Erosion can also be supply limited. In many well graded rivers, a coarse armor layers forms on top of a subsurface layer, composed of the representative reach gradation. Rivers form these coarse armor layers by static or dynamic armoring (Parker, 2008). Static armoring comes from differential transport of finer materials, where finer particles transport, leaving the coarse particles behind until the immobile coarse particles armor the bed, precluding future erosion. Static armoring processes often dominate downstream of dams, where attenuated flow regimes are competent to move finer particles but not coarser particles. Dynamic armoring can also form armor layers in systems where large flows are competent to move all grain classes. Dynamic armoring forms coarse cover layers while all grain classes are mobile because equilibrium transport of graded material requires over representing coarse particles at the surface, compensating for their lower transportability by increasing their availability (Parker, 2008).
In either case, armor layers decrease total transport because the surface particles available for transport tend to be coarser and more difficult to move. This is also a physical limiter on the transport capacity.
HEC-RAS includes three algorithms to simulate bed sorting and armoring. All three algorithms divide the bed into an active layer and an inactive layer. The active layer is a surface layer that represents actively transporting material (or material that could be transported). The active layer gradation evolves independently and material is moved between it and the parent material in the inactive layer below it. HEC-RAS computes transport capacity based on the gradation of the active layer, not the entire bed.
