The Story Of The Story Of WavesWaves

WavesWaves

Caused byCaused by::

•WindWind

•EarthquakesEarthquakes

•Gravitational force of the Gravitational force of the Moon and Sun.Moon and Sun.

Wavelength

Wave Height

Crest

Trough

Still Water

Wave Parts

Parts of a WaveParts of a Wave

•CrestCrest – highest point of a wave – highest point of a wave

•Trough Trough – lowest point of a wave– lowest point of a wave

•Wave HeightWave Height – vertical distance – vertical distance between the crest and the troughbetween the crest and the trough

•WavelengthWavelength – horizontal distance – horizontal distance between two crests or two troughsbetween two crests or two troughs

PeriodPeriod

The time it takes for one full The time it takes for one full wavelength to pass a given wavelength to pass a given point.point.Most waves have periods ranging from Most waves have periods ranging from 2 to 10 seconds.2 to 10 seconds.

speed = wavelength period

Wind or Storm WavesWind or Storm Waves

•The most The most familiar familiar waves are waves are wind wind generated.generated.

Wind or Storm WavesWind or Storm Waves

•Three things Three things determine determine how large how large these waves these waves can become.can become.

Waves Caused by WindWaves Caused by Wind

•When wind blows across a body of When wind blows across a body of water, water, frictionfriction causes the water to causes the water to move along with the wind.move along with the wind.

•Wave Height depends on : Wave Height depends on : 1. Wind speed1. Wind speed2. Length of time the wind blows2. Length of time the wind blows3. Distance over which the wind blows3. Distance over which the wind blows

1. Wind Speed1. Wind Speed

•This is the speed that the This is the speed that the wind is blowing.wind is blowing.

•This is usually measured in This is usually measured in knots or nautical miles per knots or nautical miles per hour.hour.

2. Duration2. Duration

•Duration refers to the time Duration refers to the time over which the wind blows.over which the wind blows.

•Winds in some areas may Winds in some areas may blow strong for long blow strong for long periods of time which can periods of time which can produce large waves.produce large waves.

3. Fetch3. Fetch

•Fetch refers to the distance Fetch refers to the distance over which the wind blows.over which the wind blows.

•If a wind blows over If a wind blows over thousands of miles the waves thousands of miles the waves can get much larger than can get much larger than blowing across a small pond.blowing across a small pond.

Wave MovementWave Movement

•When a wave passes through When a wave passes through the ocean, individual water the ocean, individual water molecules move up and downmolecules move up and down but they do not move forward but they do not move forward or backward.or backward.

Winds at the ocean’s surface cause waves

Waves On The BeachWaves On The Beach

•Most of the Most of the waves we waves we see are at see are at the beach.the beach.

Waves On The BeachWaves On The Beach

•As the wave As the wave travels in travels in shallower shallower water it slows water it slows and eventually and eventually falls forward falls forward as a breaker.as a breaker.

Wave MovementWave Movement

•When a wave breaks against When a wave breaks against the shore, thethe shore, the crest crest outruns theoutruns the trough trough and theand the crest crest collapses.collapses.

•This is called aThis is called a breaker. breaker.

•The wave breaks into what is The wave breaks into what is called called surfsurf..

Wave refractionWave refraction

•Close to coast, water gets Close to coast, water gets more shallowmore shallow

•Waves are slowed downWaves are slowed down• If waves arrive at an If waves arrive at an

angle, one part is slower angle, one part is slower than the restthan the rest

•Causes waves to bend = Causes waves to bend = wave refractionwave refraction

•Waves arriving at bays are slow (deposition)Waves arriving at bays are slow (deposition)•At headlands, faster (erosion)At headlands, faster (erosion)

Wave cut cliffs

Sea Stack

Sea Arch

Sea Arches

Sea Cave

Sandy Beach

Rocky Beach

How is sediment transported along a coastline ?

Most waves move toward the shore at a slight angle.

Consequently, the uprush of water (swash) from each breaking wave is on an angle.

The direction of swash is oblique.

However, the backwash runs back to the water at a right angle.

Sediment particles are therefore transported in a zig-zag pattern along the beach.

This “beach drift” can carry sand and pebbles hundred to thousands of metres per day.

Beach drift

Longshore drift

In a similar manner, water in the shoreface zone flows toward the shore at an angle, and flows back at a right angle to the shore.

The net result is a current that flows parallel to the shore. This is called a longshore current.

The movement of shoreface sediment by a longshore current is called longshore drift.

Longshore drift

Coastal Coastal depositiondeposition

•Result of Result of longshore drift longshore drift and a lot of and a lot of sedimentsediment

•= produces = produces extensions of extensions of deposit from deposit from the shorelinethe shoreline

•May grow May grow across a bay across a bay ((baymouth baymouth barbar))

•May link an May link an island to the island to the main land main land ((tombolotombolo))

spit = curved extension

Spit

Spit

Baymouth Bar

Lagoon

Tombolo

Barrier IslandsBarrier Islands

• Wave Action creates sand barsWave Action creates sand bars

• Waves and Longshore current - Waves and Longshore current - can form barrier islands Ex. Fire can form barrier islands Ex. Fire IslandIsland

Jetty

The sand trapping effect of the jetties to the north at Charleston, South Carolina has deprived Morris Island of needed sand. As a result, the island has eroded to the point that this previously land based lighthouse is now 400 meters out to sea.

Port Washington Breakwater

Rip Currents

Breaking waves approaching the beach carry water toward the beach. The water can't just pile up there: it has to escape back out to sea somehow.

Various “paths of least resistence” (e.g low areas along sandbars) provide areas for water to flow back to the sea.

If caught in a rip current, swim parallel to shore !

How to spot a rip channel

Rip channels (as seen from the air)

Head of Rip channelRip channel

(as seen from beach)

Note: a rip current is different from undertow

Undertow results when water cannot escape as a rip current.

Remember that water that is pushed toward the beach must return to the sea somehow !

If the water can’t escape as a rip current, it returns to the sea by flowing underneath the waves.

Undertow

Swimmers are less likely to drown from undertow than in a rip current (most so-called “undertow drownings” result from swimmers losing their balance in backwash and getting pulled out in a rip current).

TsunamisTsunamis

•Tsunamis were Tsunamis were once called once called Tidal waves, Tidal waves, but they have but they have nothing to do nothing to do with the tides.with the tides.

TsunamisTsunamis

•They are They are produced by produced by earthquakes and earthquakes and other seismic other seismic disturbances. disturbances. That’s why That’s why they’re also they’re also called called seismic seismic sea waves.sea waves.

Tsunamis are very long, Tsunamis are very long, fast moving waves!fast moving waves!

•They can They can have have wavelengths wavelengths of 150 miles.of 150 miles.

Tsunamis are very long, Tsunamis are very long, fast moving waves!fast moving waves!

•They can They can travel at over travel at over 450 miles per 450 miles per hour! As fast hour! As fast as a jet!as a jet!

The Danger of The Danger of TsunamisTsunamis

•In the open In the open ocean, a ocean, a tsunami may tsunami may only have a only have a wave height wave height of a few feet.of a few feet.

The Danger of The Danger of TsunamisTsunamis

•As the wave As the wave approaches approaches shallow water, it shallow water, it builds to builds to heights that can heights that can reach greater reach greater than 100 feet.than 100 feet.

TidesTides

•Tides are Tides are caused caused mainly by the mainly by the gravitational gravitational pull of the pull of the moon.moon.

Other Things That Other Things That Influence TidesInfluence Tides

•The gravitational The gravitational pull of the sun.pull of the sun.

•The distance of The distance of the sun and the the sun and the moon from earth.moon from earth.

•Winds and other Winds and other factors.factors.

•Tides tend to be high where Tides tend to be high where the gravitational force the gravitational force between the earth and the between the earth and the moon are the strongest, along moon are the strongest, along with the opposite side of the with the opposite side of the earth where the centrifugal earth where the centrifugal force is strongest.force is strongest.

Tidal RangeTidal Range

Spring TidesSpring Tides•Tides have the greatest Tides have the greatest range during spring tides.range during spring tides.•This occurs during the This occurs during the

new and full moons.new and full moons.

Tidal RangeTidal Range

Neap TidesNeap Tides•Tides have the least Tides have the least amount of range during amount of range during

neap tides.neap tides.•This occurs during the first This occurs during the first

and third quarters.and third quarters.