Riprap Design Basics: Stone Size, Filter Layer and Placement

Riprap is a layer of large stones protecting slopes and shorelines from flowing water and waves. Its apparent simplicity should not mislead; most riprap failures come from unsuitable stone size or a missing filter.

How stone size is set

The required size comes from hydraulic design. In common relationships, stable stone size rises roughly with the square of flow velocity, and its weight roughly with the sixth power. Doubling velocity therefore calls for a far heavier stone. Design guides such as HEC-23 and HEC-11 provide these calculations.

Stone properties

• Durability: stone must stay stable through wetting, drying and weathering.
• Density: denser stone is more stable at the same size.
• Shape: angular pieces interlock better than rounded ones.
• Grading: a range of sizes reduces voids and keeps the layer stable.

The role of the filter layer

Without a filter, underlying soil washes out between the stones and the layer settles. A granular filter or geotextile stops this migration while still letting water pass. Granular filter design is explained in Drainage and filter materials.

Placement notes

1. Place stones in a controlled way so segregation does not occur.
2. Set layer thickness in proportion to the largest stone.
3. Continue the toe to sufficient depth against scour.
4. Secure the edges of the layer to prevent erosion behind it.

Maintenance

After a flood or storm, check for displaced stones and settlement. A small failure does not spread if repaired early. See also the specifications of riprap and the requirements for slope protection and water structures.