Sand transport by non-breaking Sand transport by non-breaking waves and currentswaves and currents

Highlights from the SANTOSS projectHighlights from the SANTOSS project

Jan S. Ribberink, Tom O’DonoghueJan S. Ribberink, Tom O’Donoghueand many others and many others

SANTOSS project funded by UK’s EPSRC (GR/T28089/01) and SANTOSS project funded by UK’s EPSRC (GR/T28089/01) and Dutch research organisation STW (TCB.6586)Dutch research organisation STW (TCB.6586)

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Contents presentation

• Introduction: Project background, aims and approach • Wave deformation: acceleration skewness (PhD-UAb)• Progressive surface waves vs. oscillatory flows (PhD-UT)• Process-based modelling (PhD-UT/Deltares) • Practical sand transport model• Conclusions

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INTRODUCTION

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Research questions

• How is sand transport affected by wave deformation (asymmetry and skewness) ?

• How is sand transport affected by wave-induced mean flows (e.g. boundary layer streaming)

• How to develop a practical sand transport formula for this environment (wave+ current, different bed regimes, variable grain size)

Background SANTOSS (2005-2009)

shoreface

surfzone

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Research approach (1)

1. Data integration

Bring together data from large-scale oscillatory flow experiments conducted in The Netherlands, the UK and elsewhere ( Database)

2. New experiments

1. Acceleration effects (UAb – PhD )

2. Sheet flow under surface waves (UT - PhD)

3. Exp’tal data analysis

Identify and parameterise the most important physical processes determining transport in sheet flow conditions

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4. Process modelling

Models from Deltares, UWB and LU - use process models for understanding, parameterise important processes.

5. New transport model

“semi-empirical” model: “explicitly accounts for the most important physical processes through parameterisations based on the experimental data and sound understanding of the physical processes” (UT and UAb)

1. Data integration

2. New expts

3. data analysis

Research approach (2)

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WAVE DEFORMATION

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Wave deformation: wave skewness

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Acceleration skewness Dominic van de A (UAb)

max

max min

uu u

Near-bed horizontal orbital flow: velocity and acceleration skewness

max

max min

uR

u u

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Research Facility

Aberdeen oscillatory flow tunnel (AOFT)

- 16m long, 0.3m wide, 0.75m high

- T ≈ 5-12s, amax = 1.5m

500mm

7m

fixed bed

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Reynolds stress asymmetry

T = 7s, u0max= 1.1m/s. d50=5. 65mm

β = 0.75

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‘Onshore’ net Sand Transport

• Net transport against acceleration skewness

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PROGRESSIVE SURFACE WAVES vs. OSCILLATORY FLOWS

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Oscillatory flows Progressive surface waves

wave tunnel

wave flume

Horizontal oscillatory flow Horizontal + vertical orbital flow Non-uniform flow

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‘Large-scale’ wave flumes

Delta Flume 2006

GWK Hannover 2007, 2008

Sand transport processes under progressive surface waves

Jolanthe Schretlen (UT)

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10 mm

UVPsVectrino

CCMs

UHCM

TSS

30° 10°

Beach

Wave paddl

e

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Intra-wave boundary layer velocities

H = 1.5 mT = 6.5 sFine sand

Sheet flow layer

onshore

offshore

250

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Wave-mean flow (streaming)

xp

zwu

zu

zzu

z tt

1~~

]~~[])[(

Mean oscillatory turbulent Reynolds stress

Wave ReynoldsStress

‘offshore streaming’ (asymmetric waves)

‘onshore streaming’

Trowbridge & Madsen (1984)Davies and Villaret (1999)

Longuet-Higgins (1958)

Tunnels

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Streaming in Tunnel and Wave flume

Urms = 0.86-0.89 m/sT = 5 sR = 0.54 - 0.6

-10

-5

0

5

10

15

20

25

30

35

-0,1 -0,05 0 0,05 0,1

<U> m/s

z

mm tunnel - f ine

flume - f ine

250

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Medium sands Flume vs Oscillatory Flow Tunnel

-20

0

20

40

60

80

100

120

0 0.05 0.1 0.15 0.2 0.25 0.3

U3 (m3/s3)

net t

rans

port

(10

-6 m

2 /s)

Sand transport rates (medium sand)

Tunnels

GWK flume

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Sand transport rates (fine sand)

Fine Sands Flume vs Oscillatory Flow Tunnel

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 0.05 0.1 0.15 0.2 0.25 0.3

<U 3> (m3/s3)

Net

Tra

nspo

rt (1

0-6

m2 /s

)

GWK flume

Tunnels

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PROCESS-BASED MODELLING

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Wave boundary layer models with hydrostatic pressure distribution (1DV)

Single-phase models (UWB, Deltares/UT-PSM: Wael Hassan) Two-phase flow model (UL)

Full water depth model, non-hydrostatic for waves (1DV) Single-phase models (Deltares/UT-PSM)(Semi) two-phase model (Deltares/UT-PSM+)

Process-based modellingWael Hassan (UT), Wouter Kranenburg (UT), Rob Uittenbogaard (Deltares)

PhD: Wouter Kranenburg

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Mean Sediment flux (single-phase WBL)

0.0

1.0

2.0

3.0

4.0

-50.0 -30.0 -10.0 10.0 30.0 50.0

Total sediment-flux 10-3 (m/s)

z (c

m)

D = 0.15 mm

D = 0.28 mm

-1.0

0.0

1.0

2.0

3.0

4.0

-50.0 -30.0 -10.0 10.0 30.0 50.0

Total sediment-flux 10-3 (m/s)

z (c

m)

D = 0.15 mm

D = 0.28 mm

(Hassan and Ribberink, 2010)

250

onshore

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Mean transport rate (single-phase WBL)

0

50

100

0.2 0.4 0.6 0.8 1.0Urms (m/s)

<qs>

10-6

(m2/s)

RANS-model with S

RANS-model with HRANS-model with H&S

RANS-model without H&Smeasured D = 0.32 mm

-300

-250

-200

-150

-100

-50

0

50

0.2 0.4 0.6 0.8 1.0Urms (m/s)

<qs>

10-6

(m2/s)

RANS-model with S

RANS-model with H

RANS-model with H&S

RANS-model without H&S

measured D = 0.13 mm

250

medium sand fine sand

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Mean transport rate (single-phase, FWD)

250

(Kranenburg et al., 2010)

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PRACTICAL SAND TRANSPORT MODEL

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Requirements • Simple formula for application in morphodyamic model • Cross-shore and alongshore transport (wave + current) • Wave shapes (velocity, acceleration-skewed)• Range of grain sizes (fine -> coarse sand) • Bed regimes: sheet-flow (flat beds) and rippled beds

New practical sand transport model Jebbe van der Werf (UT), Dominic van de A (UAb), Rene Buijsrogge (UT), Jan Ribberink (UT)

shoreface

surfzone

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Transport model conceptTransport model concept

δ

zb

Outer layer he

Wave boundary layer

<U>.<C>

<U(t).C(t)>

2-layer schematization

Transport formula

Suspension adv-diff

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c cc tcc

cc

Basic model

350( 1)

s c tc t

q T TT Ts gd

ˆcu

t tt ctt

tt

timeonshore

u

Tc

Half wave-cycle concept

<uδ>

Tt

ˆtu

(Dibajnia and Watanabe, 1996)

Phase-lag effect

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Comparison with database Velocity-skewed wave (+current) data

wave alone &wave+current

ripples(52)sheet flow (86)

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Acceleration-skewed waves

‘Acceleration’ corrections

• Bed shear stress (τc > τt , friction factor )• Phase-lag (Pc < Pt , settling periods)

max

max min

uu u

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Comparison with database Acceleration-skewed wave (+current) data

wave alone &wave+current

medium sand(36)

fine sand(21)

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Transport model: acceleration skewness effect

Velocity + (acceleration) skewed wave (r = 0.62, b = 0.7)

-1.5E-04

-1.0E-04

-5.0E-05

0.0E+00

5.0E-05

1.0E-04

1.5E-04

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Urms [m/s]

Calc

ulat

ed: Q

sx [m

2/s]

velocity + acceleration skewed

velocity + acceleration skewedvelocity skewed

velocity skewedmedium sand

fine sand

R = 0.62β = 0.7T = 6.5 s

onshore

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Progressive surface waves

Reb w uw

w

• Additional onshore mean bed shear stress

Settling phases

wave Reynolds stress τwRe Lagrangian drift

w

• Lagrangian drift grain motion (Pc < Pt , settling periods)• Grain settling velocity (ws,c > ws,t , Pc < Pt )

‘Surface wave’ corrections

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Comparison with database All (surface) waves (+current)

All waves (206)

% factor 2 : 76 %r2 : 0.81

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Transport model: progressive surface wave effects

onshore

R = 0.62β = 0.5T = 6.5 sh = 3.5 mFine sand

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Comparison with ‘current alone’ data

Current alone(137)

% factor 2 : 87 %r2 : 0.73

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Main conclusions

• Through lab research and process-based modelling new insights were obtained in the influence of wave shape and surface wave processes on the sand transport process.

• Acceleration skewness of wave-induced oscillatory flows leads to additional ‘onshore’ sand transport in the sheet flow regime.

• For identical oscillatory flow the sand transport under progressive surface waves is more onshore than in wave tunnels, caused by the combined influence of surface wave processes such as WBL streaming and Lagrangian grain motion effects.

• Based on the new insights and a new large dataset an improved sand transport formula was developed. The model performs well in a wide range of conditions: different wave shapes, wave+current, current alone, range of grain sizes, different bed regimes.

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THE END