Assessing the impacts of submerged breakwaters on coastal erosion at Songdo Beach, South Korea, using hydrodynamic observations and remote sensing

Coastal landforms are continuously reshaped by natural forcings such as typhoons, waves, tides, and sea-level rise, as well as by human interventions including coastal protection structures. Rapid morphological changes can lead to shoreline erosion, retreat, and infrastructure damage, highlighting the need to quantitatively assess both the effectiveness and unintended consequences of submerged breakwaters.

This study investigates short- and long-term morphological responses at Songdo Beach (Busan, South Korea), where a semi-enclosed nearshore zone has been formed by an east–west oriented submerged breakwater system. We integrated Real Time Kinematic (RTK) drone-based surveys with in situ hydrodynamic observations. High-resolution aerial surveys were conducted on six occasions, before and after the landfall of Typhoon Khanun (7 and 10–12 August 2023) and approximately two years later (20 August and 29 September 2025), enabling assessment of event-scale changes and subsequent recovery. In addition, an Acoustic Wave and Current Profiler (AWAC) was deployed inside the breakwater system from November 2023 to August 2024 (~10 months) to continuously measure wave height, wave period, current velocity, and current direction.

The observations indicate that mean current velocities inside the breakwater system were higher than those offshore, likely due to flow acceleration through breakwater gaps and around breakwater heads. After the typhoon, sediment loss was pronounced near the lateral beach sections close to the breakwater ends, whereas the central section in the lee of the breakwater showed net deposition. This spatial heterogeneity suggests that, while the submerged breakwater attenuates wave energy, it also redistributes nearshore currents, enhancing localized erosion–deposition patterns.

By integrating hydrodynamic measurements with high-resolution remote sensing, this study provides a quantitative assessment of how submerged breakwaters influence coastal dynamics and morphological evolution. The results emphasize that coastal protection design should consider not only erosion mitigation but also the risk of secondary erosion and long-term instability. Under increasing extreme wave events and expanding coastal development, these findings support more sustainable and adaptive coastal management strategies.