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WAVESWAVES
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Waves• Wave - disturbances of the water surface (energy
transmitted through matter)
• Manifestation of energy propagating on the ocean surface
• Waves can propagate along any density interface
• Caused by three things
– 1. wind (most familiar)– 2. attraction of sun and moon - tides– 3. sudden movements of ocean bottom - earth-
quakes, volcanic explosions
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Wave types
• Ideal waves are simplest to understand using mathematics
• Simple wave can be made by steadily bobbing the end of a pencil in a basin of water - the waves move away from the disturbance and are called progressive waves
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Progressive wave
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• Complex waves - described by combining many simple waves
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Wave components: S&A 114 (p 249)
• wave height H - distance from low part of trough to tip of crest
• wave length L - distance from crest to crest
• wave period T - time (sec) it takes for successive crests to pass a fixed point
• wave steepness H/L
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• Speed (C) = L/T - longer waves travel faster than shorter waves (think of speed as mph or feet per second)
• T 90 Fig 8.8 p 237 this figure is a standard curve, and shows the reader how he might determine any one of the three questions he may ask knowing the answers to at least two of them.
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Water parcels move in circular orbits, returning to their original position (almost) as
each wave passes
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Wave types (two types)
• Deep-water waves • 1. are unaffected by the bottom Fig 8.7 p 236
S&A 115• 2. water depth is >1/2 wave L• 3. all wind generated waves, as they move across
the ocean, are Progressive waves• 4. below >1/2 L, water is moved very little by
wave passage• 5. speed determined by the wave period
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Deep/Shallow Waves• Fig 8.7 p 236 S&A 117• shallow water waves
– C and L depend upon depth only– H increases
• Intermediate waves (transitional)– C and L decrease as depth decreases– H increases
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Deep/Shallow Waves cont
• S&A 117 p 250• Deep water waves
– Depth > l/2 wave length– C and L are constant and depend upon T– only H is constant
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S&A 117
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Wave types cont.• Shallow-water waves are affected by the bottom
S&A 117 Fig 8.7 p 236 – 1. depth to the bottom is < 1/20 of the wave length
– 2. wind-generated waves that have moved into shallows near shore areas
– 3. tsunamis (seismic sea waves) generated by earthquakes in the ocean floor
– 4. tide waves (generated by gravitational attraction of the sun and moon)
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Constructive interferenc.
• Transparency MasterWhen two light waves of the same wavelength (colour) combine exactly in phase (in step) their amplitudes add to produce a large (brighter) wave of maximum intensity
• Fig 8.14 p 241
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Waves classified by size Fig 8.9 p 237
• Capillary waves - ripples - surface tension of water works to restore the smooth ocean surface
• Gravity waves are most common – as the capillary waves catch more wind, transferring
energy to the wave, gravity waves develop
– gravity on the wave is more important than surface tension
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Wind generated waves• most commonly observed S&A 116• Energy from the wind increases wave height, length,
and speed• With increased wind speed, waves (called sea) are
sharp-crested and chaotic Table 8.1 p 239 (next slide)
• Heavy seas Fig 8.12 & 8.13 p 240 (2nd slide)• After moving away from the winds that generated
them, waves are called swell - more regular, longer, smoother crested than sea S&A 116
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Table 8.1 p 239
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Aircraft carrier USS BenningtonFig 8.13
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Wave steepness• Speed = length/period (period - time (sec) it takes for
successive crests to pass a fixed point, and does not change.)
• Steepness = height/length 4 = 8/2 3 = 6/2• if the ratio of height to length > 1:7 the wave breaks• L = 70 H = 10 ratio is 1:7 (0.14)• L = 60 H = 8 ratio is < 1:7 (0.13)• L = 70 H = 12 ratio is > 1:7 (0.17) wave breaks
• < 120o wave becomes unstable S&A 116
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Wind generated waves cont.
• Three factors controlling wave size (energy
transfer from wind to water) T91 Fig 8.10 p 238
• 1. Length of fetch - refers to the distance over which the wind blows in one direction
• 2. speed of wind
• 3. duration of wind - refers to the time (hours) that the wind blows in one direction
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• White caps - as wind increases waves gain energy and their steepness increases
• - when steepness reaches a critical value breakers form (white caps)
• - energy (from the wind) is released
• - speed = meters/sec
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• ENERGY IS NEVER DESTROYED IT IS TRANSFERRED FROM ONE THING TO ANOTHER
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• Wave Speed T 93
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Change in wave due to bottom
• Fig 8.7 p 236 S&A 117
• as depth decreases
• deep-water waves -> intermediate waves -> shallow-water waves
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Shallow-water waves
• Movement affected by the bottom – 1. wave length and speed decrease– 2. wave period remains constant
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Shallow-water waves cont.
• Movement– 3. wave refraction (bending): Fig 8.19 a p 247 S&A
118 occurs because the part of the wave still in deeper water moves faster than the part that has entered the shallower water
– a. orthogonal lines (wave ray) (in red/yellow) show direction wave travels - right angle to wave crest
– b. crest line rotates so that it is more parallel to the bottom depth contours Fig 8.19 b S&A 118
– c. waves in bays - waves in the center of the bay do not shorten as much; therefore, they have less height and less energy S&A 118
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Shallow-water waves cont.
• 4. wave diffraction - wave energy being transferred around or away from barriers - may spread to most protected areas
• Once through the opening, the wave crests decreases in height and radiate out and away from the gap
• Surf - mix of breakers along shore or over a submerged bank or bar
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Surf
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Breakers
• Breaking waves (breakers) dissipate energy - found on all beaches Fig 8.19 p 247 in book– 1. surf produces effects on beaches - beach
topography changes through out the season– 2. long shore currents - transport sediment parallel
to the coast– 3. rip currents - a narrow stream of return flow
through the breaker zone Box 10.1 p 290 book– 4. moves sand along beaches– 5. swell - wind speed decreases, wave continues at
same speed, wave steepness decreases, wave becomes long-crested
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Rogue waves Box 8.1 p 243
• really big waves - tend to occur more frequently in locations that are downwind from islands or shoals, and where storm-driven waves move against strong ocean currents (south eastern coast of Africa) Agulhas Current Fig 7.5 p 198
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Rogue wave
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Internal waves Fig 8.1 P 232
• 1. Occur at density discontinuities – pycnocline - boundary of two fluids of different
densities (remember the oil and vinegar)
• 2. Move slower than surface waves• 3. have longer periods than most wind
waves• 4. height is limited by the thickness of the
surface layer
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Internal wave
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Standing waves
• wave form does not move - example: fill a shallow bowl with water then tilt, water flows to opposite side, then back again. Occur in nearly enclosed basins
• Important in tides
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Standing waves cont.
• Anatomy of a standing wave
• node - water is motionless– maximum horizontal movement beneath nodes
• antinode - movement is maximum– movement is vertical beneath antinodes
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Fig 8.21 p 248
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Storm surge• Read p 179 (book)• large waves that move with the wind that caused
it• Forerunner comes first - can cause water level to
drop• Surge - comes when the storm center passes (2-5
hrs) - sharp rise in water level• Cause flooding - extremely high waves are
generated - sea level continues to rise and fall as storm caused oscillations pass
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Storm surge
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Storm surge
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Tsunami
• caused by fault movement, or displacement, in earth's crust along a fracture Fig 8.22 p 249 Remember the tsunami March 2011. This caused the nuclear melt down of the nuclear reactors on the north end of Japan.
• sudden change in water level at the ocean surface above Box 8.2 read the 2004 account in book p 253
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Energy
• can be extracted from waves, read account in book p 256- 257
• know each of the energy capturing methods this will be on the next exam
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The Pelamis Wave Energy Converter Fig
8.28 p 257
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Ocean & Sea Wave Power
LIMPET Fig 8.27 p 256
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The end of the WAVEThe end of the WAVE