It is very important to understand the mechanism of estuary topographic change for setting up estuary management and treatment methods. In this study, the effects from the land side such as rainfall, river discharge, sediment discharge and the sea side such as tide, tidal current, wave, bottom material(surface sediments) related to the topographic changes of the Nakdong river estuary were investigated and analyzed. Based on the analyzed data, hydrodynamic modeling, topographic modeling, and wave induced current modeling were performed to understand the variation patterns from past to present.
As the results of the topographic changes on the Nakdong river estuary, the deltas of the group of alluvial island such as Sinhodo, Myeonghodo, and Eulsukdo formed before 1916 year, were formed by sediment discharge at a location where a slow current velocity occurred in the middle of the river. Jinudo, Daemadeung, and Maenggeummeorideung in front of these are called the first sandbar group. Jangjado and Baekhapdeung the second sandbar group. Sinjado and Doyodeung the third sandbar group. Especially, the construction of river estuary barrage has affected the sediment environment caused the change of sediment supply and transport in the river estuary. In other words, It is considered that the construction of the river estuary barrage has not only slowed the deposition of sediments by slowing down the sediment supply but also accelerated the topographic changes due to the hydraulic effects such as wave and tide.
The average annual discharge through the river estuary barrage for the last 13years is 19,274.56×106㎥. According to the discharge, the annual average suspended load is 3,153,995㎥/yr, and the bed load 30,410㎥/yr, which is 1.0% of the suspended load.
The sandy contents of surface sediments has been increasing since the construction of river estuary barrage. These results indicate that river sediments were discharged to the offshore on the flood time and the muddy sediments of offshore were moved and sorted by tidal current on ordinary time before construction of the estuary barrage. In addition, it is considered that the sandy sediments moved in front of the sandbar group, the muddy sediments moved to the offshore by tidal current.
The mixing branch point before the construction of the river estuary barrage was the front of the Daemadeung, but it was found that the effects of fresh water appears up to the Sinjado and Baekhapdeung. The seawater circulation is governed by the behavior of the river discharge rather than by tidal current. It was confirmed that the flow moved repeatedly upward and downward along the main waterway without being completely mixed, because the freshwater and seawater were stratified.
From the analysis of the wave observations of the wave buoys in Geojedo from 2008 to 2017, the ENE series of winter has the highest rate of occurrence by 2008 and 2009, the SW series of summer has the highest rate from 2010 to 2017. This result seems to be related to recent seawater temperature rise. It suggests that the trend of hydraulic impact on the estuaries due to the wave will increase according to recent research results.
As the results of analysis of environmental factors correlation related to topographic changes in the river estuary, the river discharge and sediment discharge are directly proportional to the change of sandbar area. The results of surface sediment analysis shows that the high content of sand is distributed at the region far away from the estuary mouth. It seems that high resistance to the external force due to the instantaneous discharge through river estuary barrage and increase of incident wave height due to the gradual increase of the mean sea level and extreme highest tide level. As per the tidal current observations, the currents is also controlled by the river discharge and wave of the external environmental factors. The ebb time is more prevalent than the flood time, and the overall change of current is shown by the discharge through the river estuary barrage. These results are considered to be dominant in the topographic change of the rear side of the sandbar. As the result of the analysis of wave for the last 10 years, the ENE series in winter has the highest occurrence rate in 2008 and 2009, but the SW series in summer has the highest occurrence rate from 2010 to 2017. Therefore, it is necessary to reconsider the summer S series wave.
The change of currents after construction of river estuary barrage was analyzed for the pre-post construction of Busan new port. Simulation results show that the current velocity decreases widely nearby the sea side of the Sinjado, Jangjado, Baekhapdeung, and Doyodeung and increases nearby the waterway Sinjado, Janjado, and Dadeadeung.
As per the results of the topographic change, the river estuary showed a general deposition dominant phenomenon. These results are influenced by the decrease of the current velocity in ordinary time without the influence of the wave, sediment discharge due to summer river rainfall, the appearance of the Doyodeung, and the wave from offshore in summer and winter rough time. As the result, it is expected that new sandbar will appear on the offshore, and deposition will be strengthened in the tideland near Jinudo and Sinjado, and in Dadaepo and its north side coast.
It is also inferred that there will be development in the east-west direction of Jinudo and Sinjado, and littoral movement to the waterway and Dadeapo beach. It is considered that the topographic development of the south side and the east-west side of on the barrier island will be continued.
Generally, the contribution analysis shows that i) the increase in current velocity at the narrow waterway was greater than the decrease at the tideland; ii) river discharge was greater at the main waterway than at both the tideland and the front waters of the barrier islands; and iii) the wave impact was the largest in the winter with higher contribution to the erosion. Compared with deposition dominance, which is the result of the topographic change prediction, the impact of each external force condition gives larger erosion, but the combined impact of each external force condition shows deposition dominant. Therefore, the topographic changes of the Nakdong river estuary are considered to be the result of various complex external factors. Since the impact of each external force condition shows a different contribution to each area, above results should be considered when establishing the estuary management plan, but it must be understood as the result of complex interaction.