Chapter 10 Ocean Waves Part 1 ftp://ucsbuxa.ucsb.edu/opl/tommy/Geog3awinter2011/

Why are surface waves important?

Importance of Surface Waves Ships at sea (Perfect Storm) Military naval operations Marine organisms and animals (surfing critters) Transport of sand and sediments Beachfront damage Recreation (How many surfers in class?) Energy source

How are surface waves measured?

Other navigation aids? Read Hawaii by James Michener.

Measuring Waves Current meters Tide gauges and wave staffs Bottom pressure sensors Wave rider buoys Coastal radar Satellites: Altimeters Microwave Scatterometers

FIGURE 11-18: Changes in hydrostatic pressure occur with the passing of a wave and can be used to detect surface waves using pressure sensors on the sea floor. Most importantly, these pressure sensors can detect the passing of a tsunami. They are the first line of defense in tsunami warning systems. Bottom pressure sensors are used to measure passages of tsunamis at sea. Can resolve 1cm wave height variations in 6000 m depth waters.

TOPEX/POSEIDON satellite altimeter to measure wave heights to within a few cm’s.

How can we define and quantify waves and their characteristics?

A = amplitude A = H/2 S = steepness S = H/L c = wave speed c = L/T where T = wave period h = water depth A Ideal Wave or Sinusoid Form See Figure also.

FIGURE 10-5: Concept of wave period and wave frequency. Wave period T in seconds Wave frequency (1/T) in cycles per second or Hertz (Hz) T = L/c 1/T = c/L

What are the classes of surface gravity waves?

See Table 10.1 Long wavelengths and Periods Short Wavelengths and Periods

What causes surface gravity waves?

Surface waves are caused by disturbances such as: + Winds and atmospheric pressure + Earthquakes, submarine landslides, and volcanoes + Ship movement + Meteors

See Table 10.1

FIGURE 10-12: Sustained winds over large expanses of the ocean such as those produced by storm systems generate most of the waves observed along the shore.

What are restoring forces?

Wave restoring forces: + Gravity + Surface Tension + Coriolis Effect

The passage of wave energy causes water parcels to have positive or negative buoyancy relative to sea level. The force of gravity restores fluid particles to their resting level, which is why ocean surface and internal waves are called gravity waves. Gravity as a restoring force

See Table 1 Tsunami Tides

How do waves develop and evolve? What is wave “fetch”?

Capillary and gravity waves Figure 10-13

Capillary waves ruffle the surface of an estuary.

Surface slicks appear where natural and man-made organic substances prevent the propagation of capillary waves, creating a smooth, glassy appearance to the water’s surface.

Figure Development of of waves. Miles Theory.

FIGURE 10-12: Sustained winds over large expanses of the ocean such as those produced by storm systems generate most of the waves observed along the shore.

Conceptual model of wave development from capillaries to ripples to ocean swell. Note fetch, windsea, and groundswell. Windsea Groundswell

What affects surface gravity wave phase speed, c? Recall c = L/T where L is wavelength and T is wave period

FIGURE 10-7: A family of curves based on Equation 10.2 illustrating the mathematical relationship between wave speed, wavelength and water depth. In shallow waters, wave speed for nearly all wavelengths is governed solely by water depth. In deeper waters, wave speeds generally increase with increasing wavelength. Wave speed versus wavelength for different water depths h = 1m h = very deep Shape of curve called hyperbolic tangent (tanh).

Shallow versus Deep Water Waves

“Deep” water waves “Deep” water waves are surface waves that travel in waters with depth h > L/2 where L = wavelength Note: Wave speed depends on L and longer waves travel faster. These waves have circular orbital motion decreasing rapidly with depth.

Fig For deep-water wave case, longer wavelength and period waves travel faster. Therefore, these waves disperse meaning move away from each other.

Fig Concept of wave groups and group velocity for deep-water waves. More on sets of waves in context of surfing later on.

Wave orbitals for deep water waves

Orbital motion of a wave

FIGURE 10-9: Wave orbitals for (a) deep-waves. Deep-water waves are defined as waves traveling in water depths greater than ½ the wavelength. Deep-water Case: h > L/2 case

FIGURE 10-9: Wave orbitals for (a) deep-, (c) intermediate and (d) shallow-water waves. Deep-water waves are defined as waves traveling in water depths greater than ½ the wavelength. Intermediate water waves are defined as waves traveling in water depths less than ½ the wavelength but greater than 1/20th the wavelength. Shallow water waves are defined as waves traveling in depths less than 1/20th the wavelength.

“Shallow” water waves “Shallow” water waves are surface waves that travel in waters with depth h < L/20. Note: Tsunamis are “shallow” waves even though they often travel in the deep ocean as their wavelengths are long! Note: Speed depends on depth and waves in deeper water travel faster. Note: Orbitals are elliptical

Figure 10-8: Mathematical formulations and limits of applicability for deep, intermediate and shallow-water waves. L L h > L/2 h < L/20 1/2 k = 2  /L

Transition from shallow to deep waves See Figure 10-8 As waves progress toward shoreline c = (gh) 1/2 c = (gL) 1/2 /(2  ) 1/2 = gT/(2   = 3.14 tanh function

Wave orbitals for shallow water waves

FIGURE 10-9: Wave orbitals for (a) deep-, (c) intermediate and (d) shallow-water waves. Deep-water waves are defined as waves traveling in water depths greater than ½ the wavelength. Intermediate water waves are defined as waves traveling in water depths less than ½ the wavelength but greater than 1/20th the wavelength. Shallow water waves are defined as waves traveling in depths less than 1/20th the wavelength. Shallow-water Case: h < L/20

End Part 1 of Waves