Rapidly flowing avalanches are highly destructive natural phenomena, especially when they interact with buildings or other man-made structures. Here we report a new experimental study of the interaction of a rapid granular flow with a solid barrier, which is of a comparable height to the flow depth. Our experiments show that the flow detaches from the top of the obstacle as a coherent granular jet, the motion of which is well described by theory for an inviscid jet of fluid. As well as giving fundamental new insights into the behaviour of granular flows, the results have important practical consequences for the design of dams used to provide protection from snow avalanches.