Transcript Chapter 8
CHAPTER 8 Waves and Water Dynamics Waves are visual proof of the transmission of energy across the ocean Origin of waves Most waves are wind-driven Moving energy along ocean/air interface Wind main disturbing force Boundary between and within fluids with different densities ○ Air/ocean interface (ocean waves) ○ Air/air interface (atmospheric waves) ○ Water/water interface (internal waves) – movement of water of different densities Atmospheric Kelvin-Helmholtz waves are caused when a certain type of cloud moving horizontally one way interacts with a stream of air moving horizontally at a different speed. Eddies develop, making beautiful, unusual, curling waves of cloud. http://www.siskiyous.edu/shasta/map/mp/bswav.jpg Internal waves Associated with pycnocline Larger than surface waves – up to 100 m Caused by tides, turbidity currents, winds, ships Possible hazard for submarines Fig. 8.1a Internal waves (wavelength about 2 km) which seem to move from the Atlantic ocean to the Mediterranean Sea, at the east of Gibraltar and Ceuta http://envisat.esa.int/instruments/images/gibraltar_int_wave.gif Other types of waves Splash wave Coastal landslides, calving icebergs Seismic sea wave or tsunami Sea floor movement Tides Gravitational attraction among Moon, Sun, and Earth Wake Ships Wave motion Waves transmit energy by oscillating particles Cyclic motion of particles in ocean Particles may move ○ Up and down ○ Back and forth ○ Around and around Particles in ocean waves move in orbital paths Progressive waves • • Waves that travel without breaking Types • Longitudinal – push/pull waves in direction of energy transmission • sound • Transverse – back and forth motion • Only in solids • Orbital • Combination of longitudinal and transverse • around and around motion at interface of two fluids Orbital or interface waves Waves on ocean surface at water/air interface Crest, trough, wave height (H) Wavelength (L) Orbital waves Wave characteristics Wave steepness = ratio of wave height to wave length H/L ○ If wave steepness > 1/7, wave breaks Wave period (T) = time for one wavelength to pass fixed point Wave frequency = # of wave crests passing fixed location per unit of time, inverse of period or 1/T Circular orbital motion Water particles move in circle Movement up and down and Back and forth Orbital motion Diameter of orbital motion decreases with depth of water Wave base = ½ L Hardly any motion below wave base due to wave activity Types of Waves dependent on interaction with bottom Deep-water waves No interference with ocean bottom Water depth is greater than wave base ( > 1/2L) Wave speed (celerity) proportional to wavelength Longer the wave, the faster it travels Shallow-water wave Water depth is < 1/20L Wave “feels” bottom, because water is shallower than wave base Orbits are compressed elliptical Celerity proportional to depth of water The deeper the water, the faster the wave travels Transitional waves Characteristics of both deep and shallow-water waves Celerity depends on both water depth and wavelength Wave development Most ocean waves wind-generated Capillary waves (ripples) formed first Rounded crests, very small wavelengths Provide “grip” for the wind Increasing energy results in gravity waves Symmetrical waves with longer wavelengths Wave energy Factors that control wave energy Wind speed Wind duration Fetch – distance of uninterrupted winds Maximum wave height caused by wind that is known: ○ Reliable measurement Measured on US Navy tanker caught in typhoon ○ Wave height 34 m or 112 ft Fig. 8.10 Wave energy Fully developed sea Maximum wave height, wavelength for particular fetch, speed, and duration of winds at equilibrium conditions Swell Uniform, symmetrical waves that travel outward from storm area Long, rounded crests Transport energy long distances http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter1 Swell Longer wavelength waves travel faster and outdistance other waves Wave train = group of waves with similar characteristics Sorting of waves by their wavelengths is wave dispersion Wave train speed is ½ speed of individual wave Wave interference patterns Different swells coming together Constructive interference In-phase wave trains with about the same wavelengths Add to wave height Rogue waves – unusually large waves ○ Rare but can happen and be unusually large http://www.ethnomusic.ucla.edu/courses/ESM172a Wave interference patterns Destructive interference Out-of-phase wave trains with about the same wavelengths At least partially cancel out waves Mixed interference Two swells with different wavelengths and different wave heights Wave height is extremely variable ~50% of all waves are less than 2 m (6-7 ft) 10-15% are greater than 6 m Up to 15 m in Atlantic and Indian oceans Up to 34 m in Pacific - long fetch (speed at 102 km/hr) Largest rogue wave can sink largest vessels Largest = 34 m (120 ft) high (above theoretical max) 1:1200 over 3x average height; 1:300000 over 4x height Waves hitting current may double height suddenly and break Most common near strong currents, long fetches, storms Rogue waves that rise as high as 10-story buildings and can sink large ships are far more common than previously thought, imagery from European Space Agency satellites has shown. A rogue wave is seen in this rare 1980 photo taken aboard a supertanker during a storm near Durban, South Africa. (Reuters) http://www.allhatnocattle.net Storm surges • Large wave moving with a storm (not just hurricanes) • Low pressure above water water level rises at center • Up to 3-4 m higher than normal • Preceded by low sea-level in front of storm • Added to increased wind waves + high tide most damage Hurricane Katrina – 2005 Record storm surge in Pass Christian, MS - ~27.8 ft Waves approach shore Deep-water swell waves shoal Transitional waves Become shallow-water waves (< L/2) Wave base “touches” sea bottom Waves approach shore During transition to shallow-water waves Wave speed and wavelength decreases Wave height and steepness increases Waves break Period remains constant Breakers in surf zone Different types of breakers associated with different slope of sea floor Spilling Plunging Surging http:// www.mikeladle.com Spilling breaker Water slides down front slope of wave Gently sloping sea floor Wind “onshore” Wave energy expended over longer distance http://www.winona.edu/geology/oceanography Plunging breaker Curling crest Moderately steep sea floor Wind “offshore” Wave energy expended over shorter distance Best for surfers http://www.seagrant.umn.edu/seiche/2002 Surging breaker Breakers on shore Steepest sea floor Energy spread over shortest distance Challenging for surfers http://www.bbc.co.uk/wales/surfing/images/ecards/400_232/hawaii/sandy_beach_bridgey.jpg Wave refraction As waves approach shore, they bend so wave crests are nearly parallel to shore Wave speed proportional to depth of water (shallow-water wave) Different segments of wave crest travel at different speeds Sets – series from relative calm to largest waves • Interference in wave train cancel some, adds to others • Destructive interference lull “between sets” • Rip currents are wave energy escaping shoreline Stream of water returning out to sea through surf zone Flows up to a few hundred meters offshore then dissipates http://www.ripcurrents.noaa.gov/overview.shtml http://www.ocean.udel.edu/mas/wcarey Wave energy distribution at shoreline • • Energy focused on headland • Headland eroded Energy dissipated in bay • Bay filled up with sediment Fig. 8.17b Tsunami or seismic sea wave Sudden changes in sea floor caused by Earthquakes, submarine landslides, volcanic eruptions Long wavelengths ( > 200 km or 125 m) Shallow-water wave characteristics (<L/2) Speed proportional to water depth so very fast in open ocean • • • • Not steep when generated (low H/L ratio) Crest of only 1-2 ft over 16 min period Move very fast -- up to 212 m/sec (470 mile/hr) As crest arrives on shore, slows but grows in height quickly • • • • • Sea level can rise up to 40 m (131 ft) when tsunami reaches shore Fast, onrushing flood of water rather than a huge breaker Series of waves Warning initial rushing out of water from shore Tsunami or seismic sea wave Most occur in Pacific Ocean (more earthquakes and volcanic eruptions) Damaging to coastal areas Loss of human lives Krakatau eruption (1883) in Indonesia created tsunami that killed more than 36,000 people Aura, Japan (1703) tsunami killed 100,000 people Indonesia (Dec. 26, 2004) tsunami killed over 229,000 around Indian Ocean Speed of tsunami Undersea earthquake at 6:59 AM Scale of tsunami damage on Sumatran coast in Aceh province Landsat image before tsunami: 13-Dec. 2004 ** Note sediment covered area impacted by tsunami 1-5 km inshore Landsat image after tsunami: 29www.jpl.nasa.gov/news Dec. 2004 Tsunami watches and warnings Pacific Tsunami Warning Center Seismic waves forecast possible tsunami Issues tsunami watches and warnings Increasing damage to property as more infrastructure constructed near shore Evacuate people from coastal areas and send ships from harbors Water “sucked” out before first http://www.drgeorgepc.com/tsuStationsTravelChart.jpg Waves as a source of producing electricity Lots of energy associated with waves Mostly with large storm waves How to protect power plants How to produce power consistently Environmental issues Building power plants close to shore Interfering with life and sediment movement Offshore power plants? Wave power plant at Islay, Scotland Fig. 8.25b Ocean Literacy Principles 1.c – Throughout the ocean there is one interconnected circulation system powered by winds, tides, force of the Earth’s rotation, the Sun, and water density differences. The shape of ocean basins and adjacent land masses influence the path of circulation. 5.h - Tides, waves, and predation cause vertical zonation patterns along the shore, influencing the distribution and diversity of organisms.