendicular to that when the current is parallel increases with F, and is equal to about 4 at F = 100. Horizontal spreading of the waste field is governed by buoyancy forces rather than ambient turbulence. For F ≥ 1 the initial surface plume spreading is found to be linear, and independent of L/H and Re for 3.7 < L/H < 15, and 2,900 < Re < 13,000. Beyond this initial linear spreading zone the rate of plume growth decreases. It is speculated that regimes may exist where the surface width grows as the 2/3 or 1/5 power of downstream distance; the results are not adequate to confirm these growth laws. It is believed that ambient turbulence has no significant effect on diluting the waste within several diffuser lengths from the source. The results have been presented in a manner which makes them immediately applicable for improving outfall designs, and demonstrates the error frequently made in assuming two-dimensional flow fields. This assumption is incorrect even if the diffuser length is an order of magnitude greater than the water depth.

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