Three-dimensional Numerical Simulation of Flow Field and Water Tem-perature Structure in Channel-type Reservoirs under Flood Conditions

[Objective] This study aims to provide theoretical support and technical basis for ecological regulation and water quality safety management of reservoirs during flood season by revealing the coupling disturbance mechanisms of different flood processes on the flow and thermal structures in different reservoir zones. [Methods] Taking the BLH Reservoir, a typical channel-type reservoir of northern China, as an example, a three-dimensional hydrodynamic-thermal model based on Delft3D was developed by comprehensively inputting measured reservoir topography, hydrological, and meteorological data for modeling, calibration, and validation. The model’s accuracy was evaluated using water level and water temperature data of a typical flood year from January to December. Four flood inflow scenarios (S1-S4, with peak discharges of 7 487, 6 150, 4 743, and 2 670 m3/s, respectively) were designed to analyze flow velocity, thermocline thickness, and thermal stratification responses. The RWCS/H index was used to assess the intensity of thermal stratification and its spatiotemporal variation trends. [Results] 1) Model validation showed that the water level simulation had an R2 of 0.995, the water temperature in front of the dam had an R2 of 0.989, the water temperature at the reservoir center had an R2 of 0.982, indicating high adaptability of the model in capturing the spatiotemporal responses of hydrodynamics and thermal structure. 2) Under flood conditions, significant variations in flow field structure were observed, particularly in the lake zone where a pronounced vertical velocity gradient formed. During flood peak stages, the flow velocity in the bottom layer increased from 0.008 m/s to 0.078 m/s, reflecting the enhanced disturbance of subsurface flow on vertical thermal exchange. The vertical water temperature structure showed that as inflow volume increased, the surface-bottom temperature differences across all zones decreased, and thermal stratification became weaker. Under the extreme S1 scenario, the thermocline in the river zone was completely disrupted, with the temperature difference dropping to 0.29 ℃ and RWCS/H decreasing to zero, resulting in nearly uniform vertical water temperatures. RWCS/H in the transition and lake zones declined markedly, reaching minimum values at 70 h and 100 h after the flood peak, respectively, demonstrating a spatial propagation lag in flow-induced disturbances. 3) Furthermore, under high flood conditions, density currents penetrated deeper into the water body, generating large-scale disturbances that significantly weakened thermocline stability. This intensified bottom sediment disturbance and elevated the risk of pollutant release. [Conclusions] The three-dimensional hydrodynamic-thermal numerical model established in this study demonstrates good capabilities in simulating and predicting hydrothermal dynamics in channel-type reservoirs under flood conditions, accurately capturing the water temperature response processes influenced by density currents. The greater the flood intensity, the more thoroughly the water temperature stratification is disrupted, the more concentrated the flow channels become, and the more homogenized the water temperature structure tends to be. The RWCS/H index effectively reflects the intensity of thermal stratification disturbances and reveals their spatial differences and propagation lag characteristics. This study, for the first time, systematically quantifies the coupling variation patterns of flow fields and water temperature structures in channel-type reservoirs under different flood magnitudes, filling a gap in related researches. The findings provide scientific support for reservoir ecological regulation, water quality prediction, and disaster prevention decision-making, particularly applicable for managing sudden flood events under extreme climate conditions.