Riprap
Riprap is loose, angular stone laid where moving water meets bare soil: at a downspout, under a swale inlet, along a channel too steep for grass. It does not stop the water; it stops the water from taking the soil. The two levers are stone size and channel slope — if the water is too fast for the stone, don't buy bigger feelings, buy bigger rock.
Sizing by situation
| Where | Typical stone | Why that size |
|---|---|---|
| Downspout splash pad | 2–4 in | Breaks the fall; the water's energy dies on the first layer |
| Swale inlet pad | 3–6 in | Inlets concentrate flow; the pad takes the jet |
| Channel bottom, 2–4% grade | 4–8 in | Continuous flow starts moving smaller stone |
| Channel bottom, >4% grade | 8–12 in | At this point check dams should be doing the slowing, not the stone alone |
| Outlet / pipe discharge | 6–12 in | Exit velocity is the highest energy on the site |
These are yard-scale starting points, not an engineered channel design. Slope and flow volume move the numbers; your stormwater manual is the authority — read it.
Rules that decide whether it survives
- Angular, not rounded. River rock is round; round stones say goodbye to each other at the first current. Crushed, angular stone locks.
- Graded, not one size. A range of sizes packs and interlocks. All 8-inch stones leave 2-inch gaps for the water to mine.
- Filter underneath or the soil leaves through the gaps. On erodible soil, a geotextile or a graded gravel layer separates stone from dirt. Without it, the rock sinks into the yard and the yard reappears on top of the rock.
- Thickness ≥ 1.5–2× the largest stone. One stone deep is a costume, not armor.
What riprap cannot fix
Stone at the bottom of a gully with ten times its design flow does not protect the gully; it decorates it. Armor handles energy, not volume. When the water is too much, the fix is upstream — a smaller catchment, a split flow, storage — not a bigger pile at the wound.
Where to go next
- Build check dams — riprap's partner on steep channels.
- Grade — the number that picks the stone size.
- Grass or rock — the channel-lining dispute.
Sources
- Minnesota Stormwater Manual, riprap and channel stabilization — stormwater.pca.state.mn.us
- USDA NRCS — nrcs.usda.gov
- US EPA NPDES, erosion control — epa.gov/npdes