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  • 1

    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

    z

    :

  • 2

    1/ 3( 1)

    * 502s g

    D d

    =

    ( ) 50b

    s gd

    =

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

    Shields 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)

  • 3

    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

  • 4

    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

  • 5

    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)

  • 6

    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?

  • 7

    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

  • 8

    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

  • 9

    Asymmetric waves

    Onshore transport

    Same mechanism as for skewed waves?

    Why / why not?

  • 10

    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

  • 11

    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

  • 12

    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 H :

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

    Transport increases when wave height increases!

    Transport is maximum when = 45

  • 13

    Sediment balance

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

    mass balance equation (Exners 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

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