AbstractsPhysics

Theoretical breaking criteria for progressive surface gravity waves are examined, and laboratory and field experiments concerned with breaking waves are reviewed with respect to the testing of these breaking criteria. The measurements of Komar and Simmons are presented here for the first time. Only three theoretical breaking criteria have been proposed for maximum steady waves in water of constant depth: (1) the kinematic breaking criterion, in which the horizontal partical velocity at the crest just equals the wave phase velocity, (2) the reversal of the vertical particle velocity near the crest as the ratio of wave height to water depth, H/h, increases, and (3) the reversal of the vertical pressure gradient beneath the crest as H/h increases. Although most theoreticians have applied the kinematic breaking criterion in conjunction with relatively simple wave theories (based on the motion being inviscid, irrotational, incompressible, surface tension free, and two dimensional), they do not always obtain identical results; for example, theoretical estimates of the particle acceleration at the crest range from zero to g, the gravitational acceleration. For shoaling waves, the kinematic breaking criterion and the presence of a vertical surface are suggested as breaking criteria. Unfortunately, these criteria were applied to the long wave theory which is considered inadequate near the breaking position. The re-examination of experiments on breaking waves shows that past measurements are not sufficient for testing any of these breaking criteria. In particular, the following improvements should be made: (1) standardize definitions of wave and breaking parameters, (2) apply or design, if necessary, more accurate techniques to measure water particle velocities and accelerations, and (3) monitor the fluid motions from which the breakers cannot be separated (e.g. backwash, solitons, reflected waves, edge waves and rip currents). Studies specifically designed to obtain the necessary measurements for testing the theoretical breaking criteria are needed.