Analysis of Beach Changes and Sediment Transport Due to Construction of a Detached Breakwater and a Submerged Breakwater at Yeomjeon Beach

To investigate sediment-transport mechanisms associated with the construction of a detached breakwater and a submerged breakwater at Yeomjeon Beach on the central coast of Gangwon Province, Korea, beach changes were periodically monitored from February 2021 to December 2022, encompassing the construction and completion of the structures. Surface sediment sampling and shoreline surveys were conducted, and the results were analyzed with wave data from a buoy at nearby Gyeongpodae Beach. The study area is influenced by high northeasterly (NE) waves and seasonally reversing NW–SE residual currents, both generally strongest in winter. The shoreline advanced by up to 94.0 m in the lee of the structures but retreated by up to 49.4 m elsewhere. Surface sediments were sandy at most stations, and Grain Size Trend Analysis (GSTA) showed that transport directions varied among zones during construction but became spatially reorganized in response to the structural configuration after completion. Although changes in water depth and mean grain size at some stations appeared to be affected by anthropogenic disturbances, sediments generally became coarser in erosional zones and finer in depositional zones. Shoreline-change magnitude (RMS) was primarily associated with high winter waves and relatively long-period waves, and the cross-shore component of wave energy flux had a stronger influence on shoreline change than the longshore component. After completion, the mean shoreline advanced, but continued erosion along unprotected sections resulted in a sediment imbalance across the beach. These findings suggest that spatial variations in sediment transport and redistribution should be considered in the planning and design of coastal structures.