# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 3 Results  
### General Pattern of Shoreline Recovery  
In Figure 6, time variations of the four parameters measured in the site are compared for about 33 months, since the initiation of the VMS measurement in November 2018. The significant wave height, \(H_{m0}\), and the significant wave period, \(T_{s}\), were measured at M1 (Figure 3). The time variations of \(H_{m0}\) and \(T_{s}\) in Figure 6a show complex patterns, as they highly fluctuate, which may cause difficulties in understanding the impact of these wave parameters on the shoreline change. Therefore, another parameter, wave power (\(P_{w}=\frac{\rho\sigma^{2}}{64\pi^{2}}H_{m0}^{2}T_{s}\)) was calculated (Figure 6b). \(P_{w}\) is an index that directly measures the wave energy flux and can be used as a parameter that combines the effects of the wave height and period. The time variation of \(P_{w}\) is more clearly distinct from those of \(H_{m0}\) and \(T_{s}\). Its magnitude is usually less than 10 kWatt/m, with an average value over the  
Figure 5: **(a,b)** Snapshots of VMS images captured from one of the cameras on S1 and S2, **(c)** orthogonal image in which the pieces of image data captured by the cameras in the VMS system were combined, **(d)** the baseline system to estimate the shoreline positions and beach width. The inner line is set along the landward end of the backshore of the beach, and the baselines are set to start from the inner line and extend seaward perpendicular to the shoreline. A total of 34 baselines are set in Yeongildae Beach, as marked with red solid lines. The shoreline positions are determined by comparing the color of image pixels along each baseline, and the beach width can be calculated from the area of the polygon (marked with blue lines in the figure), which is composed by connecting the crossing points of the baselines to the inner line and shoreline positions horizontally. 33 months of \(\sim\)0.9 kWatt/m. However, there were times when \(P_{w}\) sharply increased, exceeding 20 kWatt/m, with its maximum value reaching \(\sim\)75 kWatt/m. The mean beach width, \(\langle y\rangle\), was estimated by integrating the beach width, \(y\), along each baseline, from baseline #1 to #34. Here, the y magnitude of each baseline was obtained by subtracting the average value during the first 2 years from the original VMS beach width (i.e., y is the average removed beach width). As shown in Figure 6d, the beach was significantly eroded when TAPAH (T1917) attacked the site in September 2019. During the period of \(\sim\)2 days, \(\langle y\rangle\) was reduced by \(\sim\)12 m. In this study, the time of 23 September 2019 was set as T1, as it was the initial time for the erosion to occur due to the typhon. Since the severe erosion, the beach width hardly recovered, as \(\langle y\rangle\) could not reach the original value in T1 (marked with the dashed magenta line in Figure 6d) until 1 March 2021, and this time was set as T3 as an indication of the time of shoreline recovery, although \(\langle y\rangle\) touched the original value only for a short time at T3 and decreased again sharply after that. The time variation of \(\langle y\rangle\) from T1 and T3 shows an interesting pattern. Once the severe erosion occurred in T1, \(\langle y\rangle\) tended to increase slowly (i.e., with much lower speed than the erosion in T1). However, there were times when \(\langle y\rangle\) decreased back to the level when the most severe erosion occurred in T1. This accretion and erosion pattern was repeated until 5 October 2020 (this time was set as T2). After T2, \(\langle y\rangle\) continued to increase gradually (without severe erosion) until T3, when \(\langle y\rangle\) recovered to the level before the erosion in T1. Once it reached the maximum level in T3, \(\langle y\rangle\) decreased again, and the gradually increasing process stopped. After this, \(\langle y\rangle\) remained level, without showing a clear pattern of erosion/accretion until July 2021, the time of the most recent data.