  We extend previous analyses of the origins of complex transport dynamics in
poroelastic media to the case where the input transient signal at a boundary is
generated by a multimodal spectrum. By adding harmonic and anharmonic modal
frequencies as perturbations to a fundamental mode we examine how such
multimodal signals affect the Lagrangian complexity of poroelastic flow. While
the results apply to all poroelastic media (industrial, biological and
geophysical), for concreteness we couch the discussion in terms of unpumped
coastal groundwater systems having a discharge boundary forced by tides.
Particular local regions of the conductivity field generate saddles that hold
up and braid (mix) trajectories, resulting in unexpected behaviours of
groundwater residence time distributions and topological mixing manifolds near
the tidal boundary. While increasing spectral complexity can reduce the
occurrence of periodic points, especially for anharmonic spectra with long
characteristic periods, other signatures of Lagrangian complexity persist. The
action of natural multimodal tidal signals on confined groundwater flow in
heterogeneous aquifers can induce exotic flow topologies and mixing effects
that are profoundly different to conventional concepts of groundwater discharge
processes. Taken together, our results imply that increasing spectral
complexity results in more complex Lagrangian structure in flows through
poroelastic media.
