This paper presents the results of a laboratory experiment of swash hydrodynamics on a coarse sand barrier beach backed by a lagoon. Boundary layer dynamics have been analyzed using the high-resolution near-bed velocities measured by Acoustic Doppler Velocity Profilers deployed in the swash zone. Swash events have been ensemble-averaged in order to study mean hydrodynamic patterns. A proposed velocity gradient criterion allowed identification of the boundary layer growth during the backwash phase, but it was unable to characterize boundary layer variability during uprush. Cross-shore velocity profiles were well represented by the logarithmic model for a large portion of the ensemble-averaged swash duration. Uprush and backwash logarithmic-estimated friction factors were of the same order of magnitude with a strong variability related to the boundary layer growth during the backwash. The momentum integral method provided smaller bed shear stresses than the logarithmic model, a result possibly related to either the assumptions involved in the momentum integral method or to an underestimation of the boundary layer thickness during uprush. A decrease of friction coefficients for increasing Reynolds numbers at the early backwash was observed. This behavior is consistent with traditional results for steady and uniform flows in a transitional regime.

Boundary layer dynamics in the swash zone under large-scale laboratory conditions

Ruju A.
;
2014-01-01

Abstract

This paper presents the results of a laboratory experiment of swash hydrodynamics on a coarse sand barrier beach backed by a lagoon. Boundary layer dynamics have been analyzed using the high-resolution near-bed velocities measured by Acoustic Doppler Velocity Profilers deployed in the swash zone. Swash events have been ensemble-averaged in order to study mean hydrodynamic patterns. A proposed velocity gradient criterion allowed identification of the boundary layer growth during the backwash phase, but it was unable to characterize boundary layer variability during uprush. Cross-shore velocity profiles were well represented by the logarithmic model for a large portion of the ensemble-averaged swash duration. Uprush and backwash logarithmic-estimated friction factors were of the same order of magnitude with a strong variability related to the boundary layer growth during the backwash. The momentum integral method provided smaller bed shear stresses than the logarithmic model, a result possibly related to either the assumptions involved in the momentum integral method or to an underestimation of the boundary layer thickness during uprush. A decrease of friction coefficients for increasing Reynolds numbers at the early backwash was observed. This behavior is consistent with traditional results for steady and uniform flows in a transitional regime.
2014
Boundary layer; Laboratory experiments; Logarithmic model; Momentum integral model; Swash;
Boundary layer; Laboratory experiments; Logarithmic model; Momentum integral model; Swash
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11584/282890
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