Riprap is large, angular stone placed to protect soil from moving water. It appears at culvert outlets, along channel banks, at basin spillways, and anywhere flow concentrates enough to scour. It looks simple, and it fails predictably when it is designed carelessly.
Stone size is selected from the design flow velocity, the depth, and the slope. The specification is a gradation — a median stone size with limits above and below — not a single dimension. Stone that is too small moves in the first large storm; a uniform gradation leaves voids that flow exploits.
Angular quarry stone interlocks. Rounded river rock rolls. On any slope where stability matters, the shape of the stone is as important as its size.
Water passing through the riprap will pull fine soil out from underneath it until the layer settles and unravels. A properly graded granular filter or a geotextile filter fabric under the stone prevents that migration while still allowing water to pass.
A riprap blanket is typically placed at one and a half to two times the maximum stone dimension in thickness, so the layer is at least two stones deep. It must also extend far enough — up the bank above the design water surface, and downstream past the zone of turbulence at an outlet. Aprons cut short simply move the scour hole a few feet.
The upstream edge is keyed into the bank and the toe is carried below the anticipated scour depth. Undermining at the toe is the most common way a well-built apron comes apart.
Done properly, riprap is a permanent, low-maintenance solution. Done casually, it is a pile of rock in a ditch.