Chapter 9: Waves and Water

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16 Νοε 2013 (πριν από 3 χρόνια και 6 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
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