We perform mathematical anaysis of the biofilm development process. A model describing biomass growth is proposed: It arises from coupling three parabolic nonlinear equations: a biomass equation with degenerate and singular diffusion, a nutrient tranport equation with a biomass-density dependent diffusion, and an equation of the Navier-Stokes type, describing the fluid flow in which the biofilm develops. This flow is subject to a biomass-density dependent obstacle. The model is treated as a system of three inclusions, or variational inequalities; the third one causes major difficulties for the system's solvability. Our approach is based on the recent development of the theory on Navier-Stokes variational inequalities. on the other hand u ε := ρ ε * u is the local spatial-average of u(x, t) by means of the usual mollifier ρ ε (x) (see Section 2 for details).(ii) The nutrient concentration w(x, t) is non-negative and has the threshold value 1, i.e. 0 ≤ w(x, t) ≤ 1. Also, we suppose that there is no nutrient supply from the exterior. The diffusion coefficient d(u) depends on the biomass density u and c d ≤ d(r) ≤ c d , |d(r 1 ) − d(r 2 )| ≤ L(d)|r 1 − r 2 |, ∀r 1 , r 2 ∈ R, (1.2)