1Chapter 10 WavesCapillary Waves, Wind Waves, Tsunamis, Internal wavesbig wavessmall waveshuge wavesrogue wavesWave directionWavewave energy –NOT the water particles –moves across the surface of the seawave form moves and with it, energy is transmittedFig. 10-2, p. 266Direction of wave motionABWavelengthHeightStill water levelCrestTroughFrequency: Number of wave crests passing point A or point B each secondOrbital path of individual water molecule at water surfacePeriod: Time required for wave crest at point A to reach point Bmore like a real wavemore like a sine waveAnatomy of a WaveParts of a Wave* Wavelength* height * Crest* trough * amplitude * Frequency - # of waves passing a fixed point in a given length of time* Period - time for successive crests or troughs (1 wavelength) to pass a fixed pointDirection of wave motionWave-lengthStill water levelCrestTroughCrest1/2 wave-length depth2Stokes drift (mass transport)No mass transportWaveWaveClosed orbit after one periodOpen orbit after one periodIf we had this below, then there would be no net mass transport and no contribution of waves to the surface currents BUTin reality orbits are not exactly closed and waves DO contribute to mass transportHow do a waves form?• Wind blowing across calm water – if gentle breeze Æcapillary waves. Generating force = wind; restoring force = surface tension (cohesion); grow up to a wavelength of about 2 centimeters• As wind speed increases - wave becomes larger. Generating force = wind; restoring force Æ changes from surface tension to gravityTypes of waves - (1) progressive & (2) standing waves(1) progressive = have a speed and move in a direction• surface waves: deep-water & shallow-water waves • big’ waves: large swells, tsunamis & episodic waves • internal waves at the pycnocline(2) standing waves or seiches - do not progress, they are progressive waves reflected back on themselves and appear as alternating troughs and crests at a fixed position called antinodes, oscillating about a fixed point called node. They occur in ocean basins, enclosed baysand seas, harbors and in estuaries. Seismic disruptionDisturbing forcelandslidesGravity WindRestoring forceGravitySurface tensionType of waveTide Tsunami Seiche Wind waveCapillary wave (ripple)24 hr.Amount of energy in ocean surface100,000 sec (1 1/4 days)10,000 sec (3 hr)1,000 sec (17 min)100 sec 10 sec 1 sec 1/10 sec 1/100 secPeriod (time, in seconds for two successive wave crests to pass a fixed point)1 10 100Frequency (waves per second)12 hr.Period (& wavelength) and Wave EnergyProgressive WavesDeep- to Shallow-Water WavesHLAKeep in mind: wave energy, NOT the water particles move across the surface of the sea. Wave propagates with C, energy moves with VWave Speed is C - Group Speed is Vwave speed = wavelength / period or C = L / TT is determined by generating forceso it remain the same after the wave formed, but C changes. In general, the longer the wavelength the faster the wave energy will move through the water. Wave Speed3Deep Water WavesTgTLgLC 56.1225.12====ππFor example, for a 300 meters wave and 14 sec period, the speed is about 22 meters per second• Period to about 20 seconds• Wavelength to at most 600 meters (extreme)• Speed to about 100 kilometers/hour (70 mi/hr) (extreme)Deep Water Waves * surface waves progressing in waters of D larger than 1/2 L * as the wave moves through, water particles move in circularorbit * diameter of orbits decreasewith depth, orbits do not reach bottom,particles do not move below a depth D = L/2 * The wave speed can be calculated from knowledge of either thewavelength or the wave period: C = 1.56 m/s2 T or C2= 1.56 m/s2 L * Group Speed (which really transport the energy) is half of thewave speed for deep-water waves: V = C/2Shallow-Water WavesSeismic Sea Waves – Shallow-Water Waves• Period to about 20 minutes• Wavelength of about 200 kilometers• Speed of about 750-800 km/hr (close to 500 mi/hr!!)DgDC 1.3==Shallow-Water Waves• surface waves generated by wind and progressing in waters of Dless than (1/20) L • wave motion: as the wave moves through, water particles move inellipticalorbits • diameter of orbits remains the samewith depth, orbits do reach thebottom where they ‘flatten’ to just an oscillating motion back and forth along the bottom * The wave speed and the wavelength are controlled by the depth D of the waters only: * Group Speed (which transport the energy) is the same as the wave speed for shallow-water waves: V = CDgDC 1.3==Wind Speed: velocity at which the wind is blowingFetch: distance over which the wind is blowingDuration: length of time wind blows over a given areaWind Blowing over the Ocean Generates WavesWaves development and growth are affected by:Larger Swell Move Faster Æwaves separate into groupswave separation is called dispersion• Storm centers and dispersion• Winds flow around low pressure • Variety of periods and heights are generated Æ grouped into wave trainsWaves with longer period (T) and larger length move faster - these get ahead of the ‘pack’. Wave sorting of these free waves is dispersion4543217654 365432876587657654376 54654321Wave Train (‘pack’, group)• wave 1 transfers ½ of its energy to water (gets orbital motion going) and ½ to wave 2 (to keep that going)• wave 1 disappear – later 2 and 3 and so on will disappear also as wave 6, 7, etc. form• waves 1, 2, 3, etc. move at their deep-water wave speed C but the wave train moves at ½ of C = V, the group velocity, speed at which energy moves forwardDispersion only affects deep-water waves, as depth decreases waves become shallow-water waves, they slow down until C=VWave size increases with increased wind speed, duration, and fetch. A strong wind must blow continuously in one direction for nearly three days for the largest waves to develop fully.Fetch: uninterrupted distance over which the wind blows without significant change in direction.Wind Speed, Fetch & DurationPacific Ocean: wind speed of 50 mi/hr, blowing steadily for about 42 hours over a region of size 800 miles will results in 8 meters waves – can get to 17 meter waves! (see Table 10.2)7 across1 high120°Wave Height, Wavelength & Wave SteepnessTypical ratio wave height to wavelength in open ocean = 1:7 = wave steepness – angle of the crest = 120°Exceed these conditions and wave will break at sea ÆwhitecapsWave Height is controlled by (1) wind speed, (2)