Numerical Modelling of the swash-zone has received considerable attention in recent years in an effort to better predict the hydrodynamics of this coastal zone subsection. State of the art numerical models typically employ the well-known Navier-Stokes equations or one of its variations. Consequently, suitable boundary conditions are needed for such models to ensure that real-life applications are simulated whilst also reducing the computational effort. This paper presents an empirically derived sea-ward boundary condition at the initial still water line position of the zone. The boundary condition is developed via laboratory experiments which simulate regular linear incident wave climates upon impermeable and variable beach slopes. The developed boundary condition favorably mimics experimental data and better predicts swash flow depths and velocities at the initial still water line when compared to more well-established models.