The interaction between low-order roughness and large-scale topography is experimentally investigated using high-resolution particle image velocimetry in a refractive-index-matched channel at two bulk Reynolds numbers differing by an order of magnitude, $\text{Re}=4.0\times {10}^3$ and $4.0\times {10}^4$. A two-dimensional sinusoidal roughness is superimposed on a two-dimensional wavy wall to isolate the effects of multiscale surface modulation on near-wall turbulence. The added roughness reorganizes the flow by inducing larger coherent structures near the surface. Two-point correlations and integral length scales confirm enhanced near-wall coherence, while quadrant-hole analysis reveals a marked suppression of ejection events. This suppression weakens the local turbulence production term $-\langle u^{\prime }{v}^{\prime }\rangle dU/dy$, leading to attenuation of the separated shear layer that develops downstream from the topographic crest within the adverse-pressure-gradient region. The resulting flow exhibits up to 25% lower peak turbulent kinetic energy and Reynolds shear stress relative to the smooth-wavy configuration. Even modest low-order roughness can therefore reorganize the coupling between near-wall and outer-layer motions, a mechanism that persists across an order of magnitude in Reynolds number and offers valuable insight for modeling transport and drag over natural and engineered surfaces.

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• Boundary layers