A semianalytical model for sheet flow layer thickness with application to the swash zone

A new semianalytical model for the time-dependent thickness of the sheet flow layer that includes the effects of pressure gradients, bed slope, boundary layer growth, and bore turbulence is presented. The shear stress and boundary layer growth are computed using the boundary layer integral method. The model is expressed as two coupled ordinary differential equations that are solved numerically given a prescribed time series of free-stream velocity, horizontal pressure gradient and bore turbulence, which together represent the hydrodynamic forcing. The model was validated against two data sets of sheet flow layer thickness collected in oscillatory flow tunnels and one data set collected in the swash zone of a prototype-scale laboratory experiment. In the oscillatory flow tunnel data sets, sheet flow is mostly generated by shear stress, with pressure gradients providing an important secondary forcing around flow reversal. In the swash zone, pressure gradients and shear stresses alone are not sufficient to generate the large sheet flow layer thickness observed at the initial stages of uprush. Bore turbulence is most likely the dominant generation mechanism for this intense sheet flow.