Experimental study of the swash of single and successive solitary waves

Abstract:

The swash zone is a complex flow environment involving wave breaking, wave-generated turbulence, bed-generated turbulence and interactions of successive waves. The high rates of sediment transport in the swash zone make it an important region for study. Amongst the flow properties that are of interest, the bed shear stress is perhaps the most important to measure since it has an influence on the run-up height, sediment transport and wave damping. However, the study of the swash zone in the laboratory remains challenging due to its transient nature, shallow depths and entrained air. The current study is conducted using a large-scale wave flume in which solitary waves are used as a convenient way to generate swash events on a plane concrete slope of steepness 1:12. Both single waves and successive waves are used to study a single swash event and the influence of one swash event on the next. The bed shear stress, near-bed flow velocity, bed pressure and local water depth are measured at nine locations in the cross-shore direction. The bed shear stress is measured directly using a custom-built shear plate sensor (Pujara & Liu, 2014). It is found that while the magnitudes of the bed shear stress are greatly increased by wave breaking, non-breaking waves also exhibit much larger magnitudes of onshore-directed bed shear stress in the region that is initially dry. Furthermore, the swash of a single solitary wave is found to be a gravity-dominated flow apart from the tip region near the moving shoreline during uprush and near the receding shoreline during downrush. In these regions, the bed shear stress, non-hydrostatic flow conditions and streamwise pressure gradients are also important. In the case of successive swash events, it is found that when the waves are separated by one wave period in the offshore region, the downrush of the first swash event is met by the uprush of second swash event creating a separation point.