Protect both ends of a culvert with rock, because the pipe delivers water faster than the ditch did and the extra speed digs out the outlet and undermines the pipe. Place riprap of 15-30 cm rock for at least 1 m upstream and 1 m downstream, add a stone or timber headwall to hold the fill at the inlet, and extend the rock further if you see it moving after a storm. Leave the inlet open or use widely spaced bars, since a fine grate that blocks with debris traps anyone pulled against it.
Part of Culverts
The ends of a culvert are the parts that fail first. Inlet protection keeps the fill from washing out where water enters, and outlet protection spreads and slows the jet that leaves the pipe.
The Pipe Speeds The Water Up, So The Outlet Takes The Damage
A ditch carries water at a modest speed over a wide cross-section. A pipe gathers the same flow into a smaller opening and releases it as a jet. The first thing to do is put a number on that jet.
Worked Example: How Fast The Water Leaves The Pipe
Take the 0.6 m pipe from Culvert Sizing, flowing at 0.58 m3/s in its largest design storm.
- Pipe area: pi x 0.6^2 / 4 = 0.283 m2.
- Outlet velocity if the pipe runs full: 0.58 / 0.283 = 2.05 m/s.
The same flow in the 50 cm x 40 cm ditch at brim-full is 1.17 m/s at 0.5 percent, with an assumed 1:1 side slope (see Ditch Construction). The outlet speed is therefore about 1.75 times the ditch speed, and the energy scales with the square of speed, so the jet carries roughly three times the erosive energy per kilogram of water. A bare earth outlet scours into a pit, the pit undercuts the pipe end, and the end drops and breaks.
What You Need
- Rock — angular rock 15-30 cm across, the size the parent article gives for riprap at culvert outlets. See Riprap.
- Gravel — a layer under the rock so fine soil does not wash out between the stones.
- Headwall material — stone or timber to retain the fill around the pipe mouth.
- A shovel and a rake — for shaping the apron.
Method: Protect Each End
Step 1: Build the inlet headwall. Retain the fill around the pipe mouth with a stone or timber wall so the embankment cannot slump into the opening. Keep the wall short of the opening so it does not narrow it.
Step 2: Lay the inlet apron. Spread a gravel layer, then place rock over it for 1 m upstream. Fit stones by hand so they interlock rather than tipping them in a heap.
Step 3: Lay the outlet apron. Do the same for 1 m downstream, but expect the outlet to need more. Make the apron slightly wider than the pipe so the jet spreads.
Step 4: Set a toe. Dig a trench at the downstream end of the apron and bury the last row of rock in it. Without a toe the water scours beneath the apron and the whole mat slides.
Step 5: Dissipate energy. A shallow depression of rock just below the outlet turns the jet into turbulence in a pool instead of letting it cut a gully. Fill it with rock and check it after every storm.
Step 6: Inspect after storms. If the rock has moved, the apron was too short or the stones too small. Extend the apron downstream and use larger rock. This is the reliable test; a formula for rock size is outside what this page can source.
Keep Fine Grates Off Inlets
Debris will come. A fine grate catches it, backs the water up and then holds anything against the bars with the full pressure difference across them. A 1 m head across 0.25 m2 is 1,000 kg/m3 x 9.81 m/s2 x 1 m x 0.25 m2 = about 2,450 N, equal to the weight of 250 kg. Prefer an open inlet with a line of posts upstream that turns large debris aside, or a grate with widely spaced sloping bars that you clear before storms.
Common Mistakes
- Protecting only the inlet — the outlet takes the faster water and scours worse.
- Dumping rock in a heap — loose rock rolls out. Place and interlock it.
- No gravel under the rock — fine soil washes out through the gaps, and the rock settles into holes.
- No toe — the mat slides when scour undercuts its downstream edge.
- A tight grate on a pipe people can reach — see below.
Safety
A blocked, submerged inlet holds a body against it
Pressure from the water backed up behind a blocked grate pins a person who is pulled against it; at 1 m of head over 0.25 m2 it is about 2,450 N. Never wade to a flowing culvert inlet to clear it. Clear debris with a long rake from the bank when the flow has dropped, or leave the inlet open.
Rock work at the water's edge
Placing riprap means lifting heavy stones on wet slopes beside flowing water. Wear stout footwear, lift with a partner for stones you cannot lift one-handed, and work when the water is low. About 15 cm of fast-flowing water can knock an adult off their feet.
What’s Next
- Learn next: Riprap — placing rock armour on banks and slopes
- Combine with: Safe Outlets — where the water goes after it leaves the apron