  We conducted direct numerical simulations of turbulent open channel flow
(OCF) and closed channel flow (CCF) of friction Reynolds numbers up to
$\mathrm{Re}_\tau \approx 900$ in large computational domains up to $L_x\times
L_z=12\pi h \times 4\pi h$ to analyse the Reynolds number scaling of turbulence
intensities. Unlike CCF, our data suggests that the streamwise turbulence
intensity in OCF scales with the bulk velocity for $\mathrm{Re}_\tau \gtrsim
400$. The additional streamwise kinetic energy in OCF with respect to CCF is
provided by larger and more intense very-large-scale motions in the former type
of flow. Therefore, compared to CCF, larger computational domains of $L_x\times
L_z=12\pi h\times 4\pi h$ are required to faithfully capture very-large-scale
motions in OCF -- and observe the reported scaling. OCF and CCF turbulence
statistics data sets are available at
https://doi.org/10.4121/88678f02-2a34-4452-8534-6361fc34d06b .
