Waves
Most waves like those you see crashing on beaches are created by wind in areas of storms. The strength of waves depends on three factors:
- Wind strength - stronger winds mean bigger waves (duh)
- Wind duration - the longer the wind blows, the bigger the waves (also duh)
- Fetch - the greater the distance over which the wind blows, the bigger the waves (not quite so duh)
Waves create no net transport of water -- just circular motion of water as wave passes through. They can be described by the following parameters:
- Crest (high point) and trough (low point)
- Height - difference in elevation between crest and trough
- Wavelength - Distance over which wave repeats (distance from crest to crest or trough to trough)
- Period - time wave takes to travel one wavelength. Period is CONSTANT; it does not vary from when a wave is created to when it breaks.
- Celerity - speed of travel of a wave, equal to wavelength divided by period
In shallow water waves begin to drag on the bottom of the ocean. This first occurs when water depth decreases to one half of the wavelength. As waves interact with the ocean bottom in shallow water, their characteristics change as follows:
- shorter wavelength
- slower speed
- greater height (waves break when height/wavelength = 1/7)
- same period
"Tidal" waves, tsunamis, seismic sea waves
- Usually derived from seismic events (earthquake, volcanic eruption)
- Very long wavelengths (up to 100-200 km)
- Always act as shallow water waves (drag on the ocean bottom)
- Travel very quickly (200-300 miles per hour not unusual)
- Are small (e.g. 5 feet) in ocean, but get very tall (30-40 feet) when striking coasts
Waves interact with their surroundings in many ways. Three important wave behaviors are:
- Refraction -- bending of waves toward slower velocity areas. Refraction tends to make waves approach the shore more perpendicularly even if they come in at an angle. Refraction also
- Reflection -- bouncing of waves off hard near-vertical surfaces. Can create interference (see below)
- Diffraction -- spreading of waves that travel through a narrow opening. Diffraction causes even protected harbors to experience wave motion.
- Interference -- waves can add to each other or cancel each other out. In stormy seas, hundreds or thousands of waves are generated and interfere with each other, causing swell, where water elevations can change drastically very quickly.