WAVES

Understanding wave physics is important for human life (and not just for surfing)

1. A wave is the transmission of energy through matter – in this case through water

2. Two important types of waves are deepwater and shallow-water waves

3. Do you know what a tsunami is?

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A wave is transmission of energy through matter

When energy moves through matter, like water, matter moves and returns to its initial position

The energy is transmitted to its adjacent surroundings – in this case, adjacent water particles

Transmission of energy through the water moves the water particles in a circle motion known as orbital motion

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Wave

Orbitalmotion

Progressive waves are waves that “move” forward across a surface.

As waves pass, wave form and wave energy move forward, but not the water.

Water molecules move in an orbital motion as the wave passes.

Diameter of orbit increases with increasing wave size and decreases with depth below the water surface.

7-2 Wave Motions

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Photo: Navy

Some important features of a wave

H:L is the ratio of wave height to wavelength

Period (T) refers to the time it takes for the same spot on two consecutive waves to pass the same point

Speed = L/T

Wavelength is the distance between identical points on two waves, from crest to crest

Crest is the highest point of the wave

Wave height is the distance fromtrough to crest Trough is the

lowest point of the wave

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There is more than one type of wave

Note: D = water depth; L = wavelength Deepwater waves occur when water depth

is greater than 1/2 wavelength (D > 1/2 L)

Shallow-water waves occur when water depth is less than 1/20 wavelength (D < 1/20 L)

Shallow and deepwater waves can occur at the same time7

Wave base is the depth to which a surface wave can move water.

If the water is deeper than wave base:– orbits are circular– no interaction between the bottom and the wave.

If the water is shallower than wave base– orbits are elliptical– orbits become increasingly flattened towards the

bottom.

7-2 Wave Motions

Deep- and Shallow-Water Motion

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How does a wave break?

H:L ratio> 1:7

As a deepwater wave moves toward shore,its wavelength decreases

The height of the wave increases

The top part of the wavemoves faster than the bottom

When the wave meets the oceanfloor, the bottom of the wave slows

The faster-moving top of the wavecrashes over causing the wave to break.This happens as the wave depth gets closer to the wave height.(H:L ratio is greater than 1:7)

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What is a tsunami?

A typical tsunami wavelength is several hundred miles long, as an example let’s say wavelength is 400 miles

The deepest part of the ocean is 7 miles deep

Based on this information, do you think a tsunami is a shallow or deep water wave?

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What is a tsunami?

L = 400 miles, D = 7 miles D < 1/20 L Tsunamis are shallow water waves! The

ocean would have be deeper than 200 miles on average for a tsunami to be a deep water wave!

A tsunami is a shallow-water wave triggered by displacement of a large amount of water

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Generation of a Tsunami

Generation of a Tsunami

Tsunami damage

Why is it important to know so many details about waves?

Waves can be very destructive

A tsunami is one type of destructive, shallow-water wave

Studying the causes and forces underlying waves can help scientists predict the timing and magnitude of these events and potentially save lives!19

VIDEOS

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