Field-Based Investigation of Flow-Regime-Dependent Hydrodynamic Characteristics in a Complex Artificial Lake

Artificial reservoirs with meandering planforms and tributary junctions often exhibit complex three-dimensional flow structures that vary depending on hydrologic conditions. In such systems, interactions between inflow regimes and water-intake facilities can influence both hydraulic behavior and water-supply stability. This study examines the flow-regime-dependent hydrodynamic characteristics of Lake Paldang, a large regulated reservoir that serves as a primary drinking-water source for approximately 25 million people in the Seoul metropolitan area, Korea.

Field measurements were conducted under contrasting hydrologic conditions, including flood and normal-flow periods, focusing on major bend sections downstream of tributary confluences. Three-dimensional velocity fields were obtained using an Acoustic Doppler Current Profiler (ADCP), while vertical profiles of water-quality parameters were measured using multi-parameter sondes (YSI-EXO2). Spatial patterns of flow and mixing were analyzed by integrating cross-sectional velocity distributions, electrical conductivity fields, and the positions of maximum-velocity lines (MVL). These datasets were used to examine secondary-flow structures, lateral mixing behavior, and transport pathways of distinct water masses.

The results indicate that during flood conditions, increased discharge from main tributaries enhances curvature-induced momentum, leading to a pronounced lateral separation of the MVL and the development of a dominant single secondary-circulation cell. Under these conditions, the flow field exhibits river-like characteristics, suggesting an increased potential for asymmetric sediment transport and bank-related hydraulic stresses. In contrast, during normal-flow periods, overall flow velocities decrease and multiple smaller secondary cells emerge across the channel section, reflecting more lacustrine hydraulic behavior with reduced lateral momentum and weaker organized circulation.

In addition, under specific hydrologic regimes, tributary inflows characterized by relatively high electrical conductivity were observed to migrate along near-surface pathways toward water-intake zones. This behavior suggests that seasonal flow conditions may influence not only mixing efficiency but also the potential exposure of intake facilities to pollutant-rich inflows.

Overall, the findings suggest that regulated artificial lakes can alternate between riverine and lacustrine hydrodynamic states depending on flow regimes, with direct implications for mixing processes, sediment dynamics, and intake management. The study highlights the value of high-resolution field measurements for understanding regime-dependent hydraulic behavior and supports the need for adaptive monitoring and operational strategies to enhance the resilience of reservoir-based water-supply systems.