  Boundary layer flow over a realistic porous wall might contain both the
effects of wall-permeability and wall-roughness. These two effects are
typically examined in the context of a rough-wall flow, i.e., by defining a
``roughness'' length or equivalent to capture the effect of the surface on
momentum deficit/drag. In this work, we examine the hypothesis of Esteban et
al. (2022), that a turbulent boundary layer over a porous wall could be
modelled as a superposition of the roughness effects on the permeability
effects by using independently obtained information on permeability and
roughness. We carry out wind tunnel experiments at high Reynolds number ($14400
\leq Re_{\tau} \leq 33100$) on various combinations of porous walls where
different roughnesses are overlaid over a given permeable wall. Measurements
are also conducted on the permeable wall as well as the rough walls
independently to obtain the corresponding lengthscales. Analysis of mean flow
data across all these measurements suggests that an empirical formulation can
be obtained where the momentum deficit ($\Delta U^+$) is modelled as a
combination of independently obtained roughness and permeability lengthscales.
This formulation assumes the presence of outer-layer similarity across these
different surfaces, which is shown to be valid at high Reynolds numbers.
Finally, this decoupling approach is equivalent to the area-weighted power-mean
of the respective permeability and roughness lengthscales, consistent with the
approach recently suggested by Hutchins et al. (2023) to capture the effects of
heterogeneous rough surfaces.
