  The Whitham equation is a model for the evolution of surface waves on shallow
water that combines the unidirectional linear dispersion relation of the Euler
equations with a weakly nonlinear approximation based on the KdV equation. We
show that large-amplitude, periodic, traveling-wave solutions to the Whitham
equation and its higher-order generalization, the cubic Whitham equation, are
unstable with respect to the superharmonic instability (i.e. a perturbation
with the same period as the solution). The threshold between superharmonic
stability and instability occurs at the maxima of the Hamiltonian and
$\mathcal{L}_2$-norm. We examine the onset of wave breaking in traveling-wave
solutions subject to the modulational and superharmonic instabilities.
  We present new instability results for the Euler equations in finite depth
and compare them with the Whitham results. We show that the Whitham equation
more accurately approximates the wave steepness threshold for the superharmonic
instability of the Euler equations than does the cubic Whitham equation.
However, the cubic Whitham equation more accurately approximates the wave
steepness threshold for the modulational instability of the Euler equations
than does the Whitham equation.
