Rip Rap 101: Protecting Slopes, Culverts, and Stream Banks

Rip rap is large, irregular armor stone, typically ranging from fist-sized to well over a foot across depending on the application, placed to absorb and dissipate the direct energy of moving water. Unlike crusher run’s compacted stability or clean gravel’s drainage function, rip rap’s job is pure physical mass and interlock against force.

Water exiting a culvert pipe is concentrated and moving fast — exactly the conditions that erode soil quickest. A rip rap apron at the outlet spreads and slows that energy before it can dig out a hole under the pipe or wash out surrounding ground, which is one of the more common causes of culvert failure over time.

On steep cut or fill slopes, especially those with concentrated flow paths, rip rap holds the ground in place against both direct rainfall impact and channelized runoff. It’s typically placed over a filter fabric or bedding layer that keeps the underlying soil from washing out from beneath the rock itself.

Along stream banks, rip rap resists the ongoing erosive force of flowing water and holds the bank’s shape rather than letting it slowly undercut and collapse — this often requires permitting depending on the waterway, which is worth checking before starting work near a stream.

Sizing depends on the velocity and volume of water the rock needs to withstand — a low-flow drainage swale needs much smaller stone than a culvert handling heavy stormwater. Undersized rip rap defeats the purpose and will eventually move just like any other rock; oversizing wastes material and money. This is typically judged on-site based on the specific flow conditions.