In this paper we explore essentially nonlinear disturbances produced in an incompressible boundary layer by a roughness on the wall. The scale of the stationary roughness is supposed to be large enough so that generated waves are governed by the forced Benjamin–Davis–Acrivos (fBDA) equation. The disturbance patterns for a wide range of roughness sizes are analyzed revealing the remarkable phenomenon of bifurcations. A very specific oscillation motion over the obstacle is found. It appears to be the basic mechanism causing the periodic generation of solitary waves upstream and downstream. The general structure of disturbances in space at different values of time is discussed. The asymptotic analysis of the solution, when the intensity of the external agencyQbecomes a small parameter, is given. The quadratic term of an expansion based on this parameter is responsible for the redistribution of solution mass between regions located ahead and behind the obstacle, inevitably leading to the gradual growth of nonlinear effects. According to the asymptotic consideration, the timeTc*determining the onset of the nonlinear stage of disturbance development isO(Q−3), this estimation correlates well with numerical results. ©1996 American Institute of Physics.