Chapter 32: Waves, Breakers and Surf

Generation and propagation

Wave height depends on wind speed, duration and fetch; a fully developed sea is reached when further duration or fetch adds nothing. Locally generated sea is steep and confused; swell that has travelled out of the generating area is longer, more regular and lower. Deep-water waves travel at a celerity proportional to their period, so longer swell outruns shorter and arrives first — a classic early warning of a distant storm. Significant wave height is the mean of the highest third; the maximum in a storm may approach twice it, and rogue waves can exceed that.

• Fetch, wind speed and duration set the sea state
• Longer period = faster = arrives first
• Significant height is a statistic, not a limit

Shoaling, refraction and breakers

As depth decreases below about half the wavelength the wave slows, shortens and steepens until it breaks near a depth of about 1.3 times its height. Refraction bends wave crests toward the shallows, concentrating energy on headlands and shoals and creating dangerous breaking seas over bars and in the mouths of harbours even in moderate offshore conditions. Diffraction and reflection produce confused clapotis near breakwaters. Synchronous and parametric rolling arise when the encounter period matches the vessel's natural roll period — alter course or speed to break the resonance.