Development of a Real-Time Flood Forecasting System by Hydraulic Flood Routing

The objective of this study is to develop a predictive model for a flood forecasting system in the downstream of the Nakdong river basin. A flood forecasting and warning system in the Nakdong river basin has been implemented and operated on the basis of the hydrologic flood routing method since 1987. However, the backwater effects caused by tides or reservoirs has revealed serious shortcomings in the hydrologic flood routing method in the prediction, of water level changes. This is why it is imperative to develop a hydraulic flood forecasting model which accounts for those backwater effects. In the Nakdong river, ranging from the gauging station at Jindong to the Nakdong estuary barrage, the hydraulic flood routing model (DWOPER) based on the Saint Venant equation was calibrated by comparing the calculated river stage with the observed river stages on the basis of four different flood events in the past. The upstream boundary condition was specified by the measured river stage data at Jindong station and the downstream boundary condition was given according to the tide level data observed downstream from the the Nakdong estuary barrage. The lateral inflow from tributaries were estimated by the rainfall-runoff model. In the calibration process, the optimum roughness coefficients for proper functions of channel reach and discharge were determined by minimizing the sum of the differences between the observed stage and the computed stage. In addition, the forecasting lead time on the basis of each gauging station was determined via numeric simulation technique. The calibration results suggested that the unsteady hydraulic flood routing model simulated with the optimum roughness coefficient showed better performance than the one simulated with the existing non-optimum roughness coefficient.