  Direct numerical simulations are conducted to study the receptivity and
transition mechanisms in a solitary wave boundary layer developing over
randomly organized wave-like bottom topography. The boundary layer flow shows a
selective response to broadband perturbations from the bottom, and develops
streamwise-elongated streaks. When the streaks reach high amplitudes, they
indirectly amplify streamwise-elongated vortices through modulating small-scale
fluctuations and pressure fields. These stronger vortices in turn stir the
boundary layer more effectively and further intensify streaks via the lift-up
mechanism. This nonlinear feedback loop increases the sensitivity of the
boundary layer to the roughness level and yields dramatic variations among
cases sharing the same Reynolds number with differing roughness height. Three
different local breakdown scenarios are observed depending on the amplitude of
the streaks: (i) two-dimensional wave instabilities in the regions with weak
streaks; (ii) inner shear-layer instabilities in the regions with
moderate-amplitude streaks; (iii) rapidly growing outer shear-layer
instabilities in the regions with highly elevated high-amplitude streaks. Inner
instabilities have the slowest growth rate among all transition paths, which
confirms the previous predictions on the stabilising role of moderate-amplitude
streaks (\"Onder & Liu, J. Fluid. Mech., vol. 896, 2020, A20).
