What is Riprap and How Does It Stop Erosion?
Riprap (also spelled rip-rap or rock armor) is heavy, angular quarry-blasted stone placed along riverbanks, highway drainage ditches, culvert spillways, and bridge abutments to shield vulnerable soil from high-velocity stormwater scouring.
The irregular, faceted shapes of crushed quarry rock interlock when dumped or hand-placed. When stormwater rushes down a culvert outfall, the jagged voids between stones break up water velocity, dissipating kinetic energy before it can wash away native topsoil.
Alaska Pro-Tip: In Southcentral Alaska, spring breakup runoff from melting Chugach snowpacks creates torrential runoff down drainage easements. Using Class II riprap (6" to 12" stone) at culvert outfalls prevents yard and driveway washouts.
Standard DOT and USACE Riprap Classifications
State Departments of Transportation (DOT) and the U.S. Army Corps of Engineers (USACE) categorize riprap based on median stone diameter (D50):
| Riprap Grade / Class | Median Stone Size (D50) | Recommended Minimum Layer Depth | Typical Application |
|---|---|---|---|
| Class I | 6 inches | 9 to 12 inches | Roadside swales & small residential ditches |
| Class II | 9 inches | 12 to 18 inches | Culvert outfalls & residential creek banks |
| Class III | 12 inches | 18 to 24 inches | Fast stormwater channels & retention pond inlets |
| R-4 / Heavy | 15 inches | 24 to 30 inches | Bridge scour protection & river revetments |
| R-5 / Quarry Spalls | 18+ inches | 30 to 36 inches | Coastal wave protection & major floodways |
Frequently Asked Questions
Common questions regarding this calculation and code compliance
One cubic yard of standard crushed granite or basalt riprap weighs approximately 1.35 to 1.45 US tons (2,700 to 2,900 lbs), depending on stone angularity and void ratio.