# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 1 Introduction
Short-term prediction models such as XBeach [15] calculate the rapid changes in the seabed and dunes during storms and the post-storm recovery process in the time scale of months [9]. Due to the high computational cost, however, the application of process models is still restricted for year-scale predictions. For such long-term predictions, line models [16] based on the one-line theory [17] or statistical models such as Monte Carlo have been applied [18]. Another tool that is useful for the long-term observation of the beach process is remote sensing. The satellite imagery can be regularly collected over a long period, and it has been used to derive shorelines and identify their spatial patterns [19; 20]. However, it is sensitive to cloud cover and has a limited temporal resolution. Similarly, aerial photos are useful to detect geographical changes over decades, although they cannot provide detailed information on detected variation [21]. On the other hand, airborne and in situ LiDAR also provides detailed information on the beach geography, for comparison and validation of other measurements [22; 23]. Despite their advantages, the application of aerial photos and LiDAR data is restricted in measuring the beach recovery process continuously due to the high cost. For this reason, Video Monitoring Systems (VMSs) installed at beaches have been successfully applied to observe the recovery process in specific sites [8; 9]. The VMS is not only useful for long-term monitoring [24] but also for short-term observation of the recovery process after storms [5] and for estimation of the wave parameters [25] as well as wave-induced currents [26]. In this study, we also investigated a recovery process after an attack of storm waves, when a severe erosion occurred, by analyzing the shoreline variations in a pocketed beach located inside a bay in the southeast coast of Korea. Our study focused on the conditions that determined the erosional or depositional process at the beach using data sets from a VMS and wave measurements. As previously described, it is a natural process that beaches undergo erosion by attacks of storm waves, and then they are slowly recovered under milder conditions. However, the criteria that decide the shoreline erosion and accretion are still unclear. Although such information obtained at a beach is site-specific and thus cannot be generally applied for other beaches, the analytical methods may still be applied to beaches with different conditions. In addition, if a criterion is provided for a specific beach, then plans can be designed to protect the beach from severe damage that can lead to a loss of its value. Another focus was examination of the locality in the recovery pattern between different locations along the beach. Previous studies using the VMS generally focused on the time scale of the recovery process [8]. However, it is also important to analyze the discrepancy in the recovery process between different locations, if it exists, because such disequilibrium may lead to the concentration of damage at specific locations if subsequent storms attack the site before fully recovered. The results of the present study are, therefore, useful not only to understand the criteria for erosion/accretion processes at the study site, but also to analyze the causes of disequilibrium during the recovery process. These outcomes are then applied to suggest measures for conservation of the beach, considering the characteristic erosion and recovery pattern, generally or locally. The paper is organized as follows. The information of the study site and the damage as a result of Typhoon TAPAH (T1917) are described in Section 2.1. The VMS used in this study and its data are introduced in Section 2.2. The general trend of the beach process is described in Section 3.1, and the locality (discrepancies in the process between areas of the beach) is analyzed in Section 3.2. The discussion on the results is provided in Section 4, and the conclusion of the study in Section 5.