A Two-Dimensional Wave Flume Investigation into the Effect of Multiple Vertical Steps on the Form of Breaking Waves

Abstract

A two-dimensional wave flume investigation was conducted to examine the effect of multiple steps on the form of breaking waves. The study was initiated to gain a better appreciation of the design tolerances for artificial surfing reefs. The geotextile bags used to form an artificial surfing reef create multiple steps in the seabed; reducing the size of these steps improves the quality of waves for recreational surfing but comes at considerable economic cost. Sets of steps were placed on a 1 : 15 slope, and image analysis methods were used to quantify the effects of increasing step size on the form and intensity of the breaking waves. The vortex area, angle, and length-to-width ratio were quantified for various step sizes. It is shown that breaking intensity decreases with increasing step size. Variations in wave reflection were measured using a three-element surface-piercing wave gauge. Wave reflection coefficients were found to be unchanged for all step sizes implemented in this experiment, and reflection was not found to significantly affect the form of the breaking waves in the current tests. The quality of the surfing waves for recreational surfing was seen to degrade measurably with increasing step size, with a higher degree of variability and irregularity in the vortices of the plunging breakers. A qualitative assessment of the quality of the breaking waves suggests that step size should be limited to approximately 23% of the design wave height.