Modelling the fate and transport of negatively buoyant storm–river water in small multi-basin lakes

The dynamics of negatively buoyant river plumes in a small multi-basin kettle lake with steep bathymetry (Toolik Lake, AK) are simulated using a Cartesian hydrodynamic model based on the solution of the three-dimensional shallow water equations. To validate the model, model predictions are compared with results from previous analytical and laboratory studies and with field observations. The grid resolution adopted for the Toolik Lake model is 0.5 m (= Δz) in the vertical and 20 m (= Δx) in the horizontal, so that the ratio of the bottom slope S0 to Δz/Δx is lower than 4 in 99% of the computational domain. With that resolution, the model represents correctly the rate of mixing between lake and river water and the speed of propagation of downslope gravity currents. The model provides accurate predictions of the temperature structure (RMSE = 0.25 °C) and of eddy diffusivities at the depths of the intrusions of incoming water. Measurements and modelling show similar fractions and depth distribution of river water on a cross-basin transect, which suggests that the mixing dynamics of the plume as it transits between basins are well resolved. Thus, the stage is set to quantify the ecological consequences of storm events in small lakes with several interconnected basins using coupled biological measurements and 3D modelling.