Violent breaking wave impacts. Part 1: Results from large-scale regular wave tests on vertical and sloping walls

As part of an investigation into the detailed characteristics of wave impacts, experimental data are presented for the impact pressures and forces generated by waves up to 1.7 m high breaking onto a vertical wall and a wall inclined at 27o to the vertical. Particular attention is given to the influence of entrained and entrapped air and, by selecting regular wave conditions that produce impacts, trends are identified for highly variable phenomena that could easily be missed when masked by the even greater variability associated with irregular waves.

The characteristics of the impacts are found to depend on the breaker conditions and four different types of impact are identified and discussed. The distinctive features of low-aeration and high-aeration impacts are considered in some detail and it is shown that a high level of aeration does not always reduce the peak pressure although it tends to increase both the rise time and duration.

Consequently, high levels of aeration can increase both the force and impulse on the structure. Alternate compression and expansion of the air can also generate potentially detrimental sub-atmospheric pressures. The largest impact pressures tend to occur around still water level and distributions in terms of both the incident wave height and the peak pressure are presented in dimensionless terms for each type of impact.

Although the highest pressures are associated with low- and high-aeration impacts, the results suggest that slightly-breaking and broken wave impacts should not be neglected and percentage exceedance curves are included to give an indication of the frequency of extreme values. Whilst nominally identical waves often produce very different spatial and temporal pressure distributions, the impulse on the structure during an impact is far less variable and is shown to be broadly proportional to the peak momentum flux of the incident waves.

The pressures, forces and impulses associated with an impact on the sloping wall tend to be lower than those on the vertical wall in the same circumstances. However, the total impulse caused by a whole wave event (impact plus quasi-hydrostatic phases) is found to be independent of wall slope in accordance with basic principles.

The paper concludes by demonstrating that, for the conditions investigated, the impact can account for up to ~30% of the total impulse.