  We investigate the role of the chiral anomaly in hydrodynamic and crossover
regimes of transport in a Weyl or Dirac semimetal film. We show that the
magnetic-field-dependent part of the electric conductivity in the direction of
the magnetic field develops an unusual nonmonotonic dependence on temperature
dubbed the anomalous Gurzhi effect. This effect is realized in sufficiently
clean semimetals subject to a classically weak magnetic field where the
electron-electron scattering dominates and the relaxation of the
valley-imbalance charge density happens at the boundaries. Moreover, the
conditions for the realization of the conventional Gurzhi effect in
hydrodynamic and crossover regimes of transport in three-dimensional Dirac and
Weyl semimetals are determined.
