Numerical Analysis of Sediment Transport Rates from Rip Currents at an Open Inlet between Low Crested Breakwaters (LCB): The Role of Infra-Gravity Waves

Low Crested Breakwaters (LCB), the most preferred structural type in the coastal zone management project by the Korean Ministry of Ocean and Fisheries, has been designed to mitigate beach erosion by high waves occurring only a few times a year. Now that moderate sea conditions are dramatically different from those in rough seas, these poor design practices could interrupt the grand circulation process of natural beaches over the course of a year. As a result, the beach stabilizing effect of LCB often falls short of our expectations. In this study, in order to test this hypothesis, 3-D numerical simulation was carried out to analyze the rip currents and its associated sediment transport at the open inlet between LCB when LCB is subject to infra-gravity waves, which play an indispensable role in the beach restoring process in a mild sea with an annual prevalence rate of over 80%. Numerical results show that in the case of LCB with lower crest freeboard, rip currents gets increased by 2.6 times when compared to the one before the deployment of LCB since the water mass influx toward the down-wave side of LCB by overflowing LCB by the preceding waves is redirected toward the open inlet. The sediment transport rate was estimated using Bailard's model, the most referred cross-shore sediment model in the literature. It was shown that the primary sediment transport mode at the open inlet between LCB was bed load, and sand of 5.62 ×10-5m3 / m was leaving the inner zone of LCB per unit wave period.