  We present a numerical study of convection in a horizontal layer comprising a
fluid-saturated porous bed overlain by an unconfined fluid layer. Convection is
driven by a vertical, destabilising temperature difference applied across the
whole system, as in the canonical Rayleigh-B\'enard problem. Numerical
simulations are carried out using a single-domain formulation of the two-layer
problem based on the Darcy-Brinkman equations. We explore the dynamics and heat
flux through the system in the limit of large Rayleigh number, but small Darcy
number, such that the flow exhibits vigorous convection in both the porous and
the unconfined fluid regions, while the porous flow still remains strongly
confined and governed by Darcy's law. We demonstrate that the heat flux and
average thermal structure of the system can be predicted using previous results
of convection in individual fluid or porous layers. We revisit a controversy
about the role of subcritical "penetrative convection" in the porous medium,
and confirm that such induced flow does not contribute to the heat flux through
the system. Lastly, we briefly study the temporal coupling between the two
layers and find that the turbulent fluid convection above acts as a low-pass
filter on the longer-timescale variability of convection in the porous layer.
