Search citation statements
Paper Sections
Citation Types
Year Published
Publication Types
Relationship
Authors
Journals
Cited by 3 publications
References 1 publication
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
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
A field experiment is described in which three two‐component electromagnetic flowmeters were used to measure simultaneously the longshore (υ) and onshore‐offshore (u) velocity components along a line normal to the shoreline and up to 100 m offshore. Spectral analysis of the data from this experiment reveals the presence of a set of discrete spectral peaks of low frequency (0.014–0.05 Hz) which dominate over the wind wave peak close to the shoreline and which decay in amplitude with distance from the shore. The amplitudes of the velocity components for each of the four lowest‐frequency (and clearest) peaks have been plotted against distance from the shoreline and are found to compare satisfactorily with the calculated edge wave amplitude variation for the beach. Phases between u, υ at one flowmeter and between u, u at different distances offshore have also been found from cross spectra and confirm that the peaks are due to edge waves. It is suggested that each of these low‐frequency peaks corresponds to a progressive edge wave mode at the cutoff frequency for the beach. Ball (1967) and Guza and Inman (1975) calculate that the cutoff frequency υ for an edge wave of mode n should be proportional to [n(n + 1 )]1/2, where the constant of proportionality is dependent on the subaqueous beach profile. The frequencies of the four lowest‐frequency peaks agree well with the predicted frequencies for the lowest modes of cutoff edge waves (n = 1, · · ·, 4), and the observed offshore decay of amplitude at each frequency indicates that assigning the peaks to these modes is correct. Energy exchange between these cutoff modes, through nonlinear interaction, is also suggested by the results. The significance of observing progressive edge waves on this beach is briefly discussed.
Surf beat, wave motion at relatively low frequency (periods of 30–200 s), is often observed on beaches. However, even with modern instrumentation it is difficult to describe the spatial variation of the low‐frequency motion; consequently, the relative importance of a number of suggestions which, at least in theory, provide mechanisms for the generation of low‐frequency energy has never been established. Recent observations (e.g., Huntley, 1976) have reinforced the idea that edge waves, the free wave modes trapped at the shoreline, are a major component of low‐frequency energy. One of the most interesting explanations of surf beat suggests that the beating between particular pairs of incoming waves leads to resonant growth of edge wave modes, which may then dominate the low‐frequency spectrum (Gallagher, 1971). Empirical evidence is essential, as any theoretical development breaks down when the incoming waves break, a fundamental problem with Gallagher's (1971) model. To investigate the importance of this resonant interaction, the general interaction conditions were therefore used to design laboratory experiments in which both resonant and nonresonant conditions were expected. The experimental results show that the response at the beat frequency is stronger when the resonance conditions for edge wave growth are satisfied and that the response is in the form of the theoretically predicted edge wave mode, even when the incident waves are breaking. These results strongly suggest that surf beat is predominantly an edge wave phenomenon.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
Contact Info
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Product
Resources
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.
