Experimental modeling of bed morphological changes and toe erosion of emerged breakwaters due to wave-structure interactions in a deltaic coast

Large-scale coastal erosion in the Mekong Delta has been dramatically increasing in severity in recent decades. Several effective hard engineering solutions have been implemented in this delta to efficiently prevent coastal erosion and stimulate sedimentation while supporting local ecosystem conservation. These measures include Pile-Rock Breakwaters (PRBW), Hollow Triangle Breakwaters (HTB) and Semicircular Breakwaters (SBW). However, research on sediment transport, morphological changes and toe erosion for these offshore breakwaters is very limited and is currently in the initial stages of understanding the specific conditions of sediment characteristics and foundations. The objective of this study was to reproduce the morphological changes and toe erosion of three breakwaters due to wave-structure interactions. This was investigated using 2D physical models with 3000 irregular waves during 8 experimental hours (equal to 15,000*Tp). To extract the bed morphological changes and toe erosion, specialized laser measurements (SW50M laser ruler) and analysis of high-speed video recording by image digitalization were applied. The experimental results show that the shape and structural design of offshore breakwaters can have a significant influence on the bed morphology on both the seaside and the leeside. We found that generally, the toe of the construction on the seaside was eroded due to the occurrence of reflected waves and that the flow is narrowed while passing through the construction, increasing the flow velocity and causing toe erosion. Additionally, the accretion of sediment at the leeside of the breakwaters was found to be mainly driven by the transport of sediment through the construction. Comparing the breakwater designs, the experimental results showed that the HTB has the maximum and fastest accretion rates behind the breakwater. The SBW has high wave energy dissipation efficiency, although the toe erosion rate is faster than the other classes of breakwaters. The PRBW shows the fastest toe erosion rate in front of the structure and causes accretion at the leeside of the construction but at a lower rate than the HTB. The findings from this study will help practical designers to reinforce the foot of construction when designing breakwaters and inform stability calculations. We recommend applying these three classes of breakwaters, especially the HTB and SBW, for stimulating sedimentation for mangrove restoration in the mud-coast delta