  We study the formation of shocks on the surface of a granular material
draining through an orifice at the bottom of a quasi-two dimensional silo. At
high flow rates, the surface is observed to deviate strongly from a smooth
linear inclined profile giving way to a sharp discontinuity in the height of
the surface near the bottom of the incline, the typical response of a choking
flow such as encountered in a hydraulic jump in a Newtonian fluid like water.
We present experimental results that characterize the conditions for the
existence of such a jump, describe its structure and give an explanation for
its occurrence.
