Riprap
"A riprap layer on the downstream slope of an embankment dam can generally provide some protection against the initiation of embankment erosion during overtopping flow. Riprap is generally composed of high quality quarried rock (often granite, volcanics, or limestone), or occasionally concrete rubble and is dumped or manually placed over a suitable bedding layer. With riprap in place, the overtopping flow is conveyed both through and above the riprap layer, thus preventing erosion by reducing flow velocities and shear stresses along the surface of the erodible embankment materials. Riprap is generally considered to be lower-priced than many other erosion protection alternatives when suitable borrow sources are available nearby." [1]
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