  We experimentally investigate how a long granular pile confined in a narrow
channel destabilizes when it is inclined above the angle of repose. A uniform
flow then develops, which is localized at the free surface. It first
accelerates before reaching a steady uniform regime. During this process, an
apparent erosion is observed and the thickness of the flowing layer increases.
We precisely study the evolution of the vertical velocity profile in this
transient regime. The measurements are compared with the prediction of a
visco-plastic model [P. Jop, Y. Forterre and O. Pouliquen, Nature 441, 727
(2006)]
