Waves approaching shore and cross-shore sediment transport - Lecture 4 Flashcards
Differences between orbital motions and wave motions
Distinct from each other and two different things
Orbitals throughout and diminishes with depth
Why do large wavelengths travel faster (assuming T is constant?)
Because there is a direct relationship between wavelength and velocity
Short wavelength - small velocity
Large wavelength - high velocity
What is the wave velocity equation
Wave velocity = wavelength / period
rate = distance / time
If wavelength decreases, then T must get really small (less time between crests)
What happens to wavelengths travelling whilst keeping speed constant
Wavelength goes down, period must be small
Wavelength goes up, in order to keep same velocity, wave period must get really big (more time between crests)
What happens to waves approaching shore (deep water)
In deep water (depth >0.5 wavelength), seabed is far below wave motion - no interaction between wave and bed
What happens to waves approaching shore (shallow water)
In shallow water <0.5, wave motion gets distorted - in a fully shallow water wave, wave motion constant over full depth
What is the behaviour of shallow water orbital motions?
Bottom of bed drags due to less depth, orbitals flatten out as less room to move in, less squeezing in front of wave = slower propagation, energy is conserved
Number of crests onshore and offshore
Stays the same - energy is conserved, can’t destroy it
What is the speed and distance between offshore crests
Far apart but come by very quickly
What is the speed and distance between onshore crests
Wavelengths decrease - close together and pass under more slowly, running out of depth and conserve energy therefore height must go up as wavelength goes down
Shoaling
Entering shallow water = speed decreases, wavelength decreases, height increases and orbital velocity increases
Orbital velocity increases onshore - why?
Trough slows down more than crest, crest squeezed into sharper wall, height increases - distortion of wave shape
Forward motion accelerates, velocity asymmetry (forward velocity) increases
What happens to velocities when waves are breaking?
Orbital velocity > wave velocity
Crest velocity > trough velocity = plunging (too fast)
Less difference = spilling
How does the bed slope influence waves breaking?
Gradual bed slopes = more spilling
Steep slopes - long wavelength = plunging
= the bigger the wavelength, the different parts of the wave feel the effects of the bed differently
What is hugely important to sediment transport and why?
Velocity asymmetry - there is a threshold for sediment transport at higher velocities (above trough as there is friction and not enough velocity to transport sediment)
How does the momentum of the wave affect sediment transport?
Wave moving through water medium, transferring momentum into water and bed - transfer of momentum causes currents moving sediment around
Definition of a force (in terms of this module, TOBY)
A change in momentum
What happens to the momentum of the water as the breaking waves shrink?
Momentum is transferred INTO water rather than through it and also transferred into bed - losing energy
What happens to wave when breaking waves shrink?
Wave height decreases, energy is lost and dissipated into the bed and sediment is transferred
What are components of cross-shore forcing?
Waves approach shore at an angle
Piles water against shore until gravity/pressure gradient pulls it back = wave force
What is the undertow of cross-shore forcing?
Residual net effect - return flow near bed
