Rock Armour — also known as riprap or armourstone — is a layer of large, durable, interlocking rock placed to protect shorelines, river banks, embankments, culvert outfalls and structures from erosion and scour. The mass and interlock of the individual stones dissipate wave and flow energy that would otherwise strip away the underlying soil.
Where rock armour is used
Typical UK applications include coastal defence revetments, river and canal bank protection, bridge pier and abutment scour protection, culvert inlet and outfall aprons, reservoir spillways and the toes of embankments and reinforced slopes. Rock armour is frequently specified alongside geotechnical works where watercourses interact with new infrastructure.
Grading and standards
Armourstone in the UK is specified to BS EN 13383 (Armourstone), which defines standard heavy and light gradings by mass — for example 60–300 kg or 300–1000 kg. Stone size is selected from the design flow velocity or significant wave height, commonly using methods in CIRIA C683 (The Rock Manual). The rock must be dense, durable and resistant to abrasion and freeze-thaw; granite, basalt and hard limestone are common sources.
Rock armour vs gabions
Gabions confine smaller, cheaper stone inside wire mesh baskets, achieving stability through the combined mass of the basket rather than individual stone weight. Gabions suit engineered, regular-profile walls and channel linings where large stone is uneconomic; loose rock armour suits exposed coastal and high-energy fluvial environments where mesh would corrode or abrade. Rock armour also tolerates settlement and can be topped up over its life.
Installation and filter layers
Rock armour must be placed over a filter system — a graded granular underlayer, a geotextile, or both — to prevent the underlying fine soil washing out through the voids between stones. Placement is normally by long-reach excavator with individual stone keying to achieve a dense, interlocked matrix at the design thickness (typically 1.5–2 times the nominal stone diameter).
Related ViaCon solutions
ViaCon supplies geotextiles, gabions and erosion control systems used with and as alternatives to rock armour. See our GeoTechnical Solutions. Related glossary entries: erosion control, gabion and geotextile.
Frequently asked questions about rock armour
What is rock armour?
Rock armour, also called riprap or armourstone, is a protective layer of large, durable rock placed on banks, coasts, embankments and around structures to dissipate wave or flow energy and prevent erosion and scour.
Is rock armour the same as riprap?
Yes. Riprap is the common international term and rock armour the usual UK term for the same erosion protection technique. Armourstone refers to the rock material itself, specified to BS EN 13383.
What size is rock armour?
Armourstone is graded by mass to BS EN 13383 — light gradings such as 60–300 kg for rivers and heavy gradings of 300–1000 kg or more for coastal works. The required size is calculated from design flow velocity or wave height, typically using CIRIA C683 methods.
Does rock armour need a geotextile underneath?
Yes, in almost all cases. A geotextile filter layer, a granular underlayer or both must be installed beneath rock armour to stop the underlying fine soil being washed out through the voids between stones, which would cause the armour layer to settle and fail.