Evaluation of shear stress computation at a tidal inlet using

different methods A. Pacheco, J.J. Williams, Ó. Ferreira, J.A. Dias

Presentation Outline

1. ObjectiveCompute shear stress using different methodologies

2. Study AreaAncão Inlet, Ria Formosa Multi-inlet System

Slide 2/8

3. MethodologyEquipment, data processing

4. ResultsPTs, ADV and ADCP - Shear stresses, drag coefficient

5. DiscussionBest method to parameterise ST based on free-stream profile?

6. Conclusion

tfs 0000

1. OBJECTIVE

Skin-friction Form drag Sediment transport

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effective in moving the sand grains

THRESHOLD OF MOTIONBEDFORMSIncrease the form drag

Decrease the ST capacitya momentum transfer to mobilise the grains

compare several methodologies used to compute shear stresses and drag coefficients at a highly dynamic tidal inlet using different equipments

Analyse how bedforms formation/destruction contribute to it uncertainty

2. STUDY AREA

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TIDESMesotidalSemi-diurnal

WAVESModerate to highBimodal (76% W-SW; 24% E-SE)

08/97 11/05

ADCP transect

3. METHODOLOGY

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Pressure transducersBoat mounted ADCP

Ihg w 0

ADV 10MHz

)(2)1(1

)]1)(1[(5.0 nzhnununun

iiziziuiuhU

)0/(/*)( zzInuzU

Orthogonal flow components- BURST MODEADV’s – HIGH FREQUENCY

Burst time averaged 0 and u*

Compare with different methods individualise wave and current turbulence

TWO METHODSTKE and RS methods - Wave motion - MA and SSM

)()( zECzTKE 5.022

)( vwuvzRS

h

u*

U

u*

Time-averaged 0

0.4-4.6 Nm-2 for MA TKE; 0.8-5.6  Nm-2 for SSM TKE;

0.2-3.5 Nm-2 for RS;0.2-4.6 Nm-2 for Water slope.

4. RESULTS

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Maximum 0 obtained with TKE;

TKE

Smooth water levels - good agreement;

Boundary interference ?

CD

0.00590.0054 (RS)0.00930.0072 (MA TKE)

Mega-ripples=1.3m; =0.23m

=1.7m; =0.16m (Van Rijn, 1993)

00.06-2.1 Nm-2

maximum stress occurring before ebb peak

CD0.00170.0001

2

)/0(1

hzIn

DC

RS better approaches water slope;

5. DISCUSSION

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• ADV and PT point specific; ADCP cross-section integrate value

u* and 0 - RS method better agree with Log Profile tidal cycle results;

• good agreement between RS and the water slope method (especially at flood);

Ebb – Boundary interference? Development of bedform crests?

In theory

RS method should be valid even under waves (Soulsby and Humphery, 1990)u and v time-series components are 180º out of phase with w – presence of waves have little effect

• TKE-derived values are slightly higher;

Rely on accurate determination of the inertial subrange of the velocity spectrumTask NOT TRIVIAL – involves some subjectiveness – Soulsby and Humphery, 1990

Average values u*, 0 and CD can differ by a maximum factor of 4

CAUTIONEstimation of both bedload and suspended sediment transport

3 EQUIPMENTS – 4 METHODS;

6. CONCLUSION

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Average values u*, 0 and CD differed by a maximum factor of 4

Highly dynamic tidal inlet - mixed tide-wave dominated behaviour

Wide range of shear stress values and velocities

Estimation of skin-friction and form drag - problematic when waves are present

Estimate time-averaged shear stresses and velocities using different approaches

IMPORTANTAccurate determine the form drag component of total roughness

Dependent on the formation/destruction of bedformsCan significant influence the suspended sediment capacity

RS MethodParameterise the best estimates of current-only skin friction shear velocity

using free-stream current data

IDEM project – Inlet Dynamics Evolution and Management at the Ria Formosa (POCI/MAR/56533/2004)

PhD grant number SFRH/BD/28257/2006

Financed by