Waves

Waves Wave – a disturbance caused by the movement of energy from a source through some medium (solid, liquid, or gas)

Waves Disturbing force - Energy that causes ocean waves to form Wind is the most common force that creates waves, but they can also be due to seismic disturbances (tsunamis) and gravitational forces (tides). Restoring force – dominant force that returns the water surface to flatness after a wave has formed in it.

Free waves vs. Forced waves Free waves – a wave that forms and then continues without any further influence from the force that formed it (wind waves, tsunami) Forced waves – a wave which is maintained by its disturbing force (tides)

Parts of a Wave Crest – highest point on a wave Trough – lowest point on a wave; valley between two waves Wavelength (L) – horizontal distance between 2 consecutive crests or troughs (measured in cm)

Parts of a Wave Cont. Wave Height (H) – vertical distance between a wave crest and trough (measured in m) Wave Amplitude – vertical distance between the still-water line and a wave crest or trough. (1/2 the wave height)

Diagram of a Wave

Diagram of a Wave

Wave Characteristics Wave Period (T) – time it takes for 2 consecutive crests or troughs (one wavelength) to pass a stationary point (measured in s). Long Period = big waves Short Period = small waves

Wave Characteristics Wave steepness (S) = measure of how high a wave can be compared to its length. S=H/L If wave steepness is greater than 1/7, the wave breaks Wave frequency = the number of waves passing a fixed point per second inverse of period (1/T) Wave speed (celerity) (C) = the distance a wave travels divided by the time it takes to travel that distance. Calculated by dividing the wavelength by the period. C=L/T

Waves (Continued)

Wave motion All waves transmit energy Particles may move Up and down (transverse wave) Back and forth (longitudinal wave) Around in circles (orbital wave)

Wave motion In the ocean (water waves), water particles move in a circle (cyclic motion) called an orbital path

Wave motion In the ocean (water waves), water particles move in a circle (cyclic motion) called an orbital path

Wave Orbital The circular motion of water particles within a wave.

Circular orbital motion Water particles move in circle Movement up and down AND Back and forth Fig. 8.4

Fig. 8.3C Orbital motion Diameter of orbital motion decreases with depth of water, until there is no movement Wave base = level below which there is little water movement due to waves (depth = ½ wavelength) Hardly any motion below wave base due to wave activity

Wave Motion Water in a wave does not move horizontally. The water is not moving toward shore; only the energy is Its vertical motion is called an orbital. Orbital motion changes shape as wave approaches shallower water.

Deep-water waves Water depth is greater than ½ wavelength (d/L>1/2) Orbital shape is circular The orbits will not touch the bottom. Wave speed (celerity) proportional to wavelength

Shallow-water wave Water depth is less than or equal to 1/20 wavelength (d/L<1/20) Orbital shape becomes elliptical Orbitals will touch the bottom and be compressed. Celerity proportional to depth of water Fig. 8.5b

Transitional waves Characteristics of both deep and shallow-water waves Water depth is greater than 1/20 but less than 1/2 of wavelength Orbitals begin to become elliptical in shape Celerity depends on both water depth and wavelength

Waves (Continued)

Waves approach shore Wave speed decreases (waves get slower) Deep-water waves  Transitional waves  Shallow-water waves Wave speed decreases (waves get slower) Wavelength decreases (waves get closer together) Wave height increases (waves get taller) Wave steepness increases (waves get steeper) Period remains constant Waves break

Shoaling waves Fig. 8.15

What makes a wave break? The 1:7 ratio – when a wave’s height is 1/7th its wavelength, the wave will break. What is the ratio of wave height to wavelength called? Wave steepness (S) = measure of how high a wave can be compared to its length. S=H/L If wave steepness is greater than 1/7, the wave breaks

How a Wave Breaks The wave approaches the shore and becomes a shallow water wave. Its orbitals flatten to ovals. The wave’s energy is packed into less water depth so the wave crests become peaked instead of rounded. The wave approaches the 1:7 ratio. The bottom of the wave slows down because of friction with the ocean floor, while the crest of the wave continues at a faster speed.

Breakers in surf zone Top of wave topples over base because of decrease in wave speed due to friction with sea floor Wave form not sustained (wave breaks) Different types of breakers associated with different slope of sea floor

What determines how big a wave will become? The strength/speed of the wind. Wind Duration – how long the wind blows Fetch – the distance over water that the wind blows in a single direction

Wave Interactions - Reflections When waves bounce from a surface back toward the source. Can produce a “seiche.”

Wave Interactions - Reflections Seiche - A wave that sloshes back and forth in an enclosed or partially enclosed body of water.

Wave Interactions - Reflections Waves and wave energy bounced back from barrier Reflected wave can interfere with next incoming wave Fig. 8.18

Wave Interactions - Refraction When waves come into shallow water and begin to “feel” the bottom, the angle of their direction changes.

Wave Interactions - Refraction As waves approach shore, they bend so wave crests are nearly parallel to shore Wave speed proportional to depth of water (shallow-water wave) Different segments of wave crest travel at different speeds

Wave Interactions - Refraction

Wave energy distribution at shoreline Wave Interactions - Refraction Wave energy distribution at shoreline Energy focused on headland Headland eroded Energy dissipated in bay Bay filled up with sediment

Wave Interactions - Diffraction When waves go through a small opening, they will bend around the edge of the opening.

Wave Interactions – Wave Interference When 2 waves meet, their energy combines to make either: Constructive interference In-phase wave trains with about the same wavelengths (waves get bigger) Destructive interference Out-of-phase wave trains with about the same wavelengths (waves cancel each other) Mixed interference Two swells with different wavelengths and different wave heights (unpredictable/mixed results)

Wave interference patterns Fig. 8.13

Interference © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

A Few More Types of Waves Standing Wave (seiche) – shallow water waves that reflect back on themselves (bays & seas). Rogue Wave – an abnormally high wave not related to local conditions; A particularly big wave appearing in a set of smaller waves. The result of constructive interference. Storm Surge - a large wave of water created from and driven by the energy of a storm

A Few More Types of Waves Tides – regular rise and fall of sea level due to the gravitational forces of moon & sun Tides are shallow water waves because their wavelength is so long. Tsunamis –very long waves created by earthquakes on the seafloor that travel at ~400 mph and build in height when they reach coastal areas. Tsunamis are shallow water waves because their wavelength is so long. NOT tidal waves! March 2011 tsunami

Tsunami or seismic sea wave Sudden changes in sea floor caused by seismic activity (Earthquakes, submarine landslides, volcanic eruptions) Very long wavelengths (> 200 km or 125 m) Shallow-water wave Speed proportional to water depth so very fast in open ocean Sea level can rise up to 40 m (131 ft) when tsunami reaches shore

Tsunami or seismic sea wave Fig. 8.20a

Tsunami or seismic sea wave Most occur in Pacific Ocean (more earthquakes and volcanic eruptions) Damaging to coastal areas Increasing damage to property as more infrastructure constructed near shore Loss of human lives Example: Aura, Japan (1703) tsunami killed 100,000 people Example: Sumatra, Indonesia (2004) tsunami killed more than 225,000 people in eleven countries (one of the deadliest natural disasters in history)

Wave Classification

Tsunami watches and warnings Pacific Tsunami Warning Center Uses data from pressure sensors on bottom of ocean, seismographs, and tidal times to forecast possible tsunami Evacuate people from coastal areas and send ships from harbors

Waves as a source of producing electricity 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 at Islay, Scotland Fig. 8.25b

Global coastal wave energy resources Fig. 8.26