WAVES
lWaves are of
practical significance to us...they may swamp a small boat,
lsmash
supertankers,
l damage
offshore structures,
lforce
commercial vessels to slow their speed,
ldamage shore
structures,
WAVES
lmake students
skip school when they want to surf and
ldetermine
what adaptations organisms along the shoreline must have in order to exist
there.
WAVES
lWaves are
mechanical energy that has been transferred from wind, earthquakes, landslides,
or other waves to the ocean water.
lMost of the
transfer is by wind and waves travel outward from the energy source. As more
energy is supplied, waves become larger.
WAVES
lProperties of waves
l3 factors that determine size of wind
generated waves.
l1. time of contact
l2. velocity of wind
l3.
fetch-distance over which wind is in contact with water.
WAVES
lAny one of these factors can limit the wave height.
lIf the wind speed is low, it doesn't matter how far and how
long the wind blows over the water, no large waves will be produced.
WAVES
lIf
the wind speed is great but short, again, no large waves will be formed and
strong wind over a short area will also not produce large waves.
lWhen
no single limiting factor is present, large waves can form at sea. (40-50' S).
WAVES
lA typical fetch for a local storm is about 500 miles and with
the storm moving,
l and the storm winds circulation around the low pressure area,
WAVES
l The winds can continue to follow the waves on the side of the
storm which increases fetch and duration of time over which the wind can add
energy to the waves.
lWaves up to 49 feet are not uncommon and the wave lengths can
be between 330-600 feet.
WAVES
lGiant waves over 100' are rare but a Navy Tanker (USS
Ramapo)in 1933 encountered a Typhoon and riding on the downside to ease the
ride, was overtaken by waves that when measured against the ships
superstructure by the officer on watch were 112'high.
WAVES
lThe period was 14.8 seconds and the wave speed was 90'/sec (60
mph). While conditions to produce waves of this size occur, none have been well
documented (or have survived).
WAVES
lPARTS
OF THE WAVE
lThe portion of the wave that is elevated above the undisturbed
sea surface is the crest.
lThe portion that is depressed below the surface is the trough.
WAVES
lThe distance between two successive crests or two successive
troughs is the length of the wave or wavelength,
l and the height of
the wave is the vertical distance from the top of the crest to the bottom of
the trough.
WAVES
lThe amplitude is equal to 1/2 the wave height or the
distance from the crest or trough to the still water or equilibrium surface.
lThe period is the time required for two successive
crests (or troughs) to pass a point in space.
WAVES
lThe relationship between wavelength and wave periods allow
math approximations to be made giving more insight to the behavior and
properties of waves.
WAVES
lWAVE
MOTION
lThe particles of water get set into motion when a wave passes
across the water surface.
lThe ocean wave does not represent a flow of water but a flow
of motion or energy from its origin to its eventual dissipation at sea or loss
against land.
WAVES
lAs a wave crest approaches, the surface water particles rise
and move forward.
lImmediately under the crest the particles have stopped rising
and are moving forward at the speed of the crest..
WAVES
lWhen the crest passes, the particles begin to fall and to slow
their forward motion.
l It reaches a maximum falling speed and zero forward speed
when the midpoint between crest and trough passes.
WAVES
lAs the trough advances, particles slow in falling rate and
start to move backward until at the bottom of the trough they reach the maximum
backward speed and neither rise or fall.
WAVES
lAs the remainder of the trough passes, the water particles
begin to slow their backward speed and start to rise again, until the mid-point
between the crest and trough passes.
WAVES
lNow they start their forward motion and continue to rise with
the advancing crest.
lThe motion creates a circular path or orbit for the
water particles.
lThis is the motion that causes a boat to bob!
WAVES
lThe
surface water particles trace an orbit whose diameter is equal to the wave
height.
lThe
same motion is transferred to the water particles below but less energy of
motion is found at each succeeding depth.
lThe
diameter of the orbits decrease and become smaller and smaller as the depth
increases.
WAVES
l
At a depth of 1/2 the wavelength, the orbital motion has decreased to almost
zero.
WAVES
lWAVE
SPEED
lIt
is possible to relate the wavelength and period of the wave in order to
determine the wave's speed.
l
The speed of the wave (C) is equal to the length of the wave (L) divided by the
period (T):
lSpeed = length/period or C=L/T
WAVES
lParticles in
the ocean are set into an elliptical motion as wind energy acts on water.
lThe energy of
the particles move (is transferred) through the ocean, not the particles.
lTheir
movement makes the waves shape.
WAVES
lThe vertical height-from the top of the crest to bottom of
trough is the wave height.
The time between successive crests/troughs passing a fixed
point is the period of a wave.
WAVES
lSharp peaks are called seas and as waves move out of their
area.
lThe crests become rounded forming a swell, a long, low
wave that can travel thousands of miles.
WAVES
lAs the wave
approaches the shallow water, it changes shape.
lthe wave
length decreases.
lthe height
increases as particles encounter resistance from the bottom.
WAVES
lThe pathway
of the particles become more elliptical as it gets closer to the coastline.
l ..bottom resistance slows the waves.
l ..shortens
wave length when depth is 1/2 wavelength.
WAVES
lWhen depth
decreases less than 1/2 wavelength (or 1.3x height)
l the frictional drag along the bottom
land forward
motion of the wave and steepness of the crest causes the wave to break or
collapse against the shore.
WAVES
lStored energy is released as the water
falls against the shore.
WAVES
lBREAKERS
lBreakers are formed in the surf zone because the water
particle motion at-depth is affected by the bottom, slowed down, and compressed
vertically. The orbit speed of the particles near the crest are not slowed too
much so
.
WAVES
lparticles move faster toward the shore than the wave itself.
lThe crest can curl and eventually break (fall over).
lThere are two types, plungers and spillers.
WAVES
lPlunging breakers are usually found on a steep beach, the
curling crest outruns the rest of the wave curves over the air below it and
breaks with a sudden loss of energy and a splash.
l Spilling waves occur at flatter beaches and consists of
turbulent water and bubbles flowing down the collapsing wave face.
WAVES
lMarine
organisms along the ocean are affected by wave actions.
l Sandy and
rocky shores, exposed to the direct assault of strong waves are known as a high
energy environment, as opposed to beaches in protected estuaries, bays
and lagoons which is a low energy environment.
WAVES
lWinter
usually has higher crests and shorter wave lengths than summer thus release
more energy on the shore.
WAVES
lSTORM CENTERS
lMost waves at sea are progressive wind waves.
lThey are build up by the wind, restored by gravity and travel
in a particular direction.
lThese waves are formed in local active storm centers or by
steady winds of the trade wind and westerly wind belts.
WAVES
lAn active storm may be large, with unsteady winds and varying
directions and strength.
lThe winds in the storm flow in a circular pattern around the
low-pressure storm center creating waves that move outward and away from the
storm in all directions.
WAVES
lIn the storm center, the sea surface is jumbled with waves of
all heights, lengths and periods.
lThere are no regular patterns.
l Sailors call this a sea.
l As waves are being generated, they are forced to get larger
by the input of energy forced waves.
WAVES
lDue to variations in the winds of the storm area, energy at
different intensities is transferred to the sea surface at different rates,
resulting in waves with a variety of periods and heights.
WAVES
lOnce a wave is created with its period, the period doesn't
change.
lThe speed may change but the period remains the same.
lThe period is a constant property of the wave until the wave
is lost by breaking at sea, through friction, or crashing against the shore.
WAVES
lWATER
TRANSPORT
lWaves transport water toward the beach in the surf zone.
lThere is a drift of water in the direction the waves are
traveling and is intensified in the surf zone and with the waves approaching
the beach at an angle, the transport of water moves both toward and along the
beach.
WAVES
lThis water must flow seaward again and will in a quieter zone
with smaller waves.
lBecause these regions may be some distance apart, and narrow,
the water may flow out quickly forming a rip
current.
WAVES
lREFRACTION
lWaves usually approach the shore at an angle and when one end
of the crest comes in and feels the bottom and the other end is still in deeper
water, the shallow water end slows and because the deep water part is still
traveling the same speed, the wave crests bend, or refract.
WAVES
lDIFFRACTION
lWhen a wave passes its energy though a narrow opening, some
wave energy will pass through to the other side and once through the energy
radiates out and away from the gap.
WAVES
lREFLECTION
lA steep, vertical barrier in water deep
enough to prevent waves from breaking will reflect the waves..
WAVES
lSTORM SURGES
lPeriods of excessive high water due to changes in the
atmospheric pressure and the wind's action on the sea surface are called storm surges or storm tides.
lThese are not typical waves but share characteristics of
curving sea surfaces and produce like effects to that of tsunamis.
WAVES
lSWELL
lOnce energy or generating forces no longer effect the waves,
the forced waves become free waves moving at speeds due to their periods
and wavelengths.
lSome waves produced have long wavelengths and long periods and
have a greater speed than those with short.
WAVES
lThey gradually move through and ahead of the slower ones and escape
the storm and appear as a regular pattern of undulating crests and troughs
moving across the sea surface.
lOnce away from the storm these waves are called swell.
WAVES
lThey
carry considerable energy which they lose very slowly..
lThe movement of the faster through and ahead of the slower
waves is called sorting or dispersion.
l Groups of these faster waves move as wave trains or packets
of similar waves with about the same period and speed (sets).
WAVES
lTSUNAMI
lEarthquakes are often responsible for producing seismic sea
waves or tsunamis. They are called
tidal waves (incorrectly) and formed if in an area the earths crust suddenly
raised or lowered.
WAVES
lEnergy is
transferred to water as the coastal plates shift.
lTravel
through the sea at 100+ mph with a wave length of 100miles when reaching the
coast, wave lengths shorten and heights can increase to 100'.
WAVES
lThe displacement causes a sudden rise or drop of the sea level
and gravity causes the water to quickly fill it in. Waves with long wave
lengths are produced (100-200km) and periods of 10-20 min
WAVES
lHOW A WAVE FORMS
lTo create a
wave, a generating force is required.
lThis is a
result of a pulse of energy which produces waves (throw a stone etc.)
lThe waves
produced by the generating force moves away from the point of disturbance.
WAVES
lWhen the rock
hits the surface, it disturbs and displaces the water surface.
l As the rock
sinks, the displaced water flows back into the space from all sides and its
momentum forces it upward resulting in a higher surface.
WAVES
lThe elevated
water falls back causing a depression below the surface, which is filled and
l in turn sets
up a series of waves that move outward and away from the point of disturbance
luntil they
are dissipated through friction among the water molecules.
WAVES
lThe force
that causes water to return to the level of undisturbed surface is the restoring
force.
lThis has to
do with the surface tension (elastic
quality of the surface due to the cohesive behavior of the water molecules.)
lThis
affects smaller waves, but larger waves are pulled back by the force of gravity
and are called gravity waves.
WAVES
lThe most
common generating force for water waves is moving air or wind.
WAVES
lAs the wind
blows across a smooth water surface, the friction or drag between air and water
tends to stretch the surface resulting in wrinkles
l. Surface tension acts to restore a
smooth surface.
lThe wind and
surface tension create small waves called ripples or capillary waves.
WAVES
lYou can see
these as wind moves over a smooth surface of a pond or lake, and are called cat's paws as they move across the
surface keeping pace with the wind.
WAVES
lAs the wind
blows, energy is transferred to the water over large areas, for varying lengths
of time, and at different intensities.
lAs waves
form, the surface becomes rougher, and its easier for the wind to grip the
roughened water surface and add energy .
WAVES
lAs the wind
energy is increased, the oscillations of the water surface becomes larger and
the restoring force changes from surface tension to gravity.
lA wave is a
result of the interaction between a generating
force and a restoring force.
WAVES
lGenerating forces include any occurrence
that adds energy to the sea surface:
l wind,
llandslide,
lsea-bottom faulting or slipping,
lmoving ships,
land even thrown objects.
WAVES
lEPISODIC
WAVES
lLarge waves that suddenly appear at sea unrelated to local
conditions are called epsodic waves.
lIt occurs due to the combination of intersecting wave trains,
depths and currents.
WAVES
l Not much is known and when they do occur and swamp ships,
witnesses are often removed.
lThey occur near the continental shelf in water about 600' deep
and in some areas with prevailing wind, wave and current patterns
WAVES
lThere
is a maximum height for any given wavelength.
lThis
value is determined by the ratio of the wave's height to the wave's length and
is the measure of steepness of the wave:
lSteepness
= height/length or S=H/L
WAVES
lIf the height to length ratio exceeds 1:7, the wave is too
steep, the crest angle will be sharper than 120' and the wave is unstable and
will break.
l A wave length of 70m will cause a wave to break when it
exceeds 10m (1:7).
WAVES
lWhitecaps have very short wavelengths(about 1m) and break
because the wind increases their height rapidly...quicker than the wavelength
increases.
l Also when wave trains pass through each other, the quick
increase in height can cause these waves to break (even in the middle of the
ocean).