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6-95 Appendix A • At low flows, the boundary is stationary; as the flow increases, how- ever, the smallest particles begin to move. As the flow increases fur- ther, larger particles also begin to move but at a lower velocity. Fi- nally, the discharge can increase to a point where the entire boundary is moving, although the larger particles move more slowly than the smaller. As the flow decreases, the process reverses itself; but if the smaller particles are not replaced, the bed is left degraded and coarser. Armoring occurs when smaller particles are transported from the bound- ary but not replaced and coarser particles are exposed but not trans- ported. Whether true armoring occurs depends on whether the exposed coarser particles originated at their present position or upstream. If they originated upstream, what has occurred is not armoring but sediment transport by unsteady flow. • A design of a stable channel that depends upon armoring for stability can be a contradiction unless the armor surface has already been estab- lished and will not be disturbed during construction. Otherwise, degra- dation must occur before a complete armor surface can exist, and result- ing eroding bed material contributes a downstream sediment load to the sys- tem. Furthermore, this degradation causes undercutting of the toes of the bank, which can lead to bank sloughing. Ultimate design value of armoring may be that it is the last line of defense against the more extreme events that otherwise may completely unravel a channel and possibly lead to eco- logical disaster or catastrophic failure of important cultural features. Math Models The math models developed by Shields and Strickler provide the basis for the armoring design procedure. The procedure was verified by lane's field work. The designer must analyze (1) the active or driving forces and (2) the passive or resisting forces. The analysis of active forces consists of determining the hydraulics or depth of flow and determining the bound- . ary roughness shear or tractive stress. The latter determination is nec- essary because not all energy loss is due to boundary roughness. Bends or changes in cross-sectional area cause energy loss through internal fluid shear.