Recent catastrophic events caused by tsunamis, storm surges, flood waves, and the failure of dams (e.g. in Ukraine and Libya) have shown to be a significant threat to densely populated coastal communities. Interactions with built environments can lead to violent wave impacts that may have severe consequences, including significant infrastructural damage and potential loss of life. The frequency of these water-related disasters is increasing globally due to climate change and sea level rise, resulting in a rising demand for deeper knowledge related to the physical processes of such hazards.

The dynamic behaviour of these type of wave phenomena is described by long-period, high translatory waves, where the on-shore propagation or inland inundation is associated with sudden free-surface deformations. This results in a steeping of the slope at the leading edge, causing non-linear flow behaviour to prevail and inducing the wave to collapse. The breaking process generates a breaking roller at the wave front, containing a rapidly fluctuating mixture of air and water, associated with a strong recirculation. The high degree of air-water interaction in these unsteady flows has a significant impact on the flow properties as it influences many dynamic processes, including viscous and surface tension effects at air-bubble level, as well as larger scale gravitational effects associated with the turbulent flow and eddy formation (Brocchini and Peregrine, 2001). New innovative measurement techniques have allowed experimental studies to more precisely quantify the air-water interactions in multiphase flows. However, most experimental research focused on air-water flow properties in hydraulic jumps and other steady flows (e.g. spillway flows, plunging jets). Currently, limited research is available for unsteady flows and mostly based on small datasets and limited flow conditions. This lack of availability and diversity of experimental data restricts the understanding of how these multi-phase flows behave under different conditions, hence the need for future research.