  Using 3D direct numerical simulations of the Navier--Stokes equations, we
study the effect of wall roughness on the onset of turbulence in channel flow.
The dependence of the friction factor on the Reynolds number, $\mathrm{Re}$, is
found to follow a generalized Forchheimer law, which interpolates between the
laminar and inertial asymptotes. The transition between these two asymptotes
occurs at a first critical $\mathrm{Re}$, $\mathrm{Re}_{\mathrm{c}}$, that
depends nontrivially on the roughness amplitude. We identify the transition
from subcritical to supercritical onset by looking at the dependence of the
velocity fluctuations on Re for different roughness amplitudes. We find that
this second critical $\mathrm{Re}$ is comparable in magnitude to
$\mathrm{Re}_{\mathrm{c}}$, implying that transitional flow is an integral part
of flow in open fractures when $\mathrm{Re}$ and the roughness amplitude are
sufficiently high.
