Waves and Water Dynamics Essentials of Oceanography

Pebble in Still Water What happened when I dropped the pebble into the still tank? Do the particles at the point of disturbance move outward too? http://www.brighthub.com/education/k-12/articles/39375.aspx#ixzz1A5lRdeRe The surface of the water gets disturbed and it travels outward from the point of disturbance. This is a wave. Do the particles at the point of disturbance move outward too? Read more: http://www.brighthub.com/education/k-12/articles/39375.aspx#ixzz1A5lMzmS1

Cork in the disturbance How does the cork move? A disturbance travels from one point to another point away from the source, but there is no transport of material or medium itself.

What Causes Waves? Waves are created by releases of energy (disturbances) including: Wind Movement of fluids of different densities Mass movement into the ocean (splash waves) Underwater sea floor movement (tsunami) Pull of the moon and sun (tides) Human activities

Most Ocean Waves Are Wind-generated

Anatomy of a Wave Crest- maximum elevation about still water level Trough- max depression below still water level Wavelength (L)- crest to crest, or trough to trough Height (H)- vertical distance crest to trough Amplitude- H/2, still water to crest or trough Period (T) - time for one wavelength to pass a point Frequency – number of wave crests passing a point in 1 second

Anatomy of a Wave

Wave Characteristics and Terminology (Continued) If wave steepness exceeds 1/7, the wave breaks Period (T) = the time it takes one full wave—one wavelength—to pass a fixed position

How do waves move? 1. The direction of propagation of wave, that is, the direction in which the disturbance travels. 2. The disturbance is transferred due to the oscillation of the particles of the medium involved. The direction of these oscillations is the second direction. Longitudinal Waves : When these two directions are parallel. Transverse Waves : When these two directions are perpendicular to each other. Read more: http://www.brighthub.com/education/k-12/articles/39375.aspx#ixzz1A5o63iOX

Types of Progressive Waves Longitudinal Back-and-forth motion Transverse Side-to-side motion Orbital Combination

Circular Orbital Motion As a wave travels, the water passes the energy along by moving in a circular orbit Floating objects also follow circular orbits

Orbital Motion in Waves Orbital size decreases with depth to zero at wave base Depth of wave base = ½ wavelength, measured from still water level

What about Surfing? Here's what happens when the wave gets to water that's about half the depth of its wavelength: The swells slow down as the water beneath them gets shallower. As a result, the waves get closer together, much the way a line of cars will get closer together if the car in front starts to slow down. The leading edge of the swell becomes increasingly vertical as it slows while the trailing edge continues to look like a rounded slope. The waves get taller as the solid surface under them and the waves' energy pushes the water upwards. The exact shape of the ocean floor has a dramatic difference on how the waves break. If the shore slopes gently upward, the wave will gently spill over as it crests. A steep slope can cause waves that break suddenly and dramatically. The cresting waves can travel for thousands of miles before reaching shore and becoming conducive to surfing.

Wave Characteristics Period (T) = time between wave crests Is a constant for any wave Function of the disturbing force If wind energy decreases Speed and wavelength can decrease Period does not change Wavelength  Size of the orbits Water depth relative to wavelength  Shape of the orbits

Deep- and Shallow-water Waves Deep-water waves Water depth > wave base Shallow-water waves Water depth < 1/20 of wavelength

Deep Water Waves Orbits don’t reach the seafloor Water depth > ½ wavelength If L = 20 m, depth > 10 m Only wind waves can be deep water waves Wavelengths for tsunamis and tides are so long (100’s – 1000’s km) Seafloor maximum depth = 11 km Ocean depth would need to be >50 km for these to be deep water waves Body surfing St Aug. 2 m  wavelength > 40m Earth’s circ = ~40,000 km tide wavelength = 20,000 Water depth would have to be 10,000 km

Shallow Water Waves Orbits reach and interact with the seafloor Water depth < 1/20 wavelength Orbits “feel” the bottom Orbits flatten  elliptical Water at seafloor moves back and forth 1/20 wl > water depth >1/2wl

Wave Speed (S) General formula: Deep-water waves: Wave speed (S) in meters per second = 1.56 T in seconds Wave speed (S) in feet per second = 5.12 T in seconds Shallow-water waves: (d = water depth)

Producing Waves Disturbing Force = energy input Wind = wind waves* Landslide/faulting = tsunami Moon/sun gravitation = tides Restoring Force = returns water to flatness Tends to overcompensate leading to oscillations Surface tension- for small waves (capillary waves) Gravity for larger waves- nearly friction-free travel long distances

History of a Wave Wind waves = wind energy transferred to the water Wind blows on the water – friction causes stretching of the surface Ripples (capillary waves) form Wind’s energy transferred to the water to drive the wave forward Larger surface area more energy transferred = bigger waves

Factors Controlling Wave Height Wind strength Strong wind = more energy = taller waves Wind duration How long the wind blows in one direction Waves produced by strong winds will dissipate when wind stops Fetch - distance wind blows in one direction Longer continuous input = taller waves

Fully Developed Sea Factors that increase wave height: Increasing wind speed Increasing duration (time) of wind Increasing fetch (distance) A fully developed sea is the maximum height of waves produced by conditions of wind speed, duration, and fetch

Largest Wind-generated Waves Authentically Recorded In 1935, the vessel USS Ramapo experienced large waves while crossing the Pacific Ocean Wave height was measured at 34 meters (112 feet)

Waves at the Shore

Waves Undergo Physical Changes in the Surf Zone

Surf Zone As waves approach the shoreline: Become shallow water waves Slow down due to friction Become closer together Steepen Break Lose energy