# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 3 Results
### General Pattern of Shoreline Recovery
The pattern of \(\langle y\rangle\) shows a high correlation with that of \(P_{w}\), especially when the erosions occurred from T1 to T3. To increase visibility, four red rectangles are marked to show the time periods when \(\langle y\rangle\) and \(P_{w}\) are significantly correlated. In these four  
Figure 6: Time variations of (**a**) the significant wave height, \(H_{m0}\), and the significant wave period, \(T_{s}\), (**b**) the wave power, \(P_{w}\), (**c**) the amplitude of the near-bed velocity, \(U_{m}\), and (**d**) the mean beach width, \(\langle y\rangle\), averaged over the 34 baselines. The red rectangles mark the 4 time periods when \(P_{w}\) exceeded 20 kWatt/m and \(\langle y\rangle\) was decreased, indicating the high correlation between \(P_{w}\) and \(\langle y\rangle\). The T1 in (**d**) denotes the time just before the beach was severely eroded by Typhoon TAPAH (T1917) in September 2019, T2 denotes the time when the recovery process started in October 2020, and T3 denotes the time when \(\langle y\rangle\) was recovered to the level of T1 in March 2021.  
periods, the beach width showed a significant decrease (or the increasing trend in \(\left\langle y\right\rangle\) was halted, as shown in T3), and the maximum magnitude of \(P_{w}\) was observed to exceed 20 kWatt/m, which was significantly higher than observed at other times. In addition to the wave power, the amplitude of the near-bed velocities, \(U_{m}\), was estimated using the formula (\(U_{m}=0.0116H_{m0}/T_{s}\)) by [PERSON] (1986) [28]. \(U_{m}\) could be an important factor for sediment motion because cross-shore sediment fluxes are calculated from bed shear stresses that are based on near-bed velocities [29]. As shown in Figure 6c, the time variation pattern of \(U_{m}\) was similar to that of \(P_{w}\); hence, \(U_{m}\) increasing with increasing \(P_{w}\) might trigger the sediment motions. However, the correlation between \(\left\langle y\right\rangle\) and \(U_{m}\) is less relevant compared to that between \(\left\langle y\right\rangle\) and \(P_{w}\), that is, \(U_{m}\) peaked on 3 September 2020, not on 23 September 2019, when \(\left\langle y\right\rangle\) declined considerably. Furthermore, \(U_{m}\) was an indirect parameter estimated from the empirical formula using the measured wave heights and periods from the wave sensor. Therefore, it may not be appropriate to apply \(U_{m}\) to directly measure the erosion/accretion processes in the beach face. In contrast, \(P_{w}\) was directly estimated from the wave measurements and thus can be suggested as a controlling indicator in this study. The correlation between the beach width and wave power can be more clearly investigated in Figure 7, in which the time variation of \(P_{w}\) is also compared with that of \(\left\langle y\right\rangle\) for the same observation period. To increase the visibility, however, the range of \(P_{w}\) in the \(y\)-axis is adjusted so that its upper limit is set to 30 kWatt/m (Figure 7a). In addition, four green rectangles are added in the figure to mark the times when the magnitude of \(\left\langle y\right\rangle\) increased (i.e., when the shoreline was advanced). Red/green rectangles are marked over the whole observational period of 33 months so that the erosion/accretion trend can be examined even during the time before Typhoon TAPAH (T1917). Compared to the pattern in the red rectangles, the times of shoreline accretion are also closely related to the wave power in the green rectangles. The maximum magnitude of \(P_{w}\) was generally higher than 20 kWatt/m in the periods of five red rectangles, whereas it was less than 10 kWatt/m in the periods of four green rectangles. In particular, at four out of five periods of the red rectangles, the maximum magnitude of \(P_{w}\) exceeded 30 kWatt/m and could reach up to \(\sim\)75 kWatt/m. During the longest time of the gradual shoreline accretion for \(\sim\)4 months, which started from T2, the maximum magnitude of \(P_{w}\) was generally lower than 10 kWatt/m.