Hydrodynamic and water quality modeling for gate operation: A case study for the Seonakdong River basin in Korea

The Seonakdong River is an impound stream whose flowrate is controlled by the Daejeo and Noksan gates. Therefore, constant natural flow of the Seonakdong River does not occur. In contrast, pollutant loadings occur constantly occurring as water flows into the river from the watershed. The Daejeo gate is located at the upstream head and the Noksan gate is at the downstream end of the Seonakdong River. The Seonakdong River is an estuarine tributary of the Nakdong River. It is a reservoir-like river used for agricultural irrigation, with the Noksan gate at the river mouth serving to prevent the intrusion of saline water. In this study, a steadystate model, QUAL2E, and an unsteady-state model, CE-QUAL-RIV1, were chosen for comparative simulations. The results of the simulations of river water quality changes were verified with respect to short-term variations of water quality due to the increasing water flow from the Daejeo gate and the discharging flow from the Noksan gate. Field measurements were performed to monitor the flowrate and water quality during the operation of the Daejeo and Noksan gates in the Seonakdong River. The general trend observed is that the water quality worsens after the opening of the gates. The reduction in water quality ranges from 3% to 38%. These results show that the gate operators should carefully select the most appropriate operating plan to obtain better water quality in the Seonakdong River. The numerical simulation shows that a discharge of 300 CMS, which is a peak inflow from the Daejeo water gate to the river, can make it possible to achieve the target water quality of the Seonakdong River if the Daejeo water gate can remain open in connection with the Noksan water gate operation.