Laboratory and numerical experiments have revealed physical reasons for the permeability of the thermal bar to horizontal transport. The thermal bar is understood as a front associated with the temperature of maximum density (Tm = 3.98 °C for fresh water). Laboratory experiments were performed in a 2-metre-long non-rotating channel with a sloping bottom, filled with water with T < Tm and naturally heated from above. Analysis of Particle Image Velocimetry (PIV) images revealed water dynamics in the presence of Tm. It was revealed that the compensating flow in intermediate layers is responsible for the horizontal exchange across the thermal bar front. We applied a 3D non-hydrostatic MIKE3-FlowModel ( to investigate the permeability of the spring thermal bar in basins on the scale of lakes and a laboratory flume. We performed an analysis of the concentration distribution of 12 passive tracers released at different locations in the flow domain. Scaling analysis, corroborated by the results of laboratory and numerical experiments, predicts the discharge across the thermal bar as Q = 0.1[g × Δρ/ρ0]1/2h3/2, where h is the depth of the upper thermo-active layer, ρ0 is a maximum density and Δρ is a characteristic horizontal density difference. A combined analysis of data shows that this law is obeyed.

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