Armor Limited Clear-Water Contraction Scour at Bridges

A simplified model of streambed armoring is used to extend Laursen's well-known equation to include the limiting effects of a coarse surface layer when calculating clear-water contraction scour at bridges. An empirical relation for selective entrainment of gravel from naturally sorted riverbed material is used to estimate the smallest nontransportable particle in the armor layer. If the smallest nontransportable particle approaches the largest-sized particles in the bed-material mixture, evidence indicates that the armor layer will be unstable and particles of all sizes will be nearly equally mobile. However, if the armor layer is stable, clear-water contraction-scour depth estimates might be significantly less than for nonarmored conditions. To calculate armor-limited contraction-scour depths, two equations need to be solved simultaneously, one for the depth of the so-called active layer provided by the armor layer model, and one for depth of clear-water scour in a long constriction. The simple computational procedure will be especially useful for evaluating clear-water contraction scour at flow relief bridges crossing floodplains composed of well-graded alluvial till where a coarse protective surface layer is likely to form.