Interflow dynamics and three-dimensional modeling of turbid density currents in Imha Reservoir, South Korea

Abstract

This study reports a detailed research identifying the turbid density flow regimes and propagation dynamics of density currents in Imha Reservoir in South Korea during Typhoon Ewiniar. We employ a high resolution 3-D numerical model (FLOW-3D), based on nonhydrostatic Navier-Stokes equations, to investigate the propagation of density flows resulting from the complicated reservoir morphometry and various mixing processes. The 3-D numerical model was modified to simulate particle-driven density currents. The particle dynamics algorithm builds upon the original FLOW-3D code in two ways: (1) improve the original buoyant flow model to compute the changes in density via particle deposition; and (2) include multiple sediment sizes in mixtures as a function of particle size. The influences of inflow characteristics and seasonal changes of thermal structure of the reservoir on the turbid density currents intruding into Imha Reservoir are studied. A series of numerical simulations of lock-exchange are validated with laboratory experiments on: (1) gravity currents propagating into a two-layered fluid; (2) gravity currents propagating into a stratified fluid; and (3) particle-driven gravity currents. The model predictions of propagation speed compared very well with laboratory experiments and analytical solutions. Two numerical approaches (Reynolds Averaged Navier-Stokes model and large-eddy simulation) are equally effective and robust in predicting propagation speed and interfacial instability compared to the laboratory experiments. The simulation of gravity currents intruding into a stratified fluid matched the theoretical solution derived from an energy model. The modified FLOW-3D model successfully captured the decreasing propagation speed due to the different deposition rates of different particle sizes, compared to experimental measurements. We extended our simulations to include the effects of particle sizes on the propagation dynamics of gravity currents. The type of gravity currents depends on particle sizes and can be subdivided into three zones: (1) When ds, is less than about 10 μm, the particle-driven gravity currents behave like IGC (Intrusive Gravity Currents) and all sediments can remain in suspension. Thus the suspended sediments can increase the density of the currents enough to travel a longer distance; (2) When ds > 40 μm, particles will rapidly settle, resulting in a decrease in excess density of the gravity currents. So, such density currents lose their momentum quickly and rapidly vanish; and (3) When 10 μm ds 40 μm, some particles will settle quickly, but others remain suspended for a long time, affecting the propagation dynamics of the currents. Modeling gravity currents in this regime particle sizes must account for particle dynamics and settling. We applied the FLOW-3D coupled with the particle dynamics algorithm to Imha Reservoir in South Korea. The model application was validated against field measurements during Typhoon Ewiniar in 2006. In the field validation, absolute mean error (AME) and root mean squared error (RMSE) for the prediction in water temperature profiles were calculated to be 1.0 oC and 1.3 oC, respectively. For turbidity predictions, AME and RMSE were 37 and 47 NTU (nephelometric turbidity units) between the simulated and the measured turbidity at stations G3, G4, and G5. We showed the influence of inflow characteristics (discharge, temperature, sediment concentration, and particle size distribution) on the fate of density currents in Imha Reservoir. Two threshold values in particle size (10 μmand 40 μm ) were identified, consistent with previous findings from the simulations of Gladstone's experiments. The simulations indicate that when the particle sizes ds are less than 10 μm, most of the sediment inflows at the inlet point (G2) will be transported to Imha Dam (G4) in suspension by interflows. When the particle sizes ds are greater than 40 μm, they will rapidly settle before reaching the dam. Therefore, highly concentrated turbid interflows could only occur when ds is less than the threshold value of 10 μm. The numerical results also present three flow regimes determining the intrusion types of density currents: (1) river inflows will form interflows when the sediment concentration Ci is less than 2000 mg/l; (2) when Ci is between 2000 mg/l and 3000 mg/l, they will form multiple intrusions (i.e., interflows and underflows); and (3) when Ci is greater than 3000 mg/l, they will plunge and propagate as underflows. These threshold values (2000 mg/l and 3000mg/l) can be used to practically predict the formation of turbid density currents, flow type, and intrusion level in Imha Reservoir.