The importance of exchange flows through channels or straits, which are interconnecting water bodies of different densities, lies in the fact that water mass characteristics in sub-basins are determined by these flows, extend of which depends on the exchange rate between the interconnected water bodies. The knowledge of vertical diffusivity, k, along a channel/strait is therefore crucial as this gives us an idea about the amount of heat, mass and dissolved solids transported across it and provide parameterisations for circulation models. From comparison with numerical results of an exchange through a channel at various values for Gr, Pr and aspect ratio, we infer the effective value for k as it appears in the Grashof number.