Wave Height and Wave Velocity Measurements in the Vicinity of the Break Point in Laboratory Plunging Waves

Abstract

Results are presented of laboratory experiments undertaken to study the dynamics of wave propagation and transformation within the surf zone. The study involved measuring the external flow characteristics of regular plunging waves propagating along a 20 m long flume fitted with a 1:20 plane slope. To achieve this, monochromatic waves of frequency 0.4 Hz and a deep water wave height of 12 cm were generated by a servo-controlled piston-type wave maker. A set of calibrated parallel-wire capacitive wave gauges were employed to measure statistics of the free surface elevation along the slope in order to get an insight into the wave breaking behavior. To characterize the wave field, free surface elevation measurements were made in the vicinity of the break point. The measured time series data were analyzed at each flume position to obtain statistics of the mean water level, wave height, and wave velocity along the flume. Results show that as the wave propagates from deep water towards shallow water, there is an increase in the wave height, reaching a maximum height of about 21.5 cm at the break point, and then decreases sharply thereafter. Wave phase velocity calculations at different flume positions were made from the measured time series. Cross correlation techniques were used to determine the phase difference between the reference wave near the generator and the wave at various points along the flume. The local wave velocity was obtained by taking the phase difference between two points spaced 0.2 m apart. A comparison was made between the measured wave phase velocity, its linear shallow water ((gh)) approximation and the roller model concept wave velocity (1.3(gh)), at various points along the flume. The measured wave velocity c was found to lie in the range (gh)<c<1.3(gh) for most of the positions except near the break point. After the break point, the measured wave velocity is up to 38% higher than the theoretical value predicted using the roller model concept. Also noted is the variability of the phase speed in the breaking region. The present experiments of quantifying the mean macroscopic properties of breaking waves are a necessary prerequisite for more detailed experiments involving internal fluid velocity measurements that will follow.