Riprap - Wikipedia

Riprap, also known as rip rap, rip-rap, shot rock, rock armor or rubble, is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour and water or ice erosion. Common rock types used include granite and limestone. Concrete rubble from building and paving demolition is sometimes used.

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Source: Riprap - Wikipedia