WAVE IMPACT PRESSURE ON VERTICAL WALLS UNDER BREAKING WAVES OF VARIOUS TYPES

Laboratory experiments were conducted to improve our understanding of the physics and characteristics of impact pressures due to collisions of breaking waves against a vertical wall. Measurements of impact wave pressures were performed simultaneously with observation of high-speed video pictures of the violent wave motion at the collision. The physics and characteristics of the impact pressure significantly depend on the colliding conditions of breaking waves. These were studied for the following colliding conditions: flip-through, collision of the vertical wave front, and plunging wave collision. When a small amount of air is entrapped between the breaking wave and the wall at the collision, the impact pressure increases considerably. The highest pressure, of very short duration, is observed when a vertical wave front strikes the wall while trapping a small amount of air in the form of either bubbles or a thin lens-shaped pocket. The impulsive pressure, occurring in the vicinity of the still water level, is transmitted downwards through the water body with the sound velocity. The larger the amount of the entrapped air at impact of the plunging breakers, the lower the magnitude and the longer the rise or compression time of the impact pressures. When plunging and curling of the breaking wave develop well, a thick air pocket is trapped, and damped pressure oscillations, due to the air pocket pulsation, appear immediately after the peak of the impact pressure. The oscillation frequencies are lower the greater the amount of entrapped air, and are almost equal to the resonant frequency of pulsating air pockets. The damping mechanisms, however, still remain unknown. Agreements between the measured and predicted oscillation frequencies suggest that adiabatic processes of the air pocket play an essential role in the physics of high impact pressure.