  Surface distortions to an otherwise planar channel flow introduce vorticity
perturbations. We examine this scenario in viscoelastic fluids, and identify
new mechanisms by which significant vorticity perturbations can be generated in
both inertialess and elasto-inertial channel flows. We focus on the case where
the lengthscale of the surface distortion is much longer than the channel
depth, where we find significant departure from plane shear (Page & Zaki, J.
Fluid Mech. 801 2016) due to the non-monotonic base-flow streamwise-normal
elastic stress. In inertialess flows, a purely elastic response results in
streamlines deforming to match the bottom topography in the lower half the
channel. However, the vanishing stress at the centreline introduces a blocking
effect, and the associated $O(1)$ jump in normal velocity is balanced by a
narrow, large amplitude streamwise-oscillating `jet', resulting in a localised,
chevron-shaped vorticity perturbation field. In elasto-inertial flows,
resonance between the frequency of elasto-inertial `Alfven' waves and the
frequency apparent to an observer moving with the fluid results in vorticity
amplification in a pair of critical layers on either side of the channel. The
vorticity in both layers is equal in magnitude and as such the perturbation
vorticity field penetrates the full channel depth even when inertia is
dominant. The results demonstrate that long-wave distortions - which are
relatively innocuous in Newtonian fluids - can drive a significant flow
distortion in viscoelastic fluids for a wide range of parameter values.
