# Video Monitoring of Shoreline Positions in Hujeong Beach, Korea
## 3 Experiment  
The image data collected by the video monitoring system have been processed to calculate the area of Hujeong Beach, \(A_{b}\), since December 2016 by positioning the coastline from the averaged images. In Figure 4, the time variation of the calculated beach area is plotted until October 2018. During the period of approximately two years, \(A_{b}\) generally decreased, although there were periods when \(A_{b}\) significantly increased from October 2017 to February 2018. From this time series of \(A_{b}\), we selected four terms (T1, T2, T3, and T4) that showed a specific pattern of beach area variation. T1 was chosen during the period from 9 December 2016 to 18 March 2017, when \(A_{b}\) significantly fluctuated with an increase in the resulting area at the end of the term. In T2 (8 June 2017 to 29 October 2017), \(A_{b}\) steadily decreased, losing a total beach area of \(\sim\)10%. T3 was selected because \(A_{b}\) increased rapidly during a relatively short period from 29 October 2017 to 23 December 2017. On the contrary, \(A_{b}\) significantly decreased during T4 from 13 March 2018 to 15 August 2018. The start and end times of each period are listed in Table 1, and the measurements of \(A_{b}\) at the corresponding time steps are listed in Table 2.  
Figure 3: Comparison of shoreline positions estimated with the video monitoring system (red solid line) with land topography (green and yellow contours) measured by a ship-mounted mobile LiDAR on 12 March 2017. The seaward boundary of the colored contours determines the shoreline positions. In Figure 5, the shoreline positions in Hujeong Beach, \(y_{t}\), are compared at three different times: (1) 9 December 2016, when the video monitoring data started to be measured; (2) 23 December 2017, when \(A_{b}\) became maximum at the end time of T3; and (3) 9 September 2018, when \(A_{b}\) was minimum at the end time of T4. It is interesting to find out that there are locations (L1, L2, L3, and L4) where \(\Delta y_{t}\) was greater than the other areas. The reason for this non-homogeneous \(y_{t}\) evolution along the shoreline is probably related to the nearshore sandbars and rocky seabeds that are observed at 3-5 m water depth. For example, Figure 5b shows a satellite image around the four locations from L1 to L4 on 20 April 2018 (T4), captured from Google Earth. Google Earth images are constructed based on the data from Landsat satellites. Landsat images were employed to show the shallow water seabed topography when the images are reflected through the water column under clear weather conditions [14,15]. The Google image in Figure 5 shows that crescenctic sandbars actively develop in the nearshore area of the beach, specifically near L1 and L2. Besides the sandbars, underwater rocks are abundantly observed around L3 (black dots inside rectangle R1). In addition, small rocky islands and underwater rocks around the islands are found to be located near L4 within rectangle R2. These sandbars and rocky seabeds, along with the incoming waves, affected the shoreline change pattern during the periods, which will be examined in the next section. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Dates** & **T1** & **T2** & **T3** & **T4** \\ \hline Start & 9 December 2016 & 8 June 2017 & 29 October 2017 & 13 March 2018 \\ \hline End & 18 March 2017 & 29 October 2017 & 23 December 2017 & 15 August 2018 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Start and end dates of the four selected periods. Figure 4: Time variation of beach area, \(A_{b}\), from 1 December 2016 to 7 October 2018. T1, T2, T3, and T4, marked by the blue vertical lines, denote the time periods selected for the analysis during which \(A_{b}\) shows a specific variation pattern. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Beach Area (m\({}^{2}\))** & **T1** & **T2** & **T3** & **T4** \\ \hline Start & 99,557 & 107,674 & 99,680 & 110,841 \\ \hline End & 108,247 & 99,680 & 113,904 & 88,914 \\ \hline \hline \end{tabular}
\end{table}