Excitation of seiche observed in a small harbor

Abstract

Seiche measured within a small (0.6 by 0.6 km), shallow (12-m depth) harbor is dominated by oscillations in several narrow infragravity frequency bands between approximately 10 '3 and 10 '2 Hz. Energy levels within the harbor are amplified, relative to just outside the harbor in 8.5-m depth, by as much as a factor of 20 at the lowest (grave mode) resonant frequency (~ 10 '3 Hz) compared to amplifications of roughly 5 at higher resonant frequencies (~10 '2 Hz). At nonresonant frequencies, energy levels observed inside the harbor 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 (~10'2-Hz) seiches is predicted within a factor of about 2. However, at the grave mode (10 '3 Hz), the observed amplification decreases with increasing swell and seiche energy levels, possibly owing to the sensitivity of this highly amplified mode to dissipation not included in the inviscid model. The energy levels of higher-frequency seiche within the harbor were predicted from the offshore sea and swell spectra by the ad hoc coupling of the linear model for the amplification of harbor modes with a nonlinear model for the generation of bound infragravity waves outside the harbor. The predictions are qualitatively accurate only when the swell is energetic and bound waves are a significant fraction of the infragravity energy outside the harbor. 18,210 OKIHmO ET AL.: EXCITATION OF SEICHE OBSERVED IN A SMALL HARBOR lO 1 lO o 10-• 0-2 _ ,'" • ß ß ,-ß ß -.,, ß ß -, ß _ ,, ß -,,,' ß iiI ß ß ß _ . ß ,,' ß -, _ . ,, ß ß 71 IllIll

BOUSS-2D: A Boussinesq Wave Model for Coastal Regions and Harbors

“…To overcome this deficiency, Okihiro, Guza, and Seymour (1993) used an ad-hoc coupling of a nonlinear model for the generation of bound long waves outside a harbor with a linear model for the amplification of long waves inside the harbor. The complex nature of bathymetry outside Barbers Point Harbor makes it difficult to quantify the relative amount of long-wave energy outside the harbor that is freely propagating into the harbor.…”

Low Frequency Wave Resonance in Fringing Reef Environments

“…Harkins and Briggs, 1995;Miles, 1974;Okihiro et al, 1993;Wu and Liu, 1990) on beaches (e.g. Karunarathna et al, 2005;Özkan-Haller et al, 2001;Suhayda, 1974) as well as in the laboratory (e.g.…”

Statistical properties of coastal long waves analysed through sea-level time-gradient functions: exemplary analysis of the Siracusa, Italy, tide-gauge data

“…Despite the recognised importance of longwave studies and coastal resonances, direct measurements of long waves are not common, and they have been carried out especially for research purposes (Okihiro et al, 1993;Okihiro and Guza, 1996;Lara et al, 2004;Bellotti and Franco, 2011;Guerrini et al, 2014). In recent years, the upgrade of the traditional coastal tide gauge networks, that usually recorded data every 10 or 15 min, to faster sampling and recording rates (1-10 s) has allowed the use of sea-level data to study waves in the longwave range, which are however usually analysed only for extreme events such as tsunamis (Rabi-novich and Thomson, 2007;Bressan and Tinti, 2011;Rabinovich et al, 2013).…”