Armor (hydrology)

In hydrology and geography, armor is the association of surface pebbles, rocks or boulders with stream beds or beaches.

Most commonly hydrological armor occurs naturally; however, a man-made form is usually called riprap, when shorelines or stream banks are fortified for erosion protection with large boulders or sizable manufactured concrete objects.

Hydrological modeling indicates that stream armor typically persists in a flood stage environment.

For example, if there is a large piece of sediment that sits on the bed armor layer of the river it can change the threshold for critical flow.

This effect can create a positive loop, with the critical flow disrupting smaller sediments downstream which repeat the process.