Tidal studies in the One Tree Island lagoon

Abstract

The response of the water levels in the lagoon of One Tree Island to the external tides has been studied in two series of observations in May-June 1982 and in August 1983. The tide at the eastern end of the lagoon consistently reaches maximum height 20 min after the external maximum at this point and there is evidence that the internal maximum level is 2 cm higher than the external level. The response at several points inside the lagoon is the same. Studies of the velocity profiles with height and of the mean velocity at one point on the inner edge of the reef on the leeward side show that the tide 'turns', i.e. outflow begins, regularly 45 min before the local high water. If the outflow is wind-assisted, this effect is even more pronounced. The flow across the reef is by no means uniform, however, and at points on the windward side of the reef the inflow may continue for ~2 h after high water. The water-surface gradient across the reef was monitored at one location on the leeward side. Under light to moderate wind conditions, the velocity of the flow across the reef corresponded closely to this gradient, suggesting that inertial effects are small compared with those arising from turbulent or eddy viscosity. These results are compared with theoretical models for the tidal levels within an idealized circular lagoon surrounded by an annular reef. Some of the features of the observed tides can be reproduced using a simple axisymmetric model for which the effective viscosity is of the order of 10^-2-10^-3 m² s^-1. However, the windward-leeward asymmetry in the flow velocities requires the inclusion of an asymmetric forcing term representing surface wind stresses on waves breaking on the windward margin of the reef. By a suitable choice of this term, it is shown that all the features of the observed tides can be qualitatively reproduced by the model.