Experimental study of Breaking Wave Pressures

Abstract

ABSTRACT A laboratory investigation of the impact pressures that often result when waves break against coastal structures was conducted using periodic waves in a channel 2 ft × 2 ft in cross-section and 100ft in length. The model structure used in the study, a vertical wall 6 in. wide supported in the center of the channel, was 'instrumented with six miniature piezoelectric transducers capable of measuring rapidly applied pressures. Preliminary photographic studies of breaker geometry and kinematics revealed that the presence of the relatively narrow test structure did not significantly alter the breaker form and the data thus obtained could be used to quantitatively characterize the eight wave forms investigated. Pressure measurements verified the variability of peak pressure magnitude observed by other investigators. The presence of air in significant quantities, entrapped between the advancing wave and' the structure, was also observed. Two classifications of impact pressure, termed "ordinary" and "significant", could be defined, hence raising a question about the nature of the impact pressures studied by previous investigators. Significant pressures were relatively large in magnitude and acted over a considerable portion of the structure simultaneously. Ordinary pressures, while at times quite large, occurred more frequently but acted over only a small area at any instant. For significant impact pressures it was found that the average value of the dimensionless maximum pressure, −pmax/?Yb increasing steepness of the incident wave and that for any given incident wave form the maximum pressure was inversely proportional to the time required for the pressure to reach a maximum. A comparison of the instantaneous spatial pressure distribution, determined using the six pressure transducers, with the pressure distribution currently in use for breakwater design reveals that the two differ in shape. The difference may arise because previous investigators have been limited by using only one, or at most three, movable transducers and have therefore measured the pressures caused by a number of successive waves rather than the distribution of pressure resulting from a single wave. INTRODUCTION Seawalls and vertical wall breakwaters subjected to breaking wave action occasionally experience high pressures of short duration often termed "shock pressures". The occurrence of these pressures depends critically upon the location of a structure relative to the breaking position of the wave and, because of this dependence, occurs only infrequently against full scale breakwaters. Measurements made by de Rouville, Besson and Petry on a full scale breakwater at. Dieppe revealed that only 2 to 3 percent of the waves hitting the structure resulted in impact pressures11[The term impact pressure rather than shock pressure will be used. herein to describe the high pressures caused by breaking waves when they encounter a structure.] Early full scale measurements of breaking wave forces by Stevensonl2 and Gaillard13 failed to detect the short duration impact pressures because of the slow response of their measuring equipment. Larras in a laboratory study subsequent to the full scale measurements by de Rouville, Besson and Petryll at Dieppe, was the first to identify the impact pressure as an elastic phenomenon.