  The stability and transition in the bottom boundary layer under a solitary
wave are analysed in the presence of finite amplitude disturbances. First, the
receptivity of the boundary layer is investigated using a linear input-output
analysis, in which the environment noise is modelled as distributed body
forces. The most dangerous perturbations in a time frame until flow reversal
are found to be arranged as counter-rotating streamwise-constant rollers. One
of these roller configurations is then selected and deployed to nonlinear
equations, and streaks of various amplitudes are generated via lift-up
mechanism. By means of secondary stability analysis and direct numerical
simulations, the dual role of streaks in the boundary-layer transition is
shown. When the amplitude of streaks remains moderate, these elongated features
remain stable until the adverse-pressure-gradient stage and have a dampening
effect on the instabilities developing thereafter. In contrast, when the
low-speed streaks reach high amplitudes exceeding 15% of free-stream velocity
at the respective phase, they become highly unstable to secondary sinuous modes
in the outer shear layers. Consequently, a subcritical transition to
turbulence, i.e., bypass transition, can be already initiated in the
favourable-pressure-gradient region ahead of the wave crest.
