Geoscience Reference
In-Depth Information
B
A
Crest
Wavelength, L
Wave height, H
Still water
level
Trough
Trough
Water
Depth, d
Deep
water
Shallow water
Seabed
Schematic representation of wave characteristics.
Fig. 8.2
oscillations in the surf zone, and the distance of wave
run-up
on a beach. Because most waves affecting a
coastline vary in period and height over a short period
of time, oceanographers talk about a range of wave
heights included in wave spectra. For engineering
purposes, the highest one-third or one-tenth of waves,
called the 'significant wave height', is used. Wave
period determines wave energy,
power
, and shape in
shallow water. The latter aspect determines whether
wave-generated currents at the bed move seaward or
shoreward. If seaward, then the beach erodes. The
following equations define wave energy and power for
a single wave in deep ocean water:
increasingly shallow water towards shore. However,
the speed at which the waveform is traveling decreases
with decreasing water depth. To maintain a constant
energy flux as a wave shoals into shallower water, wave
height must increase. The fact that wave velocity slows
as water depth decreases also determines wave re-
fraction. If one part of a wave crest reaches shallower
water before another part, then the wave crest bends
towards the shallower water segment. The process is
analogous to driving one wheel of a vehicle off a paved
or sealed road onto a gravel verge or shoulder. The
wheel that hits the gravel first slows down dramatically,
while the wheel still on the road continues to travel at
the original road speed. The vehicle as a result veers
towards the slower rotating wheel and careers off the
road. For waves, the effect produces a bending of
the wave crest towards shallow water, an effect that is
particularly prominent around headlands or reefs that
protrude out into the ocean.
Diffraction is the process whereby energy moves lat-
erally along a wave crest. The effect is noticeable when
a wave enters a harbor between narrow breakwalls.
Once the wave is inside the wider harbor, the crest
width is no longer restricted by the distance between
the breakwalls, but gradually increases as energy freely
moves sideways along the wave crest. In some circum-
stances, wave height can increase sideways at the
expense of the original wave height. Such an effect
often accounts for the fact that beaches tucked inside
harbor entrances and unaffected by wave refraction
experience considerable wave heights and destruction
during some storms.
The destructiveness of a wave is a function of the
wave height and period. Height determines wave
energy, the temporary elevation of sea level, the degree
of overwashing of structures, the height of water
gH
2
L
E = 0.125
(8.4)
P =
C
g
E
(
CT
)
-1
(8.5)
where
E = wave energy
P = wave power
H = wave height
L = wavelength
C
g
= the group velocity
= density of water
Group velocity is the speed at which a group of waves
moves forward. In deep water, it is exactly 0.5 times the
wave velocity such that individual waves tend to move
forward through a set of waves and disappear at the
front. In very shallow water, the group velocity is equal
to the wave velocity. There is a significant difference
between wave energy and power. Wave energy is the
capacity of a wave to do work. Power is the amount of
work per unit time. The difference can be explained
simply by comparing a greyhound to an elephant. A
greyhound is light and when it rubs up against you, you
hardly feel it. An elephant is bulky and when it rubs up
against you, you are pushed aside. The elephant,
