Sand Motion over Vortex Ripples induced by Surface Waves Jebbe J. van der Werf Water Engineering...

Sand Motion over Vortex Ripples induced by Surface Waves

Jebbe J. van der WerfWater Engineering & Management, University

of Twente, The Netherlands

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

Surface waves and oscillatory flow

background experiments flow sand dynamics transport modelling conclusions

shoreface

surf zone

wave boundary layer

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

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

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

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

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

Suspended sand concentration measurement

• Transverse suction system

background experiments flow sand dynamics transport modelling conclusions

Suspended sand concentration measurement

• Transverse suction system• Optical concentration meter

background experiments flow sand dynamics transport modelling conclusions

Suspended sand concentration measurement

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

background experiments flow sand dynamics transport modelling conclusions

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

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

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

Time-averaged flow field

background experiments flow sand dynamics transport modelling conclusions

Time- and ripple-averaged flow

background experiments flow sand dynamics transport modelling conclusions

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

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

Horizontal suspended sand fluxes

background experiments flow sand dynamics transport modelling conclusions

Horizontal suspended sand fluxes

background experiments flow sand dynamics transport modelling conclusions

Horizontal suspended sand fluxes

background experiments flow sand dynamics transport modelling conclusions

Horizontal suspended sand fluxes

background experiments flow sand dynamics transport modelling conclusions

Horizontal suspended sand fluxes

background experiments flow sand dynamics transport modelling conclusions

Horizontal suspended sand fluxes

),(~),(~),(),()(

),('),('),(~),(~),(),(),(),,(),,(),,(

),('),(~),(),,(

),('),(~),(),,(

zxczxuzxczxuz

zxczxuzxczxuzxczxuzxtzxctzxutzx

zxczxczxctzxc

zxuzxuzxutzxu

current-related wave-related

background experiments flow sand dynamics transport modelling conclusions

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

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

Net sand transport ratebedload transport

dominant

suspended load transport dominant

background experiments flow sand dynamics transport modelling conclusions

Net sand transport rate

background experiments flow sand dynamics transport modelling conclusions

50DP

bedload transport dominant

suspended load transport dominant

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

• 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

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

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

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