Overview
The lower part of the Han River, which flows through Seoul, Korea, experienced excessive toxic cyanobacterial growth in 2015. To understand algal dynamics and develop better management alternatives, the Environmental Fluid Dynamics Code (EFDC) was applied to model algal bloom occurrence patterns in this reach of the river.
Model Setup
A 71 km long river section was represented with 1,175 horizontal 2-D grid elements. The grid system was found adequate, as the maximum Courant–Friedrichs–Lewy condition and orthogonality deviation were 0.5 and 20.1, respectively. Chlorophyll-a (Chl-a) was chosen as the primary indicator of bloom likelihood, and the model was calibrated and verified against three years of field data collected from 2013 to 2015. To improve Chl-a prediction accuracy, three dominant algal groups — diatoms, green algae, and cyanobacteria — were considered, with optimum growth temperature ranges selected from field data.
Key Findings
Calibration accuracy was verified for physical variables such as mean water level and temperature, as well as for other water quality variables across the study area. Different maximum growth rates were required for the upstream and downstream regions to match field observations, indicating that more than three algal groups are needed to improve Chl-a calibration accuracy. While region-specific growth rates can overcome this limitation in the current model, the results suggest EFDC could be improved to include additional phytoplankton groups.