  We study the interplay of clumping at small scales with the collapse and
relaxation of perturbations at much larger scales. We present results of our
analysis when the large scale perturbation is modelled as a plane wave. We find
that in absence of substructure, collapse leads to formation of a pancake with
multi-stream regions. Dynamical relaxation of plane wave is faster in presence
of substructure. Scattering of substructures and the resulting enhancement of
transverse motions of haloes in the multi-stream region lead to a thinner
pancake. In turn, collapse of the plane wave leads to formation of more massive
collapsed haloes as compared to the collapse of substructure in absence of the
plane wave. The formation of more massive haloes happens without any increase
in the total mass in collapsed haloes. A comparison with the Burgers' equation
approach in absence of any substructure suggests that the preferred value of
effective viscosity depends primarily on the number of streams in a region.
