Boundarywaters

     I don't know much about waves except for the fact I'm not extremely comfortable in them.  I've gotten alot better about paddling in waves over the years but I still prefer a relatively calm body of water for canoeing.  
     I've been watching the movement(or should I say lack there of?) of the Voyageur Hudson Bay Expedition Crew and I've been keeping an eye on thewave conditions on Lake Winnipeg.  There are terms such as knots, dominant wave period, wave height and pressure listed on this website and somehow they all correlate with the conditions on the North portion of Lake Winnipeg where the Voyageur Crew has been attempting to paddle.      
     As of 9am on the 27th day of June the air temperature where the Voyageur Crew is located is 48 degrees, the water temperature 50 degrees, wind is 16 knots North(true-not sure what that means), pressure 29.95, wave height 3.9 feet and dominant wave period 5 feet.  What does this all mean?  I'm not 100 percent sure but what I determine from the information is if the wind is blowing from the north at 16 knots(roughly 16 miles per hour) and I'm wanting to paddle north there wouldn't be much forward progress with 4 foot waves coming at me.  Thus, the reason for the lack of movement from the Voyageur Crew.
     The canoeing crew is paddling when the wind and waves allow them to. That is a good thing.  Here's some wave information from an ocean website that may or may not make sense to you.

WAVES

A wave is a progression of energy from one point to another

The wave does not move forward only the energy in it progresses

Wave is almost friction free

Properties of waves
 

Wavelength:	Distance from one crest to the next
Period:	Time taken for one wave to pass a fixed point
Frequency:	Number of waves per second that pass a fixed point
Velocity:	Speed with which the waves are moving past a fixed point

Orbital motion

• The size of the orbit of the water particles increases with wavelength
• The orbit size decreases rapidly with depth
• Orbit size decreases to 1/23 of surface value at a depth equal to 1/2 of wavelength
• Only "feel" waves to a depth of 1/2 of their wavelength

Classification of waves: according to the way they are formed or destroyed

Major formation forces:

• Wind
• Atmospheric pressure
• Landslides or other earth movements
• Gravitational attraction
  =>Wind waves have the most energy in surface ocean

Restoring forces: Try to flatten out the waves

• Surface tension for very small waves (<0.6 inches)
• Gravity, for everything bigger

Deep and shallow water waves

• Relationship between wavelength and water depth determines wave characteristics
• Deep water wave: water depth >1/2 wavelength orbits die away above bottom
• Shallow water wave: water depth <1/20 wavelength orbits are flattened at the bottom
• Transitional wave:Êwater depth >1/20 but <1/2 wavelength
• Wave "feels" bottom
• Gravity and seismic waves have very long wavelengths are always shallow water
• waves regardless of ocean depth

Wave velocity

1. Deep water waves
   • Velocity of wave energy through water determined by wavelength
   • Longer waves move faster
   • Use period as is easier to measure than length
   • Speed (m/sec) = 1.56 x wave period
   • Typical 8 second trade wind wave moves at 12.4 m/sec=28 mph
2. Shallow water waves
   • Velocity of wave is related to water depth
   • Speed (m/sec) = 3.1 x square root (depth)
   • Typical 20 minute seismic wave moves at 470 mph

When a deep water wave moves into shallow water it slows down

Trade wind wave (8 second) 28 mph in deep water in 1 metre deep water speed is 3.1 m/sec=7 mph

Wind wave formation

1. Wind attempts to "stretch" surface skin of ocean
2. Surface tension: capillary wave
3. Wind deflected upwards, adds energy to wave pushes it forward
4. Low pressure behind wave contributes to forward motion
5. Continued wind, wave period and height grow together
6. Waves are peaked in areas of formation, rounded swell away from formation regions

Wave progression

• Longest waves move away from storm fastest
• Form wave trains
• Leading waves "excite" still water ahead of wave train
• New waves forms behind wave train
• Wave train travels at half the speed of the individual waves within it

Maximum development of wind waves is the result of 3 factors:

1. Wind strength
2. Wind duration
3. Uninterrupted ength of ocean that wind blows over (Fetch
    

    
    
    

   The stronger the wind the longer the duration and fetch needed to fully develop the sea
   Rarely get fully developed seas for strongest winds
   Highest waves found around Antarctica, constant wind, uninterrupted ocean

Wave steepness and dispersal from a storm

• Maximum wave height in open ocean is 1/7 of wavelength, higher waves get whitecaps
• In region of formation seas chaotic
• Waves sorted by wavelength and speed as move away from formation region
• Waves turn to swell as they move away from region of formation height to length ratio gets smaller
• When waves overtake each other constructive interference causes very large waves
• Distant observer see longest and fastest waves first

Surf prediction

See Pat Caldwell's predictions

Example: Fall 95 storm ~ 1,000 miles from Oahu

1. Get meteorological and oceanic data from weather buoys in region
2. Storm pressure 964 mbar, winds 50 knots, fetch 1200-1500 miles, duration only 24 hrs, not enough for a fully developed sea
3. Swell 30 ft, period 12 seconds
4. Wave velocity = 40 mph, group velocity = 20 mph
5. Travel time to Oahu ~ 2days
6. Wave height attenuation 30%/day
7. After 1 day wave height 2/3 x 30 = 20ft
8. After 2 days wave height 2/3 x 20 = 12ft
9. Shoaling effect
   • 2 x at Waianae = 24ft
   • 1.2 x at Makaha = 14ft
   • 1.6 x at Sunset Beach = 19ft

Waves approaching the shore

1. As wave train approaches shore "feels" bottom at depth = 1/2 wavelength
2. Wave energy packed into shallower depth, becomes peaked
3. Wave slows, period is constant, wavelength decreases
4. Bottom of wave slows even more as gets shallower, wave crest moves ahead of base of wave
5. Wave breaks when wave height to water depth ~ 3:4

Types of wave breaks

• Type of wavebreak depends on bottom
• Plunging waves from steeply sloping bottoms
• Spilling wave from gentle slopes
• Abrupt slope change: water surges on to beach

Wave refraction

1. Wave approaching coast at an angle
2. End of wave entering shallow water slows down, rest of wave continues at full speed
3. Wave bends towards shore (towards the slowest end)

Wave diffraction

• Waves passing through a small gap diffract
• New waves reform at a point in gap and radiate out
• Occurs in harbour entrances and between islands
• Radiated waves between island groups form interference patterns
• Polynesian navigators recognised interference patterns to indicate island chains beyond the horizon

Internal waves

• There are waves below surface at regions of density gradient, e.g. pycnocline
• Waves can be large but travel slowly as density gradient is small compared with one at sea surface
• Sub surface Kelvin waves bring an end to El Nino

 

Last modified: March 2010
Department of Oceanography
Send comments to: ta@soest.hawaii.edu