Preferential flow is common in the vadose zone of bedrock aquifers. Rapid film flow down fracture walls and free-fall flow down open vertical fractures produce velocities that are commonly in the range 10-4 to 10-1 m/s.
Stephen R.H. Worthington (Tue,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: