riprap

Riprap, also known as rock armor, is an engineering measure that uses a layer or mound of large rock fragments to protect soil from erosive forces applied by water, wind, or ice. It is commonly employed along shorelines, riverbanks, culverts, bridge approaches, dam abutments, and sloped banks to dissipate energy and reduce scour.

Materials for riprap are typically natural quarried rock such as granite, limestone, or basalt, chosen for durability and angularity, or concrete rubble. Angular stones interlock better than rounded ones, improving stability. Stone size, density, and gradation are selected to withstand anticipated hydraulic loads. Smaller projects may use stones in the 4–8 inch range, while larger hydraulic forces require much bigger boulders. A protective layer, sometimes including a geotextile fabric and a bedding or filter layer, is often placed between the soil and the rock to prevent material migration.

Design and installation require a stable foundation and appropriate layering. Riprap is laid with a deliberate gradation to promote interlocking and minimize voids. A bedding or filter layer is placed on the prepared ground, after which the rock is layered to the specified thickness. Stones should be positioned to resist movement under design flow, and excessive compaction is avoided to preserve voids for drainage and energy dissipation.

Advantages include durable erosion protection, ease of replacement, and adaptability to irregular shorelines. Drawbacks include higher initial cost, the need for heavy equipment, potential movement under extreme flows, and maintenance requirements to replace displaced stones or address undermining.

Riprap is a standard technique in civil engineering, guided by project specifications and local regulations. Design methods focus on hydraulic loads, slope stability, and maintainability, with variations such as reinforced riprap and concrete armor units.