Wave speed = Wavelength Period

Transcription

1 Waves Wave speed The speed the disturbance, and wave crests, travel at. Wave speed = Wavelength Period [m/s] [m] [s] Wave speed: Deep water wave When ocean depth is greater than half the wavelength. Direction of wave

2 Wave speed The speed the wave crests travel at. Deep water wave Speed dependent on wavelength (or period). gravity Speed (m/s) = P eriod (seconds). 2 Speed (m/s) = 1.56 * wave period (seconds). Longer waves travel faster. Wave speed: Deep water wave The speed the disturbance, and wave crests, travel at. Speed = 1.56 * wave period Longer waves travel faster Brooks/Cole Storms produce waves of many different periods and heights. Wind wave periods can be up to 30 seconds.

3 Wind wave dispersion Wavelength increases as period increases. Longest waves travel fastest away from storm. Wave speed: Shallow water wave When ocean depth is shallow compared to wavelength (less than 1/20 th of wavelength) Direction of wave Wave speed: Shallow water wave When ocean depth is shallow compared to wavelength (less than 1/20 th of wavelength) Speed dependent on ocean depth. p Speed (m/s) = gravity Water depth Speed (m/s) = 3.1 * square root(depth in meters) Waves travel faster in deeper water.

4 Wave refraction results in waves being more parallel to shore. Wave reflection

5 Wave groups Deep water wave groups travel at half the speed of individual waves. New waves appear to form behind the wave group. Wave groups matlab movie Wind waves approaching shore Waves touch the bottom. Wave period remains constant, wavelength decreases. Wave speed = and eventually become Shallow water waves, so Wavelength Period (Wavelength = Wave speed * Period) Speed (m/s) = 3.1 * square root(depth in meters)

6 Wind waves approaching shore Waves touch the bottom. Wave period remains constant, wavelength decreases. Friction slows the bottom of the wave. Waves approaching shore Bottom of wave slows even more as the depth shallows, wave crest moves ahead of the base of the wave. Wave breaks when the ratio of wave height to water depth is ~3:4. How waves break depends on the beach slope. Surf prediction North shore swells come from Northern Hemisphere winter storms. South shore swells often come from Southern Ocean storms.

7 Surf prediction Network of ocean buoys measure wave height, period, and direction. pacioos.org Tsunamis Hilo, April 1946 after Alaskan earthquake (NOAA) Tsunamis Tsunamis can be caused by Earthquakes, Landslide, volcanoes, and asteroid impacts. All of these suddenly displace a large amount of water. Have periods of about 20 minutes, and wavelengths of about 200 km. Average ocean depth is 4 km, so always a shallow water water (depth < wavelength/20).

8 Earthquake generation The March earthquake had a displacement of about 20m over an area ~500 km by ~200 km. Open ocean detection In the deep ocean Tsunami waves have small amplitudes, 10 to 50 cm. Combined with their long wavelength, they are hard to detect, enter DART. Why are they dangerous? Because of their long wavelength, the volume of the displacement associated with a tsunami wave is huge. When it shoals that volume piles up.

9 TsunRunAokiUAlas.mov japan.mov

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12 Kapiolani Central Medical Union Center Preschool and Kindergarden Pawaa Washington Middle Palama Market Lunalilo Elementary HOBRON LN KAIOO DR Mccully Shopping Ctr NIU ST PAU ST KUAMOO ST Duke Paoa Kahanamoku Beach Park NAMAHANA ST OLOHANA ST UNIVERSITY AVE KALAIMOKU ST LAUIKI ST LAUNIU ST McCully Ala Wai Elementary LEWERS ST ALOHA DR DON HO LN LAAU ST 'Iolani School DUKES LN DATE ST Royal Hawaiian Market International Place Market Place Waikiki Shopping Plaza Text Royal Hawaiian Shopping Center NAHUA ST WALINA ST Kaimuki High KANEKAPOLEI ST OLOKELE AVE LUKEPANE AVE Kuhio Beach Park EKELA AVE MAKALEKA AVE ALA WAI BLVD OHUA AVE PAOAKALANI AVE WAI NANI WAY Jefferson Elementary Kapiolani Beach Park Jefferson Orthopedic Unit Kapiolani Park Beach Center Waikiki (Hq.) Hawaii Center for the Deaf & Blind Waikiki Elementary How to escape? Get to higher ground. Or go above the 3rd floor in a concrete building. If possible don t get into your car, traffic jams are inevitable. If the water recedes, don t follow it. Run the other way. Know the evacuation zones are your home and work. Waikiki Evacuation map E H1 FWY PUNAHOU ST KAHEKA ST BINGHAM ST S BERETANIA ST KANUNU ST YOUNG ST ELSIE LN PONI ST ALA MOANA PARK DR ALEXANDER ST KAPIOLANI BLVD KONA ST MAHUKONA ST ARTESIAN ST VILLA LN ATKINSON DR COYNE ST MALANAI ST NANEA ST KALAUOKALANI WAY FERN ST KAHAKAI DR ALA MOANA HOLOMOANA ST Ala Wai Yacht Harbor POHA LN ALGAROBA ST WAIOLA ST CITRON ST MCCULLY ST ENA RD S KING ST ISENBERG ST PAOA PL KEONIANA ST COOLIDGE ST LIME ST MALUHIA RD HAUSTEN ST KAPAAKEA LN Ala Wai Clubhouse KAHUNA LN SARATOGA RD KALIA RD KUILEI ST KAMOKU ST BEACH WALK KAIOLU ST KAAHA ST Iolani School MAHIAI ST KAIPUU ST NOHONANI ST Ala Wai Golf Course CLEGHORN ST KOA AVE TUSITALA ST KEALOHILANI AVE MOKIHANA ST PALIULI ST WINAM AVE PALANI AVE KUHIO AVE CARTWRIGHT RD LEMON RD WILLIAMS ST AINAKEA WAY MOOHEAU AVE HOOLULU ST MARTHA ST HERBERT ST CASTLE ST BROKAW ST KAPAHULU AVE HOLLINGER ST Honolulu Zoo DUVAL ST ESTHER ST 8TH AVE 7TH AVE CATHERINE ST MONSARRAT AVE FRANCIS ST LAKIMAU ST GEORGE ST KAINA ST Legend Kapiolani Park 9TH AVE UPPER ST LEAHI AVE PAKI AVE HAYDEN ST 10TH AVE HINANO ST MAKINI ST 11TH AVE Tsunami Evacuation Line Tsunami Evacuation Zone Fire Station Ambulance Hospital Police KAUNAOA ST NOELA ST KANAINA AVE HOLEI ST NIKOLO ST JAMES ST Public and Private School Shopping Mall Major Street Street Beach Park OWENA ST GAIL ST WELA ST EDNA ST WAUKE ST Ala Moana State Rec Area Notes: Remain at least 100 feet away from inland waterways and marinas connected to the ocean due to wave surges and possible flooding. If possible, remove or deploy vessels to deep water (at least 200 fathoms). Map 1: Waikiki KALAKAUA AVE Structural steel or reinforced concrete buildings of six or more stories provide increased protection on or above the third floor. The evacuation zone is a guideline and should be considered the minimum safe evacuation distance. Scale in Feet Prepared by: DPP (HOLIS) Data Source: City and County of Honolulu GIS These maps do not consider the destructive effects of a locally generated tsunami. If you feel shaking, move inland immediately, well away from the evacuation zone. Note: Data represented on this map is not intended to replace site survey. Copyright City & County of Honolulu Date Prepared: Aug. 25, 2010 All Rights Reserved 2010 Some key ideas Deep water wave, D > L/2, speed: c=1.56*t; Longer waves travel faster than shorter waves; Wave groups travel at half the speed of individual waves. Shallow water wave, D < L/20, speed: c=3.1*sqrt(h); They feel the bottom; They refract. Period remains constant when wave approach the shore. Swim across shore to get out of a rip current. Tsunamis are shallow water waves. They have periods of around 20 minutes. Get out of their path!