Waves

Waves Waves are energy moving through a substance a. Sometimes this movement is visible (waves on ocean surface)

Wave Motion Progressive Waves Waves are a transmission of energy by cyclic movement through a substance The medium itself (water) does NOT travel Wave motion is NOT water FLOW, but is a flow of energy Progressive Waves Are waves in which the waveform can be observed to travel Can be longitudinal (push-pull), transverse (side to side), or orbital (interface waves) Orbital waves are the most common type at the sea surface Transmit energy along the interface of two fluids of different density (water and air)

Wave Characteristics Height (H) - distance between the crest and trough Wavelength (L) - distance between 2 successive crests or troughs Steepness - the ratio of wave height to wavelength (H/L)(slope) Period (T) - time for one full wave to pass a fixed point Speed (S) - the wavelength divided by the period (L/T) Frequency (f) - the number of wavelengths that pass a fixed point per unit time (1/T) and S = L*f

Waves Waves are energy moving through a substance a. Sometimes this movement is visible (waves on ocean surface) Waves are formed by some “disturbing force” – and weakened by a “restoring force”

Waves Waves are energy moving through a substance a. Sometimes this movement is visible (waves on ocean surface) Waves are formed by some “disturbing force” – and weakened by a “restoring force” 3. The depth of the water changes the behavior of the wave

(transitional) Wind waves in deep water: Period – up to 20seconds Wavelength – as much as 2000 feet (though ususally less) Speed – up to 70mph (though usually less) Seismic waves Period – up to 20 minutes Wavelength – 125 miles Speed – around 470mph ---- longer wavelengths usually move faster. ---- In deep water, speed is directly proportional to wavelength

Waves Waves are energy moving through a substance a. Sometimes this movement is visible (waves on ocean surface) Waves are formed by some “disturbing force” – and weakened by a “restoring force” The depth of the water changes the behavior of the wave Wind is a common disturbing force that generates waves transfer of wind energy into water small amounts of energy produce small waves known as ‘capillary waves’ – rippling of water larger energy produces “wind waves” – this happens when the wavelength is > .68” (the point at which gravity – and not capillary action – is the dominant restoring force)

Wind waves start small but as the crest is pushed up, it interacts even more with the wind – transferring more and more energy to the wave. Changes in pressure around the crest create a vacuum in front of it, and push it from behind.

Swells are long wavelength wind waves. The wavelengths are so long that they are sometimes imperceptible. Long wavelengths mean they move very fast.

Groups of swells travel in wave trains. Individual waves do not persist. Speed is ½ that of any individual wave. (V)

Waves can be HUGE: Largest wave ever recorded = 112 feet!

Major Factors that Increase Wave Energy Wind Speed Duration - amount of time that wind blows in one direction Fetch - the distance over which wind blows in a single direction Fully-Developed Sea - when the maximum fetch and duration are achieved for a given wind speed

Interference alters waves When waves meet they interact to either add to or subtract from one another. ----- This is “interference”. Constructive vs Destructive interference

Fig 9.13 in book

Rogue Waves Very rare large waves, probably due to unusual constructive interference Most frequent downwind from islands or shoals where storm waves encounter strong ocean currents Very dangerous to shipping or people on the shoreline Can be up to 20 - 30 m tall (8 story building!)

Deep water waves transition to shallow water waves and ‘break’. As the wave approaches the shore the cyclical motion of molecules is slowed by availability of less water space and by friction with the bottom Molecules on top are still cycling more rapidly and the wave breaks as the top wave is outrunning the bottom of the wave. Motion of molecules changes from cyclical to longitudinal

Surf Breaks happen approximately when wavelength:depth is 3:4 Influenced by slope, contour, composition of beach.

Closed orbits occur in the nearly frictionless open ocean Open orbits occur near shore.

Surf Wave Refraction - bending of a wave front Waves seldom approach a shoreline at 90° angle Part of the wave that contacts bottom is bent or refracted Orthogonal lines of equal energy are always bent toward shallower water Energy often focused on headlands Bays receive lower, dispersed energy that may enhance sediment deposition

Wave Diffraction - bending of waves around objects Bending occurs on a smaller scale than refraction Any point on a wave front can be a source which can propagate in any direction

Storm Surge “bulge” of water suddenly driven ashore Usually produced by a low-pressure system (hurricane) that creates a hill of water that moves along with the storm As the storm approaches shore, the hill moves onshore and causes the sea-level to rise Can be very destructive in low-lying areas

Storm Surge “bulge” of water suddenly driven ashore Usually produced by a low-pressure system (hurricane) that creates a hill of water that moves along with the storm As the storm approaches shore, the hill moves onshore and causes the sea-level to rise Can be very destructive in low-lying areas NOT technically a wave.

Tsunamis Waves caused by seismic activity such as an earthquake, volcanic explosion, disturbance of the seafloor, etc. Wavelengths can exceed 200 km (125 miles) Speed is determined by water depth and can be as fast as 700 km/h (435 miles/h) Very small surface waves at sea, but large when they reach land The leading edge can be a trough or a crest Usually, the water level falls before a tsunami comes ashore Tsunami warning system in the Pacific Ocean

Animation of Tsunami Generation Tsunamis Animation of Tsunami Generation http://www.onr.navy.mil/focus/ocean/motion/waves3.htm

Tsunami

Tsunamis Tsunami Warning System Waves MSCI 210 – Chapter 8 – Waves and Tides

Global Ocean Wave Model www.oceanweather.com/data/global.html Ocean Weather Buoy

The Beaufort Scale and Sea State Waves The Beaufort Scale and Sea State MSCI 210 – Chapter 8 – Waves and Tides

Internal Waves Waves that travel along the density interface in the ocean Heights may exceed 100 m (330 ft) The greater the density gradient, the faster the wave will move Internal waves often move along the pycnocline May be caused by tidal forces, turbidity currents, storms, etc. Require little energy input Move slower than surface waves - behave like shallow-water waves

Power from the Sea