Overview
During the Four Major River Restoration Project, three multi-purpose weirs were built downstream of the Daecheong Reservoir in Korea’s Geum River, transforming large areas of flowing ecosystem into deep, wide, stagnant environments. Every summer, thermal stratification occurs near the Baekje Weir — which has a mean depth of 4.0 m — and surface algal blooms dominated by buoyant cyanobacteria have frequently formed since the project.
Model Setup
The objective was to investigate the relationship between flow velocity and the thermal stability of the water body using a three-dimensional hydrodynamic model (EFDC+), calibrated against thermistor chain data obtained in 2014. The new Sigma-Zed vertical grid system of EFDC+, which minimizes pressure gradient errors, was used to better simulate the thermodynamics of the water body.
Key Findings
The model reasonably simulated the vertical profiles of observed water temperatures. The vertical mean flow velocity and the Richardson Number, representing the stability of the water body, were estimated for various management water level and flow rate scenarios. The results indicated that the thermal stability of the water body is mostly high enough to establish stratification and largely depends on flow velocity. The critical flow velocity that can avoid a persistent thermal stratification was found to be approximately 0.1 m/s.