Hydrodynamics of Dam-Break Waves on Grass Slopes: Experiments and Comparison with Overtopping Flows

Highly unsteady air-water flows are common in coastal and hydraulic engineering, particularly during dike breaches and wave overtopping events, where sudden releases of water propagate down on slopes. Despite their relevance in erosion processes, structural stability and human safety, the hydrodynamic behaviour and multiphase characteristics of these unsteady flows remain poorly understood. This study presents new experiments on dam-break waves propagating over smooth and grass-covered slopes with a 1:3 inclination, representative of typical dikes in north-western Europe. The experiments were conducted using a dam-break facility equipped with ultrasonic distance sensors and high-speed imaging to capture wave-front celerities and flow depths. Results show that surface roughness substantially influences wave propagation, leading to reduced celerities and enhanced air entrainment. On vegetated slopes, a highly aerated “ white-water ” front developed, extending over a distance equal to 5 times the dam’s impoundment depth, followed by a gradual decay of aeration. The measured wave front celerities agreed well with previous analytical solution for dam-break flows on slopes using friction factors consistent with steady flows on artificial grass. Ensemble-averaged flow depth data from up to 320 repetitions revealed statistically robust patterns of free-surface fluctuations. Results also provided a comparison between dam-break waves and unsteady flows generated with the Wave Overtopping Simulator (WOS), showing similarities and highlighting differences. Overall, these findings improve our understanding of multiphase unsteady flows on vegetated slopes and provide valuable data for developing more accurate predictive models for flood impacts and coastal structure design.