# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 5 Conclusions  
In this study, we employed a data set of VMS and wave parameters measured over 33 months to investigate the erosional and depositional processes in Yeongildae Beach, located inside Yeongil Bay on the southeastern coast of the Korean Peninsula. The beach was severely eroded when Typhoon TAPAH (T1917) hit the site in late September 2019, during which the beach width retreated \(\sim\)12 m on average; it took about 1.5 years for the beach width to be recovered to the level before the attack of the typhoon. The study then analyzed the recovery process, during which shoreline erosion and accretion repeated, corresponding to the wave conditions. The study site is a pocketed beach, as the breakwaters of two ports were built at both ends of the beach, blocking the input or loss of sediments in the longshore direction crossing the lateral boundary of the beach. The beach is also located at the west corner, inside the bay, and the distance from the beach to the mouths of the bay is 8-15 km. Due to its location on the beach, the wave energy was lower and the sediment size was finer, compared to those observed on the open coast outside the bay, because the waves were attenuated when reaching the site. The results of the analysis corresponded to the general understanding of beach processes--that beach widths decreased/increased under high/low wave energy conditions. In this study site, however, the pattern of shoreline evolution was found to be highly correlated with \(P_{w}\), the wave power that combined the impacts of wave height and period, rather than the wave propagating direction, which was observed to be similar for high-powered waves. In particular, the beach width generally increased when \(P_{w}\) was no greater than 10 kWatt/m and tended to decrease under the condition of \(P_{w}\) greater than 20 kWatt/m. However, most erosional events occurred when \(P_{w}\) exceeded 30 kWatt/m. Especially after TAPAH (T1917), it took a long time (\(\sim\)1.5 years) for the beach width to be fully recovered to the previous level because high-powered waves occasionally disturbed the recovery trend. Therefore, the full recovery of the beach width could be reached over the last five months in the recovery period, during which \(P_{w}\) was kept continuously lower than 10 kWatt/m. The locality of beach recovery process was also analyzed by examining the discrepancy in the erosion/accretion pattern between different locations within the beach. It was observed that the erosion by TAPAH (T1917) was most severe in the southern part of the beach, whereas the change in the beach width was minimal in the northern part during the same period. This pattern of locality was similarly observed under other storm conditions. Considering the wave propagation direction was similar (\(\sim\)NE) for the storm waves, it is likely that a shadow zone was formed in the northern end due to the breakwaters of Duho Port, such that the sediments in the lee area of the breakwaters were protected because the wave energy was attenuated. On the contrary, the breakwater of Pohang Port located in the southern end did not form a shallow zone, and thus the storm waves directly attacked the southern part, causing erosion. In the phase of recovery under milder wave conditions, the shadow zone did not play an important role, and the shoreline was advanced almost equally at all parts of the beach. However, it was also observed that there was a period just after TAPAH (T1917) when erosion occurred in the southern part while accretion occurred in the northern part, indicating a possible alongshore sediment movement from the south to the north. The results from this study can be usefully applied to design the protection and prevention plans of the beach from storm damage, considering the general and local erosion and recovery patterns. For example, the criteria that determined the trends of erosion and accretion based on wave power can be used to effectively estimate the size of coastal structures, such as detached breakwaters for shore protection.