The influence of air and scale on wave impact pressures

Both laboratory and field tests that are described provide new information on the characteristics of wave impacts. Laboratory drop tests conducted usi ng seawater and freshwater demonstrate that maximum impact pressures and ri se times are influenced by both the level of aeration and the violence of t he impact. A relationship is derived which enables the reduction in impact pressure caused by aeration to be estimated. This relationship is shown to provide a better means of predicting impact pressures in laboratory seawate r wave tests from freshwater tests than either the Froude or Cauchy laws. M easurements are presented which show that, due to the different properties of seawater and freshwater, aeration levels are higher in seawater breakers than in freshwater breakers, even at a 1:25 model scale. The ways in which this affects the temporal variation in pressure and the scale relationship s are discussed in some detail. Aeration and pressure measurements are also presented for full-scale wave impacts on a breakwater exposed to Atlantic waves. Attention is drawn to the likely role of expelled air and data inclu ded which indicate that the equivalent of up to 55% of entrained air does n ot necessarily prevent the occurrence of high impact pressures with short r ise times. (C) 2001 Elsevier Science B.V. All rights reserved.