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. 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

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

2. STUDY AREA Slide 4/8 TIDES Mesotidal Semi-diurnal WAVES Moderate to high Bimodal (76% W-SW; 24% E-SE) 08/9711/05

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

h u*u* U u*u* Time-averaged  Nm -2 for MA TKE; Nm -2 for SSM TKE; Nm -2 for RS; 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  (RS)  (MA TKE) Mega-ripples =1.3m;  =0.23m =1.7m;  =0.16m (Van Rijn, 1993)  Nm -2 maximum stress occurring before ebb peak C D  RS better approaches water slope;

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;

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

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