WAVES

Result of interaction between generating force and restoring force.
Generating force - wind, landslide, faults or slipping .

Restoring force - surface tension, gravity

WAVES -

1. Transfers disturbance from one part of medium to another.

2. Disturbance propagated thru medium without any overall movement of medium itself (floating cork)

3. Disturbance propagated without significant charge of wave form.

4. Appears to be propagated at constant speed.

If 2 fluid layers (sea surface and wind ) having different speeds are in contact, and there is frictional stress between them, there is transfer of energy.

Wind generated waves.

- after a period of calm, wind starts to blow, rapidly increases to gale and continues to blow a gale force for some time

- no significant wave growth occurs instill wind speed = 1 m/s.

- then small steep waves form as wind speed increases

- even after wind reaches constant gale force, waves continue to grow until they reach size and wavelength (and therefore speed) of 1/3 wind speed - beyond this, waves continue to grow, but slower. period does not change.

WAVE GROWTH STOPS BELOW WIND SPEED

1. some wind energy transferred generating surface current.
2. some wind energy dissipated by friction.
3. energy lost from larger waves by white capping (i.e. blowing off tip of wave crest because it is moving faster than the wave itself).
When waves are being generated and forced to increasing size by wind they are called forced waves. When they move from storm center, longer period waves are swell. - can travel thousands of miles.

Size of waves in deep water depends on (depth > 1/2 wavelength

1.Wind speed.
2. Length of time wind is blowing
3. fetch ( unobstructed distance of sea over which wind blows).
Some of highest waves are produced in Southern Ocean, 40, 50 S roaring 40) - intense storms and winds and long fetch.

Waves 10 - 15m in high occurs under severe storm conditions wavelengths are between 100-200 m, about the length of a freighter.

- freighter suspended between 2 crests can great its back.
- largest wave documented was 34.2m (112 ft) measured in Pacific.
 

Maximum height for wave is determined by ratio of height and length steepness = height / length

If S = 7 crest angle < 120 and wave is unstable - collapses and breaks

- Most deep water waves height below max.
- When they do exceed steepness, top is usually blown off by wind

Shallow water waves when depth < 1/2 wavelength, wave changes form

1. speed = wavelength / period
2. period does not change
When wave slow as it feels bottom, reduction of wavelength results in increase in height and steepness. When depth < 1 / 20 wavelength, wave becomes shallow water wave - length and speed controlled by water depth.

speed = g x depth.


Refraction - when deep water waves approach shore, casually not straight on - one end reaches shallow water fist, moves more slowly than portion in deeper water.

- wave crest bends and orients parallel to shore.
 

Surf zone - area on coast where waves break.

Since waves usually approach beach at angle, there is a net transport of water towards the beach and along the beach - water accumulates on beach and flows along the beach until it can flow out beyond surf zone.

Return flow usually occurs in quieter water with smaller wave lengths i.e. in areas with depressions or troughs in sea floor, - may be narrow, therefore return flow is fast forming rip currents.

TSUNAMIS

- generated by earthquakes - large area of sea floor suddenly raised or lowered - causes rise or fall in sea surface above it.

- produces wave with long wavelengths 100 200 m and long periods 10-20 min.

- since avg. depth of ocean 400 m (4km) depth < 1 / 20 wavelength - shallow water waves and speed depends on depth. Move about 200 m/s (400 mph).

In open ocean, height typically 1 -2 m distributed over entire wavelength. When it reaches island or coast, energy is compressed into smaller volume as depth decreases wave height increase rapidly - tremendous amount of water races over land, - can destroy buildings, strand large ships
 

Krakatoa, 1883, erupted - Tsunamis generated had period of 1 - 2 hrs and other island 33 miles away hit by waves 30 m height - waves recorded in Cape Hope (7800 mile ) and Panama (11,400).
 

Storm Surge

- under low pressure area of hurricane and typhoons -
- sea surface rises forming a dome which travels across the sea under the storm
- raises water level at coast as storm approaches land.
- causes extensive flooding of low lying areas.