Numerical analysis of bed elevation and bank line changes at the confluence of Nakdong and Geumho Rivers in Korea

Abstract

The river confluence forms complex flow pattern because of different inflow discharges from the main stream and tributary. Complex flow feature at the confluence affects geomorphological changes. Especially rapid and continuous flow changes produce unstable condition in the channel geometry. Therefore, it is important to analyze geomorphological changes such as bank erosion and deposition as well as bed changes at the confluence channel for river maintenance and management, especially before and after the construction of river structures like weirs and bridges. Progressive erosions including headcutting and bank erosion are expected in the confluence in Nakdong River and Geumho River, Korea because a large weir (Gangjeong-Goryeong Weir) has been constructed in the upstream channel during Four Major Rivers Restoration Project. Therefore, flow changes, bed elevation changes, and bank-line changes have been simulated in this study using the 2-dimensional numerical model of CCHE2D (Center for Computational Hydroscience and Engineering 2-Dimension) to analyze variations after the weir construction. The largest flood event before and after the weir construction is selected for hydraulic condition in the model. The measured data for flow and sediment discharge at the Dongchon Station in the Geumho River and Waegwan Station in the Nakdong River have been used for boundary conditions. Numerical simulations of this study reproduce similar variation to the actual condition regarding geomorphological changes in the river confluence. The study results should provide fundamental information to establish many countermeasures for channel stability against excessive erosion and deposition of bed and bank at the confluence section.