Chapter 9: Waves and Water Dynamics

Fig. 9-10

WAVE CHARACTERISTICS

Waves are moving energy

Forces cause waves to move along air/water interface or within water Wind (most surface ocean waves) Movement of fluids with different

densities Internal waves often larger than surface

waves Mass movement into ocean(e.g. land

slide) Splash waves

Seafloor movement Tsunami or seismic sea wave

Gravitational attraction Earth, Moon, Sun Tides

Human activities Wakes of ships Explosions

Progressive waves Longitudinal

“Push-pull” Transverse

Side-to-side or up-and-down Orbital

Circular orbit Ocean surface waves

Types of waves

Fig. 9-3a

Wave characteristics

Crest, trough Wave height is proportional to

energy Wave length Wave height/wave length = wave

steepness Waves break when H/L is 1/7

Wave period, frequency

Wave characteristics Wave base is 1/2 wave length

Negligible water movement due to waves below this depth Fig.9-6a

Deep-water wave

Depth of water is greater than 1/2 wavelength

Speed of wave form (celerity) is proportional to wavelength

Shallow-water wave Water depth is less than 1/20 wavelength Friction with seafloor retards speed Wave speed (celerity) is proportional to

depth of water Orbital motion is flattened

Transitional waves

Water depth is 1/2 to 1/20 of wavelength

Characteristics of deep and shallow-water waves

Wave speed (celerity) is proportional to both wavelength and depth of water

Three types of waves

Wave equations

Wave speed = wavelength/period S = L/T

Frequency = 1/period F = 1/T

Wave speed (m/s) = 1.56 x period S = 1.56 x T

Surface ocean waves

Most wind-driven Small wind-driven waves

Capillary waves Larger wind-driven waves

Gravity waves

Sea

Storm at sea creates waves Wave energy depends on

Wind speed Fetch Duration

Chaotic mixture of different wavelengths and wave heights

Wave dispersion

Longer wavelength waves outdistance shorter wavelength waves

Waves travel in groups or trains with similar characteristics

Swell made up of waves of similar wavelength and period

Wave interference Constructive

Wave heights increase Destructive

Wave heights decrease Mixed

Wave heights vary in wave train (surf beat)

Interference illustrated

Fig. 9-14

Rogue waves

Unusually large waves Constructive interference Waves meet strong ocean current

Fig. 9-16

Shoaling waves Waves reach surf zone

Wave speed decreases Wave length decreases Wave height increases

Wave steepness 1/7, wave breaks Surface tension no longer able to hold wave together

Breakers

Spilling Gentle beach slope

Plunging Moderately steep slope

Surging Abrupt slope

Wave refraction Shoaling waves bend so wave fronts

approach a shore nearly parallel

Fig. 9-19a

Wave energy focused on headland

Wave energy dispersed over bay

Fig. 9-19b

Wave diffraction

Wave energy transferred around or behind barriers

Fig. 9-20

Wave reflection

Waves bounce back from steep slopes or seawalls

Reflected wave may constructively interfere with other waves

Standing waves

Two waves with same wavelength moving in opposite directions

Node – no vertical movement Greatest horizontal movement

Antinode – greatest vertical movement

Fig. 9-22

Tsunami or seismic sea wave

Caused by sudden changes in volume of ocean basin Mainly submarine faults Volcanic eruptions Submarine landslides

Fig. 9-23a

Tsunami Very long wavelength Travels fast Raises sea level as crest shoals

Trough causes sea level to fall Disastrous for infrastructure at

coasts Possibly much loss of life

Tsunami warning system

Monitor seismic activity Monitor changes in unusual

wave activity Warning

People evacuate