EXCITATION OF SEICHE OBSERVED IN A SMALL HARBOR

Seiche measured within a small (0.6 by 0.6 km), shallow (12-m depth) h arbor is dominated by oscillations in several narrow infragravity freq uency bands between approximately 10(-3) and 10(-2) Hz. Energy levels within the harbor are amplified, relative to just outside the harbor i n 8.5-m depth, by as much as a factor of 20 at the lowest (grave mode) resonant frequency (approximately 10(-3) Hz) compared to amplificatio ns of roughly 5 at higher resonant frequencies (approximately 10(-2) H z). At nonresonant frequencies, energy levels observed inside the harb or are lower than those outside. These amplifications are compared to predictions of a numerical model of seiche excited by linear, inviscid long waves impinging on a harbor of variable depth. The amplification of higher-frequency (approximately 10(-2)-Hz) seiches is predicted wi thin a factor of about 2. However, at the grave mode (10(-3) Hz), the observed amplification decreases with increasing swell and seiche ener gy levels, possibly owing to the sensitivity of this highly amplified mode to dissipation not included in the inviscid model. The energy lev els of higher-frequency seiche within the harbor were predicted from t he offshore sea and swell spectra by the ad hoc coupling of the linear model for the amplification of harbor modes with a nonlinear model fo r the generation of bound infragravity waves outside the harbor. The p redictions are qualitatively accurate only when the swell is energetic and bound waves are a significant fraction of the infragravity energy outside the harbor.