Sediment transport Sediment transport GEO3-4306: Coastal Morphodynamics Sediment transport This...

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Sediment transportSediment transport

GEO3-4306: Coastal Morphodynamics

Sediment transport

This lecture

• background• modes of sediment transport• cross-shore transport• longshore transport• sediment balance

Boundary layer � stress

u

zτ ∂

∂:

2

1/ 3( 1)

* 502s g

D dν

−=

( ) 50

b

s gd

τθ

ρ ρ=

−

s = 2.65, g = 9.81 m/s2, ν = 1 x 10-6 m0.5/s, d50 = 250 x 10-6 m � D* ≈ 6 � θ = 0.05

Shield’s parameter

Sediment transport

• As soon as waves feel the sea bed, sediment will be in motion

• Waves stir the sediment

Transport modes

• Bed load (grain-to-grain interactions)• Suspended load (’in the fluid’ – turbulence

versus gravity)

Sediment transport

• Moving sediment can be organized in small bedforms (e.g., ripples, mega-ripples)

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Example of wave ripples in the shoaling zone

Sediment transport

sediment flux = velocity * concentration

*q u c=

AB

CD

Processes relevant to cross-shore sediment transport

E

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Location A (deep water)

• symmetric waves• inactive bed• transport is zero

AB

CD

Processes relevant to cross-shore sediment transport

E

Location B

• skewed waves• ripples on sea bed

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AB

CD

Processes relevant to cross-shore sediment transport

E

Location C

• skewed waves• bound infragravity waves• sheet flow (flat bed)

Location C

• skewed waves• bound infragravity waves• sheet flow (flat bed)

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Shoaling zone (1)

• Skewed waves stir AND transport sediment

• Near the bed, c in phase with u � onshore transport

• Higher up in the vertical, c much lower and phase shift between u and c � no or small offshore transport

• Overall effect: onshore transport

Net effect of bound infragravity waves?

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Shoaling zone (2)

• Bound infragravity waves transport sand stirred by gravity waves

• Large concentrations under high waves in the group coincide with bound infragravity trough (offshore infragravity orbital motion)

• Overall effect: offshore transport

Shoaling zone (3)

• Skewed waves: onshore transport

• Bound infragravity waves: offshore transport

• onshore >>> offshore

AB

CD

Processes relevant to cross-shore sediment transport

E

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Location D

• asymmetric waves • undertow

8.7 m

3.6 m

2.9 m

2.4 m

1.7 m

1.0 m

A

B

C

D

A

C

D

very large sediment concentrations under plunging breakers

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Asymmetric waves

• Onshore transport

• Same mechanism as for skewed waves?

• Why / why not?

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Large sediment concentrations and undertow � transport direction?

Breaking wave zone

• Breaking, asymmetric gravity waves stir sediment

• Large concentrations (breaking-induced turbulence)

• Sediment transport:• onshore by asymmetric waves• offshore by undertow

• In general:• few breaking waves � onshore• many breaking waves � offshore

AB

CD

Processes relevant to cross-shore sediment transport

E

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u

c

u*c

Location E

• infragravity waves• (undertow)

Swash zone (during storms)

• Water motion dominated by infragravity waves

• Infragravity waves stir AND transport sediment

• Large concentrations (breaking-induced turbulence)

• Sediment transport:• unclear• field experiments: onshore and offshore

• Potential offshore contribution by undertow

AB

CD

In summary

E

A: no transportB: little transport (skewed waves and ripples)C: onshore transport in shoaling zone (skewed waves)D: on/offshore transport in breaking zone (asym. waves/undertow)E: on/offshore transport in swash zone (infragravity waves)

transportratesincrease

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In case of rip currents

• Sediment is stirred by gravity waves, transported by currents• Skewed waves only play minor (onshore) role in between the rip currents• Other mechanisms not too important (undertow does not exist!)

Alongshore sediment transport

• Gravity waves stir sediment

• Breaking-induced alongshore currents transport the sediment• littoral drift

Cross-shore integrated formula

1.5 2.5 sin cosbr b bq g Hρ α α:

• Shore-normally incident (α = 0), transport is 0!

• Transport increases when wave height increases!

• Transport is maximum when α = 45°

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Sediment balance

• Bed level changes are a result of gradients in the sediment transport rates

• mass balance equation (Exner’s equation)

yx qq z

x x t

∂∂ ∂+ =∂ ∂ ∂

Sediment balance

• if you want to know how an area will change, determine input and output

• difference between input and output is change in transport