  We investigate the formation of fingered flow in dry granular media under
simulated rainfall using a quasi-2D experimental set-up composed of a random
close packing of mono-disperse glass beads. Using controlled experiments, we
analyze the finger instabilities that develop from the wetting front as a
function of fundamental granular (particle size) and fluid properties
(rainfall, viscosity).These finger instabilities act as precursors for water
channels, which serve as outlets for water drainage. We look into the
characteristics of the homogeneous wetting front and channel size as well as
estimate relevant time scales involved in the instability formation and the
velocity of the channel finger tip. We compare our experimental results with
that of the well-known prediction developed by Parlange and Hill [1976]. This
model is based on linear stability analysis of the growth of perturbations
arising at the interface between two immiscible fluids. Results show that in
terms of morphology, experiments agree with the proposed model. However, in
terms of kinetics we nevertheless account for another term that describes the
homogenization of the wetting front. This result shows that the manner we
introduce the fluid to a porous medium can also influence the formation of
finger instabilities.
