Waves approaching shore and cross-shore sediment transport - Lecture 4 Flashcards

1
Q

Differences between orbital motions and wave motions

A

Distinct from each other and two different things

Orbitals throughout and diminishes with depth

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2
Q

Why do large wavelengths travel faster (assuming T is constant?)

A

Because there is a direct relationship between wavelength and velocity
Short wavelength - small velocity
Large wavelength - high velocity

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3
Q

What is the wave velocity equation

A

Wave velocity = wavelength / period
rate = distance / time
If wavelength decreases, then T must get really small (less time between crests)

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4
Q

What happens to wavelengths travelling whilst keeping speed constant

A

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)

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5
Q

What happens to waves approaching shore (deep water)

A

In deep water (depth >0.5 wavelength), seabed is far below wave motion - no interaction between wave and bed

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6
Q

What happens to waves approaching shore (shallow water)

A

In shallow water <0.5, wave motion gets distorted - in a fully shallow water wave, wave motion constant over full depth

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7
Q

What is the behaviour of shallow water orbital motions?

A

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

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8
Q

Number of crests onshore and offshore

A

Stays the same - energy is conserved, can’t destroy it

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9
Q

What is the speed and distance between offshore crests

A

Far apart but come by very quickly

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10
Q

What is the speed and distance between onshore crests

A

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

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11
Q

Shoaling

A

Entering shallow water = speed decreases, wavelength decreases, height increases and orbital velocity increases

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12
Q

Orbital velocity increases onshore - why?

A

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

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13
Q

What happens to velocities when waves are breaking?

A

Orbital velocity > wave velocity
Crest velocity > trough velocity = plunging (too fast)
Less difference = spilling

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14
Q

How does the bed slope influence waves breaking?

A

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

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15
Q

What is hugely important to sediment transport and why?

A

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)

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16
Q

How does the momentum of the wave affect sediment transport?

A

Wave moving through water medium, transferring momentum into water and bed - transfer of momentum causes currents moving sediment around

17
Q

Definition of a force (in terms of this module, TOBY)

A

A change in momentum

18
Q

What happens to the momentum of the water as the breaking waves shrink?

A

Momentum is transferred INTO water rather than through it and also transferred into bed - losing energy

19
Q

What happens to wave when breaking waves shrink?

A

Wave height decreases, energy is lost and dissipated into the bed and sediment is transferred

20
Q

What are components of cross-shore forcing?

A

Waves approach shore at an angle

Piles water against shore until gravity/pressure gradient pulls it back = wave force

21
Q

What is the undertow of cross-shore forcing?

A

Residual net effect - return flow near bed