Effect of Abrupt Topographical Characteristic Change on Water Quality in a River

Abstract

In the past, water quality management was regulated on the basis of pollutant concentrations in rivers. However, the recent implementation of total maximum daily load (TMDL) in the Nakdong River, Korea, has achieved preservation of water quality with balanced watershed development based on a scientific approach. However, eight large weirs have been installed as part of the extensive river regulation works, in which the river geometry has also changed dramatically due to dredging. Consequently, the river environment has been changed, including the hydraulic characteristics, such as the residence time, longitudinal and transverse section, water depth, energy gradient, and flow rate. The water quality characteristics experienced the greatest impact from changes in the hydraulic characteristics of the river. Due to considerable difficulty in the actual implementation of the water quality management plan with respect to the changes, it was considered important to predict the water quality change likely to result from hydraulic characteristic changes in the river. In this study, the hydraulic coefficients reflecting the hydraulic characteristics were calculated and implemented in the water quality model according to the changes in the river environment. The estimated pollutant loads in subwatersheds were applied in the water quality model according to the TMDL of the Nakdong River. The hydraulic coefficients, indicating the hydraulic characteristics before and after the changes to the river environment, were applied in the water quality model with complete calibration and verification. The model simulation results were compared for analysis of the effect of an abrupt river environment change. The simulation results show that the river flow rate was reduced after the change to the river environment due to the effect of the hydraulic structures. As a result of the water stagnation, the BOD5 and Chl-a concentrations increased by 9.4% and 12.2%, respectively, whereas the T-N and T-P decreased by 6.1% and 5.0%, respectively. The impact of a drastic topography change on water quality and its improvement can be quantitatively evaluated by water quality modeling. In particular, BOD has proven to be the most effective for improving water quality when a river is in its natural state rather than when the pollution discharge loads are reduced or when the water quality, discharged from the water-treatment facility, is improved. This study analyzes the relationship between the improvement in water quality and the recovery of running water in each section of the Nakdong River. The current study also proposes a scientific alternative for river rehabilitation, which can improve the quality of stagnant water sections and enable a river to recover to its natural state.