Week 2 - Physical processes and external agents acting along coastal environment (WAVES) Flashcards
what is a perfect wave
Propagating oscillation with transport of energy
No transport of mass
what is wave frequency
The number of waves passing a point in a given time
what is wave period
The time is takes for two successive crests to pass a given point
how is the relationship between wave frequency and wave period
High frequency = shorter period
Low frequency = longer period
what is wave speed
The number of waves passing a point in a given time
what is the equation for wave speed
Wave speed (m/s) aka c: celerity = wavelength (m) x frequency (Hz)
Wave celerity: the speed at which an individual wave advances or ‘propagates’
v = λ x f
Example:
Frequency (f) = 10Hz
Wavelength (λ) = 2m
v = 10 x 2
v = 20 m/s
how are wind waves generated
Energy transfer from wind to water = generation of wind waves
3 conditions for effective wave generation:
Wind needs to be strong (wind speed)
Wind needs to blow for a long enough time
The length of water over which wind blows (fetch) is long enough
what are swell waves
Self-sustaining
Generated by energy beneath the ocean’s surface
No longer needs local winds
Long wavelengths and periods
When at the end of a local wind source, wind waves become swell waves
describe deep water waves
Water depth > ½ wavelength
Ocean bottom does not affect wave
Wave particles follow circular orbits
Wave celerity is independent of the bottom and proportional to wavelength
describe shallow water waves
Water depth < 1/20th wavelength
Ocean bottom strongly affects wave
Wave particles follow elliptical orbits
Orbits become flatter and flatter
As waves travel from deep to shallow water, they undergo transformations. what are these transformations?
Wave celerity
Friction
Shoaling
Breaking
Refraction
Diffraction
Reflection
As waves travel from deep to shallow water, they undergo transformations. describe wave celerity
Shallower water = slower waves
Very shallow = distortion (crest moves faster than the rest of the wave
Shoaling (higher waves)
As waves travel from deep to shallow water, they undergo transformations. describe friction
Energy is lost
Waves dissipate
Everything slows down
Wave distortion
Wave through slows down more than crest
As waves travel from deep to shallow water, they undergo transformations. describe shoaling
Wave speed decreases
Wave period stays constant
Wavelength decreases
‘footprint’ of the wave decreases -> wave height needs to increase to keep the same energy (conservation of energy)
As waves travel from deep to shallow water, they undergo transformations. describe breaking
When waves enter very shallow water (near shoreline) they break for two reasons:
Wave becomes so steep it cannot support its own weight and it collapses (very short and high)
The speed of water around the crest of the wave exceeds the wave speed
The water in the crest overtakes the wave form and causes it to fall over and break
Type of breaking depends on beach topography
As waves travel from deep to shallow water, they undergo transformations. describe refraction
Bending of waves because of varying water depths underneath
The part of a wave in shallow water moves slower than the part of a wave in deeper water
The wave crest in deeper water catches up so that the wave crest tends to become parallel to the shore
Waves arrive orthogonal to the coast
As waves travel from deep to shallow water, they undergo transformations. describe diffraction
Propagation of a wave around an obstacle
Tip of the obstacle scatters incoming wave in all directions
As waves travel from deep to shallow water, they undergo transformations. describe reflection
Waves bounce back from an obstacle e.g., shoreline, harbour
The proportion of incoming waves reflected back from the beach depends on the beach slope
Mild slope = practically no reflection
Steeper profiles e.g., cliff = more reflection
Given the same slope, longer waves are reflected more
The beach appears steeper to longer waves than to shorter waves
