Structural and hydraulic response of emerged low-crested cube-armoured breakwaters

Abstract

In this study, the structural and hydraulic behavior of statically stable emerged low-crested cube-armoured breakwaters were investigated by performing physical model tests. This research considered cube armour layers with irregular placement in a double layer and cube armour layers with a regular placement in a single layer. For the single layer placement, two different packing densities were tested (ψ=0.59 and 0.67) because optimizing the packing density is relevant for the ease of construction, lowering the environmental impact and for the costs of cube armour layers. Low-crested breakwaters with both regular and irregular placements were considered with three different crest widths. The armour stability, wave transmission and wave reflection of emerged type low-crested breakwaters using concrete blocks were investigated experimentally under irregular wave conditions. The results show that for cube-armoured low-crested breakwaters with irregular placement, the cube dimensions can be significantly reduced compared to conventional rubble mound breakwaters with higher crests. The structural stability of low-crested breakwaters with single-layer regular placement showed different responses compared to the irregular two-layer case due to the geometric discontinuity at the intersection of the front slope and crest. Although the regularly placed single armour layer is more stable than the irregularly placed double armour layer with the same packing density (ψ=0.59), damage to the crest is more critical, causing instability of the structure for the single armour layer. For the single-layer regular placement with the smaller packing density (ψ=0.59), damage occurred on the front slope and crest, while for the higher packing density (ψ=0.67) relatively limited damage was observed on the crest only. For the hydraulic behavior of low-crested breakwaters with cube amour layers, the incident wave conditions, the freeboard (R_c) and crest width (B) are the most important parameters. It is evident that for low-crested breakwaters, the wave transmission increases but the reflection decreases compared to conventional breakwaters with a high crest. Based upon a re-evaluation of expressions given in literature on wave transmission and wave reflection at rubble mound low-crested breakwaters, new formulas for cube-armoured structures have been obtained. Hence, this study provides further information and guidance on applications for engineers and researchers on the structural and hydraulic response of low-crested cube-armoured breakwaters because increasing water levels, wave heights and storm frequencies due to global climate change are driving the need for more resistant coastal structures, adaptation measures and improvements in the design of low-crested breakwaters.