Fixing Gully Erosion on a Graded Slope So It Stays Fixed

After one hard storm, the small rut on your graded slope is now a trench you can stand in. Gully erosion moves fast once it starts, and the way most people try to fix it, by dumping fill into the hole, usually makes the next storm cut it deeper.

I have repaired gullies on everything from a 3:1 backyard bank to a 40 foot cut slope beside a county road. The pattern is always the same: the gully is not the problem, it is the symptom. Fix where the water comes from and how it leaves, and the hole in the middle becomes a simple backfill job.

How Gully Erosion Starts on a Graded Slope

Water running off a slope starts as a thin sheet. Any low spot, tire track or equipment cleat mark gathers that sheet into a small channel called a rill. Rills are a few inches deep and you can smooth them out with a rake. Leave them through a few storms and they join up, deepen and become a gully, typically defined as anything too deep to erase with normal tillage, often 12 inches or more.

On freshly graded ground the usual triggers are:

• A roof downspout, culvert or driveway edge dumping concentrated water at the top of the slope.
• Track marks running up and down the slope instead of across it.
• Bare soil left more than a few weeks without seed, mulch or blankets.
• A swale at the top that was graded with no outlet, so it overflows at one weak point.

Walk the slope in the rain if you can. You will see exactly where the sheet flow bunches up.

Step One Is Upstream, Not in the Gully

Before you touch the gully, cut off its water supply. If the slope above is collecting runoff from a roof, parking area or road, that flow needs a new path.

1. Redirect point sources. Extend downspouts with solid pipe to a stable outlet, a rock lined channel or a dry well. A 1,500 square foot roof sheds roughly 930 gallons in a 1 inch rain; that cannot all go down one bank.
2. Build a diversion berm or swale across the top of the slope at a slight grade, about 1 to 2 percent, carrying water to a protected outlet at the side.
3. Armor the outlet. Where diverted water leaves, line the channel with 4 to 8 inch stone over filter fabric so you do not just start a new gully 30 feet over.

Skip this step and every repair below will fail within a season.

Repairing the Gully Itself

Once upstream water is under control, the repair method depends on size.

For backfilling, do not just push loose dirt in. Lay fill in 6 to 8 inch lifts and compact each one with a plate compactor or by tracking over it with a machine. Loose fill soaks up water and slumps, and you are back where you started. The same lift method we use for building pads applies here; see compacting fill in lifts for a building pad.

Key the fill into the gully walls. Cut small steps into the sides of the gully so the new soil locks into undisturbed ground instead of sitting in a smooth trough that water can slip under.

Where Common Advice Goes Wrong

The standard homeowner tip is to fill a gully with rock or broken concrete. I see it everywhere, and it rarely works on its own. Water keeps flowing through and around loose rock, carrying soil out from underneath. In a year you have a rock filled trench with a void beneath it, and often a second gully cutting along one edge.

Rock has a place, but as part of a designed channel or a check dam, set on filter fabric, with its center lower than its edges so water spills over the middle and not around the ends. If you just want to fill a hole, use compacted soil and protect the surface.

The other bad habit is seeding bare fill and hoping. Seed alone washes off in the first heavy rain. On anything steeper than about 4:1, put an erosion control blanket over the seed and staple it every 3 feet. Our comparison of rip rap versus erosion blankets on a steep bank helps you pick the right cover.

Protecting the Repair Until Grass Takes Over

The first 6 to 10 weeks decide whether the repair holds. In that window:

• Install straw wattles across the slope above the repair, spaced about 10 to 20 feet apart on the contour. Here is how to set straw wattles on a graded slope so they do not get undercut.
• Run a silt fence at the bottom if the slope drains toward a neighbor, street or stream.
• Water the seed lightly if rain does not come within a week, enough to keep the top half inch moist.
• Walk it after every storm over half an inch and patch any small rills right away with a rake and a handful of seed.

Once grass reaches 3 to 4 inches with good coverage, the roots do most of the holding.

FAQ

What is the difference between a rill and a gully?

A rill is a small channel a few inches deep that you can smooth with a rake. A gully is deeper, usually a foot or more, and needs backfill or structural repair. Rills left alone become gullies.

Can I fill a gully with dirt?

Yes, but only after redirecting the water causing it, and only if you compact the fill in thin layers and protect the surface. Loose dirt dumped in a gully washes out in the next big storm.

How much does it cost to fix gully erosion?

A small gully under a foot deep can be fixed for a few hundred dollars in fill, seed and blanket. Deep gullies needing rock channels or pipe drains often run $2,000 to $10,000 depending on access and length.

Do I need a permit to repair gully erosion?

Small repairs on your own property usually do not, but disturbing more than an acre or working near a stream or wetland can trigger state or local permits. Check with your county before moving large amounts of soil.