Criterion Delineating the Mode of Headcut Migration

Abstract

Two modes of headcut migration are generally recognized: (1) Rotating headcuts that tend to flatten as they migrate; and (2) stepped headcuts that tend to retain nearly vertical faces. A mathematical description of the sediment detachment potential immediately upstream and downstream of the headcut is used to delineate these modes of migration. The delineating parameter is the ratio of the time required to erode the headcut face from above to the time required to undermine the headcut face from below. This erosional time‐scale ratio is a dimensionless function of flow, sediment, and geometry parameters. For the limiting case of homogeneous cohesive soils, the time‐scale ratio is a simple function of a Froude number and the aspect ratio of drop height to normal flow depth. This relationship is calibrated using original laboratory experiments of headcut migration in initially vertical headcuts and verified by independent field experiments of headcuts propagating in four different homogeneous cohesive soils.