INSTRUMENTATION FOR THE MEASUREMENT OF VOID-FRACTION IN BREAKING WAVES - LABORATORY AND FIELD RESULTS

We report on the development and use of an impedance probe to measure the volume fraction of air (void-fraction) in bubble plumes generated by breaking waves. The void-fraction gauge described here is found to be most useful in the initial period after breaking when large void-fractions prevail. We describe the instrumentation at length and report on its use in the laboratory and in the field. The instrument is found to be capable of rendering the space-time evolution of the void-fraction field from controlled laboratory breaking waves. Field results show measurements of void-fractions (up to 24%) which are several orders of magnitude greater than time averaged values previously reported [1], [2]. Preliminary measurements show that the fraction of breaking waves per wave, based on the detection of breaking by the void-fraction gauge, is dependent on significant wave height and wind speed. The dependence on wind speed is compared with data of previous investigators [3]. Underwater video photography from the field clearly shows the formation and evolution of distinct bubble plumes and the presence of large bubbles (at least 6-mm radius) generated by breaking.