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-spee d video pictures of the violent wave motion at the collision. The phys ics and characteristics of the impact pressure significantly depend on the colliding conditions of breaking waves. These were studied for th e following colliding conditions: flip-through, collision of the verti cal wave front, and plunging wave collision. When a small amount of ai r 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 t he wall while trapping a small amount of air in the form of either bub bles or a thin lens-shaped pocket. The impulsive pressure, occurring i n the vicinity of the still water level, is transmitted downwards thro ugh the water body with the sound velocity. The larger the amount of t he entrapped air at impact of the plunging breakers, the lower the mag nitude and the longer the rise or compression time of the impact press ures. When plunging and curling of the breaking wave develop well, a t hick air pocket is trapped, and damped pressure oscillations, due to t he air pocket pulsation, appear immediately after the peak of the impa ct pressure. The oscillation frequencies are lower the greater the amo unt of entrapped air, and are almost equal to the resonant frequency o f pulsating air pockets. The damping mechanisms, however, still remain unknown. Agreements between the measured and predicted oscillation fr equencies suggest that adiabatic processes of the air pocket play an e ssential role in the physics of high impact pressure.