Waves

1. Waves

2. In 1872, Thomas Stevenson witnessed waves lift a concrete breakwater and sweep it towards the shore When storm subsided divers looked at the breakwater and discovered the waves had not only lifted the 800 ton breakwater but also the rocks that it was attached to The total mass moved was 1350 tons or 13,500 oceanography professors 5 years later, in the same place, the newer, heavier breakwater was also swept away = 2600 tons

3. WHAT MAKES WAVES ? All waves are caused by a disturbance Involve a transfer of energy to the water Waves can occur where two fluid substances meet: AIR-WATER (air moving over sea surface causes ocean waves) AIR-AIR (atmospheric wave – seen in ripple effects in clouds) WATER-WATER (in between water layers of different densities ) = INTERNAL WAVES These are usually associated with a pycnocline Can be 100m high Caused by tidal movement, turbidity currents or winds

6. WHAT MAKES WAVES ? Also mass movements into the ocean e.g1. landslides e.g2. icebergs and disintegrating ice shelf = SPLASH WAVES Also tides Form large, low, predictable waves Also human activities e.g1. wakes of ships e.g2. explosions

7. WHAT MAKES WAVES ? Also sea floor movements e.g1. underwater avalanches (turbidity currents) e.g2. volcanic eruptions e.g3. fault slippage = SEISMIC SEA WAVES or TSUNAMI

8. HOW WAVES MOVE SIMPLE PROGRESSIVE WAVES are waves that move through a material without ‘breaking’ There are three main types of this kind of wave Longitudinal Transverse Orbital

9. LONGDITUDINAL WAVE TRANSVERSE WAVE

11. WAVE MOTION Waves can travel great distances e.g. waves generated in Antarctica were detected in the Pacific – 10,000 km away Although the waveform travels – the water itself does not move

13. Top of crest to top of crest = wavelength Crest to trough = wave height Halfway between trough & crest = still water level or zero energy level

14. WAVE CHARACTERISTICS WAVE STEEPNESS = Wave Height (H) Wavelength (L) If wave steepness exceeds 1/7the wave BREAKS e.g. a wave 7m long can be up to 1m high The time it takes for one full wave (one wavelength) to pass a fixed position =WAVE PERIOD Typically 6 to 16 seconds

15. WAVE CHARACTERISTICS FREQUENCY (f) = 1 Period (T) e.g. A wave has a period of 12 seconds Frequency = 1/12= 0.083 = 5 waves per minute

16. The orbital motion of the water dies down at a depth = wavelength (L) / 2 Measured from the still water level = WAVE BASE Therefore submarines etc do not have to dive to particularly greatest depths to avoid even the greatest storms

17. WAVE SPEED If the water depth is greater than L/2 = DEEP WATER WAVES i.e. all open ocean waves In these waves WAVE SPEED (S) = Wavelength (L) Period (T) Wave speed should really be called CELERITY (C) as although the waveform moves forwards, the water molecules themselves do not travel forwards Generally, the longer the wavelength, the faster a wave travels

18. SHALLOW WATER WAVES If the water depth is less than L/20 = SHALLOW WATER WAVES or Long Waves Shallow water waves are said to TOUCH BOTTOM or FEEL BOTTOM Because the ocean floor interferes with their orbital motion Gravity has more effect on these waves and their speed S = √gd Where g = the acceleration due to gravity & d = depth → Wave speed = 3.13√d meters per second

19. SHALLOW WATER WAVES Shallow water waves include: Wind generated waves that have moved into shallow areas Tsunami Tides Tsunami & Tides have extremely long wavelengths → greater than the ocean depths Instead of a circular orbit, water particles in shallow water waves move in very flat ellipses

21. TRANSITIONAL WAVES Waves that have some deep water and some shallow water characteristics = TRANSITIONAL WAVES Their wavelengths are between 2 - 20 times the water depth Their speed partially depends on wavelength, partially on depth

23. WIND-GENERATED WAVES As the wind blows over the sea the pressure and stresses this causes result in small rounded waves with V-shaped troughs = CAPILLARY WAVES Intermolecular forces in the water try to pull water molecules together (i.e. capillarity) – restoring a flat calm sea As the wind increases the capillary waves increase in size

24. CAPILLARY WAVES

25. WIND-GENERATED WAVES As the wind increases, it “CATCHES” the waves transferring more energy into them = GRAVITY WAVES These waves are symmetrical with rounded crests and troughs Gravity waves have a wavelength greater than 1.74 cm At this size gravity is the main force that acts to reduce the waves and still the sea (not capillarity) The length of gravity waves = 15-35 x their height

26. WIND-GENERATED WAVES As the wind increases, more energy is transferred = TROCHOIDAL WAVES These waves have pointed crests and rounded troughs Increases in wind force increase the height, length and speed of the waves Waves reach their maximum height and length when wind speed = wave speed

27. Trochoidal wave

29. WIND-GENERATED WAVES Factors that effect the amount of energy in waves include: Wind speed The length of time that the wind blows in one direction The FETCH – the distance over which the wind blows in one direction

30. The area over which the wind in acting = the SEA AREA

31. WIND-GENERATED WAVES Wave height is related to the energy in a wave Wave heights in a sea area are usually less than 2m But heights of 10m with periods of 12 seconds are not uncommon As waves gain energy their steepness increases Until height/length =1/7 Then open ocean breakers form = WHITECAPS

33. WIND-GENERATED WAVES The areas between 40 & 60oS have particularly strong winds Roaring Forties Furious Fifties Screaming Sixties

35. WIND-GENERATED WAVES The largest recorded wind-generated wave occurred in 1935 The US Navy tanker USS Ramapo was in 108 kmph (67 mph) winds in the West Pacific The wave sizes were calculated using geometry Height = 34 m Taller than an 11 storey building

36. SWELL As waves generated in a SEA AREA move to the edges, wind speeds reduce The waves eventually move faster than the storm producing the wind Wave steepness decreases Waves become long-crested waves = SWELLS

38. SWELL Swells can move with little loss of energy →travel for long distances So there can be waves at distant shores where there is no wind Waves with longer wavelengths travel faster Therefore they leave the sea area first These are followed by shorter, slower WAVE TRAINS or groups of waves

40. WIND-GENERATED WAVES This progression of long fast waves to short slow waves = WAVE DISPERSION i.e. sorting out waves by their wavelength In the sea area (where the wind is blowing) - waves of many different wavelengths can be found – “choppy” Long (therefore fast) wavelength waves “outrun” shorter wavelength waves in leaving the sea area The distance over which waves turn from “choppy” sea to uniform swell = THE DECAY DISTANCE

41. INTERFERENCE When swells from different storms run into each other they clash or INTERFERE This leads to INTERFERENCE PATTERNS If the crests of waves occur at the same time (IN PHASE) the crests add together =CONSTRUCTIVE INTERFERENCE If the crest of one wave coincides with the trough of another wave (OUT OF PHASE) they cancel out =DESTRUCTIVE INTERFERENCE

43. INTERFERENCE The more common occurrence is a combination of the two = MIXED INTERFERENCE This type of interference explains why waves in an area on the ocean occur at various heights The sequence of high and low waves e.g. a high wave every three low waves = SURF BEAT On the ocean, swells can sometimes approach from several directions and interact

45. ROGUE WAVES Occasionally interaction between wave systems and constructive interference can result in unusually large waves = ROGUE WAVES or SUPERWAVES In the open ocean 1 wave in 23 will be double the average height 1 in 1175 will be 3 times the height 1 in 300,000 will be 4 times as high In 2000 a NOAA research vessel was sunk when hit by a 4.6m wave in fairly calm waters Up to 1000 ships a year are possiblylost due to rogue waves

46. In hurricane Ivan a giant wave 27.7m was produced

47. SURF The zone of coastline where waves break, loosing their energy = THE SURF ZONE As deep water waves of swell approach the coast the water gets shallower = SHOALING When they reach waters of depth less than ½ their wavelength → transitional waves NB this decrease in depth can also be caused by submerged rocks or wrecks – Therefore sailors should avoid a patch of breaking waves in an otherwise normal sea

48. SHOALING WATER WAVES The shoaling water’s decreasing depth interferes with the movement of water particles at the base of the wave → wave speed decreases The following wave (still moving at its original speed) moves closer to the slowed wave (like cars at a stop sign) → wavelength decreases But the energy in the wave remains the same – this energy has to go somewhere → wave height increases

49. SURF THERFORE → an increase in height but a decrease in wavelength → an increase in WAVE STEEPNESS (H/L) When wave steepness reaches 1/7 → wave BREAKS If surf is made of swell that has traveled from a distance, breakers will usually form NEAR THE SHORE in SHALLOW water The surf will consist of parallel lines of regular breakers

50. SURF If the surf consists of waves produced by local winds → waves will not have been sorted into swell → unstable, high energy, deep water waves (with high steepness) These waves will break shortly after “touching bottom” Breakers will occur at a DISTANCE from the shore The surf will be ROUGH, CHOPPY and IRREGULAR