Sand Motion over Vortex Ripples induced by Surface Waves Jebbe J. van der Werf Water Engineering & Management, University of Twente, The Netherlands

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


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


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


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


Experimental facilities

• Oscillatory flow tunnels• Flow equivalent to near-bed horizontal flow

generated by full-scale surface waves


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


Suspended sand concentration measurement

• Transverse suction system



• Transverse suction system• Optical concentration meter



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


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


Instantaneous flow field


D50 = 0.44 mm

T = 5.0 s

η = 0.08 m

λ = 0.41 m

Instantaneous flow field


D50 = 0.44 mm

T = 5.0 s

η = 0.08 m

λ = 0.41 m

Time-averaged flow field


Time- and ripple-averaged flow


Instantaneous suspended concentration field

D50 = 0.44 mm

T = 5.0 s

η = 0.08 m

λ = 0.41 m


Instantaneous suspended concentration field

D50 = 0.44 mm

T = 5.0 s

η = 0.08 m

λ = 0.41 m


Horizontal suspended sand fluxes











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

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

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

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

zxczxuzxczxuz

zxczxuzxczxuzxczxuzxtzxctzxutzx

zxczxczxctzxc

zxuzxuzxutzxu

current-related wave-related


Net horizontal suspended sand fluxes


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 (?)


Net sand transport ratebedload transport

dominant

suspended load transport dominant


Net sand transport rate


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)


• Phase-lag effects schematically included• Four transport contributions F(θ’c,θ’t,P)

New practical sand transport model

onshore flow offshore flow


New practical sand transport model


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


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)


Documents

Sand Motion over Vortex Ripples induced by Surface Waves Jebbe J. van der Werf Water Engineering & Management, University of Twente, The Netherlands