Waves Ch. 10
What are Waves? All waves are movement of energy through a medium (air, rock, water) Series of vibrations or undulations in a medium Wave types: ocean, sound, light, seismic Vocal Cords Vibrate Waving flags Earthquake
Water Waves Movement of energy through water (kinetic) Rise & fall of water (lake, sea, coffee cup) Occurs through entire medium Non-breaking Breaking waves (surf): Last gasp of waves Traveled 1000 s of miles to spend their energy on beach
Produced by: Wave Formation Generating force (water disturbing) and Restoring (water calming) force Natural Disturbing forces: Winds, storms, earthquakes, gravity Restoring forces: Surface tension/cohesion (Sm. waves), gravity (Lg. waves), Coriolis effect
Wave type Capillary (water ripples) Wind waves (Sea swell) Tsunami (Seismic sea) Tides Wave Examples Generating force Wind Wind Vertical movement of seafloor Gravitational pull of moon & sun Restoring force Cohesion of water molecules Gravity Gravity Gravity
Ocean Wave Motion Transfers energy from H 2 O particle to H 2 O particle Causes H 2 O particles to move in circles (orbits) Transmits wave energy across ocean surface Waveform moves Orbital Wave Not each H 2 O particle Slight forward displacement
Ocean Wave Characteristics Wave height (H): vertical distance between crest & trough Wavelength (L): horizontal distance between 2 successive crests or troughs Wave period (T): Time for one wave to move a distance of one wavelength (A to B) Crest (peak) Trough (valley)
Ocean Wave Characteristics Wave frequency: Number of waves passing fixed point per second Crest (peak) Trough (valley)
Wavelength Determines Size of Water Particle Orbits Decrease in orbit diameter with depth Size of orbit at surface = wave height Wave motion (orbit) nearly absent at depth of ~½ of wavelength Wave Base
Deep-Water & Shallow-Water Waves Deep Water Waves: Don t interact with seafloor Circular water molecule orbits Shallow Water Waves: Do interact with seafloor Flattened, elliptical orbits Traveling at depths that are > ½ the wave's wavelength Close to shore Shallower than (<) 1/20 wavelength
Wavelength shortens (bunches up) Wave slows down Deep water : Wind & Capillary waves Shallow water (Tsunamis, & Tides): LONG wavelengths (>62 miles), so their orbits always come in contact w/ seafloor (not deeper than 31 miles)
Wave Velocity The longer the wavelength, the faster the wave energy moves (speed) Wave Velocity Formulas: 1. Deep water: C = L/T C = speed (meters/second) L = wavelength T = time or period (sec.)
Wave Velocity Example: C (Speed) = L/T (wavelength / period-time) C = 233 meters/12 sec. C = 19.4 m/sec.
Deep-Water & Shallow-Water Waves Deep Water Waves: Wavelength to 600 m (2000 ft) Speed to 112 km (70 mph) Shallow Water Waves: Wavelength to 200 km (125 miles) Speed to 760 km (470 mph) Wind or capillary waves Wave energy moves (speed) faster w/ longer wavelengths Tsunami or Tide Waves
Wind Waves Gravity waves formed by transfer of wind energy into surface waters Height range: ~2 cm to 3 m Wavelength range: 60 to 150 m Capillary waves form 1 st (< 1.73 cm) Wind friction on ocean surface = H 2 O piles up & gravity moves it forward slightly Forms crest Surface tension restores smoothness crest trough
Capillary Waves Become Wind Waves When their wavelength > 1.73 cm (0.68 ): Gravity (atmos. pressure) > than surface tension Becomes dominant restoring Force Piled-up water flows downhill (under influence of gravity) Wave will grow if: Wind continues to blow Wave remains in water deeper than ½ its wavelength
Can travel 1000 s of km across ocean basins Break as surf on a distant shore Swell When wind slows (away from storm): Wave crests become rounded & regular Separate (dispersion) into groups w/ similar speed & wavelength Creates smooth undulation of sea surface Swell
Wave Trains Progressive groups of swell of similar origin Traveling together across ocean Leading wave s energy: ½ transferred to undisturbed H 2 O ahead forward motion ½ transferred to wave behind maintains motion
Factors Affecting Wind Wave Development Wind duration If too short, waves will not develop Wind strength (intensity or wind speed) Size of fetch: Distance that wind blows over surface w/out interruption In same direction
Wind Waves Change as Approach Shore Feel bottom when H 2 O depth ½ wave s wavelength Circular H 2 O particle orbits change to flattened ellipses Wavelength, Period (time) remains same Wave breaks when ratio of wave height to H 2 O depth 3:4 Crests become peaked, not rounded Wave height Crest moves ahead of base Wave slows down (friction)
Surf: Turbulent mass of water rushing toward shore after wave breaks Surf Zone Area between breaking waves & shore Slope of seafloor determines break characteristics
Wave Refraction: Waves approaching shore at an angle will SLOW & BEND Portion of Wave near shore slows, wave in deeper water does not slow Wave energy converging on headlands Angle Waves bend (refract) in shallow water Wrap around cliffs/headlands Energy dispersed in bays
Diffraction Wave changes shape & direction Wave train is interrupted as it moves around an obstacle Gap in breakwater or between islands Generates smaller secondary waves
Reflection Waves bounce off vertical barriers Move from obstacle in direction it came, Interfere w/ incoming waves Bluffs Seawalls Jetties The Wedge Standing waves can result w/in enclosed waters (harbors & bays) Jetty Reflected wave overlaps original wave, producing The Wedge
Tsunami (Japanese): Seismic sea waves created by seafloor movement Earthquakes, Landslides, Meteors, Volcanoes, Glaciers
Travel very fast: Up to 500 mph Can cross Pacific in 10 hrs. Nature of Tsunami Very long wavelength: ~200 km (125 mi) Very small height moving out from source Only 1-2 m high in open ocean Vessels on open sea wouldn t notice one Very long period: 5-20 min
Earthquake Generated Tsunami Plates move against each other 1 plate subducts below Crust catches, forces build up, 1 plate can tear free lurches forward = Earthquake Sumatra: ~700 miles of plate boundary ripped Ripped Boundary Epicenter Indo- Australian Plate Trench
Wave Height (Amplitude) Determined by Amount of Seafloor Displacement: 1. Energy from earthquake causes vertical movement of crust 2. Displaces water Sumatra: Crust uplifted ~15m (50ft.) Displaced 100 s of km 3 of water 3. Wave moves out from earthquake epicentre (up to 500 mph)
Tsunami Travel Long Distances w/ Little Energy Loss Change radically at shoreline: Slow down (friction with seabed) Wavelength shortens dramatically (H 2 O piles up ): Huge in wave height (shoaling) 1 st wave on shore may be trough or crest Meters 10 5 0 33 ft.
Trough: Looks like super-low low tide Sri Lanka Tsunami Ht. At Banda Aceh, Indonesia Sri Lanka Crest: Like rapidly rising tide or big wave Phuket Thailand
Low-lying Areas Along Coast: At serious risk when a rapid series of waves rush onshore Energy transferred to smaller waves Khao Lak, Thailand Krabi, Thailand Phuket, Thailand Malaysia
Gleebruk Village, Indonesia Before After
Many People Had TIME to Escape, But Did Not 30,000 died in Sri Lanka
Recent Pacific Tsunami: 1946, 1957 Aleutian (Hawaii); 1960 Chile (Chile & Hawaii); Prince Will. Sound (Alaska); Indian Ocean: 2004 (NW Sumatra) 26 Dec. 2004 NW Sumatra Max. wave ~15m (49ft) Fatalities: > 250,000
In response to Pacific tsunami: detection & warning system established U.S.A (NOAA), Chile, Japan, Australia, U.N. Satellites, tide guages, alert locals Warning: sirens, radio, t.v., education No such program exists Tide Guage for Indian Ocean yet