  In steady sediment transport, sediment deposition is balanced by entrainment
of bed particles by fluid forces or particle-bed impacts. Here we propose a
proxy to determine the role of impact entrainment relative to entrainment by
the mean turbulent flow: the "bed velocity" $V_b$, which is an effective
near-bed-surface value of the average horizontal particle velocity that
generalizes the classical slip velocity, used in studies of aeolian saltation
transport, to sediment transport in an arbitrary Newtonian fluid. We study
$V_b$ for a wide range of the particle-fluid-density ratio $s$, Galileo number
$\mathrm{Ga}$, and Shields number $\Theta$ using direct sediment transport
simulations. We find that transport is fully sustained through impact
entrainment (i.e., $V_b$ is constant in natural units) when the "impact number"
$\mathrm{Im}=\mathrm{Ga}\sqrt{s+0.5}\gtrsim20$ or $\Theta\gtrsim5/\mathrm{Im}$.
These conditions are obeyed for the vast majority of transport regimes,
including steady turbulent bedload, which has long been thought to be sustained
solely through fluid entrainment. In fact, we find that transport is fully
sustained through fluid entrainment (i.e., $V_b$ scales with the near-bed
horizontal fluid velocity) only for sufficiently viscous bedload transport at
grain scale (i.e., for $\mathrm{Im}\lesssim20$ and
$\Theta\lesssim1/\mathrm{Im}$). Finally, we do not find a strong correlation
between, $V_b$, or the classical slip velocity, and the
transport-layer-averaged horizontal particle velocity $\overline{v_x}$, which
challenges the long-standing consensus that predominant impact entrainment is
responsible for a linear scaling of the transport rate with $\Theta$. For
turbulent bedload in particular, $\overline{v_x}$ increases with $\Theta$
despite $V_b$ remaining constant, which we propose is linked to the formation
of a liquid-like bed on top of the static-bed surface.
