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.