WAVES Wave propagation of energy through a medium

WAVES

Wave – propagation of energy through a medium. • Speed is determined by the properties of the medium. Gravity waves – sufficiently large waves where gravity acts as the restoring force (force that causes the water to return to its undisturbed level) Ripple (or Capillary) waves – surface tension acts as the restoring force

Wind is the most common generating force • Capillary waves are formed and grow to form gravity waves.

Fetch – continuous area of water over which the wind blows in essentially the same direction.

Anatomy of a Wave

Measures of a Wave Period – the time required for two successive crests or troughs to pass a point. (depends on the generating force) Wave Speed: speed = length of wave C= L/T wave period C = celerity (wave speed) L = wavelength T = Period

Dispersion – faster waves move through slower waves (Sorting) • Long period, Long wavelength = Greater Speed • Short period, short wavelength =Lesser Speed Wave trains = faster waves; packets of similar waves with the same period & speed Swell = faster, longer period waves; uniform, free waves

Wave Interaction When waves meet, they pass through each other and move on. • Constructive Interference – waves meet crest to crest, reinforcing one another (LARGE AMPLITUDE) • Destructive Interference – waves meet crest to trough, cancelling one another (SMALLER AMPLITUDE)

Wave Height- The height of a wave is controlled by the interaction of several factors: 1. Wind speed 2. Wind duration 3. Fetch 40 -50° latitude – roaring forties/furious fifties Ideal for the production of high waves… • No landmasses to interfere • High intensity storms • Strong winds of long duration (westerlies)

Episodic (Rogue) Waves • Large waves that suddenly appear, unrelated to local sea conditions • Abnormally high wave that occurs because of a combination of intersecting wave trains, changing depths and currents • Occur most frequently near the edge of continental shelves (200 m deep) and in certain areas with prevailing wind, wave & current patterns • Heights: 7 to 8 stories high (1/2 mile λ)

Episodic (Rogue) Waves North Sea ~ 33. 8 meters (111 feet) http: //www. youtube. com/watch? v=ur. P 3 k. T 6 d. Ctk Algulhas current area ~ 57. 9 meters (190 feet)

More Rogue Waves… • http: //www. youtube. com/watch? v=Wt. Gi 42 nxu 8 s Rogue wavehttp: //www. youtube. com/watch? v=y. Lzgzv. Vx. UV 4

Wave Steepness = Height Length S=H L There is a maximum possible height for any given wavelength. • If the ratio exceeds 1: 7, the wave breaks • Whitecaps – short wavelengths (1 m), wind increases their height rapidly.

Shallow Water Waves • As waves enter shallow water, they “feel” the bottom and friction reduces the forward speed of the wave • Speed reduction = wavelength reduction = increased height • The wavelength and speed are controlled by water depth.

Refraction • As waves move from deep water to shallow, they bend or refract. • Result: change wavelength and speed

Refraction

Reflection • Waves bounce off of cliffs, steep beaches, breakwaters, bulkhead or other structures that are straight. • Waves passes through incoming waves to produce an interference pattern.

Diffraction • The spreading of wave energy sideways to the direction of wave travel. • After they pass the barrier, crests decrease in height and energy is transported sideways.

Diffraction http: //www. youtube. com/watch? v=IZg. Yswtwl. T 8

The Surf Zone • Shallow area along the coast in which the waves slow rapidly, steepen, break and disappear in turbulence • If shallow water depths extend offshore for some distances, the surf zone is wider than over a sharply sloping shore. • Longer, higher waves become unstable and break farther offshore Breakers – breaking waves

The Surf Zone Plungers – break with a sudden loss of energy and a splash. • Form on narrow, steep beach slopes Spillers – less dramatic wave form consisting of turbulent water and bubbles flowing down the collapsing wave face. • Form over wider, flatter beaches where energy is extracted more slowly over shallow bottom • Spillers give surfers a longer ride, but plungers give a more exciting one

Tsunamis Tsunami ~ seismic sea wave; Japanese word meaning “harbor wave. ” • When the earth’s crust suddenly is moved up or down, it causes the sea surface to move as well. • Long wavelength (100 -200 km; 60 -120 miles) • Tsunamis are shallow water waves because the depth of the ocean (4000 m; 1300 ft) is less than 1/20 th the wavelength • Velocity = 200 m/s (400 mph)

Tsunamis • When a tsunami first forms, height = 1 -2 m but this height is over many kilometers. • Energy is distributed from the surface to the floor over the length of the wave • As it reaches the coast, it slows (50 mph), its wavelength decreases, and energy is compressed into a smaller water volume. • Result: wave height builds rapidly • Loss of energy is rapid when wave breaks.

Tsunamis