Darcy's law is the basic equation
that describes fluid flow through porous media. There are many ways to
write Darcy's Law, and a few of them will be presented here. The first
equation contains a velocity term, v (L/T), a conductivity term, K (L/T),
a head term, h (L), and a distance term, l (L):

v = -K(∆h/∆l)

This is the Darcy
velocity (or Darcy flux) which is defined as the flow per unit cross
sectional area of the porous medium. Since you have a porous media the water
must move through the pores, around the solid particles, at a speed greater
than the flux. This speed is called the average linear groundwater velocity
and is calculated by dividing the Darcy flux by the porosity of the media.