WAVESWAVES

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

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

Progressive wave

• Complex waves - described by combining many simple waves

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

• 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.

Water parcels move in circular orbits, returning to their original position (almost) as

each wave passes

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

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

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

S&A 117

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)

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

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

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

Table 8.1 p 239

Aircraft carrier USS BenningtonFig 8.13

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

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

• 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

• ENERGY IS NEVER DESTROYED IT IS TRANSFERRED FROM ONE THING TO ANOTHER

• Wave Speed T 93

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

Shallow-water waves

• Movement affected by the bottom – 1. wave length and speed decrease– 2. wave period remains constant

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

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

Surf

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

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

Rogue wave

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

Internal wave

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

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

Fig 8.21 p 248

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

Storm surge

Storm surge

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

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

The Pelamis Wave Energy Converter Fig

8.28 p 257

Ocean & Sea Wave Power

LIMPET Fig 8.27 p 256

The end of the WAVEThe end of the WAVE