STREAMBED ARMORING

The development of armor layers in nonuniform sediments was investigated experimentally. Study objectives included extending the available data for high shear stresses and defining a criterion for the critical armoring condition. A new experimental technique incorporating an adjustable sediment support table, which provided uniform flow and constant shear stress throughout an experiment, was used. A nonuniform sediment forms stable armor layers for a range of shear stresses, bounded by an upper limit beyond which all grains are indiscriminately in motion and armor layers cannot form. The limiting stable armor layer formed at the highest shear stress is called the critical armor layer and the shear stress the critical shear stress. The dimensionless critical shear stress depends on the ratio of the maximum and median particle sizes of the sediment, decreasing as the ratio increases. The median particle size of the critical armor layer is the maximum particle size divided by 1.8, equivalent to a geometric standard deviation of 1.5. Thus, the composition of the critical armor layer corresponds to a lower limit of a nonuniform material. Uniform sediments do not armor.