With the purposes of reinforcement of capacity to cope with flood and stable supply of water, the government attempted to install 8 multi-functional reservoirs at basins of Nakdong River and elevate the capabilities to control flood and drain running water through the dredging by pursuing the ''Four Major Rivers Restoration Project''. In addition, it also tried to intensify the water-friendly function by repairing the entire river zone including some floodplains. Therefore, dramatic changes of river environments occur during short period, which may lead to modifications of hydraulic and hydrologic features and related water qualities in rivers.
This study aims to suggest a measure for enhancing the water quality through hydrodynamic and water quality simulation by using multi-level model as a method to measure the water quality. The hydraulic behavior of the river environments changed by the Four Major River Project should be understood and a hydrodynamic and water quality model which can be used to represent features of rivers, lakes and marshes has to be selected. Moreover, in order to represent the modifications of hydrodynamic and water quality of water body through multi-functional reservoirs, a multi-level model for the most accurate simulation of temporal and spacial changes and actual water body behaviors is required. This study analyzed effects of water quality of influent in Geumho River, the 1st level tributary stream of Nakdong River, located within the block studied on the mainstream and changes of water quality in the mainstream according to control of incoming pollution loading rates. The EFDC was used as the model to be applied to the simulation, and the models of geographical, water quality, hydrodynamic and weather data used as input one were established based on the results obtained through actual measurements.
The validities of values of hydrodynamic and water quality simulated through the EFDC model were judged by comparing data of actual measurements in water level and quality station operated in the block studied. Since the data of water quality used in the simulation of it could not be directly applied to the EFDC model, it was divided through the fraction of sub-items of water quality in order to make it to be adjusted for the EFDC model, by using other data of water quality measured every 8 day in the Ministry of Environment. The water quality was simulated for 11 months from November in 2013 to October in 2014, and the reproducibilities of water temperature, DO, T-N, T-P and Chl-a were measured. The simulations of water temperature, DO, T-N and T-P was found to be similar to measurements, but the Chl-a showed somewhat lower reproducibility. The reason of this results may be determined that this study examined the narrow block and the concentration of Chl-a in the downstream tends to be different from the time series data provided by the water quality station, and the model cannot catch up with exterior environments. In addition, an error of model is determined to be included because the item used in a tide simulation is assumed as one tide. If such exterior factors would be considered to measure the tide item in the future, the Chl-a model is expected to deduce more accurate values.
By applying this study scenario, DO upward/downward, T-N upward/downward, T-Pup ward/downward conditions are used to examine effects of inflow of pollutants in the Geumho River on the mainstream. It is found that the mainstream was largely affected by T-N and T-P, not by DO scenario condition. In addition, some pollutants flowed into rivers can be divided into point and non-point pollutants, and the former, the T-P flowed from the Geumho River is closely related with the growth of Chl-a, suggesting that if the load of T-P in the Geumho River can be largely decreased, then it will effectively contribute to improvement of water quality in the mainstream of Nakdong River.