The Effects of Weir Construction on the Hydrodynamics and Water Quality of South Han River in Korea

In Korea, The 4 Major Rivers Restoration Project has been conducted in recent years. The large-scale river engineering project dredged sediments and installed three multi-functional weirs in the South Han River, which is expected to change hydrodynamics and water quality of the river. The impoundment restricts water flow of the river and results in the settlement of particulate material on the river bed. The longer the hydraulic residence time taken on the upward weir, the greater the potential for incoming nutrients and sediments to settle on the river bed. The accumulated pollutants, caused by settling of particulate material, impart a consistent nutrient loading to the overlying water column. The movement of nutrients through the sediment-water interface has essential influence on the nutrient quality of overlying water. Therefore, It is important to investigate how the environmental change occurred by the project has impacts on the hydrodynamics and water quality of the river. To represent temporal and spatial variations in the hydrodynamics of South Han River with weir installation, a three-dimensional time variable model, Generalized, Longitudinal-Lateral-Vertical Hydrodynamic and Transport (GLLVHT) was selected. The GLLVHT model is imbedded within the Generalized Environmental Modeling System for Surface waters (GEMSS). The GEMSS model is designed in a modular fashion for easy coupling of existing as well as other user-defined water quality models. The computational grid of the three-dimensional model was developed using the GIS themes of shorelines and sounding depths. The horizontal grid is composed of 642 active cells at the surface layer with spacing varies from 83 to 380 m. There are 31 vertical layers with uniform thickness of 1 m resolution. The number of vertical layers in each horizontal column was determined from the depth of the river. Overall, study area is divided into 2061 grid elements. In addition, information related with three multi-functional weirs was used in this model in order to investigate the effects of weir installation and operation on hydrodynamics and water quality. To calibrate the model, model prediction for water level fluctuations and water temperature was compared with field collected data after the time of October 2011 up to present since the 4 Major Rivers Restoration project has been conducted from January 2009 to September 2011. The model results showed a good agreement with field measurements. From the model results, the followings are determined and discussed: (1) changes of hydrodynamics by weir construction (2) changes of water quality by extended retention time.