Geoffrey Bullock

Violent breaking wave impacts. Part 2: modelling the effect of air

When an ocean wave breaks against a steep-fronted breakwater, sea wall or a similar marine structure, its impact on the structure can be very violent. This paper describes the theoretical studies that, together with field and laboratory investigations, have been carried out in order to gain a better understanding of the processes involved. The waves approach towards a structure is modelled with classical irrotational flow to obtain the different types of impact profiles that may or may not lead to air entrapment. The subsequent impact is modelled with a novel compressible-flow model for a homogeneous mixture of incompressible liquid and ideal gas. This enables a numerical description of both trapped air pockets and the propagation of pressure shock waves through the aerated water. An exact Riemann solver is developed to permit a finite-volume solution to the flow model with smallest possible local error. The high pressures measured during wave impacts on a breakwater are reproduced and it is shown that trapped air can be compressed to a pressure of several atmospheres. Pressure shock waves, reflected off nearby surfaces such as the seabed, can lead to pressures comparable with those of the impact. Typical examples of pressure-time histories, force and impulse are presented and discussed in terms of their practical implications. The numerical model proposed is relevant for a variety of flows where air effects are important. Further applications, including extended studies of wave impacts, are discussed. 2009 Cambridge University Press.

Field and Laboratory Measurement of Wave Impacts

An extensive program of ongoing field, laboratory, and numerical investigations into the characteristics of wave impacts is described and early results presented. Shock pressures are found to be highly localized, both spatially and temporally, able to propagate into cracks and, in freshwater at least, capable of reaching surprising intensities. Conventional scaling of 1:4 hydraulic model data implies that ocean waves approximately 4m high could generate heads in excess of 1000m. Attention is drawn to the probability that, in practice, such extreme pressures may be constrained by the acoustic (water hammer) limit. The parameter map (Allstop & Kortenhaus, 2001) used for predicting impacts is shown not to be entirely reliable and preliminary results of the numerical models are discussed.

FIELD AND LARGE SCALE MODEL TESTS OF WAVE IMPACT PRESSURE PROPAGATION INTO CRACKS

Within a large & full scale study on wave impact induced pressures on coastal structures (BWIMCOST) an investigation of impact pressure propagation into structure cracks and fissures was carried out. The mechanism, which is held responsible for localized damage to existing blockwork breakwaters, had previously been verified in small scale model tests and a numerical model had been developed. The current investigation is the first which describes the effect at full scale, with recorded pressures of up to 199 kPa found within the cracks. The experimental results are related to their possible impact on coastal structural integrity.