Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgements
- 1 Introduction
- 2 Observation techniques
- 3 Description of ocean waves
- 4 Statistics
- 5 Linear wave theory (oceanic waters)
- 6 Waves in oceanic waters
- 7 Linear wave theory (coastal waters)
- 8 Waves in coastal waters
- 9 The SWAN wave model
- Appendix A Random variables
- Appendix B Linear wave theory
- Appendix C Spectral analysis
- Appendix D Tides and currents
- Appendix E Shallow-water equations
- References
- Index
3 - Description of ocean waves
Published online by Cambridge University Press: 03 February 2010
- Frontmatter
- Contents
- Preface
- Acknowledgements
- 1 Introduction
- 2 Observation techniques
- 3 Description of ocean waves
- 4 Statistics
- 5 Linear wave theory (oceanic waters)
- 6 Waves in oceanic waters
- 7 Linear wave theory (coastal waters)
- 8 Waves in coastal waters
- 9 The SWAN wave model
- Appendix A Random variables
- Appendix B Linear wave theory
- Appendix C Spectral analysis
- Appendix D Tides and currents
- Appendix E Shallow-water equations
- References
- Index
Summary
Key concepts
The conventional short-term description of ocean waves requires statistical stationarity. A time record of actual ocean waves (the fluctuating sea-surface elevation as a function of time at one location) needs therefore to be as short as possible. However, characterising the waves with any reliability requires averaging over a duration that is as long as possible. The compromise at sea is a record length of 15--30 min. If the record is longer, it should be divided into such segments (possibly overlapping; each assumed to be stationary).
The wave condition in a stationary record can be characterised with average wave parameters, such as the significant wave height and the significant wave period.
The significant wave height is fairly well correlated with ‘the’ wave height as estimated visually by experienced observers. This is not true for the significant wave period.
A more complete description of the wave condition is obtained by approximating the time record of the surface elevation as the sum of a large number of statistically independent, harmonic waves (wave components). This concept is called the random-phase/amplitude model.
The random-phase/amplitude model leads to the concept of the one-dimensional variance density spectrum, which shows how the variance of the sea-surface elevation is distributed over the frequencies of the wave components that create the surface fluctuations.
If the situation is stationary and the surface elevations are Gaussian distributed, the variance density spectrum provides a complete statistical description of the waves.
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- Waves in Oceanic and Coastal Waters , pp. 24 - 55Publisher: Cambridge University PressPrint publication year: 2007
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