  We propose a hydrodynamic model describing steady-state and dynamic electron
and hole transport properties of graphene structures which accounts for the
features of the electron and hole spectra. It is intended for electron-hole
plasma in graphene characterized by high rate of intercarrier scattering
compared to external scattering (on phonons and impurities), i.e., for
intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually
monopolar plasma). We demonstrate that the effect of strong interaction of
electrons and holes on their transport can be treated as a viscous friction
between the electron and hole components. We apply the developed model for the
calculations of the graphene dc conductivity, in particular, the effect of
mutual drag of electrons and holes is described. The spectra and damping of
collective excitations in graphene in the bipolar and monopolar limits are
found. It is shown that at high gate voltages and, hence, at high electron and
low hole densities (or vice-versa), the excitations are associated with the
self-consistent electric field and the hydrodynamic pressure (plasma waves). In
intrinsic and optically pumped graphene, the waves constitute quasineutral
perturbations of the electron and hole densities (electron-hole sound waves)
with the velocity being dependent only on the fundamental graphene constants.
