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Evaluation of shear stress computation at a tidal inlet using different methods A. Pacheco, J.J. Williams, Ó. Ferreira, J.A. Dias
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Presentation Outline 1. Objective Compute shear stress using different methodologies 2. Study Area Ancão Inlet, Ria Formosa Multi-inlet System Slide 2/8 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
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1. OBJECTIVE Skin-friction Form drag Sediment transport Slide 3/8 effective in moving the sand grains THRESHOLD OF MOTION BEDFORMS Increase the form drag Decrease the ST capacity a 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
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2. STUDY AREA Slide 4/8 TIDES Mesotidal Semi-diurnal WAVES Moderate to high Bimodal (76% W-SW; 24% E-SE) 08/9711/05
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ADCP transect 3. METHODOLOGY Slide 5/8 Pressure transducers Boat mounted ADCP ADV 10MHz 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
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h u*u* 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 Slide 6/8 Maximum 0 obtained with TKE; TKE Smooth water levels - good agreement; Boundary interference ? C D 0.0059 0.0054 (RS) 0.0093 0.0072 (MA TKE) Mega-ripples =1.3m; =0.23m =1.7m; =0.16m (Van Rijn, 1993) 0 0.06-2.1 Nm -2 maximum stress occurring before ebb peak C D 0.0017 0.0001 RS better approaches water slope;
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5. DISCUSSION Slide 7/8 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 spectrum Task NOT TRIVIAL – involves some subjectiveness – Soulsby and Humphery, 1990 Average values u *, 0 and C D can differ by a maximum factor of 4 CAUTION Estimation of both bedload and suspended sediment transport 3 EQUIPMENTS – 4 METHODS;
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6. CONCLUSION Slide 8/8 Average values u *, 0 and C D 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 IMPORTANT Accurate determine the form drag component of total roughness Dependent on the formation/destruction of bedforms Can significant influence the suspended sediment capacity RS Method Parameterise the best estimates of current-only skin friction shear velocity using free-stream current data
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IDEM project – Inlet Dynamics Evolution and Management at the Ria Formosa (POCI/MAR/56533/2004) PhD grant number SFRH/BD/28257/2006 Financed by
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