Laboratory Study on Cross Shore Sediment Budget over Submerged Breakwaters

Abstract

The objective of this study is to understand the characteristics of the cross-shore sediment transport over the submerged breakwater through quantitative observations of wave breaking, undertow, and the characteristics of near bottom velocity profiles such as skewness, asymmetry around the permeable or impermeable submerged breakwaters. A series of laboratory experiments were conducted in a 2D wave flume with uniformly sloping 1/20 movable bed of uniform sand grains (D50 =0.15 mm). Either permeable or impermeable breakwaters was installed and four regular incident wave conditions were selected so that generated waves cover both daily or stormy wave conditions. Boundary detection technique was used to obtain the temporal and spatial variation of bed profile and then to calculate cross shore sediment transport rate, CTR, from the images captured by multiple high speed and normal video cameras. Tracer particles were used in the experiment and PIV was applied to capture the velocity field around the submerged breakwater. The experimental results indicate that the impermeable breakwater is generally more effective than the permeable breakwater in terms of the reduction of the seaward CTR over the submerged breakwater but the amount of CTR was very sensitive to the breaking point especially in the case of impermeable breakwater. Increase in acceleration skewness caused low CTR in impermeable breakwater case even with a relatively high wave action on the lee of the breakwater.