  Gravity-driven collapses involving large amounts of dense granular material,
such as landslides, avalanches, or rockfalls, in a geophysical context,
represent significant natural hazards. Understanding their complex dynamics is
hence a key concern for risk assessment. In the present work, we report
experiments on the collapse of quasi-two-dimensional dry granular columns under
the effect of gravity, where both the velocity at which the grains are released
and the aspect ratio of the column are varied to investigate the dynamics of
the falling grains. At high release velocity, classical power laws for the
final deposit are recovered, meaning those are representative of a free-fall
like regime. For high enough aspect ratios, the top of the column undergoes an
overall free-fall like motion. In addition, for all experiments, the falling
grains also spread horizontally in a free-fall like motion, and the
characteristic time of spreading is related to the horizontal extension reached
by the deposit at all altitudes. At low release velocity, a quasi-static state
is observed, with scaling laws for the final geometry identical to those of the
viscous regime of granular-fluid flow. The velocity at which the grains are
released governs the collapse dynamics. Between these two asymptotic regimes,
the higher the release velocity, the smaller the impact on the collapse
dynamics. The criterion $\overline{V} \geq 0.4\sqrt{gH_0}$, where $H_0$ is the
initial height of the column, is found for the mean release velocity
$\overline{V}$ not to influence the granular collapse.
