Unit 7.1 - Waves

1. Unit 7.1 - Waves

2. 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 point Wave frequency - The number of waves that pass a given point in a given time Waves may be progressive or standing • Progressive waves move across the sea surface. • Standing waves oscillate about a fixed point.

3. What causes Waves? • 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)

4. Wave Velocity Wave velocity can be determined by the formula Wave 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?

5. Sea and Swell • 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.

6. Wave Motion • 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.

7. Wave Orbits • Orbits within a given wave will become smaller with depth until, at a depth of ½ wavelength the orbits no longer exist.

8. Wave motion with depth • 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

9. 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

11. Global Wave Heights Which ocean has the highest average wave heights?

12. Wave Interactions Diffraction Refraction Bending of waves around an obstacle. This is what causes the “scallop” effect along our coast. 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.

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

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

15. Breakers 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.

16. 7.2 TsunamisNational Geographic Clip Footer Text

17. Causes of Tsunamis • Tsunamis are seismic sea waves • They originate from sudden seafloor topography changes • Earthquakes(most common cause) • Underwater volcanoserupting or collapsing • Meteoriteimpact (splash waves)

18. Tsunami Characteristics • They have long wavelengths (120 – 720 km) • They behave as a shallow-waterwave • 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)

19. How Tsunamis Form

20. 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

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

22. Where do they occur? • Most occur in the Pacific Ocean. Why? • More earthquakes and volcanic eruptions in the Pacific

23. Krakatoa - 1883 • Indonesian volcanic eruption • Largest and loudest explosion in human history • death toll of over 120,000 • affected global weather for 2 years

24. Scotch Cap, Alaska/Hilo Hawaii - 1946 • Magnitude 7.3 earthquake in Aleutian Trench • Destroyed Scotch Cap Lighthouse

25. 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 ftto cause the tsunami

26. Japan - 2011 • Magnitude 9.0 earthquake off the coast of Japan • 15,839 deaths and 3,642 missing • Fukushimanuclear plants destroyed • NOAA animation of tsunami propogation

27. 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

28. Tsunami Watches and Warnings • Tsunami Watch – issued when potential for tsunami exists • Tsunami Warning – unusual wave activity verified • Evacuate people • Move ships from harbors