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This paper reports a new phenomenon in the near-free-surface region of open channel flows (OCFs): namely that higher relative population densities of spanwise vortices are observed than are seen at equivalent positions in turbulent boundary layers. The presence of additional retrograde spanwise vortices in OCFs throughout the region 
$y/h>0.2$ (where 
$y$ is the distance to the wall and 
$h$ is the water depth) prompts us to re-examine the wall-normal extent that the free-surface effect can reach, which is classically expected to be limited within the surface and blockage layers. Finally, possible mechanisms for the phenomenon of additional spanwise vortices in OCFs are provided.
We investigate the heat transfer and coherent structures in Taylor–Couette (TC) flows that undergo thermal convection driven by an axially applied temperature gradient. Direct numerical simulations are performed in a Rayleigh number range 
$10^6 \leq Ra \leq 3 \times 10^8$ for Prandtl number 
$Pr = 4.38$ and with the shear Reynolds number up to 
$Re = 10^4$. When the rotation number 
$R_f$ increases, the flows undergo a transition from buoyancy-dominated (
$R_f<1$) to shear-dominated convection (
$R_f>1$). In the buoyancy-dominated regime with weak rotations, the flow features are similar to those in Rayleigh–Bénard (RB) convection with large-scale plumes emanating from the thermal boundary layers. In this regime, the 
$Re$-dependence of heat transport 
$Nu$ is sensitive to 
$Ra$. We find that for low 
$Ra$, 
$Nu$ decreases with increasing 
$Re$ and becomes independent of 
$Re$ at high 
$Ra$. In the shear-dominated regime, the flow structures are characterised by Taylor vortices (TVs), which effectively enhance the heat transport. With sufficiently high Reynolds number for 
$2000< Re \le 10\,000$, the flow structures are dominated by turbulent TVs, and the transport scaling laws of heat and angular velocity fluxes become independent of buoyancy. We report that in this turbulent regime the axial heat-transport scaling 
$(Nu\sim Re^{0.578\pm 0.018})$ is consistent with the scaling of radial angular-momentum transport 
$(Nu_{\omega }\sim Re^{0.581\pm 0.026})$.
Smooth-walled open channel flow datasets, covering both the direct numerical simulation and experimental measurements with a friction Reynolds number 
${\textit {Re}}_\tau$ at a low-to-moderate level of 
$550\sim 2400$, are adopted to investigate the contributions of different scale motions to the mean wall-shear stress in open channel flows (OCFs). The FIK identity decomposition method by Fukagata et al. (Phys. Fluids, vol. 14, 2002, L73) combined with a scale decomposition is chosen for this research. To see whether/how the contributions in OCFs differ with those in closed channel flows (CCFs), comparisons between the two flows are also made. The scale-decomposed ‘turbulent’ contribution results of present OCFs exhibit two dominant contribution modes (i.e. large-scale motions (LSMs) and very-large-scale motions (VLSMs)) at a streamwise wavelength 
$\lambda _x=1\sim 2h$ and 
$O(10h)$, where 
$h$ is the water depth. The large scales with 
$\lambda _x>3h$ and 
$\lambda _x>10h$ are demonstrated to contribute to over 40 % and 20 % of the mean wall-shear stress, respectively. Compared with CCFs, slightly higher and lower contributions in the 
$\lambda _x>O(10h)$ and 
$\lambda _x < O(10h)$ wavelength ranges are observed in OCFs, revealing the important free-surface effects in OCFs. Possible mechanisms are discussed to lend support for the observed differences between the two flows.
