Unit 7.1 - Waves

Properties of Ocean Waves - An ocean wave is an undulation of the sea surface

Wave Period - The time it takes one wave to pass a given pointWave frequency - The number of waves that pass a given point in a given timeWaves may be progressive or standing • Progressive waves move across the sea surface. • Standing waves oscillate about a fixed point.

• Wind - most waves are wind driven progressive waves• Gravity - attraction of sun and moon cases tides, a form of waves• Submarine disturbance (earthquakes) cause tsunamis• Ships - wakes are often a problem in harbors

Wind Generation of Waves The type of wave generated by wind is determined by: - • Wind strength- • Wind duration • Fetch (distance over which - the wind blows)

What causes Waves?

Wave velocity can be determined by the formulaWave velocity =

Question: If the distance between wave crests is 10 meters, and the period between crests is 2 seconds, how fast is the wave moving?

Wave Velocity

• Large waves are generated by storms. As the storm subsides, the waves move out of the storm zone and carry a lot of energy.

• Swells, - more regular waves, beyond area of generation, form and the progressive wave move across the ocean.

• These waves may travel hundreds of miles across the ocean, only to break on some distant shore.

Sea and Swell

Two basic motions are associated with an ocean wave:

• The forward movement of the wave form (not the water).

• The orbital motion of water particles beneath the wave.

• It is wave energy, not water molecules, that moves across the sea surface.

Wave Motion

Wave Orbits• Orbits within a given wave will become smaller with depth

until, at a depth of ½ wavelength the orbits no longer exist.

Motion of Water Particles Beneath Waves

• Deep Water Waves. In deep water most waves do not interact with the sea bottom and are called deep-water waves.

• The orbits of the water molecules are circular.

• Shallow Water Waves. Waves that interact with the sea floor are known as shallow-water waves.

• The orbits of shallow water waves are elliptical

Wave motion with depth

Wave Height

• Wave height is directly related to wave energy

• Wave heights are usually less than 2 meters (6.6 ft)

• Breakers called whitecaps form when wave height reaches critical steepness

• Beaufort Wind Scale describes the appearance of the sea surface under different wind conditions

Global Wave Heights

Which ocean has the highest average wave heights?

Wave Interactions

Bending of waves around an obstacle. This is what causes the “scallop” effect along our coast.

DiffractionRefraction

Bending of waves as they approach a shore at an angle. Refraction explains why waves curve towards shore and usually break parallel to shore regardless of which way the beach is facing.

Wave Reflection occurs when incoming waves do not break, expending energy, but are reflected back only to interfere with incoming waves.

The Wedge

Wave Reflection

Waves approaching shore• Waves slow and increase in height when approaching shore

Shore breakers (surf) are collapsing waves. Breaking is determined by wave steepness. When the wave height/wave length ratio is about 1/7, waves begin to break.

Breakers

7.2 Tsunamis

National Geographic Clip

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Causes of Tsunamis

• Tsunamis are seismic sea waves

• They originate from sudden seafloor topography changes– Earthquakes (most common

cause)– Underwater volcanos erupting

or collapsing– Meteorite impact (splash

waves)

Tsunami Characteristics

• They have long wavelengths (120 – 720 km)• They behave as a shallow-water wave– Encompass the entire water column regardless of

depth– Can pass undetected under boats in the open

ocean• Their speed is proportional to water depth– They are very fast in the open ocean (400-500

mph)

How Tsunamis Form

Why are tsunamis so devastating?

• What characteristic of a tsunami makes it so much more devastating than a normal wind-driven wave of the same height?

• Its wavelength. Because a long wavelength wave contains much more water

Tsunami Destruction

• Sea level can rise up to 40 meters (131 ft) when a tsunami reaches shore causing utter destruction

Where do they occur?

• Most occur in the Pacific Ocean. Why?• More earthquakes and volcanic eruptions in

the Pacific

Krakatoa - 1883

• Indonesian volcanic eruption

• Largest and loudest explosion in human history

• death toll of over 120,000• affected global weather

for 2 years

Scotch Cap, Alaska/Hilo Hawaii - 1946

• Magnitude 7.3 earthquake in Aleutian Trench• Destroyed Scotch Cap Lighthouse

Indonesia - 2004• Magnitude 9.3 earthquake off coast of Sumatra Indonesia

released more energy than all the other earthquakes in the past 25 years combined.

• Up to 280,000 deaths – deadliest in history• An area of sea floor the size of the state of California lifted

upwards by over 30 ft to cause the tsunami

Japan - 2011

• Magnitude 9.0 earthquake off the coast of Japan

• 15,839 deaths and 3,642 missing• Fukushima nuclear plants destroyed• NOAA animation of tsunami propogation

Tsunami Warning System

• Pacific Tsunami Warning Center (PTWC) – Honolulu, HI– Uses seismic wave recordings to

forecast tsunami• Deep Ocean Assessment and

Reporting of Tsunami (DART) – System of buoys– Detects pulse of tsunami passing

Tsunami Watches and Warnings

• Tsunami Watch – issued when potential for tsunami exists

• Tsunami Warning – unusual wave activity verified – Evacuate people– Move ships from harbors