Evaluation of longitudinal / transverse dispersion coefficients and prediction of concentration at river confluence in two-dimensional solute transport analysis

Mixing characteristics of the longitudinal/transverse directions is inevitably different in rivers with a large aspect ratio. Particularly complex mixing behavior occurs in the area of the confluence where tributaries and main streams of different concentrations meet, and it is necessary to accurately implement such mixing characteristics by assigning appropriate values of longitudinal and transverse dispersion coefficients. In this study, the mixing behavior according to the different values in the longitudinal/transverse dispersion coefficient was analyzed by using the two-dimensional model (RAMS) at the confluence where three rivers (the Nakdong River, the Geumho River, and the Jincheon Creek) meet. Firstly the longitudinal and transverse dispersion coefficients were calibrated and validated based on the electrical conductivity (EC) acquired from field measurements. Through the calibration and validation, it was shown that the longitudinal dispersion coefficient was about 25 times larger than the transverse dispersion coefficient in this area. Then assuming that a hazardous substance (phenol) was introduced into the upper boundaries of the Geumho River and the Jincheon Creek due to an accidental spill, the concentration of phenol arrived at the water intake facilities was calculated by using the calibrated numerical model. As a result, characteristics such as time and peak concentration of hazardous substances reaching the water intake facilities were very different according to the ratio of the longitudinal/transverse dispersion coefficient values. In fact, this is an example that the selection of the dispersion coefficients can affect decision-making such as stopping water intake during an appropriate time at the facilities, when if phenol is introduced into a river. In the end, when using a two-dimensional mixing model in a river, it was confirmed that the provision of an appropriate value of the longitudinal/transverse dispersion coefficient was an important factor.