  Numerical simulations of the collapse and spreading of granular columns onto
an horizontal plane using the Contact Dynamics method are presented. The final
shape of the deposit seems to depend only on the aspect ratio $a$ of the
columns; these results are in good agreement with previous experimental work.
In particular, the renormalised runout distance shows a power law dependence on
the aspect ratio $a$, which is incompatible with a simple friction model. The
dynamics of the collapse is shown to be mostly controlled by the free fall of
the column. The energy dissipation at the base of the column can be described
simply by a coefficient of restitution. Hence the energy available for the
sideways flow is proportional to the initial potential energy $E_0$. The
dissipation process within the flow is well approximated by basal friction,
contrary to the behaviour of the runout distance. The mass ejected sideways is
showned to play a determining role in the spreading process. As $a$ increases,
the same fraction of initial potential energy $E_0$ drives more mass against
friction. This additional dissipation give a possible explanation for power-law
dependence of the runout distance on $a$. Beyond the frictional properties of
the material, we show that the flow characteristics strongly depend on the
early dynamics of the collapse. We propose a new scaling for the runout
distance that matches the data well, is compatible with a friction model, and
provide a qualitative explanation to the column collapse phenomenology.
