HOW DO WAVES EXERT ENERGY ON BEACHES?. Waves are generated by the friction of winds passing over water, they therefore represent an expression of differences in atmospheric pressure resulting from the interaction of the Earth system with insolation. Translatory waves. Oscillatory waves. Wind.

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HOW DO WAVES EXERT ENERGY ON BEACHES?

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HOW DO WAVES EXERT ENERGY ON BEACHES? Waves are generated by the friction of winds passing over water, they therefore represent an expression of differences in atmospheric pressure resulting from the interaction of the Earth system with insolation.

Oscillatory waves Impacts felt progressively less with depth Wavelength, λ d≥ λ/2

Oscillatory waves Friction of circular motion with the sea bed, slows lower water layers, causing wave front to steepen D< λ/2

Translatory waves Oscillatory waves Kinetic Energy Water moving d≥ λ/2

Translatory waves Oscillatory waves Wave breaks H = 0.8 d h d d≥ λ/2

WHAT CAUSES WAVE ENERGY TO CHANGE? Wind Speed. Direct linkage between waves as agent of erosion and “external” energy.

WHAT CAUSES WAVE ENERGY TO CHANGE? Wind Speed. Direct linkage between waves as agent of erosion and “external” energy. Wind Fetch. The greater the fetch then the more distance over which there is to exchange energy between the atmosphere and the ocean surface. ATLANTIC Gulf of Mexico

WHAT CAUSES WAVE ENERGY TO CHANGE? Wind Speed. Direct linkage between waves as agent of erosion and “external” energy. Wind Fetch. The greater the fetch then the more distance over which there is to exchange energy between the atmosphere and the ocean surface. Duration. The longer the wind blows in one direction the more consistently the energy (waves) will be generated and not interfere with each other.

Wind Speed June 1995 Summer Winter Summer Wave Height June 1995 Winter

Australia New Zealand ANTARCTICA Cape of Good Hope South Africa Cape Horn South America

Beach Shallow λ/2 Deep ENERGY AND SEDIMENT TRANFER ALONG BEACHES FROM WAVE ACTION WIND DIRECTION

Beach Shallow λ/2 Deep Oscillatory wave approaching from deep ocean at right angles to wind direction. Equal energy per unit length of wave front. TIME = 0

Beach Shallow λ/2 Deep Each unit length of wave front has moved an equal distance (equal velocity) in unit of time TIME = 1

Beach Shallow Unit of wavefront closest to shore has now reached the critical depth of λ/2 and wave energy begins to interact with bed. Wavefront section becomes translatory Energy released to bed material. C) Friction causes wavefront to slow (loss of energy to bed). λ/2 Deep Each unit length of wave front has moved an equal distance (equal velocity) in unit of time TIME = 1

Beach Shallow λ/2 Deep Each unit length of wave front in deep water has moved an equal distance (equal velocity) in unit of time TIME = 2

Beach Most shoreward section of wavefront has lower velocity (d<λ/2), therefore it has not travelled as far in unit time, causing apparent “bending” of less energetic wave Shallow λ/2 Deep Each unit length of wave front in deep water has moved an equal distance (equal velocity) in unit of time TIME = 2

Beach Most shoreward section of wavefront has lower velocity (d<λ/2), therefore it has not travelled as far in unit time, causing apparent “bending” of less energetic wave Shallow Unit of wavefront next closest to shore now reaches the critical depth of λ/2 and wavefront section becomes translatory λ/2 Deep Each unit length of wave front in deep water has moved an equal distance (equal velocity) in unit of time TIME = 2

Beach Shallow λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 3

Beach Most shoreward section of wavefront loses velocity and steepens Shallow λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 3

Beach Most shoreward section of wavefront loses velocity and steepens Shallow Wavefront unit slows and steepens after becoming translatory λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 3

Beach Most shoreward section of wavefront loses velocity and steepens Shallow Wavefront unit slows and steepens after becoming translatory Next wavefront unit reaches the critical depth of λ/2 and wavefront section becomes translatory λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 3

Beach Shallow λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 4

Beach Most shoreward sections of wavefront continue to lose velocity and steepens Shallow λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 4

Beach Most shoreward sections of wavefront continue to lose velocity and steepens Shallow Translatorywavefront units becoming slower and steeper. λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 4

Beach Most shoreward sections of wavefront continue to lose velocity and steepens Shallow Translatorywavefront units becoming slower and steeper. Point at which wavefront encounters critical depth, λ/2, moves down beach λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 4

Beach Shallow λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 5

Beach Most shoreward section now sufficiently grown in height to exceed critical value to break. Release of energy. Shallow H=0.8d λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 5

Beach Most shoreward section now sufficiently grown in height to exceed critical value to break. Release of energy. Shallow H=0.8d Waves Steepening λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 5

Beach Most shoreward section now sufficiently grown in height to exceed critical value to break. Release of energy. Shallow H=0.8d Waves Steepening Oscillatory/ translatory point λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 5

Beach Most shoreward section now sufficiently grown in height to exceed critical value to break. Release of energy. Shallow Translatorywavefront units becoming slower and steeper. H=0.8d Waves Steepening Oscillatory/ translatory point λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 5

Beach Shallow H=0.8d Waves Steepening λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 6

Beach Shallow Breaking wave H=0.8d Waves Steepening λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 6

Beach Shallow Breaking wave Translatorywavefront units becoming slower and steeper. H=0.8d Waves Steepening λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 6

Beach Shallow Breaking wave Translatorywavefront units becoming slower and steeper. H=0.8d Waves Steepening Oscillatory/ translatory λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 6

Beach Shallow H=0.8d Waves Steepening λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 7

Beach Most shoreward section of wave now slowed sufficient that almost parallel to beach Shallow H=0.8d Waves Steepening λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 7

Beach Most shoreward section of wave now slowed sufficient that almost parallel to beach Shallow Breaking wave H=0.8d Waves Steepening λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 7

Beach Most shoeward section of wave now slowed sufficient that almost parallel to beach Shallow Breaking wave H=0.8d Waves Steepening Oscillatory/ translatory λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 7

Beach Most shoreward section of wave now slowed sufficient wavefront is parallel to beach Breaking wave Shallow H=0.8d Waves Steepening Oscillatory/ translatory λ/2 Deep Deep water wavefront sections all move at equal velocity. TIME = 8

Beach Shallow H=0.8d λ/2 Deep 0 Time when wave changes from Oscillatory to Translatory

Beach Shallow H=0.8d λ/2 Deep 1 0 Time when wave changes from Oscillatory to Translatory

Beach Shallow H=0.8d λ/2 Deep 1 2 0 Time when wave changes from Oscillatory to Translatory

Beach Shallow H=0.8d λ/2 Deep 1 2 3 0 Time when wave changes from Oscillatory to Translatory