2003 Overview of IST Group Results on the Sediment Benchmark 3 rd IMPACT Workshop Louvain-la-Neuve University of Beira Interior João Leal (UBI) Rui Ferreira (IST) António Cardoso (IST) António Almeida (IST)

Overview of IST Group Results on the Sediment Benchmark OBJECTIVES: Comment the IST numerical results  Perform a sensitivity analysis on the empirical coefficients in order to improve the results  University of Beira Interior

CONCEPTUAL MODEL Closure equations University of Beira Interior depth of the sheet-flow layer (Sumer et al. 1996; Pugh & Wilson 1999) velocity in the sheet-flow layer (Sumer et al. 1996) Overview of IST Group Results on the Sediment Benchmark sediment transport rate (Bagnold 1966) bed shear stress (Chézy equation) EMPIRICAL COEFFICIENTS

CONCEPTUAL MODEL Coefficients evaluation: using Sumer et al. (1996) results and the non-dimensional fall velocity using Sumer et al. (1996) results and the non-dimensional fall velocity as the measure of similitude between the sediment used by those authors and the PVC cylinders used in the UCL test Bed material (-)(mm)(cm/s)(-) PVC cylinders University of Beira Interior Overview of IST Group Results on the Sediment Benchmark Wilson’s (1987) theoretical relation

RESULTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark GOOD AGREEMENT

RESULTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark GOOD AGREEMENT

RESULTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark BAD AGREEMENT (the numerical model underestimates the scour hole)

RESULTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark BAD AGREEMENT (the numerical model underestimates the scour hole)

RESULTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark BAD AGREEMENT (the numerical model predicts no scour)

RESULTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark BAD AGREEMENT (the numerical model predicts no scour)

RESULTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark BAD AGREEMENT (the numerical wave-front celerity is faster than the experimental one)

COMMENTS University of Beira Interior Overview of IST Group Results on the Sediment Benchmark Two major discrepancies were found between the UCL experimental data and the IST numerical results In the sections near the dam the scour is underestimated by the numerical model  The numerical wave-front celerity is faster than the experimental one  POSSIBLE SOLUTION: increase the coefficient of Bagnold’s sediment transport formula POSSIBLE SOLUTION: increase the friction coefficient NOTE: should the initial water depth downstream the gate be non negligible and the wave-front celerity would be slowed down

Influence h d University of Beira Interior Overview of IST Group Results on the Sediment Benchmark The increase of h d increases the wave-front height

Limitation imposed by the closure equations University of Beira Interior Overview of IST Group Results on the Sediment Benchmark (value adopted in the blind test) In order to increase friction (C f ) we will need to use smaller values of The experimental sheet-flow data by Sumer et al. (1996) and by Pugh & Wilson (1999) indicate that which implies a new limit

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark As  increases, the water levels increase in the wave- front region while the sheet-flow height decreases. The celerity is not influenced by  Influence

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark  = 2 overestimates the erosion, affecting the water level for higher times Influence

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark  = 2 gives better bed levels but worst water levels Influence

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark The increase of C f increases the scour hole, but the numerical bed level is still much higher than the experimental one Influence

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark The increase of C f allows the approximation between the numerical and the experimental wave-fronts Influence

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark For higher C f values the numerical wave-front celerity is reduced and approximates the experimental celerity Influence

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark Final Results

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark Final Results

University of Beira Interior Overview of IST Group Results on the Sediment Benchmark Final Results

CONCLUSIONS The major discrepancies between numerical and experimental results can be attenuated by changing the value of some parameters of the closure equations  The model always underestimate the scour hole that is observed downstream the dam in the experiments  The IST numerical results are generally in good agreement with the benchmark data  University of Beira Interior Overview of IST Group Results on the Sediment Benchmark

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