  In Weyl semimetals the application of parallel electric and magnetic fields
leads to valley polarization -- an occupation disbalance of valleys of opposite
chirality -- a direct consequence of the chiral anomaly. In this work, we
present numerical tools to explore such nonequilibrium effects in spatially
confined three-dimensional systems with a variable disorder potential, giving
exact solutions to leading order in the disorder potential and the applied
electric field. Application to a Weyl-metal slab shows that valley polarization
also occurs without an external magnetic field as an effect of chiral anomaly
"trapping": Spatial confinement produces chiral bulk states, which enable the
valley polarization in a similar way as the chiral states induced by a magnetic
field. Despite its finite-size origin, the valley polarization can persist up
to macroscopic length scales if the disorder potential is sufficiently long
ranged, so that direct inter-valley scattering is suppressed and the relaxation
then goes via the Fermi-arc surface states.
