Swash Hydrodynamics on a Steep Beach

New data on swash hydrodynamics on a steep laboratory scale beach are presented. Wave grouping is found to be particularly important and directly induces large amplitude low frequency shoreline motions which describe the run-up due to individual bores. Consequently, run-up spectra may be dominated by energy at low frequency harmonics, even in the absence of free incident long waves. A new semi-empirical swash kinematics model is developed which is based on the physical constraints on the swash motion and a non-dimensional shape function to describe the cross-shore swash profile. The model shows good agreement with laboratory data obtained over fixed sediment beds during both the uprush and backwash phases of the swash cycle.