Tsunamis, flood waves and dam-break waves are highly destructive waves where a better description of these waves will contribute to a safer design of hydraulic structures. These waves are characterized by a sudden increase in water elevation with a breaking roller, which is a recirculating flow mixture of air and water, at the front of the wave. This study aims to get a better understanding of the fluctuating behaviour of dam-break waves over a wet bed across scales, focusing on the roller toe, which marks the transition from the downstream water level to the incoming bore.

Dam-break wave experiments, generated by the use of a lift gate system, were conducted on two scales for Froude numbers in a range of 2.44- 7.05, resulting in a range of varying Reynolds numbers between 0.22e5- 3.49e5. A High-Speed video camera recorded the waves from the top which enabled the wave profiles of the roller toe to be distinctly extracted. Due to the high level of unsteadiness in the flow, ensemble statistics are used, leveraging statistical properties to compare wave characteristics across different flow conditions.

The bore front celerity, roller toe fluctuations, instantaneous celerity (fluctuations) and indentation coefficient give in-depth insight in the hydrodynamic behaviour of the roller toe. Comparison with the dam-break wave theory showed good agreement with the bore’s celerity based on the experimental results. The fluctuating behavior of the roller toe increased for an increasing Froude number, indicating that gravity is the dominant force driving these variations. The magnitude of fluctuations in relation to the Reynolds number showed a slightly decreasing trend, indicating the presence of scale effects and emphasizing the importance of investigating these highly turbulent flow motions. The indentation coefficient underscored the presence of turbulence as highlighting different results between wave types. Overall, this study contributes to a better understanding of highly unsteady flows.