Waves and Tsunamis: Characteristics and Terminology

1. Waves

2. Wave characteristics and terminology • Crest • Trough • Wave height (H) • Wavelength (L) • Still water level • Orbital motion

3. Wave characteristics and terminology (continued) • If wave steepness exceeds 1/7, the wave breaks • Period (T) = the time it takes one full wave—one wavelength—to pass a fixed position

4. Orbital size decreases with depth to zero at wave base • Depth of wave base = ½ wavelength, measured from still water level Calm water

6. Deep- and shallow-water waves • Deep-water waves • Water depth > wave base • Shallow-water waves • Water depth < 1/20 of wavelength • Transitional waves • Water depth < wave base but also > 1/20 of wavelength Figure 8-6a & b

7. Wave speed (S) • General formula: • Deep-water waves: • Wave speed (S) in meters per second = 1.56 T in seconds • Wave speed (S) in feet per second = 5.12 T in seconds • Shallow-water waves: (d = water depth)

8. Wave Classification • Ocean waves can be classified in various ways: • Disturbing Force- the forces which generate the waves. • Meteorological forcing (wind, air pressure); sea and swell belong to this category. • Earthquakes; they generate tsunamis, which are shallow water or long waves. • Tides (astronomical forcing); they are always shallow water or long waves.

9. The “sea” and swell • Waves originate in a “sea” area • Swell describes waves that: • Have traveled out of their area of origination • Exhibit a uniform and symmetrical shape Figure 8-9

10. Hurricane Katrina Hurricane Iniki September 1992 August 29, 2005

11. "Tsunami" - a Japanese word meaning "great wave in harbor". It is a series of ocean waves commonly caused by violent movement of the sea floor by submarine faulting, landslides, or volcanic activity. A tsunami travels at the speed of nearly 500 miles per hour outward from the site of the violent movement.

12. Tsunami • Tsunami terminology • Often called “tidal waves” but have nothing to do with the tides • Japanese term meaning “harbor wave” • Also called “seismic sea waves” • Created by movement of the ocean floor by: • Underwater fault movement • Underwater avalanches • Underwater volcanic eruptions

14. Most tsunami originate from underwater fault movement Figure 8-21a

15. Tsunami Fault displacement under water displaces water, water moves to fill vacuum, generating large waves.

16. Tsunami characteristics • Affect entire water column, so carry more energy than surface waves • Can travel at speeds over 700 kilometers (435 miles) per hour • Small wave height in the open ocean, so pass beneath ships unnoticed • Build up to extreme heights in shallow coastal areas

17. Coastal effects of tsunami • If trough arrives first, appear as a strong withdrawal of water (similar to an extreme and suddenly-occurring low tide) • If crest arrives first, appear as a strong surge of water that can raise sea level many meters and flood inland areas • Tsunami often occur as a series of surges and withdrawals

18. Tsunami since 1900 • Most tsunami are created near the margins of the Pacific Ocean along the Pacific “Ring of Fire”

19. Maui Puunene AvenueAftermath of a Tsunami in Kahului, 1960 Tsunamis struck Kahului in 1946, 1957, 1960, and 1964. The earliest historically recorded tsunami in Kahului occurred on November 7, 1837, when a large tsunami traveled 800 yards inland and destroyed a Hawaiian village.The 1960 tsunami was caused by a violent earthquake in Chile on May 22, 1960. It took approximately 15 hours for the tsunami to travel from Chile to the Hawaiian Islands. The tsunami killed 61 people in Hilo on the Big Island, but there were no other human casualties on any of the other islands. The tsunami caused moderate damage in Kahului.

20. A giant wave engulfs the Hilo pier during the 1946 tsunami. The red arrow points to a man who was swept away seconds later.

21. Tsunami warning system • Seismic listening stations track underwater earthquakes that could produce tsunami • Once a large earthquake occurs, the tsunami must be verified at a nearby station • If verified, a tsunami warning is issued • Successful in preventing loss of life (if people heed warnings) • Damage to property has been increasing

23. tides w/out tsunami Earthquake originated in Anchorage, AK Passage of a tsunami as seen in a sea level record from Hilo, Hawaii. The observed sea level shows high frequency variations with a period of approximately 20 minutes and an initial amplitude of nearly two meters (total tsunami wave height 3.7 m)

24. This mathematical simulation (above) shows the tsunami created by the Cascadia Subduction Zone earthquake on January 26, 1700, as it reaches Hawaii on its way across the Pacific Ocean (5 hrs).

25. Tsunami India Ocean Dec. 26, 2004 No tsunami ocean buoy in Indian Ocean • The most powerful earthquake in 40 years • Measured 9.0 on the Richter scale • Occurred approximately four miles below the Indian Ocean near the Indonesian Island of Sumatra.

26. Tsunami: Thailand

31. Free Waves, Forced Waves • Free waves- a wave that is formed by a disturbing force such as a storm. Waves continue to move without additional wind energy • Forced wave- a wave that is maintained by its disturbing force, e.g., tides

32. Restoring Force • Force necessary to restore the water surface to flatness after a wave has formed in it • Capillary waves- wavelength < 1.73 cm • Gravity waves- wavelength > 1.73 cm

33. Deep-water, Transitional, & Shallow-water waves Wavelength- determines the size of the orbits of water molecules within a wave Water depth- determines the shape of the orbits • Deep-water waves • Water depth > wave base • More circular orbits • Shallow-water waves • Water depth < 1/20 of wavelength • Orbits are more flattened • Transitional waves • Water depth < wave base but also > 1/20 of wavelength • Intermediate-shaped orbits

35. Wind Waves- gravity waves formed by the transfer of wind energy into water • Wave ht- usually <3m • Wave length- 60-150m • Factors that affect wind wave development: • Wind strength • Wind duration • Fetch- the uninterrupted distance the wind blows http://www.newportsurf.com/tides.html

36. Fetch

37. Interference and Rogue Waves Interference waves: when waves from different storm systems overtake one another. They add (constructive interference) or subtract (destructive interference) from the other. Constructive Destructive Mixed

38. Rogue waves: • freak waves that come out of nowhere • created by constructive interference • formed by the interaction of a wind wave and a swift surface current • common in southeastern tip of Africa

39. Waves approaching shore • Types of Breaking Waves: • Plunging breaker • Spilling breaker • Surging breaker • Factors that determine the position and nature of the breaking wave: • Slope • Contour • Composition

41. a gradual sloping bottom generates a milder wave

42. Surging Breaker • doesn't break, because it never reaches critical wave steepness • breaker diminishes in size and looses momentum • Found on beach with a very steep or near vertical slope

43. Sunset Beach Waikiki What type wave are these?

44. Wave refraction • As waves approach shore, the part of the wave in shallow water slows down • The part of the wave in deep water continues at its original speed • Causes wave crests to refract (bend) • Results in waves lining up nearly parallel to shore • Creates odd surf patterns

45. Wave Refraction

46. Wave Diffraction- Propagation of a wave around an obstacle

47. The Wedge, Newport Harbor, Ca waves Wave Reflection • Wave energy is reflected (bounced back) when it hits a solid object

48. Rip Current

50. Internal Waves- at thermocline/pycnocline layer