We consider the problem of nonlinear buoyant flow in a horizontal mushy layer during alloy solidification. We study the nonlinear evolution of such flow based on a recently developed realistic model for the mushy layer. The evolution approach is based on a Landau type equation for the amplitude of the secondary nonlinear solution, which is derived in this article. Using both analytical and computational methods, we calculate the solution to the evolution equation for both subcritical and supercritical regimes. We find, in particular, that for a passive mush, where the permeability is constant, and supercritical regime, the primary solution is linearly unstable to the secondary solution which becomes a steady stable solution for sufficiently large time, while the secondary solution decays to zero for the subcritical regime. On the other hand, for a realistic reactive mush, where the permeability is variable, the secondary flow can break down in a finite time for either supercritical or subcritical regime, which indicates existence of some kind of bursting behavior. These results are then compared to the corresponding ones based on the weakly nonlinear theory.