Waves Flashcards
What is a transverse wave?
A wave in which the medium vibrates at right angles to the direction of its propagation
What is a longitudinal wave?
A wave vibrating in the direction of propagation
What is the height in terms of waves?
Vertical distance between crest and trough
What is a wavelength?
Horizontal distance between successive crests
What is a period in terms of waves?
Time interval between successive crests passing a point
What are the forces involved in the generation of waves?
Generating force and restoring force
What does the generating force do?
Disturbs from equilibrium state e.g. wind stress
What does the restoring force do?
Restores to equilibrium state e.g. gravity (surface tension at small scales e.g. capillary)
What does energy/size of waves depend on?
- Wind speed
- Wind duration
- Fetch
- Original sea state
What does the energy/size of waves depend on for large fetch and duration?
What does the size of waves depend on?
Size of waves (Hs) depends only on wind speed (W)
How do deep water waves work?
- Water particle orbits are circular
- Forwards beneath crest, backwards beneath trough
What is true when particle orbits are closed?
- → No net flow of water
- Energy and momentum transferred by waves
What is true at the surface and for larger waves?
- Particle orbits are open
- → Net forwards flow of water (Stokes Drift)
- Mass transport of water due to waves
How do shallow water waves work?
- Particle orbits influenced by seabed
- Orbits compressed in vertical → ellipses
- To and fro motion at the seabed
What is true of waves generated by storm winds?
Storm winds are unsteady and of variable direction, which means that the waves generated are:
- ‘Forced’ waves
- Irregular
- Mixed characteristics
- Known as “SEA”
What are ‘free waves’?
Waves away from the storm centre
What is the Deep Water wave formula?
L = wavelength, T= wave period, C= phase speed or wave celerity
L = gT^2 / 2𝝅 and C = L/T gives C = gT / 2𝝅
What is dispersion?
- Longest waves travel fastest and run ahead – swell
- Shorter waves follow behind
= Waves become sorted by length = DISPERSION
What do waves tend to travel in?
Wave groups
What is true of wave groups?
- Propagate at group speed
- Arise from interaction of waves with similar characteristics
- Individual waves move through group from back to front
What is the equation for wave groups in deep water?
Phase speed (C) = speed of individual wave crest
Group speed (CG) = speed at which wave energy propagates
C↓G = C/2 → C↓G = gT/4𝜋
What happens during shoaling?
As waves enter shallowing water (from deep), the waves interact with the seabed and particle orbits are compressed = waves slow down
What is the shallow water wave formulae?
C = (square root)gD
C = L/T
(D= water depth, C= speed of individual wave crest, L= wavelength, T= wave period0
What is true of a wave crest’s orientation to the shore?
Wave crests tend to be parallel to the shore due to reaction
What is true for oblique incidence waves?
Shallow end of wave slows fist
Wave crest bends round
What does refraction do?
Refraction focuses wave energy:
- Towards shoals and headlands
- Away from deeper areas and bays
Why does breaking occur?
In very shallow water, waves become too steep
What are the types of breaker waves?
- Spilling
- Plunging
- Surging
What is true of spilling breaker waves?
Slow energy release (flat beach)
What is true of plunging breaker waves?
Rapid energy release (steep beach)
What is true of surging breaker waves?
Wave breaks at shoreline
What is true of oblique incidence in breaking waves ?
Water is pushed forwards when waves break
Oblique incidence
→ Pushes water along shore
→ Longshore current
What is true of normal incidence in breaking waves?
Water is pushed forwards when waves break
Normal incidence
→ Piles water up on the shore
→ Return flow offshore
→ Undertow or rip current
