Armor or Pavement

The necessary condition for, and the process of, evolution of a coarse layer over finer sediment underneath are presented. Flow regions for pavement and armor formation are identified. A new definition for armor and pavement is proposed. The relevation of the two flow regions contributes towards an improved understanding of the distinction between armor and pavement. With increasing shear velocity, the surface layer coarsens in the armor region and becomes smoother in the pavement region. The eroded material (EM) in the armor region becomes coarser with increasing shear velocity but remains finer than the parent-bed material (PBM). In the pavement evolution region, the EM is finer than PBM; and in the equilibrium pavement region, the composition of EM is identical to that of the PBM. The available experimental data are in agreement with the proposed hypothesis of evolution of the coarse surface layer.