# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 3 Results
### Locality in the Erosion/Accretion Process
In the northern end, \(\left\langle y\right\rangle_{m}\) was not significantly reduced by the typhoon and gradually increased after that time as it became greater than the value before the typhoon during the recovery process, since November 2020. In the middle of the beach, \(\left\langle y\right\rangle_{m}\) shows the middle course between the two previous cases. It is also noted that the pattern of \(\left\langle y\right\rangle_{m}\) is similar to that of \(\left\langle y\right\rangle\), indicating the time variation in the middle of the beach can represent the pattern for the whole beach. In Figure 8c, the beach width change pattern is compared at two additional time periods, marked with circles. The grey circle covers the period from October 2019 to January 2020, when the beach settled down after TAPAH (T1917). The purple circle covers the recovery period from October 2020 to February 2021. These two periods were chosen because the beach width change pattern shows a discrepancy between the southern and northern ends. During the period of the grey circle, the magnitude of \(\left\langle y\right\rangle_{s}\) decreased,  
Figure 8: Time variation of (a) beach widths of all 34 baselines, where the \(y\)-axis represents the baseline numbers, (b) wave power of Figure 7a, (c) the beach width, \(\left\langle y\right\rangle_{s}\), averaged for baselines #2–#5, representing the southern end area (blue), \(\left\langle y\right\rangle_{m}\), averaged for baselines #15–#17, representing the middle part of the beach (orange), \(\left\langle y\right\rangle_{n}\), averaged for baselines #30–#33, representing the northern end area (yellow), and \(\left\langle y\right\rangle\), averaged for the all 34 baselines (black), (d) \(L_{t}=\int(\partial y/\partial t)dx\), the parameter that indicates the locality of erosion/accretion pattern. The colors of the dots in (d) represent the range of \(L_{t}\) magnitude, with reds for positive and blues for negative values. The red and green rectangles in the figure are marked the same as those in Figure 7. (c) The grey and purple circles mark the periods when the time variation pattern of the beach width shows locality between the two ends of the beach. whereas that of \(\left\langle y\right\rangle_{n}\) increased, showing that the shoreline retreated in the southern end but advanced in the northern end. These results may be interpreted as an indication of a long sediment movement from the southern area toward the northern area. In contrast, both \(\left\langle y\right\rangle_{s}\) and \(\left\langle y\right\rangle_{n}\) increased during the recovery process, as marked with the purple circle, which indicates that the shoreline was advanced in both areas during the period. For a better understanding of the locality in the shoreline variation pattern, a parameter was developed using the formula \(L_{t}=\int(\partial y/\partial t)dx\), as plotted in Figure 8d. The dots in the figure show the magnitude of \(L_{t}\) at each time step, and their colors also indicate the range of \(L_{t}\) magnitude, with reds for positive and blues for negative values. In the formula of \(L_{t}\), x is the alongshore direction toward the northern end. Therefore, the negative value of \(L_{t}\) indicates that \(\partial y/\partial t\) magnitude decreased in the positive x-direction. In other words, the positive/negative values of \(L_{t}\) indicate the beach width change rate \((\partial y/\partial t)\) increased/decreased toward the northern end of the beach. For example, the minimum \(L_{t}\) value (\(\sim\!-\!4\)) during the time of TAPAH (T1917) indicates that the erosion rate was greater in the southern area and decreased in the increasing x-direction so that beach might be most severely eroded in the southern end. Figure 9a shows the distribution of \(L_{t}\) in terms of wave power. Although its probability of occurrence is low (Figure 9b), \(L_{t}\) usually became negative with increasing magnitude when \(P_{w}\) was greater than 10 kWatt/m.