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Sand Motion over Vortex Ripples induced by Surface Waves
Jebbe J. van der WerfWater Engineering & Management, University
of Twente, The Netherlands
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Outline
1. Background2. Laboratory experiments3. Flow over ripples4. Sand dynamics over ripples5. Practical sand transport modelling6. Conclusions & further research
background experiments flow sand dynamics transport modelling conclusions
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Surface waves and oscillatory flow
background experiments flow sand dynamics transport modelling conclusions
shoreface
surf zone
wave boundary layer
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Wave-generated ripples
• Cover large part shoreface bed• η = 0.01-0.1 m and λ = 0.1-1.0 m• Vortex shedding if η/λ > 0.1
λ η
background experiments flow sand dynamics transport modelling conclusions
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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
z ≈ 2 η
η
Lower layer: organised convective processes dominant
Upper layer: turbulent processes dominant
background experiments flow sand dynamics transport modelling conclusions
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Ph.D. research
1. New full-scale laboratory experiments2. Improvement ripple predictors3. Improvement practical models to predict
time-averaged concentration profile4. Development new practical sand
transport model5. Improvement 1DV-RANS sand transport
model
background experiments flow sand dynamics transport modelling conclusions
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Experimental facilities
• Oscillatory flow tunnels• Flow equivalent to near-bed horizontal flow
generated by full-scale surface waves
background experiments flow sand dynamics transport modelling conclusions
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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
background experiments flow sand dynamics transport modelling conclusions
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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
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Suspended sand concentration measurement
• Transverse suction system• Optical concentration meter• Acoustic backscatter system
background experiments flow sand dynamics transport modelling conclusions
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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
timeonshore
offshore
u
background experiments flow sand dynamics transport modelling conclusions
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Instantaneous flow field
background experiments flow sand dynamics transport modelling conclusions
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
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Instantaneous flow field
background experiments flow sand dynamics transport modelling conclusions
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
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Time-averaged flow field
background experiments flow sand dynamics transport modelling conclusions
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Time- and ripple-averaged flow
background experiments flow sand dynamics transport modelling conclusions
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Instantaneous suspended concentration field
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
background experiments flow sand dynamics transport modelling conclusions
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Instantaneous suspended concentration field
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
background experiments flow sand dynamics transport modelling conclusions
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Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
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Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
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Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
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Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
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Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
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Horizontal suspended sand fluxes
),(~),(~),(),()(
),('),('),(~),(~),(),(),(),,(),,(),,(
),('),(~),(),,(
),('),(~),(),,(
zxczxuzxczxuz
zxczxuzxczxuzxczxuzxtzxctzxutzx
zxczxczxctzxc
zxuzxuzxutzxu
current-related wave-related
background experiments flow sand dynamics transport modelling conclusions
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Net horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
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Bedload transport
• Near-bed (mm’s) transport where grain-grain interactions are important
• Net bedload in the onshore direction due to flow asymmetry
• Forcing mechanism for onshore ripple migration (?)
background experiments flow sand dynamics transport modelling conclusions
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Net sand transport ratebedload transport
dominant
suspended load transport dominant
background experiments flow sand dynamics transport modelling conclusions
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Net sand transport rate
background experiments flow sand dynamics transport modelling conclusions
50DP
bedload transport dominant
suspended load transport dominant
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Practical sand transport modelling
• Implemented in larger morphological modelling systems
• Current practical sand transport models– Quasi-steadiness: qs(t) = m |u|n-1 u
– <qs> onshore (> 0) for asymmetric oscillatory flows with larger onshore velocities
– Not valid in vortex ripple regime where net transport is generally offshore (< 0)
background experiments flow sand dynamics transport modelling conclusions
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• Phase-lag effects schematically included• Four transport contributions F(θ’c,θ’t,P)
New practical sand transport model
onshore flow offshore flow
background experiments flow sand dynamics transport modelling conclusions
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New practical sand transport model
background experiments flow sand dynamics transport modelling conclusions
10-2
10-1
100
101
-101
-100
-10-1
-10-2-10
1
-100
-10-1
-10-2
pred
icte
d no
n-di
men
sion
al s
and
trans
port
measured non-dimensional sand transport10
-210
-110
010
1
quasi-steady modelnew model
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Conclusions
1. Flow and suspended sand dynamics controlled by vortex generation and ejection
2. Net sand transport controlled by offshore-directed suspended fluxes and onshore-directed near-bed transport
3. New practical sand transport model
background experiments flow sand dynamics transport modelling conclusions
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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)
background experiments flow sand dynamics transport modelling conclusions