Announcements. Explosions at the Fukushima nuclear power plant, Japan. Next project due online Nov. 6th A week of waves

Transcription

1 Announcements Next project due online Nov. 6th A week of waves Today: Gravity waves Thursday: Tidal waves Figure from Science, 3 September, 005 Explosions at the Fukushima nuclear power plant, Japan 0 Japan tsunami photo from National Geographic

2 Waves Learning objectives: Learn how to stay alive I. Wave anatomy II. Kinds of waves III. Wave length, period, and speed A. Deep-water waves B. Shallow-water waves IV. (DWW)Wave sorting and dispersion V. Wave trains and group velocity VI. Waves heights max height, factors influencing wave height, interference VII. (SWW) Shoaling, bending and breaking waves VIII. Internal waves IX. Tsunamis Wave anatomy height amplitude wavelength period frequency speed=wavelength/period Wave motion Wave form propagates Water stays behind Wind wave formation and pressure gradients Stokes Drift: Wave orbitals do not close exactly This results in net forward motion of water (at a much slower velocity than the wave propagation speed) Kinetic and potential energy components of waves Variation in pressure due to air flow over wave Pressure gradients and gravity in waves Question: You re playing in chest-deep the water at the beach and a huge wave approaches fast. What do you do?

3 Wave Velocity (m s - ) Predicted speed (m/s) Types of waves Capillary waves: Surface tension (or cohesion -H-bonds) causes wave to propagate Generally created by wind. Less than half an inch in wavelength Gravity waves: Gravity causes wave to propagate. Wind waves: Capillary or gravity waves produced by wind shear. Propagates freely (WL: - 00m) Seiches: Gravity wave, often produced by wind, freely oscillates w/in a semi-enclosed space Tsunami: Gravity wave produced by seismic activity, landslides, meteors (WL: 00 km) Tides: Forced gravity wave. (WL: 0,000 km) Wavelength, period, wave velocity Speed: C=L/T gl d C tanh L d = 0.4 m d = 0. m d = 0. m Wavelength (m) Hyperbolic tangent Wavelength, period, wave velocity Deep-water waves: Water depth > / L C= gl/, or C=.5 L =.56T Shallow-water waves: Water depth < /0 L C= gd, or C=3. d, where d=water depth Transitional (intermediate) waves: gl d C tanh L Testing the transitional wave formula Measured Wave Speed Calculated Wave Speed Water Depth (m) Wavelength (m) Measured Wave Speed Calculated Wave Speed 3

4 Wave sorting and dispersion Since velocity square root of wavelength, after creation, waves will sort by wavelength. You re waterskiing and as you look at the boat wake you wonder: Why is your boat wake shaped like chevrons? Photo from Velocity of a wave train is always less than that of individual waves Group velocity = / wave velocity Factors influencing wave height : Wind speed : Wind duration 3: Fetch: distance of sea surface over which wind can blow 4

5 In Puget Sound, winds from north or south generally produce the largest waves. Most big wind storms come from the south Strait of Juan de Fuca can have large waves when the wind blows from the west You inherit enough money to purchase waterfront property in Puget Sound. Where should you buy? Where in the world ocean is the longest fetch located? Rouge waves rogue wave off South Africa Extremely large waves Wave interference Wave/current interaction Semester at Sea M/V Explorer, 005 5

6 Shallow-water waves: Shoaling, bending and breaking Waves break at height:water depth ratio ~3/4 (or, at a height: wavelength ratio ~/) Question: You re in a boat with a 3 draft and see a wave break ahead of you. Is the water deep enough for your boat to safely pass? As wave velocity slows with decreasing water depth, waves bend, or refract when traveling over complex bottoms Where is the safest spot on this beach to view the waves? Half Moon Bay Internal wave packets viewed from the Space Shuttle Wave refraction intensifies waves at Mavericks From: 6

7 Internal waves in shallow water, Tasmania Photo by M. Tomczak, 996 Equatorial Pacific (temperature data) Tsunamis and seiches Seiche: standing wave that rocks back and forth within an enclosed or semi-enclosed area Tsunami: long-wavelength shallow-water wave caused by a large water displacement (e.g. seismic sea waves) Washington State: Internal waves in the Strait of Juan de Fuca propagating from both ends (Edwards and MacCready 00) Harmut Peters (UW) wavelengths often > 00 km height in open ocean < m If the wavelengths are < m, how do they cause damage? Tsunami facts: Shallow-water wave velocity: C = 3. sqrt(d), where d = 4500 m, = 00 m/sec (or, 750 km/h!) Sumatra Tsunami Wave generation and propagation Upon reaching the coast, water shallows and the leading edge of the tsunami slows first. Wave height can increase to > 30m. Dec 6 th, 004 tsunami illustrates these phenomena What you d see from the beach Consequences of long wavelength 7

8 0 Tohoku earthquake and tsunami, Japan Tsunamis in Washington State Cascadia subduction zone is similar to the Sumatra subduction zone. How could you protect yourself from a Tsunami produced at the Cascadia subduction zone? Modeled tsunami hazard map of Bellingham Bay Green: m Yellow: 0.5 m Red: 5 m 8