Shorelines and Shoreline Processes - The Changing Earth: Exploring Geology and Evolution - page 409

where L is wavelength and T is wave
period—that is, the time it takes
for two successive wave crests, or
troughs, to pass a given point.
The speed of wave advance ( C )
is actually a measure of the veloc-
ity of the wave form rather than the
speed of the molecules of water in a
wave. In fact, water waves are some-
what similar to waves moving across
a grass-covered fi eld; the grass moves
forward and back as the wave passes,
but the individual blades of grass re-
main in their original position. When
waves move across a water surface,
the water moves in circular orbits but
shows little or no net forward move-
ment (Figure 16.4a). Only the wave
form moves forward, and as it does
it transfers energy in the direction of
wave movement.
The diameters of the orbits that
water follows in waves diminish
rapidly with depth, and at a depth
of about one-half wavelength ( L /2),
called wave base , they are essentially
zero. Thus, at a depth exceeding
wave base, the water and seafl oor or
lake fl oor are unaffected by surface
waves (Figure 16.4a). Wave base is
an important consideration in some
aspects of shoreline modification;
we will explore it more fully in later
sections of this chapter.

a Low tide.

Wave Generation Several processes
generate waves. Landslides into the
oceans and lakes displace water and
generate waves, and so do fault-
ing and volcanic eruptions. Some
waves so formed are huge and might
be devastating to coastal areas, but
most geologic work on shorelines
is accomplished by wind-generated
waves, especially storm waves. When
wind blows over water—that is, one fl uid (air) moves over
another fl uid (water)—friction between the two transfers
energy to the water, causing the water surface to oscillate.
In areas where waves are generated, such as beneath a
storm center at sea, sharp-crested, irregular waves called
seas develop. Seas have various heights and lengths, and one
wave cannot be easily distinguished from another. But as seas
move out from their area of generation, they are sorted into
broad swells with rounded, long crests and all are about the
same size.
The harder and longer the wind blows, the larger are the
waves. Wind velocity and duration, however, are not the only

b High tide.

◗

Figure 16.2 Low and High Tides Low tide a and high tide b in Turnagain Arm, part of
Cook Inlet in Alaska. The tidal range here is about 10 m. Turnagain Arm is a huge fi ord now
being fi lled with sediment carried in by rivers. Notice the mudfl ats in a .

where they have their greatest impact on seashores. In fact,
waves are directly or indirectly responsible for most ero-
sion, sediment transport, and deposition in coastal areas.
Wave terminology is illustrated with a typical series of
waves in

Figure 16.4a. A crest , as you would expect, is the
highest part of a wave, whereas the low area between crests
is a trough . The distance from crest to crest (or trough to
trough) is the wavelength, and the vertical distance from
trough to crest is wave height. You can calculate the speed at
which a wave advances, called celerity ( C ), by the formula

◗

C = L / T