Riprap is a layer of large angular rock, 15-30 cm across, placed on slopes where water runs fast enough to erode soil: stream banks, ditch junctions, culvert outlets and steep hillsides. Place the stones by hand so they interlock, bed them on a layer of gravel so soil cannot wash out between them, bury the bottom row in a toe trench, and make the mat at least two stones thick. The rocks absorb the energy of the water without moving.
Part of Flood Control and Drainage
Riprap is rock armour: a blanket of stone laid over soil to protect it from running water where plants alone cannot hold.
Rock Works Because It Is Too Heavy For The Water To Move
Flowing water pushes on whatever it meets. Soil and grass give way at a certain speed, but a large rock keeps its place at far higher speeds. Riprap puts a skin of such rock over the soil, so the water meets stone instead of earth. The parent article names the places it belongs: stream banks, ditch confluences, culvert outlets and steep hillsides.
Riprap is not a pile of stones. Its strength comes from the stones touching, being angular, and sitting on a filter layer that stops the soil beneath being drawn out.
What You Need
- Angular rock, 15-30 cm across — broken quarry rock or fractured field stone. Rounded river cobbles roll out of place. Include some smaller stones to fill gaps.
- Gravel — a filter layer under the rock.
- A crowbar, spade and gloves.
- A sloped, shaped surface — the bank or outlet dressed to a smooth slope before the rock goes on.
Method: Place Riprap
Step 1: Dress the surface. Grade the bank or outlet to an even slope. Remove roots and soft spots.
Step 2: Dig the toe trench. Dig a trench along the bottom of the slope, deeper than the rock is thick. The toe is where scour starts, and unsupported rock slides.
Step 3: Lay the gravel filter. Spread gravel over the dressed surface. It stops fine soil washing out through the gaps and the rock settling into holes.
Step 4: Place the rock by hand. Start at the toe and work up, setting each stone so it bears on those below and beside it. Fit smaller stones into the gaps. Do not dump rock from a cart and leave it.
Step 5: Keep it two stones thick. A single layer lets a gap expose the gravel and the soil. Two stone-thicknesses is a common minimum.
Step 6: Extend above the high-water line. Cover the bank to the highest water you expect, and wrap the rock into the bank at both ends so water cannot run behind it.
Step 7: Inspect after storms. Moved rock means the stones were too small or the mat too short. Add larger stone and extend it.
Worked Example: How Much Rock A Bank Needs
A bank section 10 m long and 1.5 m up the slope, protected with a mat 0.4 m thick (two stones of about 20 cm).
- Volume in place: 10 m x 1.5 m x 0.4 m = 6.0 m3.
- Gravel filter at an assumed 0.1 m thick: 10 x 1.5 x 0.1 = 1.5 m3.
So the rock is about 6 m3 in place, and the gravel about 1.5 m3. Loose rock in a pile or cart takes more volume than the same rock in place, so order more than the figure. Sizing the stone from the water speed by formula is a design calculation outside what this page can source; use the 15-30 cm range from the parent article and the inspect-and-extend rule above.
Common Mistakes
- Rounded stones — they roll. Use angular rock.
- Stones dumped, not placed — they leave gaps and rest on points.
- No filter layer — soil washes out and the mat sinks.
- No toe — scour undermines the base and the mat slides down.
- Ending the mat where the water speed is still high — scour starts at the end and works back. Extend further downstream.
Safety
Heavy stone beside moving water
A 25 cm angular rock weighs several tens of kilograms and rolls without warning on a wet slope. Place stone from above or beside it, never from below, and keep feet clear of stones in motion. Do not work at the edge of fast water; about 15 cm of fast-flowing water can knock an adult off their feet. Wear gloves and sturdy boots, and use a lever or two people for any stone you cannot lift easily.
What’s Next
- Learn next: Inlet and Outlet Protection — riprap at culvert ends
- Combine with: Gabion Walls — rock held in cages
- Combine with: Vegetation Cover — grass on the slope above the rock