# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 3 Results
### Locality in the Erosion/Accretion Process  
The time variation in Figures 6 and 7 shows the general pattern of the erosion and recovery processes of the averaged beach width of the 34 baselines. As described in Section 2.1 with Figures 1c and 2, however, the beach widths show high locality--i.e., a discrepancy in y between different baselines. Due to the construction of Duho Port, a shadow zone was formed in the area near the baselines #29-#34 (Figure 5), where the shoreline was advanced. In contrast, the shoreline at the area near baselines #21-#27 severely retreated even before Typhoon TAPAH (T1917) attacked the site. Similarly, the beach widths in the southern end (near baselines #11-#10) were thicker compared to those in the middle of the beach (near baselines #11-#20). This high locality in the beach width between different areas of the beach might affect the erosion and recovery processes, which is analyzed in this section. Figure 8 shows the time variation of parameters that are related to the locality in terms of wave power. In Figure 8a, the colored contours represent the beach width (\(y\)) variations of the 34 baselines, as the numbers in the \(y\)-axis denote the baseline numbers marked in Figure 5. Here, it is noted that the colors in the contours show the variation in \(y\) along each baseline but cannot be used to compare the magnitude between the baselines because each \(y\) is the average removed beach width using the first 2-year average. It is clear that the \(y\) values rapidly dropped after TAPAH (T1917). Compared to the northern area (# of baseline >25), however, the \(y\) values decreased more severely in the southern end (# of baseline <6). The locality was also observed after the typhoon, as \(y\) values were even reduced in the southern end near baselines < #5 until the recovery process prevailed, starting in November 2020. In contrast, \(y\) values slightly increased in the northern end (# of baseline >28) by that time. This pattern continued during the recovery process after November  
Figure 7: Comparison of time variations between \(P_{w}\) and \(\langle y\rangle\) as shown in Figure 6. (**a**) the wave power, \(P_{w}\), the range of the \(y\)-axis for \(P_{w}\) is adjusted from 0 to 30 kVatt/m to increase the visibility of the correlation between the two parameters. (**b**) the mean beach width, \(\langle y\rangle\), averaged over the 34 baselines, and (**c**) the mean beach widths of the southern end, the middle, and the northern end of the beach, \(\langle y_{s}\rangle\), \(\langle y_{m}\rangle\), and \(\langle y_{n}\rangle\). T1, T2, and T3 are the same time steps marked in Figure 6. The red rectangles mark the period when the maximum magnitude of \(P_{w}\) became higher than 20 kWatt and \(\langle y\rangle\) decreased. The green rectangles mark the periods when the maximum magnitude of \(P_{w}\) was no greater than 10 kWatt/m and \(\langle y\rangle\) generally increased. 2020, because the \(y\) values in the northern end became even greater than those before the typhoon since January 2021, whereas they were smaller than those before the typhoon in the southern end. This pattern of locality can be more clearly quantified through \(\left\langle y\right\rangle_{s}\), \(\left\langle y\right\rangle_{m}\), \(\left\langle y\right\rangle_{m}\), \(\left\langle y\right\rangle_{m}\), and \(\left\langle y\right\rangle\) in Figure 8c, in which the time variations of the beach widths calculated for three different groups are compared to the mean beach width. In the southern end, the \(\left\langle y\right\rangle_{s}\) rapidly decreased \(\sim\)30 m after TAPAH (T1917) and reached a minimum on 3 September 2020. The \(\left\langle y\right\rangle_{s}\) magnitude could not be recovered even during the recovery process, since November 2020, as the maximum \(\left\langle y\right\rangle_{s}\) observed in 2021 was still -10 m smaller than that before the typhoon.