# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 2 Materials and Methods  
### Study Sites  
Yeongildae Beach is in the southeast of the Korean Peninsula (Figure 1a), inside Yeongil Bay, which faces northeast with a mouth width of \(\sim\)10 km (Figure 1b). Due to its location in the west corner of the bay, the beach has been protected from severe erosion because the wave energy is generally attenuated when reaching the site. Yeongildae Beach is a \(\sim\)1.7 km long sandy beach where the sediment size ranges from about 0.15 to 0.35 mm. It is a pocketed beach facing ESE (East-Southeast), bounded by two ports--Pohang Port at the southern end and Duho Port at the northern end. Due to the breakwaters that were built for these ports, the sediments were expected to remain within the coastal cell without loss in the longshore direction. However, the beach has been classified as an erosive beach because input sources of sediment to the beach have been lost due to the construction of the ports. In addition, after the breakwaters of Duho Port were constructed, a shadow zone could have been formed in the lee area of the breakwaters, reducing the wave energy by wave diffraction [27]. The shadow zone has caused redistribution of sediments along the beach shore so that the shoreline width has become narrower near the Yeongil Bridge area but thicker at both ends of the beach near the two ports (Figure 1b). The aerial photograph and satellite image in Figure 2 compare the shorelines measured in 1977 (before the construction of the Duho Port breakwaters) and in 2019 (after construction). It shows that the shoreline severely retreated in the middle of the beach, whereas it accredited at both ends in 2019, compared to 1977, indicating the redistribution of sediment to reach an equilibrium by the changed wave energy field after the construction of Duho Port. The wave data were measured at two locations, M1 and M2, as shown in Figure 3a. At M1, a pressure transducer was moored at the bottom, at an 8.5 m depth, \(\sim\)1 km away from the shore of the Yeongildae Beach, measuring the wave height (\(H_{s}\)) and period (\(T_{p}\)). Figure 1: (a,b) Location of Yeongildae Beach, the Republic of Korea, (c) map of Yeongildae Beach with the location of the video monitoring system (S1 and S2). The pictures are captured from Google Earth. Figure 2: Comparison of shoreline positions of Yeongildae Beach: (a) aerial photograph taken in 1977 and (b) satellite image measured in 2019. Because the pressure transducer could not provide information on the wave propagation (\(D_{p}\)), it was measured at M2, where an Acoustic Wave and Current Profiler (AWAC) was moored at the bottom, at a 21.6 m depth, located in nearly the center of Yeongil Bay, -7 km away from Yeongildae Beach. The tidal range in the study area was no greater than 0.3 m, and the tide-induced current was no greater than 0.2 m/s. Therefore, the effect by the tide on the beach processes may not be significant, and thus was not considered in this study. Figure 3b shows a wave rose diagram that represents the wave conditions at M2. Inside Yeongil Bay, the waves mainly propagate from the NE direction, which is similar to the wave propagation outside the bay. Wave conditions in M2 are moderate because the significant wave heights (\(H_{s}\)) are generally less than 2.5 m, except for several extreme storm cases. The dominant wave directions in M2 are NE and NNE, which indicates that the wave propagates inside the Yeongil Bay generally in the normal direction to the mouth of the bay. Due to the geographical characteristic of Yeongildae Beach being located at the northwest end, inside Yeongil Bay (Figure 1b), the impact of storm waves has not been significant compared to other beaches located outside the bay. In September 2019, however, Typhoon TAPAH (T1917) attacked the study site. Although TAPAH (T1917) was a Category 1 typhoon (max. wind speed 35 m/s), it caused serious damage to many beaches located on the southeast coast of Korea due to the proximity of its path (Figure 4).