Transverse dispersion ($t_{\mathrm{mix}}\sim H^2/D_T$)

Spreading of solute across the mean flow direction through diffusion and pore-scale velocity variations. In a reflecting layer of depth $H$, a constant transverse dispersion coefficient gives mixing time of order $H^2/D_T$. Repeated exchange between different longitudinal speeds produces Taylor dispersion on times long compared with this mixing time.