  In wall-bounded parallel flows, sustained turbulence can occur even while
laminar flow is still stable. Channel flow is one of such flows and displays
spatio-temporal fluctuating patterns of localized turbulence along its way
from/to featureless turbulence. By direct numerical simulation, we study the
observed inconsistency between turbulence decay according to a two-dimensional
directed-percolation (2D-DP) scenario and the presence of sustained oblique
localized turbulent bands (LTBs) below the DP critical point. Above Reynolds
number Reg \sim 700 sustained LTBs are observed; most LTBs have the same
orientation so that the spanwise symmetry of the LTB pattern is broken below
Re2 \sim 1000. The frequency of transversal splitting, by which an LTB
generates another one with opposite obliqueness, so that turbulence spreading
becomes intrinsically two dimensional, increases in the range Reg < Re < Re2.
It reaches a critical rate at Re2 beyond which symmetry is restored. 2D-DP
behavior is retrieved only above Re2. A mean-field model is proposed which
qualitatively accounts for the above symmetry-restoring bifurcation by
considering interactions between space-averaged densities of LTBs propagating
in either direction.
