# Video Monitoring of Shoreline Positions in Hujeong Beach, Korea
## 5 Discussion  
The shoreline data provided by the video monitoring shows that \(\sim\)10% of \(A_{b}\) was lost during the experimental period. The loss of the total beach area indicates that the beach was, in general, under an erosional process, probably due to the blocking of sediments by the breakwater near Bugu River (Figure 1). It should be noted, however, that construction of the breakwater was completed in 1989. Considering that the shoreline data was obtained 27 years after the breakwater construction, the two-year time variation of \(A_{b}\) in Figure 4 is not sufficient to confirm that the beach is seriously erosive, and additional data may be required to support the process. As shown in Figure 9, the seabed topography was occasionally measured using echosounders in Hujeong Beach between July 2016 and July 2018. By comparing the two bathymetry data, the changes in seabed elevation could be calculated to obtain the volume changes in the seabed. The result shows  
Figure 11: **(a) Satellite image captured from Google Earth near L1 and L2 measured on 3 March 2018. (b) Same location but measured on 20 April 2018. The yellow lines denote the orientation of the horns of the sandbars. **  
that a total of \(\sim\)99,000 m\({}^{3}\) of sand were lost in the nearshore area of the beach during the period. This significant loss of sediment from the nearshore area in Hujeong Beach was continuously observed from the bathymetry data measured at other times, which supports that the beach was in an erosional process probably due to the blocking of sediments by the breakwater. In addition, Figure 12 shows an aerial photograph taken on 28 October 1980 when the intake breakwater near the Bugu River had not been constructed yet. It clearly shows that the waves obliquely approached the shore from the left side of the beach, which probably transported the sediments to the right. However, the sediments in the beach were likely in equilibrium as they were provided through the river. The equilibrium could have been broken after the breakwater was built. The shoreline positions obtained from Figure 12 implies that the beach width might have been reduced by \(\sim\)15 m on average since 1980. Although the beach was losing the sediments in general, the change in shoreline did not occur uniformly along the coast. Instead, there were locations where the shoreline significantly protruded to make it undulated. This process was closely related to the bottom topography formed by the nearshore sandbars and the underwater rocks that developed at nearshore areas along the beach. Figure 13 shows a satellite image from Google Earth near L1-L3 on 13 February 2019 (after T4). Although the time when the image was taken did not match with the time of T3, it still is a good example that implies the shoreline accretion process during moderate wave conditions. It can be observed in the figure that five crescentie sandbars developed along the shore around L1, L2, and L3. Interestingly, the waves were breaking at the same locations of the sandbar horns. It should also be noted that the shoreline was significantly advanced to the sea at the corresponding locations of the horns, which confirms the out-of-phase coupling of the sandbars with the shoreline. When the waves broke at horn locations due to shoaling, the wave energy in the lee area was reduced to cause a sediment transport gradient in the longshore direction. According to Equation (1), therefore, the shoreline would advance to the sea at these locations. A similar process might be expected in the area near L3 where underwater rocks actively develop. In the rocky area where the water depth was relatively shallower, the waves were easily breaking to cause a longshore sediment transport gradient and the consequent shoreline changes. This is confirmed in Figure 13, which shows that the waves were breaking, and the shoreline protruded at L3 where underwater rocks actively developed. Figure 12: Aerial photograph over the Hujeong Beach area, taken on 28 October 1980 before the breakwater was constructed.