Rip Rap - Erosion Control Stone for Drainage Guide

Rip rap is placed stone used to protect soil, slopes, banks, and drainage channels from erosion caused by flowing water or runoff.

What It Is

Rip rap is made of large angular rock placed over prepared soil, filter fabric, or underlayment so water can move through the voids while the stones resist being washed away. It is used where bare soil or small gravel would erode under concentrated flow.

The system matters because erosion control is not only about the stone size. Slope, subgrade preparation, filter layer, and edge containment all affect whether the rip rap stays in place over time.

Types

Common types include hand-placed rock, dumped quarry stone, graded riprap by specified size range, and specialized bedding layers beneath the visible stone. The exact stone size depends on expected water velocity and site conditions.

Where It Is Used

Rip rap is used along swales, culvert outlets, creek banks, retaining wall toes, shoreline edges, and drainage ditches. On residential properties it often appears where roof runoff, driveway drainage, or hillside flow concentrates enough force to cut the soil.

How to Identify One

Look for a blanket of large angular stone intentionally placed on a slope or at a water outlet, not decorative landscape rock scattered loosely. Displaced stones, undercut soil, washouts behind the rock, or missing fabric are signs the installation may be failing.

Replacement

Replacement usually means regrading the base, rebuilding the filter layer, and adding properly sized stone rather than simply tossing new rock on top. If runoff is concentrated by a downspout, culvert, or drain outlet, that upstream water issue should be corrected at the same time.

Rip Rap — FAQ

What is the difference between rip rap and decorative rock?

Rip rap is engineered erosion-control stone sized and placed to resist moving water, while decorative rock is chosen mainly for appearance. Decorative gravel usually washes out much more easily. The function, size, and installation method are different.

Why is my rip rap washing downhill?

The stone may be undersized, the slope may be too steep, the base may be eroding underneath, or runoff may be stronger than the installation was designed for. Once the filter layer fails, the rock often starts to unravel. Simply adding more loose stone rarely fixes the root cause.

Does rip rap need fabric underneath it?

Often yes, or it needs another designed filter layer. The purpose is to keep soil from migrating out through the voids between stones. Without that support, the rock can sink, separate, and lose effectiveness.

Can I use rip rap around a downspout outlet?

Yes, that is a common residential use. The stone still needs enough area and proper sizing to spread the water energy. If the discharge is strong, a splash block, pipe extension, or better channel detail may also be needed.