Floating pontoons to reduce wave overtopping at a vertical seawall: An experimental study

Coastal flooding caused by extreme wind, wave and water level conditions is an increasing concern, particularly for historical coastal cities where conventional flood defenses may be unsuitable due to aesthetic and cultural constraints. Floating structures have gained attention for their adaptability to sea level rise, yet previous studies have mainly examined wave transmission rather than their capacity to reduce wave overtopping. This study presents the first experimental investigation to directly measure wave overtopping for floating pontoons placed in front of a vertical seawall. Tests were conducted in a controlled wave flume environment to evaluate the effects of pontoon geometry, mooring type, and distance from the seawall on overtopping performance. The results show that floating pontoons can significantly reduce wave overtopping. Overtopping reductions of 75-98 % was achieved, with the most effective configuration combining high freeboard and large draft (1.5 m prototype scale). Wave transmission was also measured and compared with existing prediction formulas. When the transmitted wave height is used in EurOtop (2018) formula, overtopping rate is overestimated particularly when the relative crest freeboard exceeds 0.75 as differences in wave steepness, spectral period and directional spreading induced by the floating pontoon are not captured by the formula. To improve predictive capability, a new influence coefficient (gamma fp) is proposed to modify Eq. 7.5 in EurOtop (2018) for cases involving pile-guided floating pontoons. These findings provide new experimental evidence on wave--structure interaction and highlight the potential of floating pontoons as effective, adaptable, and visually compatible flood mitigation solutions for vulnerable coastal regions.