  We present an experimental and numerical study that describes the motion of
flexible blades, scaled to be dynamically similar to natural aquatic
vegetation, forced by wave-induced oscillatory flows. For the conditions
tested, blade motion is governed primarily by two dimensionless variables: (i)
the Cauchy number, $Ca$, which represents the ratio of the hydrodynamic forcing
to the restoring force due to blade stiffness, and (ii) the ratio of the blade
length to the wave orbital excursion, $L$. For flexible blades with $Ca \gg 1$,
the relationship between drag and velocity can be described by two different
scaling laws at the large- and small-excursion limits. For large excursions ($L
\ll 1$), the flow resembles a unidirectional current and the scaling laws
developed for steady-flow reconfiguration studies hold. For small excursions
($L \gg 1$), the beam equations may be linearized and a different scaling law
for drag applies. The experimental force measurements suggest that the
small-excursion scaling applies even for intermediate cases with $L \sim O(1)$.
The numerical model employs the well-known Morison force formulation, and
adequately reproduces the observed blade dynamics and measured hydrodynamic
forces without the use of any fitted parameters. For $Ca \gg 1$, the movement
of the flexible blades reduces the measured and modeled hydrodynamic drag
relative to a rigid blade of the same morphology. However, in some cases with
$Ca \sim O(1)$, the measured hydrodynamic forces generated by the flexible
blades exceed those generated by rigid blades, but this is not reproduced in
the model. Observations of blade motion suggest that this unusual behavior is
related to an unsteady vortex shedding event, which the simple numerical model
cannot reproduce. Finally, we also discuss implications for the modeling of
wave energy dissipation over canopies of natural aquatic vegetation.
