Waves

Marince Science - waves

What determines whether a wave is a deep-water wave or a shallow-water wave?

The relationship between wavelength and water depth. A wave is deep-water when depth > ½ wavelength, and shallow-water when depth <1/20 wavelength.

What happens to water-particle orbits in deep vs shallow water?

In deep water, particles move in nearly circular orbits. In shallow water, the seafloor flattens the orbits, causing particles to move mainly back and fourth.

What are wind waves and how do they form?

Wind waves are gravity waves created when wind transfers energy to the ocean surface. Most are < 3m high, with common open-ocean wavelengths of 60-150m.

What are capillary waves, and how do they develop into wind waves?

Capillary waves are tiny ripples created by wind friction and surface tension. Wind transfers energy into them, causing them to grow. Once wavelength exceeds 1.73 cm, gravity becomes the dominant restoring force and they become wind waves.

How do wind waves continue to grow?

If the wave remains in deep water and wind continues blowing, its crest rises into faster-moving air and extracts more energy. Height, wavelength, and period increase as energy increases.

What is wave steepness, and what happens when it becomes too great.

Wave steepness = wave height: wavelength. Wind waves have a maximum ratio of about 1:7. if a wave exceeds this, it breaks, releasing energy as turbulence and producing whitecaps/combers.

What is dispersion, and how does it create swell?

Dispersion is the separation of waves according to their wavelengths and speeds. Longer waves travel faster and move ahead of shorter waves, producing smooth, regular swell that can travel thousands of kilometres from storms.

What is a wave train?

A wave train is a progressing group of swell waves with the same origin and wavelength. Leading waves lose energy as they encounter undisturbed water, while new waves form behind them.

What is a group velocity?

Group velocity is the speed at which wave energy travels. In deep water, the wave train moves at ½ the speed of individual waves within the group.

What happens to waves as they enter shallow water?

Deep-water waves slow down as they enter shallow water. Their individual wave speed decreases until it eventually equals the group velocity.

What three factors control the growth of wind waves?

Wind strength, wind duration, and fetch. Stronger winds, longer-lasting winds, and longer fetch generally produce larger waves.

What is wind strength, and why does it matter?

Wind strength is the mean speed of the wind. The wind must move faster than the wave crests to continue transferring energy from the atmosphere to the ocean.

What is wind duration?

Wind duration is the length of time the wind blows. Even very strong winds won’t create large waves if they only blow for a short time.

what is fetch?

Fetch is the uninterrupted distance over which wind blows without significant changes in direction. A longer fetch allows waves to grow larger.

What conditions produce the largest wind waves?

Strong winds + long duration + long fetch. For the largest waves to fully develop, strong winds may need to blow continuously in one direction for nearly 3 days.

What is a fully developed sea?

A fully developed sea is the maximum wave size theoretically possible for a particular combination of wind strength, duration and fetch. Waves cannot grow further because energy is lost through breaking wave tops and whitecaps.

Why don’t cyclonic storms usually produce fully developed seas?

Although winds can be extremely strong, their circular direction limits fetch, preventing waves from having enough uninterrupted distance to fully develop.

Where is the greatest potential for very large waves?

The west wind drift surrounding Antarctica, where winds are strong and nearly continuous, provides exceptionally favourable conditions for large waves.

How large can waves become in the west wind drift?

Wave heights of around 11 m are fairly common, while historical observations have reported waves over 30m.

What is the largest reliably measured wave mentioned in the text?

A 34 m wave observed from the USS Ramapo in the Pacific in 1933. It remains the largest wave described as having a dependable on-site measurement.

Why can waves be very large even when the sea isn’t fully developed?

A storm can produce extremely strong winds and large waves even if its limited fetch prevents the sea from reaching its theoretical maximum.

What happens when wind energy input exceeds the ability of waves to grow?

Wave tops begin to break, producing whitecaps and dissipating excess energy, which limits further wave growth.