Riprap

Riprap is loose rock, broken concrete, or other heavy, hard material placed by hand or machine along a shoreline, streambank, or bridge pier to armor it against erosion from waves, current, or ice, rather than resisting that force with a single solid wall. Builders lay riprap over a buffer layer of geotextile fabric or crushed stone so the underlying soil cannot simply wash out from beneath the rock, and they vary the size and spacing of the stones to slow, rather than entirely block, the movement of sediment through the waterway; specially engineered concrete shapes such as tetrapods serve the same wave-breaking purpose where plain rock is unavailable or insufficient. Because riprap protects one stretch of bank by redirecting erosive energy rather than eliminating it, the reach immediately downstream often erodes faster once riprap is installed upstream, even as the rocky armor itself becomes habitat for small fish and invertebrates.