Building a Diversion Berm Above a Slope That Holds Up

A fresh slope rarely fails because of the rain that falls on it. It fails because of the water that runs onto it from the ground above, picking up speed across a driveway, a roof line or an upslope lawn and pouring over the crest in a sheet. A diversion berm stops that water before it reaches the slope face and walks it to a place that can take it.

I design these for residential and small commercial sites, and the berm itself is the easy part. The grade of the channel behind it and the outlet at the end decide whether it works. Here is how to size, build and finish one so it survives the first storm and the fiftieth.

When a diversion berm is worth building

Not every slope needs one. Look at the ground above the crest. If more than about 50 feet of lawn, pavement or roof drains toward the slope, the runoff arriving at the crest will carry enough energy to start rills within a season. Signs that you need a diversion berm include:

• Fan shaped sediment deposits at the toe after rain
• Rills starting at the crest and running straight down the face
• A driveway or parking area that sheds water toward the top of the slope
• Bare spots near the crest where seed keeps washing out

If several of these show up on a slope you just graded, the problem is upslope water, not your seed mix. Our breakdown of why a freshly graded slope fails after the first storm walks through the other causes so you can rule them out.

Sizing the berm and the channel

A diversion berm works as a pair: the raised ridge and the shallow channel on its upslope side. Water collects in the channel and flows along it, parallel to the crest, toward the outlet. These are the dimensions I start from on most residential sites with a drainage area under two acres.

Larger drainage areas need a hydraulic calculation, and many jurisdictions require one stamped by an engineer for permanent diversions. If your site disturbs an acre or more, the berm may also need to appear on your stormwater plan.

How to build a diversion berm step by step

1. Stake the line. Set stakes every 25 feet along the berm route and mark the channel grade with a laser or builder's level. A 0.5 percent grade drops 1.5 inches every 25 feet, which is easy to miss by eye.
2. Strip the footprint. Remove sod and topsoil under the berm so the fill bonds to firm subsoil rather than sliding on a grass layer.
3. Cut the channel and place the fill. On most jobs, the channel cut supplies the berm fill. Use the heavier, clay rich material for the berm core and keep rocks larger than 4 inches out of it.
4. Compact in lifts. Place fill in 6 to 8 inch lifts and compact each with a plate compactor or the tracks of a skid steer. Aim for at least 90 percent standard Proctor density.
5. Shape and check grade. Run a level along the channel flow line and fix any low spots. A single dip will pond water and soak the berm until it slumps.
6. Stabilize within 7 days. Seed the berm and channel, then cover with an erosion blanket or a heavy straw mulch. Bare berms erode from the top down within a couple of storms.

With a skid steer and a two person crew, a 150 foot berm usually takes one day. Contractor pricing in our area runs about $8 to $15 per linear foot, more when the outlet needs rock.

The outlet matters more than the berm

Every diversion moves the erosion problem to the end of the channel. If that end dumps concentrated water onto bare soil, you will get a gully there instead of rills across the face.

Good outlets include a rock apron on stable ground, a level spreader that releases water as a sheet across undisturbed vegetation, a stabilized swale, or a connection to an existing storm structure. Size rock aprons with stone at least 6 inches across and extend them at least 10 feet. If the outlet channel runs downhill steeply, add a series of rock check dams in the swale to slow the flow.

Why making it taller is usually the wrong fix

When a berm overtops, the common response is to add a foot of dirt and call it done. I see this advice on job sites constantly, and it rarely solves anything.

Overtopping almost always happens at a low spot in the channel or because the outlet is blocked. Water stacks up behind the blockage until it finds the lowest point of the ridge. Raise the whole berm and the water simply ponds deeper, saturates the fill and eventually breaches a weaker section with more force than before. Find the low spot with a level, clear the outlet, and repair the grade. Height only helps when the drainage area really outgrew the original design, which a quick runoff calculation will show.

Inspection after each storm

Walk the berm after every rain of half an inch or more during the first season. Look for sediment collecting in the channel, slumps on the downslope face, and scour at the outlet. Remove sediment once it fills a third of the channel depth. Pair the berm with a sediment barrier below the slope while vegetation establishes, using the approach in our guide to silt fence installation that actually holds sediment.

Frequently asked questions

What is the difference between a berm and a swale?

A berm is a raised ridge of compacted soil, while a swale is a shallow channel. A diversion uses both together: the channel carries water and the berm keeps it from spilling over. Many sites use a swale alone where there is enough room to grade a wide, gentle channel.

How high should a diversion berm be?

Most residential diversion berms are at least 18 inches tall after settling, with 2:1 or flatter side slopes. The required height depends on the drainage area and the channel capacity. For larger areas, an engineer should calculate it from the design storm.

Can I build a diversion berm with topsoil?

Topsoil alone compacts poorly and erodes easily, so it makes a weak berm core. Use subsoil or clay rich fill for the body and place a thin layer of topsoil only on the surface for seeding. Compact the core in lifts before adding that surface layer.