Sand Motion over Vortex Ripples induced by Surface Waves

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Sand Motion over Vortex Ripples induced by
Surface Waves

• Jebbe J. van der Werf
• Water Engineering Management, University of
  Twente, The Netherlands

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Outline

• Background
• Laboratory experiments
• Flow over ripples
• Sand dynamics over ripples
• Practical sand transport modelling
• Conclusions further research

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Surface waves and oscillatory flow
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Wave-generated ripples

• Cover large part shoreface bed
• ? 0.01-0.1 m and ? 0.1-1.0 m
• Vortex shedding if ?/? gt 0.1

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Sand transport processes over vortex ripples

• Vortex ripples strongly influence wave boundary
  layer structure, near-bed turbulence intensity
  and sand transport mechanisms

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Ph.D. research

• New full-scale laboratory experiments
• Improvement ripple predictors
• Improvement practical models to predict
  time-averaged concentration profile
• Development new practical sand transport model
• Improvement 1DV-RANS sand transport model

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Experimental facilities

• Oscillatory flow tunnels
• Flow equivalent to near-bed horizontal flow
  generated by full-scale surface waves

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Measurements

• Bed elevation using laser displacement sensor
• Particle velocities using ultra-sonic velocity
  profiler and PIV
• Net sand transport rates by mass conservation
  technique using measured masses in traps and
  volume changes
• Suspended sand concentrations

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Suspended sand concentration measurement

• Transverse suction system

background
experiments
flow
sand dynamics
transport modelling
conclusions
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Suspended sand concentration measurement

• Transverse suction system
• Optical concentration meter

background
experiments
flow
sand dynamics
transport modelling
conclusions
11
Suspended sand concentration measurement

• Transverse suction system
• Optical concentration meter
• Acoustic backscatter system

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Experimental conditions

• Regular and irregular asymmetric flow with T
  5.0-10.0 s and u 0.4-1.3 m/s
• Uniform sand with D50 0.22-0.44 mm

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Instantaneous flow field
D50 0.44 mm T 5.0 s ? 0.08 m ? 0.41 m
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Instantaneous flow field
D50 0.44 mm T 5.0 s ? 0.08 m ? 0.41 m
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Time-averaged flow field
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Time- and ripple-averaged flow
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Instantaneous suspended concentration field
D50 0.44 mm T 5.0 s ? 0.08 m ? 0.41 m
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Instantaneous suspended concentration field
D50 0.44 mm T 5.0 s ? 0.08 m ? 0.41 m
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Horizontal suspended sand fluxes
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Horizontal suspended sand fluxes
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Horizontal suspended sand fluxes
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Horizontal suspended sand fluxes
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Horizontal suspended sand fluxes
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Horizontal suspended sand fluxes
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Net horizontal suspended sand fluxes
D50 0.44 mm T 5.0 s ? 0.08 m ? 0.41 m
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Bedload transport

• Near-bed (mms) transport where grain-grain
  interactions are important
• Net bedload in the onshore direction due to flow
  asymmetry
• Forcing mechanism for onshore ripple migration (?)

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Net sand transport rate
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Net sand transport rate
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Practical sand transport modelling

• Implemented in larger morphological modelling
  systems
• Current practical sand transport models
• Quasi-steadiness qs(t) m un-1 u
• ltqsgt onshore (gt 0) for asymmetric oscillatory
  flows with larger onshore velocities
• Not valid in vortex ripple regime where net
  transport is generally offshore (lt 0)

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New practical sand transport model

• Phase-lag effects schematically included
• Four transport contributions F(?c,?t,P)

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New practical sand transport model
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Conclusions

• Flow and suspended sand dynamics controlled by
  vortex generation and ejection
• Net sand transport controlled by
  offshore-directed suspended fluxes and
  onshore-directed near-bed transport
• New practical sand transport model

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Future research

• Comparison detailed data with more sophisticated
  models, 2DV-RANS models, ?
• Development of a general practical model to
  predict sand transport in coastal waters
  (Dutch/UK SANTOSS project)