Lecture 9 - Wave and Tides

What do ocean waves primarily transport?

Energy (not water).

Do ocean waves move water across long distances?

No, water particles mostly move in circular orbits with little net movement.

What is a wave crest?

The highest point of a wave.

What is a wave trough?

The lowest point of a wave.

What is wave height?

The vertical distance between crest and trough.

What is wavelength?

The horizontal distance between two successive crests (or troughs).

What is wave period?

The time it takes for one full wave to pass a point.

What is wave frequency?

The number of waves passing a point per second.

What creates most ocean waves?

Wind blowing over the ocean surface.

What is a capillary wave?

A very small wave where surface tension is the restoring force.

What is the restoring force for most ocean waves?

Gravity.

What are wind waves?

Waves generated by wind blowing over the ocean.

What factors determine the size of wind waves?

Wind speed, duration, and fetch (distance wind blows).

What is fetch?

The distance over which wind blows across the water.

When do wind waves stop growing?

When wave speed matches wind speed or they break.

When do waves break?

When wave steepness exceeds a critical value (~1/7).

What is a fully developed sea?

When waves reach their maximum size for given wind conditions.

What is the Beaufort scale?

A scale that relates wind speed to sea conditions and wave height

What is the relationship between wind speed and wave height?

Stronger winds produce larger waves.

How are wind speed and wave height related globally?

They closely match—regions with strong winds have large waves.

Are waves stronger in summer or winter?

Winter.

What is swell?

Waves generated by distant winds.

What are wind seas?

Waves generated by local winds.

What is wave dispersion?

Waves of different wavelengths travel at different speeds.

Which waves travel fastest in deep water?

Long-wavelength waves.

Which waves arrive first at the shore?

Long-wavelength waves.

Why do waves arrive in sets at the beach?

Because waves of different speeds separate due to dispersion.

What is a deep-water wave?

A wave moving in water deeper than half its wavelength.

What is a shallow-water wave?

A wave affected by the seafloor (depth < 1/20 wavelength).

How does wave speed depend on wavelength in deep water?

Longer wavelength = faster speed.

How does wave speed depend on depth in shallow water?

Greater depth = faster wave speed.

What happens to waves as they approach shore?

They slow down, increase in height, and eventually break.

Why do waves break near shore?

Particle speed exceeds wave speed.

What is constructive interference?

When waves combine to form a larger wave.

What is destructive interference?

When waves cancel each other out.

What is group velocity?

The speed of a group of waves.

What is phase velocity?

The speed of individual wave crests.

Which is faster: phase velocity or group velocity?

Phase velocity.

What is Stokes drift?

A small net movement of water caused by waves.

Do waves move any water at all?

Yes, a small amount via Stokes drift.

What are tsunamis?

Long-wavelength waves caused by seafloor disturbances (earthquakes, landslides).

Why are tsunamis considered shallow-water waves?

Because their wavelength is so long they feel the ocean bottom.

What generates tides?

Gravitational pull of the Moon and Sun.

What is the restoring force for tides?

Gravity.

What is the typical period of tides?

About 12 or 24 hours.

What are rip currents?

Strong currents flowing away from shore caused by returning water.

How do rip currents form?

Waves push water to shore, and it flows back through narrow channels offshore.

If climate change increases wind strength, what happens to waves?

Waves become larger.