  Using a (3+1)-D hydrodynamic model CLVisc, we study the directed flow
($v_{1}$) of light hadrons produced in Au+Au, Ru+Ru and Zr+Zr collisions at
$\sqrt{s_{\textrm{NN}}}=$ 200 GeV. The evolution of tilted energy density,
pressure gradient and radial flow along the $x$-direction are systematically
investigated. Counter-clockwise tilt of initial fireball is shown to be a vital
source of the directed flow for final light hadrons. A good description of
directed flow is provided for light hadrons in central and mid-central Au+Au
and isobar collisions at RHIC. Our numerical results show a clear system size
dependence for light hadron $v_{1}$ across different collision systems. We
further study the effect of nuclear structure on the directed flow and find
that the $v_{1}$ for light hadrons is insensitive to the nuclei with quadrupole
deformation.
