Hydrostatic leakage through a basal seal ($\Omega =\lambda \Delta \rho g/(\varphi \mu b)$)

A dense porous gravity current of height $h$ places excess hydrostatic pressure $\Delta \rho gh$ on a basal seal. For seal permeability of a porous medium $\lambda$ and thickness $b$, Darcy law gives downward flux $\lambda \Delta \rho gh/(\mu b)$. Dividing the stored height-volume balance by porosity $\varphi$ gives a linear drainage term $-\Omega h$. The coefficient has inverse-time units and does not contain an extra factor of the aquifer permeability.