Characterizing the effluence near Waikiki, Hawaii with a coupled biophysical model

Aperiodic discharge from the Ala Wai Canal near Waikiki, Hawaii is often contaminated with naturally occurring harmful bacteria that may affect nearby beaches. Using a coupled hydrodynamic and biological model, we examine how tides, wind, surface waves, and light affect the fate of this effluent plume. The nearshore circulation and tidal flows constrain the freshwater plume to the surface. Winds can overcome both the stratification and ambient tidal advection to keep the plume to the west (away from major beaches) and mix the plume deeper into the water column. Surface waves due to swell act to mix the plume deep and keep the plume onshore. We find that the plume does not extend offshore in the presence of winds or waves. Using Enterococcus spp. as the bacterial agent, we find that residency is increased when mixed deeper away from the light. Comparing predicted Enterococcus spp. with in-situ samples for five distinct plume events, we find that the model accurately represents the observed bacterial concentrations when the Ala Wai is the primary source of Enterococcus spp. discharge. The results show that predictive models may be an invaluable complement to water quality sampling programs for regions of high human activity.