WAVES, ORIGIN AND CHARACTERISTICS
I.
Why study?
A.
Driving force behind all coastal processes
B.
Move sediment
C.
Straighten irregular shorelines
II.
What are waves?
A. Forms of mechanical energy
B.
Transformed wind energy, e.g., the deformed still water
surface of
the ocean
III.
How does wind set water in motion?
A. Wind blows across a flat surface
B. Successive air layers are slowed by frictional resistance
C.
Results in areas of low pressure (crest) and high pressure
(trough)
D. Crests/troughs act as grip for waves to increase in
size
IV.
Important terms for waves
A. Idealized wave crest (high), trough (low)
B. Wavelength (L)
C. Wave height : representative height (H3);
significant wave height; H3
in Atlantic is 2 m
D. Wave period (T) in seconds
E. Frequency (1/T)
F. Wave Steepness H/L
G. Progressive waves waves that transmit energy along the
interface
between two fluids of different density have particle motion that is
orbital in nature
H. Wave speed (meters/sec.) = L/T
V.
Water motion and wave movement
A. Water movement is rotational, orbit diameter is related to
wave height;
little movement occurs below
Length (L)/2
B.
Deep water waves occur when water depths are > L/2;
thus, they are
a function of:
1.
Basin depth
2.
Wave length (period)
VI.
Heavy duty math is used to explain the relationships
between L, T and H.
Simplified, fundamental equations that apply to deep water waves are:
V
= 1.56 x T where
V = speed in meters
T = wave period in secs.
L = wavelength
in meters
VII.
Waves in shallow water; formulas are different and water
movement is
different
A.
Particle movement is elliptical in orbits
B.
Much frictional resistance in depths < L/20
C. Speed calculated by:
V= Φg x D
or
V= Φ9.8 m/sec2 x D or
V= 3.1 m/sec x ΦD
where V= speed
in meters/sec
generally as waves travel from deep water into shallow water
1.
Speed decreases
2.
Height increases
3.
Length decreases
1.
Energy in wave is proportional to square of the wave
height,
e.g. E = H2
2.
Doubling wave height increases energy by a factor of 4
VIII.
Wave classification, based on period (T) and related to
length (L) by
L = 5.12 T2 (ft) or
1.56 T2 (m)
TYPE
PERIODS
1.
ripples
fractions of seconds
2.
wind chop
1-4 secs.
3.
seas 5-12 secs.
4.
swell 6-16 secs.
6.
tides 12-24 hours
Note
1-4 are wind generated, and 5-6 are impulsively generated
A.
Wind waves
1. Three factors influence growth of seas
-wind speed (constancy)
-duration
-extent of open water over which
it blows (fetch)
2. Seas building to swell
-swell waves represent decayed
seas
-uniform height and length
-travel great distances
B.
Impulsively generated waves 2 types forced and free
Causes earthquakes,
landslides, nuclear explosions, volcanic eruptions,
name of wave is tsunami or
seismic sea wave
Characteristics: L
up to 150 miles; H 1-2 in deep water, builds in
shallow water; T-=1000
secs.
Move as shallow water wave
150/20=7.5 miles, speed controlled by depth
1.
Effects of tsunamis:
a.
Open ocean no damage, slight rise in water surface, no
damage
b.
In coastal areas length decreases rapidly, height
increases
rapidly, destruction of property/erosion
C. Waves in very shallow water
As waves enter shallow water,
they are increasingly affected by the bottom.
1.
deeper water moves faster than the part in shallower water.
Result
crest rotates so that it is more or less parallel (mimic) to
the depth contours.
2. Orthognals lines separating areas on equal wave energy.
They
generally converge at headlands and thus erode; they diverge in bays
and
thus deposit.
3. Longshore drift (result of wave refraction)
a. Break
b. Swash (uprush)
c. Backwash