All waves begin as disturbances, and ocean waves form as the result of a disturbing force.
Wave Generation
- Disturbing force causes waves to form.
- Wind blowing across ocean surface
- Interface of fluids with different densities
- Air – ocean interface
– Ocean waves - Air – air interface
– Atmospheric waves - Water – water interface
– Internal waves
Types of Waves
Internal Waves
- Associated with pycnocline
- Larger than surface waves
- Caused by tides, turbidity currents, winds, ships
Wave Movement
All types of waves transmit energy—or propagate—by means of cyclic movement through matter. The medium itself (solid, liquid, or gas) does not actually travel in the direction of the wave that is passing through it. The particles in the medium simply oscillate—or cycle—back and forth, and up and down, transferring energy from one particle to another.
- Waves transmit energy
- Cyclic motion of particles in ocean
– Particles may move - Up and down
- Back and forth
- Around and around
Different types of waves move in a variety of ways. Simple progressive waves are waves that oscillate uniformly and progress or travel without breaking. Progressive waves are divided into longitudinal, transverse, or a combination of the two motions, called orbital.
Progressive Waves
- Progressive waves oscillate uniformly and progress without breaking
– Longitudinal
– Transverse
– Orbital
waves, 2 transverse waves, and 3 orbital waves.
Longitudinal Waves
In longitudinal waves (also known as push–pull waves), the particles that vibrate “push and pull” in the same direction that the wave is traveling, like a spring whose coils are alternately compressed and expanded. The shape of the wave (called a waveform) moves through the medium by compressing and decompressing as it goes. Sound, for instance, travels as longitudinal waves
- Also called push-pull waves
- Compress and decompress as they travel, like a coiled spring
Transverse Waves
In transverse waves (also known as side-to-side waves), energy travels at right angles to the direction of the vibrating particles. If one end of a rope is tied to a doorknob while the other end is moved up and down (or side to side) by hand, for example, a waveform progresses along the rope and energy is transmitted from the motion of the hand to the doorknob. The waveform moves up and down (or side to side) with the hand, but the motion is at right angles to the direction in which energy is transmitted (from the hand to the doorknob). Generally, transverse waves transmit energy only through solids, because the particles in solids are bound to one another strongly enough to transmit this kind of motion
- Also called side-to-side waves
- Energy travels at right angles to direction of moving particles.
- Generally only transmit through solids, not liquids
Orbital Waves
Longitudinal and transverse waves are called body waves because they transfer energy through a body of matter. Ocean waves are considered surface waves because they travel along the interface between two different fluids (air and water). The
movement of particles in ocean waves involves components of both longitudinal and transverse waves, so particles move in circular orbits. Thus, waves at the ocean surface are orbital waves (also called interface waves).
- Also called interface waves
- Waves on ocean surface
Wave Terminology
As an idealized wave passes a permanent marker (such as a pier piling), a succession of high parts of the waves, called crests, alternate with low parts, called troughs. Halfway between the crests and the troughs is the still water level, or zero energy level. This is the level of the water if there were no waves. The wave height, designated by the symbol H, is the vertical distance between a crest and a trough. The horizontal distance between any two corresponding points on successive waveforms, such as from crest to crest or from trough to trough, is the wavelength, L.
Wave Steepness
Wave steepness is the ratio of wave height to wavelength:
Wave steepness = Wave height (H) / Wave length (L)
If the wave steepness exceeds 1/7, the wave breaks (spills forward) because the wave is too steep to support itself. A wave can break anytime the 1:7 ratio is exceeded, either along the shoreline or out at sea. This ratio also dictates the maximum height of a wave. For example, a wave 7 meters long can only be 1 meter high; if the wave is any higher than that, it will break.
Wave Period and Frequency
The time it takes one full wave—one wavelength—to pass a fixed position (such as a pier piling) is the wave period, T. Typical wave periods range between 6 and 16 seconds. The frequency (f) is defined as the number of wave crests passing a fixed location per unit of time and is the inverse of the period:
Frequency (f) = 1 / Period (T)
- Crest
- Trough
- Still water level
– Zero energy level - Wave height (H)
Orbital Wave Characteristics
- Wave steepness = H/L
– If wave steepness > 1/7, wave breaks - Wave period (T) = time for one wavelength to pass fixed point
- Wave frequency = inverse of period or 1/T
- Diameter of orbital motion decreases with depth of water.
- Wave base = ½ L
- Hardly any motion below wave base due to wave activity
Circular Orbital Motion
The water itself doesn’t travel the entire distance, but the waveform does. As the wave travels, the water passes the energy along by moving in a circle. This movement is called circular orbital motion.
- Wave particles move in a circle.
- Waveform travels forward.
- Wave energy advances.
Deep Water Waves
If the water depth (d) is greater than the wave base (L/2), the waves are called deep-water waves
- Wave base – depth where orbital movement of water particles stops
- If water depth is greater than wave base (>½L), wave isa deep water wave.
- Lack of orbital motion at depth useful for floating runways and other structures
- All wind-generated waves in open ocean
- Wave speed = wavelength (L)/period (T)
- Speed called celerity (C)
Speed of Deep Water Waves
Shallow-Water Waves
Waves in which depth (d) is less than 1/20 of the wavelength (L/20) are called shallow-water waves, or long waves . Shallow-water waves are said to touch bottom or feel bottom because they touch the ocean floor, which interferes with the wave’s orbital motion.
- Water depth (d) is less than 1/20 L
– Water “feels” seafloor - C (meters/sec) = 3.13 √ d(meters) or
- C (feet/sec) = 5.67 √d (feet)
Transitional Waves
- Characteristics of both deep- and shallow-water waves
- Celerity depends on both water depth and wavelength
Wind-Generated Wave Development
The “lifecycle” of a wind-generated wave includes its origin in a windy region of the ocean, its movement across great expanses of open water without subsequent aid of wind, and its termination when it breaks and releases its energy, either in the open
ocean or against the shore.
- Capillary waves : As the wind blows over the ocean surface, it pushes down on and parallel to the surface, causing water to pile up into small wavelets called capillary waves, which are more commonly called ripples.
- Wind generates stress on sea surface
- Gravity waves As capillary wave development increases, the sea surface takes on a rougher appearance. The water “catches” more of the wind, allowing the wind and ocean surface to interact more efficiently. As more energy is transferred to the ocean, gravity waves develop
– Increasing wave energy - Capillary Waves
– Ripples
– Wind generates initial stress on sea surface - Gravity Waves
– More energy transferred to ocean
– Trochoidal waveform as crests become pointed
Sea
The area where wind-driven waves are generated is called the sea, or sea area.
- Sea
– Where wind-driven waves are generated
– Also called sea area
Factors Affecting Wave Energy
- Wind speed
- Wind duration
- Fetch – distance over which wind blows
Wave Height
- Directly related to wave energy
- Wave heights usually less than 2 meters (6.6 feet)
- Breakers called whitecaps form when wave reaches critical steepness.
- Beaufort Wind Scale describes appearance of sea surface.
Global Wave Heights
Beaufort Wind Scale
| Beaufort wind scale and the state of the sea | ||||
| Beaufort number | Descriptive term | Wind speed | Appearance of the sea | |
| km/hr | mi/hr | |||
| 0 | Calm | <1 | <1 | Like a mirror |
| 1 | Light air | 1–5 | 1–3 | Ripples with the appearance of scales, no foam crests |
| 2 | Light breeze | 6–11 | 4–7 | Small wavelets; crests of glassy appearance, no breaking |
| 3 | Gentle breeze | 12–19 | 8–12 | Large wavelets; crests begin to break, scattered whitecaps |
| 4 | Moderate breeze | 20–28 | 13–18 | Small waves, becoming longer; numerous whitecaps |
| 5 | Fresh breeze | 29–38 | 19–24 | Moderate waves, taking longer form; many whitecaps, some spray |
| 6 | Strong breeze | 39–49 | 25–31 | Large waves begin to form, whitecaps everywhere, more spray |
| 7 | Near gale | 50–61 | 32–38 | Sea heaps up and white foam from breaking waves begins to be blown in streaks |
| 8 | Gale | 62–74 | 39–46 | Moderately high waves of greater length, edges of crests begin to break into spindrift, foam is blown in well-marked streaks |
| 9 | Strong gale | 75–88 | 47–54 | High waves, dense streaks of foam and sea begins to roll, spray may affect visibility |
| 10 | Storm | 89–102 | 55–63 | Very high waves with overhanging crests; foam is blown in dense white streaks, causing the sea to appear white; the rolling of the sea becomes heavy; visibility reduced |
| 11 | Violent storm | 103–117 | 64–72 | Exceptionally high waves (small and medium-sized ships might for a time be lost from view behind the waves), the sea is covered with white patches of foam, everywhere the edges of the wave crests are blown into froth, visibility further reduced |
| 12 | Hurricane | 118+ | 73+ | The air is filled with foam and spray, sea completely white with driving spray, visibility greatly reduced |
Maximum Wave Height
- USS Ramapo (1933): 152-meters (500 feet) long ship caught in Pacific typhoon
- Waves 34 meters (112 feet) high
- Previously thought waves could not exceed 60 feet
Wave Damage
- USS Ramapo undamaged
- Other craft not as lucky
- Ships damaged or disappear annually due to high storm waves
Wave Energy
- Fully developed sea
– Equilibrium condition
– Waves can grow no further - Swell
– Uniform, symmetrical waves that travel outward from storm area
– Long crests
– Transport energy long distances
Fully Developed Sea
| CONDITIONS NECESSARY TO PRODUCE A FULLY DEVELOPED SEA AT VARIOUS WIND SPEEDS AND THE CHARACTERISTICS OF THE RESULTING WAVES | ||||||
| These conditions . . . | . . . produce these waves | |||||
| Wind speed in km/hr (mi/hr) | Fetch in km (mi) | Duration in hours | Average height in m (ft) | Average wave- length in m (ft) | Average period in seconds | Highest 10% of waves in m (ft) |
| 20 (12) | 24 (15) | 2.8 | 0.3 (1.0) | 10.6 (34.8) | 3.2 | 0.8 (2.5) |
| 30 (19) | 77 (48) | 7.0 | 0.9 (2.9) | 22.2 (72.8) | 4.6 | 2.1 (6.9) |
| 40 (25) | 176 (109) | 11.5 | 1.8 (5.9) | 39.7 (130.2) | 6.2 | 3.9 (12.8) |
| 50 (31) | 380 (236) | 18.5 | 3.2 (10.5) | 61.8 (202.7) | 7.7 | 6.8 (22.3) |
| 60 (37) | 660 (409) | 27.5 | 5.1 (16.7) | 89.2 (292.6) | 9.1 | 10.5 (34.4) |
| 70 (43) | 1093 (678) | 37.5 | 7.4 (24.3) | 121.4 (398.2) | 10.8 | 15.3 (50.2) |
| 80 (50) | 1682 (1043) | 50.0 | 10.3 (33.8) | 158.6 (520.2) | 12.4 | 21.4 (70.2) |
| 90 (56) | 2446 (1517) | 65.2 | 13.9 (45.6) | 201.6 (661.2) | 13.9 | 28.4 (93.2) |
Swells
As waves generated in a sea area move toward its margins, wind speeds diminish and the waves eventually move faster than the wind. When this occurs, wave steepness decreases and waves become long-crested waves called swells (swellan = swollen), which are uniform, symmetrical waves that have traveled out of their area of origination.
- Longer wavelength waves travel faster and outdistance other waves.
– Wave train – a group of waves with similar characteristics
– Wave dispersion – sorting of waves by wavelengths
– Decay distance – distance over which waves change from choppy sea to uniform swell - Wave train speed is ½ speed of individual wave.
Wave Train Movement
Wave Interference Patterns
When swells from different storms run together, the waves clash, or interfere with one another, giving rise to interference patterns. An interference pattern produced when two or more wave systems collide is the sum of the disturbance that each wave would have produced individually.
- Collision of two or more wave systems
- Constructive interference
– In-phase wave trains with about the same wavelengths - Destructive interference
– Out-of-phase wave trains with about the same wavelengths
- Mixed interference
– Two swells with different wavelengths and different wave heights
Rogue Waves
- Massive, spontaneous, solitary ocean waves
- Reach abnormal heights, enormous destructive power
- Luxury liner Michelangelo damaged in 1966
- Basis of The Perfect Storm
- Difficult to forecast
- Occur more near weather fronts and downwind of islands
- Strong ocean currents amplify opposing swells
Waves in Surf Zone
- Surf zone – zone of breaking waves near shore
- Shoaling water – water becoming gradually more shallow
- When deep water waves encounter shoaling water less than ½ their wavelength, they become transitional waves.
Waves Approaching Shore
- As a deep-water wave becomes a shallowwater wave:
– Wave speed decreases
– Wavelength decreases
– Wave height increases
– Wave steepness (height/wavelength) increases
– When steepness > 1/7, wave breaks
Breakers in Surf Zone
- Surf as swell from distant storms
– Waves break close to shore
– Uniform breakers - Surf generated by local winds
– Choppy, high energy, unstable water - Shallow water waves
Three Types of Breakers
- Spilling
- Plunging
- Surging
Spilling Breakers
- Gently sloping sea floor
- Wave energy expended over longer distance
- Water slides down front slope of wave
Plunging Breakers
- Moderately steep sea floor
- Wave energy expended over shorter distance
- Best for board surfers
- Curling wave crest
Surging Breakers
- Steepest sea floor
- Energy spread over shortest distance
- Best for body surfing
- Waves break on the shore
Surfing
- Like riding a gravity-operated water sled
- Balance of gravity and buoyancy
- Skilled surfers position board on wave front
– Can achieve speeds up to 40 km/hour (25 miles/hour)
Wave Refraction
- Waves rarely approach shore at a perfect 90-degree angle.
- As waves approach shore, they bend so wave crests are nearly parallel to shore.
- Wave speed is proportional to the depth of water (shallow-water wave).
- Different segments of the wave crest travel at different speeds.
- Wave energy unevenly distributed on shore
- Orthogonal lines or wave rays – drawn perpendicular to wave crests
– More energy released on headlands
– Energy more dissipated in bays
- Gradually erodes headlands
- Sediment accumulates in bays
Wave Reflection
- Waves and wave energy bounced back from barrier
- Reflected wave can interfere with next incoming wave.
- With constructive interference, can create dangerous plunging breakers
Standing Waves
- Two waves with same wavelength moving in opposite directions
- Water particles move vertically and horizontally.
- Water sloshes back and forth.
- Nodes have no vertical movement
- Antinodes are alternating crests and troughs.
Tsunami
- Seismic sea waves
- Originate from sudden sea floor topography changes
– Earthquakes – most common cause
– Underwater landslides
– Underwater volcano collapse
– Underwater volcanic eruption
– Meteorite impact – splash waves
Tsunami Characteristics
- Long wavelengths (> 200 km or 125 miles)
- Behaves as a shallow-water wave
– Encompasses entire water column, regardless of ocean depth
– Can pass undetected under boats in open ocean
- Behaves as a shallow-water wave
- Speed proportional to water depth
– Very fast in open ocean
Tsunami Destruction
- Sea level can rise up to 40 meters (131 feet) when a tsunami reaches shore.
Tsunami
- Most occur in Pacific Ocean
– More earthquakes and volcanic eruptions - Damaging to coastal areas
- Loss of human lives
Historical Tsunami
- Krakatau – 1883
– Indonesian volcanic eruption - Scotch Cap, Alaska/Hilo, Hawaii – 1946
– Magnitude 7.3 earthquake in Aleutian Trench - Papua New Guinea – 1998
– Pacific Ring of Fire magnitude 7.1 earthquake
Historical Large Tsunami
Global locations and corresponding table showing significant tsunami (those with large tsunami height and/or responsible for
a large number of fatalities) since 1990. Tsunami have claimed more than 300,000 lives worldwide since 1990. These killer waves are most often generated by earthquakes along colliding tectonic plates of the Pacific Rim, although the most deadly tsunami in history was the 2004 Indian Ocean Tsunami (number 8). Locations of ocean trenches are shown by dark red lines; Pacific Ring of Fire is shown by red shading.
Indian Ocean Tsunami
- December 26, 2004
– Magnitude 9.2 earthquake off coast of Sumatra
– 1200 km seafloor displaced between two tectonic plates
– Deadliest tsunami in history
– Coastal villages completely wiped out - Detected by Jason-1 satellite
- Traveled more than 5000 km (3000 mi)
- Wavelength about 500 km (300 mi)
- 230,000–300,000 people in 11 countries killed
- Lack of warning system in Indian Ocean
The Indian Ocean Tsunami was initiated by a large earthquake offshore Sumatra on December 26, 2004 (red star). By a fortuitous circumstance, the Jason-1 satellite passed over the Indian Ocean (black line) two hours after the tsunami was generated. Its radar altimeter detected the crests and troughs of the tsunami (colors), which showed a wave height of about 1 meter (3.3 feet) in the open ocean. The graph of the satellite’s overpass (below) shows the difference between the measured sea level from satellite data (black line) and the modeled wave height (blue curve).
Japan Tsunami
- March 11, 2011 – Tohoku Earthquake
– Magnitude 9.0 earthquake in Japan Trench
– Felt throughout Pacific basin
– Most expensive tsunami in history - Initial surge 15 meters (49 ft)
– Topped harbor-protecting tsunami walls
– Amplified by local topography - Killed 19,508 people
- Disrupted power at Fukushima Daiichi nuclear power plant
– Reactors exploded
– Radioactivity problem initiated
Tsunami Warning System
- Pacific Tsunami Warning Center (PTWC) – Honolulu, HI – Uses seismic wave recordings to forecast tsunami
- Deep Ocean Assessment and Reporting of Tsunami (DART)
– System of buoys
– Detects pulse of tsunami passing
The DART system consists of a deep-ocean pressure sensor that can detect a tsunami passing above. The pressure sensor relays information to a buoy at the surface that transmits the data via satellite, allowing oceanographers to detect the passage of a tsunami in the open ocean.
Tsunami Watches and Warnings
- Tsunami Watch
- issued when potential for tsunami exists
- Tsunami Warning
- unusual wave activity verified
– Evacuate people
– Move ships from harbors
- unusual wave activity verified
Waves as Source of Energy
- Lots of energy associated with waves
- Mostly with large storm waves
– How to protect power plants
– How to produce power consistently - Environmental issues
– Building power plants close to shore
– Interfering with life and sediment movement
Wave Power Plant
- First commercial wave power plant began operating in 2000.
- LIMPET 500 – Land Installed Marine Powered Energy Transformer
– Coast of Scotland
– 500 kilowatts of power under peak operating capacity
Wave Farms
- Portugal – 2008
– Ocean Power Delivery
– First wave farm - About 50 wave power development projects globally
Global Wave Energy Resources
