Internal tide effects on a sewage plume at Sand Island, Hawaii

The variability in the vertical position of a sewage plume, far from its point of discharge, is described and explained by the forcing from an internal tide. The sewage plume, discharged from the Sand Island treatment plant in the coastal waters of Mamala Bay, Oahu, Hawaii, was mapped using ship-board towyo observations collected during September 25 – October 1, 1994. Horizontal currents and temperature were measured in 1994–1995 at moorings located throughout Mamala Bay. Data at mooring D2, located close to the diffuser, suggested that the presence of an internal tide of 18 km wavelength propagating along the isobaths at speed 0.4 m s−1 was responsible for large (more than half the total depth) semi-diurnal vertical displacements of the isotherms. Comparison of the vertical far-field plume position and isotherm displacements at the mooring showed that the horizontal propagation of the internal tide was westward along the isobaths. These data challenge the classical view of a sewage plume “established” at a constant depth of equilibrium and are the first example showing the effects of internal tides on sewage plumes.