Damage mechanisms and influence of gradation for steep riprap

Model tests in a laboratory flume were performed to verify certain hypotheses put forward to explain steep riprap behavior observed in the field at several embankment dams sites. The laboratory study has focused on riprap degradation under wave attack, and on the influence of fine blocks incorporated into riprap. Unlike flatter riprap, steep riprap is characterized by a low rate of damage until a point of acceleration after which damage progresses rapidly. At that stage, damage is characterized by sliding and crumbling, which results in an upward propagation of damage toward the crest. The inclusion of a portion of small blocks into the riprap significantly reduces the stability of steep riprap. This detrimental effect is gradually reduced as the riprap slope becomes flatter.