Waves approaching shore & cross-shore sediment transport Flashcards
Why do large wavelengths travel faster?
Rate = distance/ time
Wave velocity = wavelength/ period
Keeping velocity constant…
If wavelength goes down, then T must get really small (less time between crests).
If wavelength goes up, then T must get big (more time between crests).
Define deep water
Depth is greater than half a wavelength
Define shallow water
Depth is less than half a wavelength
Why does wave motion get distorted in shallow water and not deep water?
In deep water (depth > half a wavelength), seabed is far below motion so no interaction between wave and bed.
In a fully “shallow water wave” wave motion constant over full depth.
What happens when a wave enters shallower water?
There is less depth for water motions.
If there’s less depth it means there is less room for that squeezing to happen in front of the wave = slower wave propagation (can’t move itself forward as fast).
HOWEVER wave energy must be conserved…
The number of crests is therefore the same.
Velocity is going down (less squeezing) but the wave period (number of crests/ min) is staying the same = wavelength MUST decrease, to conserve energy H must go up (only space available is up).
What is shoaling?
Number of crests stays the same (period is constant)
Propagation velocity decreases
Means wavelength has to decrease
Energy conserved, but changed wave shape means
Wave height has to increase
Meanwhile…
Orbital velocity goes up
Why does orbital velocity increase during shoaling?
When do you get wave breaking?
At the trough (bottom of the wave) there is a frictional drag on the bed (more and more).
This means the trough is slowing down more than the crest is.
The crest gets squeezed into a sharper wall as H increases = more and more distorted.
Forward velocity increases.
When the orbital velocity is greater than the wave velocity.
When H > 1/7 wavelength
What is a plunging wave?
The crest is moving too fast to keep the wave form together.
What is a spilling wave?
The crest is too steep to keep the wave form together.
The kind of wave you get (plunging vs spilling) depends on what?
The relative difference between wave wavelength vs bed slope (big range).
Gradual bed slopes = more spilling
Steep (or abrupt) slopes = more plunging
Steep slope, long wavelength = BIG plunge
Bigger wavelength -> different parts of the wave feel the effects of the bed differently.
Velocity asymmetry is hugely important to…
sediment transport
What is a force?
A change in momentum.
Why do breaking waves shrink?
Momentum transferred into water (rather passing through it)
Same as a person when a wave knocks you over, momentum is getting transferred from the wave to you.
Momentum also transferred into bed, losing energy (dissipation -> not conserving it anymore)
Means H goes down
Cross shore forcing diagram
Waves always approach shore at an angle.
Angle of approach is represented by dark blue arrow- three hash lines represent wave crests.
Different lengthed light blue arrows represent angle (alongshore and cross shore component).
What cross shore (onshore) components drive sediment transport?
Piles water against shore Until gravity (pressure gradient) = wave force Waves, even with refraction (bending), often look parallel but almost never are; even a small angle can have a major long term effect.
