# Chapter 9: Waves and Water

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16 Νοε 2013 (πριν από 4 χρόνια και 9 μήνες)

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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

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