Following the installation of 16 weirs in South Korea’s major rivers through the Four Rivers Project (2010–2012), the water residence time increased significantly. Accordingly, cyanobacterial blooms have occurred frequently, raising concerns regarding water use and the aquatic ecosystem health. This study analyzed the environmental factors associated with cyanobacterial dominance at four weirs on the Nakdong River through field measurements, and parametric and non-parametric data mining methods. The environmental factors related to cyanobacterial dominance were the seven-day cumulative rainfall (APRCP7), seven-day averaged flow (Q7day), water temperature (Temp), stratification strength (ΔT), electrical conductivity (EC), dissolved oxygen (DO), pH, and NO₃–N, NH₃–N, total nitrogen (TN), total phosphorous (TP), PO₄–P, chlorophyll–a, Fe, total organic carbon (TOC), and SiO₂ content, along with biological and chemical oxygen demands. The results indicate that site-specific environmental factors contributed to the cyanobacterial dominance for each weir. In general, the physical characteristics of EC, APRCP7, Q7day, Temp, and ΔT were the most important factors influencing cyanobacterial dominance. The EC was strongly associated with cyanobacterial dominance at the weirs because high EC indicated persistent low flow conditions. A minor correlation was obtained between nutrients and cyanobacterial dominance in all but one of the weirs. The results provide valuable information regarding the effective countermeasures against cyanobacterial overgrowth in rivers.

The impact of inflow mixing on reservoir stratification is significant for reservoirs situated in a monsoon climate area. It cause difficulty in the calibration of a two-dimensional hydrodynamic and water quality model, CE-QUAL-W2 that was recently adopted for a real-time turbidity monitoring and modelling system (RTMMS) for a reservoir in Korea. This paper presents a systematic calibration and verification processe of the model for the reservoir. A sensitivity analysis showed that wind sheltering, Chezy, and sediment heat exchange coefficients are most sensitive to stratification structure. Inflow temperature was very sensitive during a year of normal precipitation, but it is not significant during a year of drought. Residual analysis revealed that the model has shortcomings in the simulation of water temperature near the metalimnetic zone without calibration. After calibration, however, the absolute mean errors between observed and simulated values were placed within 0.116–1.190 °C. Its performance was maintained under heavy flood events during the verification stage, which implies that the model is ready to use for the simulation of turbidity plume in the RTMMS under various hydrologic conditions. The suggested model calibration strategy and relevant results may be adopted for other reservoirs located in a monsoon climate area.

The effectiveness of a proposed curtain weir to be installed in the transitional zone of a eutrophic reservoir located in monsoon areas on the control of algal blooms in the lacustrine zone where drinking water withdrawals occur was assessed with various hydrodynamic flow regimes. A two-dimensional hydrodynamic and eutrophication model that can accommodate vertical displacement of the weir following the water surface changes was developed and validated using field data obtained from two distinctive hydrological years; drought (2001) and wet (2004). The model adequately reproduced the temporal and spatial variations of temperature, nutrients and phytoplankton concentrations in the reservoir. The efficacy of the curtain weir method found to be diverse for different hydrological conditions and dependent on the inflow densimetric Froude number (Fr(i)). Algal blooming was considerably mitigated by curtailing the transport of nutrients and algae from riverine zone to lacustrine epilimnion zone during the drought year as long as Fr(i) < 1.0. However, some flood events with Fr(i) > 1.0 transported nutrients and algae built upstream of the weir into the downstream euphotic zone by strong entrainments in 2004. Numerical experiments revealed that the efficiency of the weir on the control of algal blooming becomes marginal if the Fr(i) > 3.0.

Inflow mixing affects the spatiotemporal heterogeneity of water quality in reservoirs. Reservoir water quality management requires accurate prediction of density flow regimes to understand the spatiotemporal distribution of dissolved and particulate nutrients and organics. This study aims to characterize the mixing and circulation of three rivers with different physicochemical properties in a run-of-the-river (ROR) reservoir, using high-frequency monitoring and three-dimensional (3D) hydrodynamic modeling. The Aquatic Ecosystem Model (AEM3D) was constructed for the reservoir and calibrated with high-frequency data obtained from May–June 2016, accurately reproducing the observed spatiotemporal variations of flow velocity, water temperature, and electrical conductivity (EC) in the reservoir. High-frequency data and 3D model results showed that mixing of the rivers in the ROR reservoir is governed by density flow regimes formed by influent water temperature differences. At the confluence, colder and warmer river influents formed underflows and surface buoyant overflows, respectively. The spatial arrangement of flow direction, water residence time, and EC concentration were largely controlled by the buoyancy-driven flow. Stagnant areas with long residence times corresponded with areas of observed algal blooms and hypoxia. High-frequency sensor technology, combined with 3D hydrodynamic modeling, is effective for understanding the complex flow regimes and associated water quality characteristics in ROR-type reservoirs.