The Circulation in Keehi Lagoon, Oahu, Hawaii, During July and August, 1968

Abstract

The data from seven oceanographic field surveys taken during July and August, 1968 in Keehi Lagoon, Oahu, Hawaii, and the results of an analysis of these data are presented in this report. The primary objectives of the work were to determine the volume transports to and from the lagoon and to find the circulation both in the lagoon and in the area adjacent to the entrance. The surface circulation was found to be strongly dependent upon the prevailing winds. A westward flow of surface water was observed in most areas of the lagoon except during periods of weak winds. The subsurface flow (below 2.5 meters) was strongly dependent upon the bathymetry. This flow was either to or from the lagoon depending on whether a flooding or ebbing tide was in progress. However, on the eastern side of the lagoon, the incoming transport was greater than the outgoing transport, particularly in a dredged ship channel that crosses the lagoon entrance reef. In contrast, the outgoing transport was greater than the incoming transport on the western side of the lagoon. These conditions result in a limited amount of daily flushing of the lagoon from the east to west. The tide records showed a large number of high amplitude free oscillations of the lagoon surface. The contribution to the circulation from these free oscillations was examined and found to be nominal throughout most of the lagoon, but significant at a few locations in the lagoon. The stratification in the lagoon was also examined and found to be of importance only in the dredged seaplane channels bordering the lagoon and in the area outside and west of the lagoon entrance. Two contributing factors causing the existing stratification are stream runoff from the Moanalua and Kalihi Streams, and warming of the surface water due to surface heat exchange. Most of the warming of the surface water takes place over the large centrally located mud flats in the lagoon. This warm water subsequently flows into the seaplane channels during ebbing tides and later moves westward around Ahua Point.