## 3 Results and Discussion
### Analysis of Discharge in the Nakdong River Estuary Dike  
The hydraulic factor that primarily influences the bathymetry in the estuary is the inflow of river water. The underwater topography is affected by not only the supply of sediments but also various physical environmental factors that cause erosion and deposition. The estuary, which hosts the confluence of freshwater and saltwater, can also be influenced by density gradients caused by the difference in density between freshwater and saltwater, affecting the transport of suspended particles, as well as, the bathymetry changes owing to various meteorological factors such as tides, waves, and other weather conditions. The study area experiences repeated erosion and sedimentation events owing to various factors, such as discharge from the Nakdong River estuary dike and inundation of waves from the open sea. There appears to be no unilateral tendency in the area. In particular, Area C, which is directly affected by the discharge from the Nakdong River estuary dike, is influenced by two canals (the Eulsuk Island and Myeongji shipping routes) that drain into the open sea. The seabed topography has been found to fluctuate repeatedly between erosion and deposition. Figure 11 shows the data on the discharge of the Nakdong River estuary dike, outflow of suspended sediment, and precipitation in the Busan area. Figure 10: Distribution changes in the height (H) for Shorelines 1 and 2. (**a**) Shoreline 1: Jinwoo Island; (**b**) Shoreline 2: Shinja Island. Severe distortion is possible if there is an abnormality in the shoreline data of traditionally widely used methods, such as average or standard deviation for shoreline changes. The precipitation data obtained from the Korea Meteorological Administration revealed that both discharge and suspended sediment discharge change similarly when precipitation increases or decreases. This result may be considered to be obvious given that increased precipitation leads to increased inflow to the Nakdong River, and the increased water flow can naturally explain the increase in the sediment discharge. The results in  
Figure 11: Time series of semi-annual data for the duration of the study (K-WATER, KMA). (a) Average daily discharge; (b) total precipitation; and (c) average daily suspended sediment outflow. Figure 11 show a general trend that as the discharge increases, the suspended sediment discharge also increases. However, compared with that in the second halves of 2018, 2019, and 2021, the discharge in the second half of 2020 increased by approximately three-fold, but the suspended sediment discharge decreased or remained almost the same. The seasonal effect showed a clear trend, with a decrease in the first half and an increase in the second half of the year for both discharge and suspended sediment discharge, reflecting the amount of precipitation. However, it was found that the suspended sediment discharge did not increase in proportion to the discharge. In the latter half of 2020, the average discharge of the Nakdong River estuary dike increased by approximately 2.5-fold compared with that in the same period in previous years owing to heavy rainfall that affected the Korean Peninsula during the summer of 2020. The increase in discharge is estimated to have affected the topography of the estuary. Until the first half of 2020, according to the main outflow route of the discharge from the estuary dike, the Doyo sandbar area was eroded, whereas the Shinja Island area, which was less affected by the discharge, showed a tendency to accumulate sediment. However, analysis in the latter half of 2020 showed that a considerable amount of the Doyo sandbar area had been eroded, and it is necessary to confirm whether the bathymetry will return to its previous trend of erosion in the first half of the year and sedimentation in the second half. Factors such as waves, tides, and typhoons can considerably affect topographical changes in the estuarine region.