Transcript WAVES 1: INTRODUCTION GEOL 1053 wind wave formation

WAVES 1:
INTRODUCTION
(wind wave formation)
GEOL 1053
WAVES IN THE PROCESS OF FORMATION
• Waves in the process of formation by the wind are
called, "sea." Here, seas are building up off the
Bahama Banks in the Atlantic Ocean. Note the poorly
organized waves of irregular size and spacing.
FORMATION OF WHITE CAPS
• White caps are formed as the strengthening wind begins
to have a stronger effect on the water surface
LARGE CHAOTIC WAVES IN A STORM AT SEA
Regular SWELL WAVES approaching shore
• Shoreline with approaching wave train of regularlyspaced swell waves. Note localized breaking of waves
(surf waves) on shoreline (north of Safi, Morocco).
WAVE TRAIN approaching shore
ANATOMY OF A PROGRESSIVE WAVE
DESCRIPTION OF A WAVE FORM
• crests = high, linear, subparallel ridges of a "wave
train" (= series of waves)
• troughs = low, linear, subparallel depressions
between the crests of a wave train
• wavelength (in m) = L or l (= lambda)
• wave height (in m) = H
(highest measured ~ 25-30 m)
(This is not the same as amplitude.)
• amplitude = one-half the wave height (H/2)
• velocity (in m/s) = V
(This is a length or distance divided by time.)
PERIOD & FREQUENCY OF A WAVE
•
seconds
period = “time of a wave” = T or P = ------------# of waves
• frequency = # of waves per second = F or f
# of waves
= -------------- = cycles per sec = Hertz = Hz
seconds
• Note: period =1/frequency and frequency = 1/period
ORBITAL MOTION OF WATER PARTICLES IN A WAVE
Diameter = 4%
Energy = 11%
WAVE STEEPNESS
• The limiting angle at the crest of a deep-water wave is 120 degrees.
120 deg
crest is unstable
if angle is < 120 deg.
• At this point the steepness (S) of the wave is 0.142 or a ratio of 1:7.
• To determine wave steepness, divide wave height (H) by wavelength (L).
•
wave height
H
wave steepness = S = --------------- = -----wavelength
L
• to break in deep water, steepness must exceed 0.142 which is a ratio of 1:7
H
1
S = ------ = ------ = 0.142
L
7
WAVELENGTHS & DISTURBING FORCES OF
IMPORTANT OCEAN WAVES
3 MAJOR FACTORS INFLUENCE WAVE PROPERTIES,
such as H (height), L (wavelength), V (velocity), T (period),
F (frequency), and energy:
• 1) Average velocity of wind over fetch
• 2) Fetch (distance over which wind blows)
• 3) Duration of wind over fetch
Table of values showing conditions necessary for a fully
developed sea at given wind speeds, and the parameters of
the resulting waves.
Table showing the relationship of fetch to wave height,
wavelength, period, and wave speed with wind speed held
constant at 93 km/hr (58 mi/hr).
3 MAJOR FACTORS INFLUENCE WAVE PROPERTIES,
such as H (height), L (wavelength), V (velocity), T (period),
F (frequency), and energy:
• 1) Average velocity of wind over fetch
• 2) Fetch (distance over which wind blows)
• 3) Duration of wind over fetch
SPECTRUM OF WAVE ENERGY IN THE OCEANS
Diagrammatic view of the spectrum of wave energy in the
oceans as a function of wave period. Most wave energy
is typically concentrated in wind waves. (A tsunami, a
rare event, can transmit more energy than all wind waves
for a brief time.)
Global wave height acquired by a radar altimeter aboard
the TOPEX/Poseidon satellite in October 1992. In this
image, the highest waves occur in the southern ocean,
where waves were over 6 meters high. The lowest waves
(indicated by dark blue) are found in the tropical and
subtropical ocean, where wind speed is lowest.
STAGES OF WAVES
• 1) Sea
– waves in area effected by wind
– tend to be very irregular
– composed of many waves superimposed
• 2) Swell
– far from origin (storm area)
– larger wavelength & period waves
– travel faster than smaller waves
– travel great distances (1000`s km)
– deep-water waves
• 3) Surf
– nearshore where depth decreases to L/2
– swells shoal and break
Stopped Here
LARGER WAVES HAVE MUCH MORE ENERGY
generated by
80 km/h winds
Swell
Wave
energy Waves
greater wave
height
generated by
40 km/h winds
0
0
Wavelength
Swell waves outdistance smaller
waves from a storm
storm center
wind direction
sea waves
fetch
wave crests
swell waves
THREE TYPES OF PROGRESSIVE WAVES
V depends on wave properties
V = L/P
V depends on wave
properties and water
depth, so it is
mathematically complex.
Maximum V depends on water depth
DEEP-WATER WAVES
• depth for deep-water waves is greater than the
equivalent of half the wavelength:
> 1/2 L
• velocity equals wavelength times frequency
V = L x F = m x 1/s
• because F = 1/T,
with T being period,
V = L/T = m/s
• so speed is determined by the wave’s properties
DEEP-WATER WAVE CHARACTERISTICS
TRANSPARENCIES - WAVE SPECTRA
VELOCITY OF SHALLOW-WATER
WAVES IS CONTROLLED BY DEPTH
(Velocity)2 = gD
12
or
Velocity = (gD)1/2 10
for depth = 10 m
8
Velocity
6
4
V= 4.4 m/s
for depth = 2 m
2
for depth = 1 m
(m/s)
0
0
20
40
60
80
Wavelength (m)
100
SURF WAVES
• Transformation of swells from offshore begins significantly
as they enter water depths equal to or less than L/2.
(This will occur when H/depth ratio is about 0.6 to 0.8.)
• V decreases as the front of wave “feels” bottom.
• L decreases as forward water movement slows.
• H increases as water has less space to occupy.
• T, however, remains the same!
• Intermediate-water waves form between L/2 to L/20.
• Shallow-water waves form when depth is less than L/20.
• Crest water moves faster than trough water, so wave
“breaks” or “rolls" or "spills" over.
Breaking waves along a beach, New Zealand
END OF FILE
The progress of a wave train. (a) The energy in the
leading waves (here, waves 1 and 2) is transferred into
circular movement in undisturbed water. (b) As waves 1
and 2 are drained of energy, they gradually disappear, but
the circular movement forms new waves 4 and 5 at the end
of the train.
diagram showing the crest of an internal wave
between masses of water with different
densities, especially at the base of the
pycnocline.
Strait of Gibraltar, Spain and Morocco
Recent
ERS-1
satellite Synthetic Aperture Radar (SAR) imagery with false colors added;
image from 7 January 1992. This spectacular image shows internal waves
(with a wavelength of about 2km) progressing from the Atlantic Ocean into
the Mediterranean. These internal waves are generated at a salinity
interface (halocline) between inflowing surface Atlantic waters and the
deeper return flow of saline Mediterranean waters over the Gibraltar
sill. The internal waves reach the surface some kilometers behind the
Strait; although not visible to the eye, the waves produce patterns of
still and rough water that are picked up by radar imaging. 90 x 100 km
European Space Agency, European Space Research Institute (ESRIN)
Strait of Gibraltar, Gibraltar, southern Spain, northern Morocco
Recent
High altitude oblique photograph from the Space
Shuttle (October 1984).
A spectacular set of internal waves are visible
where surface waters pass from the Atlantic Ocean into the Mediterranean
over deeper, denser waters exiting the Mediterranean. These large
wavelength internal waves are visible here in sunglint off the
thermocline despite the lack of any expression at the ocean surface.
Straits of Gibraltar and western Mediterranean Sea Modern
High
altitude oblique photograph from the Space Shuttle (October 1984). Shows
reflections of internal wave forms progressing from the Atlantic Ocean
into the Mediterranean Sea. These wave are produced on the
thermocline/pychnocline at circa 50 meter depth. The waves have
amplitudes of tens of meters, despite negligible surface expression.
They are visible here because of high water clarity, minimal surface
waves, and oblique lighting conditions.
NASA photograph,
courtesy of Johnson Space Center (STS041G-34-098)
Offshore British Columbia, Canada (54.9¡N 130.5¡W) Recent
High
altitude oblique photograph from the Space Shuttle (December 1988).
Internal waves off the west coast of Canada. Some of these large scale
waves, formed on the thermocline, show interference patterns with
internal waves that have reflected off the steep coastline.
NASA,
Johnson Space Flight Center (STS027-040-026)
Sulu Sea (southeast Asia)(8.0¡N,119.0¡E)
Recent
High altitude
oblique photograph from the Space Shuttle (8 May 1992).
Internal waves
that have formed at a density interface (pycnocline) are visible due to
the reflection of sunlight from that relatively shallow interface. Small
surface eddies are also visible in the sunglint.
Punakaiki area, north of Greymouth, Westland, South Island, New
Zealand
Modern
Wave refraction (bending of wave
trains) in a large coastal embayment.
Aerial view of wave refraction around a rocky island. Note the
nearly 90 degree rotation of wave crests and the formation of a
tombolo -- a sandy spit connecting the island and the mainland
(Green Island at Cunjurong, southern coast of New South Wales,
Australia).
“Orbital transparency experiment” or movie
transparency