  In the present treatise, a stability analysis of the bottom boundary layer
under solitary waves based on energy bounds and nonmodal theory is performed.
The instability mechanism of this flow consists of a competition between
streamwise streaks and two- dimensional perturbations. For lower Reynolds
numbers and early times, streamwise streaks display larger amplification due to
their quadratic dependence on the Reynolds number, whereas two-dimensional
perturbations become dominant for larger Reynolds numbers and later times in
the deceleration region of this flow, as the maximum amplification of
two-dimensional perturbations grows exponentially with the Reynolds number. By
means of the present findings, we can give some indications on the physical
mecha- nism and on the interpretation of the results by direct numerical
simulation in (Vittori & Blondeaux 2008; Ozdemir et al. 2013) and by
experiments in (Sumer et al. 2010). In addition, three critical Reynolds
numbers can be defined for which the stability prop- erties of the flow change.
In particular, it is shown that this boundary layer changes from a
monotonically stable to a non-monotonically stable flow at a Reynolds number of
18.
