Characterizing the effluence near WaikῙkῙ, Hawaiʻi with a coupled biophysical model

Abstract

Aperiodic discharge from the Ala Wai Canal near Waikiki, Hawaii is contaminated with naturally occurring harmful bacteria. We use a coupled hydrodynamic and biological model to examine how tides, wind, surface waves, and light affect the fate of the effluent plume. The freshwater plume is constrained to the surface and advected by the ambient and tidal flow. Winds overcome the background advection and keep the plume from the Waikiki beaches and mix the plume downward. Surface waves also mix the plume water and keep the plume near-shore. 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 outflow events, we find that the model well captures the observed bacteria when the Ala Wai is the source of discharge. The results show that such a system may be valuable to predicting times and locations for harmful effluent plumes near the Ala Wai Canal.