# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 4 Discussion  
From the shoreline erosion and accretion patterns shown in Figures 6 and 7, a consistent description can be provided for the recovery process, after the severe erosion by TAPAH (T1917) occurred in T1. The beach width decreased under high wave conditions and increased under low conditions, which corresponds to the general understanding of the shoreline evolution as previously described in Section 1. However, the criteria that determine the range of accretion and erosion are still unclear. The results in the present study can provide these criteria, although their application should be only confined to this study site. Once the severe erosion by TAPAH (T1917) occurred in T1, the recovery process mainly started in T2, about 1 year after the erosion, and continued until T3, about 5 months after T2. The factors that affected short-term (days) processes can be further examined by posing a question about the recovery process--why didn't the recovery process start  
Figure 10: Beach width variation along the baseline at three different times. Data was calculated by averaging the beach widths over one week, as their centered times were on 23 September 2019 (navy), 5 October 2020 (red), and 1 March 2021 (green). These three times correspond to T1, T2, and T3, as marked in Figures 6 and 7.  
earlier rather than starting \(\sim\)1 year, until T2, after the typhoon? The answer may be simple: there were earlier times in which the recovery actually occurred under mild wave conditions. However, it could not continue because another set of high waves attacked the site, restraining the shoreline accretion. Some of the high waves were powerful enough to reverse the accretion trend into erosion. Therefore, the recovery process could not continue further, while the accretion and erosion processes were repeated until T2. During the period of \(\sim\)5 months from T2 to T3, the wave power remained low so that the recovery process could continue without severe disturbances. The next question on the process is which wave condition determines the trend of erosion or accretion? Based on the results of this study, a rough estimation can be provided: the shoreline was advanced when the wave power was no greater than 10 kWatt/m. In contrast, the erosion likely occurred when the wave power became greater than 20 kWatt/m. In fact, most rapid erosions occurred when the wave power exceeded 30 kWatt/m. During the total observational period of 33 months, the maximum wave power reached up to 75 kWatt/m, whereas the average wave power during the period was \(\sim\)0.9 kWatt/m. When the wave power was too low in this site, the sediments did not move, and neither erosion nor accretion could occur. However, the criteria for this 'no sediment motion' condition could not be determined based on the results of this study. It should be noted that there are critical cases in which the criteria may not be applicable. For example, once the beach width reached the minimum value at T1, its magnitude did not fall below this value during the rest of the observation period. This indicates that the erosional status reached a critical limit with this minimum width, and thus further erosion was prevented unless more extreme events occurred to beak the equilibrium again. On the other hand, there was a period of \(\sim\)3 months from mid-February to mid-May 2019 (between the first green rectangle and the first red rectangle in Figure 7) when the wave power was generally no greater than 10 kWatt/m, but the beach width did not increase. This is likely because the beach width was thicker than the other period, and thus additional accretion of the shoreline might be disturbed, i.e., the beach width already reached saturation. The results from locality analysis on the erosion/accretion process showed that the southern part of the beach was more severely eroded, especially when high-powered waves afflicted the site, whereas the shoreline in the northern part of the beach was relatively well protected.