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Evaluation of shear stress computation at a tidal inlet using different methods

Evaluation of shear stress computation at a tidal inlet using different methods. A. Pacheco, J.J. Williams, Ó. Ferreira, J.A. Dias. Slide 2/8. Presentation Outline. 1. Objective Compute shear stress using different methodologies. 2. Study Area Ancão Inlet, Ria Formosa Multi-inlet System.

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Evaluation of shear stress computation at a tidal inlet using different methods

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  1. Evaluation of shear stress computation at a tidal inlet using different methods A. Pacheco, J.J. Williams, Ó. Ferreira, J.A. Dias

  2. Slide 2/8 Presentation Outline 1. Objective Compute shear stress using different methodologies 2. Study Area Ancão Inlet, Ria Formosa Multi-inlet System 3. Methodology Equipment, data processing 4. Results PTs, ADV and ADCP - Shear stresses, drag coefficient 5. Discussion Best method to parameterise ST based on free-stream profile? 6. Conclusion

  3. BEDFORMS Increase the form drag effective in moving the sand grains THRESHOLD OF MOTION a momentum transfer to mobilise the grains Slide 3/8 1. OBJECTIVE Decrease the ST capacity 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 Form drag Skin-friction Sediment transport

  4. 08/97 11/05 TIDES Mesotidal Semi-diurnal WAVES Moderate to high Bimodal (76% W-SW; 24% E-SE) Slide 4/8 2. STUDY AREA

  5. Boat mounted ADCP Orthogonal flow components- BURST MODE ADV’s – HIGH FREQUENCY Burst time averaged 0 and u* Compare with different methods individualise wave and current turbulence TWO METHODS TKE and RS methods - Wave motion - MA and SSM Pressure transducers ADCP transect ADV 10MHz Slide 5/8 3. METHODOLOGY

  6. RS better approaches water slope; Maximum 0 obtained with TKE; h 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. TKE Boundary interference ? u* 0 0.06-2.1 Nm-2 maximum stress occurring before ebb peak CD 0.00170.0001 U CD 0.00590.0054 (RS) 0.00930.0072 (MA TKE) • Mega-ripples • =1.3m; =0.23m  =1.7m; =0.16m (Van Rijn, 1993) u* Slide 6/8 4. RESULTS Smooth water levels - good agreement;

  7. 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 Slide 7/8 5. DISCUSSION • good agreement between RS and the water slope method (especially at flood); Ebb – Boundary interference? Development of bedform crests? • TKE-derived values are slightly higher; Rely on accurate determination of the inertial subrange of the velocity spectrum Task NOT TRIVIAL – involves some subjectiveness – Soulsby and Humphery, 1990 • ADV and PT point specific; ADCP cross-section integrate value u* and 0 - RS method better agree with Log Profile tidal cycle results; 3 EQUIPMENTS – 4 METHODS; Average values u*, 0and CD can differ by a maximum factor of 4 CAUTION Estimation of both bedload and suspended sediment transport

  8. Slide 8/8 6. CONCLUSION 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 IMPORTANT Accurate determine the form drag component of total roughness Dependent on the formation/destruction of bedforms Can significant influence the suspended sediment capacity Average values u*, 0and CD differed by a maximum factor of 4 RS Method Parameterise the best estimates of current-only skin friction shear velocity using free-stream current data

  9. Financed by IDEM project – Inlet Dynamics Evolution and Management at the Ria Formosa (POCI/MAR/56533/2004) PhD grant number SFRH/BD/28257/2006 Thank you

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