Long-period waves in an island reef harbor

Abstract

Surface waves of periods between 2 and 600 min are of common occurrence in coastal embayments and harbors. These long-period waves may affect water circulation and exchange, may cause moored ships to range, and may contribute to extreme low or high water levels. Long-period waves, excluding those caused by wave groups, are generally unimportant unless they coincide with a harbor resonance. Studies of harbor resonance are reviewed. Data for Seeadler Harbour, Manus Island are presented, and it is shown that long waves of well-defined periods occur in bursts of three to seven waves every few days during the monsoon season. Some of these bursts match abrupt changes in wind stress, but others cannot be explained by this mechanism. Through spectral and frequency-time analyses, the principal long waves present are identified. Of the five long waves present, three are shown to be resonant oscillations whereas an oceanic source is postulated for the other two. Long waves at the fundamental period of seiching of reef or island lagoons may generate currents and mass flows of water as great as the tides, and may have a significant influence on lagoon flushing and reef entrance sediment transport.