## 3 Results and Discussion
### Bathymetric Changes in Channel Sections  
The sandbars in the study area are not connected to the mainland at either end, forming embayments. Therefore, channels are formed between each sandbar, and erosion and deposition trends in these channels were analyzed by dividing them into four sections from Area A to D, as shown in Figure 5. Each survey was conducted along three to five survey lines, spaced 100 m apart, in the north-south (vertical) and east-west (horizontal) directions, as shown in Table 4.  
Figure 4: Overall bathymetric data (depth) for each survey time obtained in the survey area (October 2018–October 2022). Area A is characterized by a narrow channel where the waterway narrows owing to the nearby bridge, river, and port outflow. Both outflow from the inner part of the coastal shoal and inflow from the outside of the southern part of the shoal occur as the water flows into the estuary. Therefore, there is a higher possibility of changes in the underwater topography compared with that in the other areas. Similar to Area A, Area B also experiences heavy ship traffic, whereas Areas C and D are channels through which the main stream of the Nakdong River is discharged. Area C is the most strongly affected by discharge from the Nakdong River, whereas Area D can also be considered a major tributary, despite its narrow waterway, owing to discharge from the Nakdong River estuary dike. As shown in Figure 6, erosion and deposition appear to occur repeatedly in the survey area, with erosion observed in April after a period of low rainfall and deposition observed in October after a period of heavy rainfall. In Area B, there is a tendency for the southern pit to move westward and the northern pit to move eastward. This area, which is the main waterway of the West Nakdong River, is subject to repeated erosion and sedimentation owing to various factors, such as discharge from the Nakdong River estuary dike and the intrusion of waves from the open sea. In some areas where the flow rate decreases, sedimentation occurs owing to the transport of suspended sediment from inland by the West Nakdong River. In other areas with relatively fast flow rates, erosion occurs. This tendency is particularly intensified by external forces, such as heavy rainfall and typhoon waves, increasing the vulnerability of terrain changes by external force. The analysis of the survey lines in Areas C and D revealed that in Area C, there is a tendency for erosion to occur to the east of the survey line. This tendency can be attributed to the influence of the discharge from the Nakdong River, which causes the main channel to move in the north and east directions toward the Nakdong River estuary. The seasonal phenomenon of erosion in the first half of the year and sedimentation in the second half of the year, which has been observed in the past, continued in the study period. In addition, owing to the increase in discharge from the Nakdong River during the typhoon season in 2020, the eastern part of the main channel in the Nakdong River estuary showed a tendency toward \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Survey Area**} & \multicolumn{2}{c}{**Survey Line**} \\ \cline{2-3}  & **Vertical** & **Horizontal** \\ \hline A & AV01 (West)–AV04 (East) & AH01 (South)–AV04 (North) \\ \hline B & BV01 (West)–BV05 (East) & BH01 (South)–BV05 (North) \\ \hline C & CV01 (West)–CV04 (East) & CH01 (South)–CV04 (North) \\ \hline D & DV01 (West)–DV04 (East) & DH01 (South)–DV04 (North) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Survey lines in each study area. Figure 5: Survey areas for channel analysis. Areas A–D are defined in Table 4.  
erosion. The southern part of the main channel, which is the inlet for the discharge from the Nakdong River, did not show a substantial change in shape, but erosion occurred overall, resulting in an increase in the width of the channel.