# Video Monitoring of Shoreline Positions in Hujeong Beach, Korea
## 4 Results
During T2, therefore, wave forces acted uniformly along the coast regardless of the existence of underwater rocks under normal direction approaching wave conditions. The rose diagram in Figure 7 shows that extreme wave conditions with \(H_{w}>4\) m were observed (\(\sim\)5% of total waves). These high waves probably contributed to the erosive process. It should be noted, however, that the beach area gradually decreased during T2 instead of having a specific time of rapid reduction in \(A_{b}\), and thus, the impact of extreme waves on the shoreline change is still not clearly identified based on the wave data. Figure 6: Comparison of \(y_{t}\) between two different time steps measured at the beginning of T1 (9 December 2016) and at the end of T1 (18 March 2017) along the coastline. The red and blue colors filled in-between the two lines denote, respectively, the advance and retreat of the shoreline during T1. The numbers denote the magnitude of the gained and lost area in the corresponding colors (m\({}^{2}\)). The rose diagram shows the \(H_{w}\) and \(D_{w}\) distributions measured during the period. Figure 7: Same comparison as that in Figure 6 but measured at the beginning (8 June 2017) and end (29 October 2017) of T2. Figure 8 shows the shoreline positions at the beginning and end time of T3 (29 October to 23 December 2017) with the corresponding rose diagram of wave conditions. It is interesting to find out that \(A_{b}\) increased \(\sim\)15% during the three-month period. Specifically, the shoreline significantly advanced at the four locations from L1 to L4. The wave conditions in T3 were similar to those in T1 as most of the incoming waves propagated obliquely from the north of the beach (\(D_{w}\sim\)NNE). Therefore, shadow zones were formed to cause the shoreline accretion at L3 and L4 where underwater rocks developed, as was observed in the case of T1. One of the differences in the shoreline evolution pattern between T1 and T3 is that the shoreline was significantly advanced at L1 and L2 in T3, which was not observed in T1. For example, \(\Delta y_{t}\) became maximum at L2 and it was also dominant at L1 in T3, while \(\Delta y_{t}\) showed no clear changes at these locations in T1. As already mentioned, the significant shoreline changes at L1 and L2 were related to the development of nearshore sandbars. Figure 9 shows the seabed elevation changes in Hujeong Beach between 18 November 2016 (before T1) and 12 March 2017 (after T1) [17]. The bottom topography was measured using echosounders and a mobile LiDAR. The changes in seabed elevation show that nearshore sandbars actively developed parallel to the shoreline, in the area with water depth shallower than 5 m. The development of sandbars can be confirmed with the Google image shown in Figure 5b because crescenti sandbars are observed near the locations of L1 and L2 on 20 April 2018 (T4). The figure also shows that the shoreline is advanced to the offshore at the locations where the horns of the sandbars developed near L1 and L2, although the underwater rocks do not develop in these areas. It should be noted that these locations of the shoreline accretion correspond to the locations where the horns of the sandbars developed, which shows a good example of out-of-phase coupling between the sandbars and shoreline [18; 19]. Therefore, the dominant shoreline accretions at L1 and L2 indicate that nearshore crescenti sandbars might be actively developed at T3, and the shoreline was advanced at the locations corresponding to the sandbar horns. Compared to T1, the wave energy in T3 was lower as the waves with \(H_{w}>2\) m were \(\sim\)7% of total incoming waves in this period, which might provide favorable conditions to form crescenti sandbars as a result of positive feedback between the nearshore flow circulation, sediment transport, and the morphology itself [18]. Figure 8: Same comparison as that in Figure 6 but measured at the beginning (29 October 2017) and end (23 December 2017) of T3.