Incipient Motion during Static Armoring

Abstract

Critical bed shear stresses for grains in a mixed-size bed are known to depend on the ratio between the size of dislocated grains and an average grain size for the bed. However, laboratory experiments with sediments consisting solely of two fractions show that changes in armor-area concentration have a marked additional impact on critical stresses for the fines. If the bed area is covered up to 30% by static armor grains, critical stress for the mobile fraction is about the same as for unisize sediments, while as the specific armor-area approaches unity, critical stress increases by a factor of five. To a minor extent, critical stress also depends on a grain Reynolds number and the grain size ratio between the two fractions.