# Video Monitoring of Shoreline Positions in Hujeong Beach, Korea
## 4 Results  
In Figure 6, \(y_{t}\) at two times--beginning and end times of T1--are compared. The red and blue colors filled in-between the two lines denote the advance and retreat, respectively, of the shoreline during T1. In the figure, a rose diagram that shows the distribution of the propagating wave heights, \(H_{w}\), and directions, \(D_{w}\), is also drawn to support the understanding of the shoreline evolution pattern. \(H_{w}\) and \(D_{w}\) were estimated from the wave spectra based on the wave data measured by KIOST using Nortek Acoustic Waves and Currents (AWAC) at the location of \(\sim\)19 m depth and \(\sim\)1.0 km away from the coast. For the three-month period of T1 when \(A_{b}\) significantly fluctuated by repeatedly losing and gaining the total beach area, \(\Delta y_{t}\) shows an inconsistent pattern along the coastline because the shore significantly advanced at L3 and L4 while it decreased at L1 and L2. In this period, \(\sim\)50% of the waves approached the shore in NNE direction (slightly inclined to the north) and \(\sim\)30% of the waves approached the shore in the normal direction (\(D_{w}\) = NE). Regarding the propagating wave heights, \(\sim\)20% of them were high waves with \(H_{w}\) > 2 m. Therefore, the shoreline accretions at L3 and L4 were caused when shadow zones were formed by the obliquely approaching waves in the lee area of the underwater rocks developed in R1 and the rocky island in R2. These inclined waves could produce longshore sediment transport to the southeast direction along the beach. In the shadow zones behind the rocks, however, the sediment transport rates were reduced to cumulate sediments, specifically in L3 and L4, which can be easily understood from a simple consideration of the continuity equation for the one-line theory [16] as  
\[h_{p}\frac{dy}{dt}+\frac{dQ}{dx}=0 \tag{1}\]  
Figure 5: **(a)** Comparison of shoreline positions, \(y_{t}\), measured on 9 December 2016 (start date of data collection), 23 December 2017 (when \(A_{b}\) became maximum at T3), and 9 September 2018 (when \(A_{b}\) was minimum at T4). L1, L2, L3, and L4 denote the locations where the shoreline positions, \(y_{t}\), significantly changed during the period. **(b)** Satellite image on 20 April 2018 (T4), captured from Google Earth. The white rectangles, R1 and R2, denote the locations where underwater rocks develop near L3 and L4, respectively. The figure also shows the development of nearshore sandbars, indicated with the white lines. where \(h_{p}\) is the active profile height, \(y\) is the shoreline position, and \(Q\) is the longshore sediment transport. According to Equation (1), therefore, the shoreline would advance (\(\Delta y>0\)) if the sediment transport rate decreases (\(\Delta Q<0\)), as observed in L3 and L4. At locations near L1 and L2, \(y_{t}\) slightly advanced to the sea. This shoreline change may be related to sandbar development as shown in Figure 5b, which will be discussed later in this section. Figure 7 compares the shoreline positions at the beginning and end time of T2 with a rose diagram of the wave conditions. It shows that the shoreline generally retreated for a \(\sim\)4.5-month period by losing \(\sim\)7% of the beach area, which can be estimated from Figure 4. The loss of sediments during this period is unexpected considering the relatively lower wave energy compared to T1 because \(\sim\)95% of the incoming waves had \(H_{w}\) lower than 2 m. As for \(D_{w}\), the directional spectrum is wider compared to T1 but the majority of the waves approached the shore in the normal direction, which reduced the amount of sediments that moved in the longshore during the period. Instead, more sediments were probably transported in the cross-shore direction. For example, the shoreline at the end time of T2 became flatter compared to that at the beginning time, when the shoreline was more severely undulated.