Why do offshore breakwaters trap low levels of fine-grained sediments? Insights from Vietnam

Abstract Offshore structures were originally designed to protect sandy beaches, but they are increasingly being built to protect mangrove-dominated muddy coasts from further erosion. So far, these structures have had limited success in trapping sediments. It is unclear whether these offshore breakwaters were installed in the most suitable locations for sediment accumulation. We selected hollow triangular offshore breakwaters in Tan Thanh, Tien Giang, Vietnam (herein called the Tan Thanh HTBs), Tien Giang in Vietnam as a case study to partially address this question. Using the MIKE 21/3 Coupled Model FM, we developed sedimentation scenarios related to the positions of the Tan Thanh HTBs taking into account detailed field observations made during site visits. Our findings showed that the Tan Thanh HTBs were not installed adequately due to their positions in areas with low total gross depositional accumulation (150–200 kg/m 2 in the dry season and 200–250 kg/m 2 in the wet season), low suspended sediment concentrations (0.1–0.2 kg/m 3 in the dry season and 0.3–0.4 kg/m 3 in the wet season), and in the wrong sediment transport direction. This partly explains the low amount of sediment accumulation in the areas in front and behind the Tan Thanh HTBs. The findings of the study will serve as a technical reference for future lessons and replication.