Year after year, Donnelly, Minn., farmer Dave Liebl would close the gully that formed in one of his fields, only to have it reopen. “I’d dig it, and it would look smooth. Then after a hard rain, there’d be another gully in the same place.” The ditch below the field had silt several feet deep.
“Our land is washing away,” Dave’s wife, Karen, says. “This is crazy. Let’s do something.”
The Liebls installed two water- and sediment-control basins in the field to slow the speed of water flowing through the draw. These short, earthen dams bridge the drainage-way, preventing runoff water from cutting a new channel. The dams, which are 3-4 ft. high, are broad enough to be farmed on both banks. Now, the Liebls’ soil stays put, and the field — minus the gully — is more farmable. “It works so well, I can’t say enough good about it,” Dave says.
Water- and sediment-control basins, or WASCOBs, are a time-tested way to halt gully erosion with little or no loss of productive cropland, says Jeff Hellermann, NRCS district conservationist in Stevens County, Minn. They also improve drainage water quality and boost crop production in fields with irregular topography, he says.
Hellermann is seeing renewed interest in the practice these days, in part because the region has experienced more pounding rainfalls in the last few years. These intense rainstorms carve out small gullies, which then transport field runoff laden with sediment, fertilizer and other pollutants.
“A lot of growers disk them closed,” says Scott Wallace, NRCS district conservationist in Peoria County, lll. “But they tend to show up again and again, even with no-till.” Illinois Soil Conservation transect surveys estimate that gully erosion occurs on about 20-25% of fields in the state.
“Ephemeral gullies , are a substantial source of sediment leaving fields,” says Matt Helmers, Iowa State University (ISU) agricultural and biosystems engineer. “And we’re seeing more of them in recent years.”
The Upper Midwest’s climate is getting wetter, and the number of severe downpours is on the rise, notes Agronomist Richard Cruse, director of ISU’s Iowa Water Center. “Today’s soil erosion rates are expected to increase disproportionately as precipitation continues to rise,” he says. “A 20% increase in precipitation has been shown to increase erosion rates by an estimated 37%.”
Helmers adds: “We are taking out a lot of grassed waterways that might have provided some protection from gully erosion.”
Grassed waterway alternative
In small watersheds of fewer than 40 acres, WASCOBs can achieve the same goals as grassed waterways, Hellermann says.
They work best in hilly fields with irregular slopes not uniform enough for contour practices, such as terraces. The embankments are often built in a parallel series across an area of concentrated water flow.
The structure includes an earthen dike high enough to hold back runoff water from a 10-year storm event. Most dikes are about 4 ft. tall with broadly sloping banks so they can be farmed on both sides. Taller, steeper dikes are planted with grass.
After a cloudburst, water and sediment from upslope are trapped behind the dike. The sediment settles out as the pooled water drains slowly into a discharge pipe and tile line. The field does not need to be tiled, but an adequate, stable outlet must be available, Hellermann notes. The ponding area is designed to drain within 36 hours to avoid permanent crop damage.
Dave and Karen Liebl considered a grassed waterway to solve their erosion problem. But “that would have taken quite a bit of land out of production,” Dave says. Grassed waterways are generally 30 - 50 ft. wide, and they tend to have quackgrass, Dave says. “And you have to be really careful when spraying.”
With today’s wide sprayer booms and faster speeds, it can be hard to avoid damaging grass waterways with herbicide, Hellermann agrees. “Little by little, the edges get sprayed with glyphosate, and after awhile they’re too narrow. Water then flows around the remaining grass, causing gulley erosion.”
Designed to be farmed over
In a productive field, “I’d go with a sediment dam versus a grassed waterway,” says Alberta, Minn., farmer Jim Krosch. In 2004, he installed four, 4-ft.-tall sediment-control dams to intercept runoff in a field that falls 8 ft. in half a mile.
“Water would cut gullies in the field every year. We’d work them shut with a digger; then they’d wash out again. We were losing a lot of soil. Now there’s very little erosion off the field.”
Krosch crops all his sediment-control dams, including a large, L-shaped dam in another field that’s about 6 ft. high. “I’ve never had a problem farming around them,” he says. “When the rest of the field has been ready to work, the basins have been, too.”
One year, he did have some crop loss when an intake pipe got plugged up with residue and failed to drain properly. Another time, water overtopped one of the berms, damaging it.
WASCOBs don’t stop sheet and rill erosion, so at least 30% residue cover is needed to reduce sediment loading in the basin, Hellermann says. Krosch, who farms very heavy clay loam soils, also suggests installing some subsurface tile at the top of the drainage-way to cut surface flow volume and extend the life of the WASCOB. Eventually, the ponding area fills up with sediment and has to be scraped out.
Downsides of WASCOBs
The cost of a WASCOB varies with the site, but a typical structure that can be farmed over costs around $5,000, including the tile line, Hellermann says.
Most farmers prefer to install broad-based dikes that can be cropped, if the site permits, says Illinois’ Scott Wallace, “but the downside is the amount of soil it takes to build them. A narrow, all-grass ridge takes two-thirds less soil and is much cheaper to install. It’s just as effective as a broad-based dam and more durable.”
Sediment dams are also quite a bit more expensive than a grassed waterway, especially if built in a series, although many farmers consider WASCOBs a better value, long-term, Wallace says. And there are a variety of federal, state and local cost-share incentives that can cover up to 75% of installation. Subsidies vary by state, so check with your local NRCS office.
Field logistics are another challenge, Wallace says. Often, the dikes can’t be aligned with the preferred farming direction. They pose an obstacle for wider planters and combines, too, he says, “although we try to make them fit equipment sizes.”
Gullies that straddle property lines can also be a hurdle. Doug Backman, an Alberta, Minn., farmer, is leading an effort to fix a long gully that cuts through a quarter section of land he bought last year.
The channel, which collects water from a 100-acre watershed, begins on his neighbor’s land, angles across Backman’s new farm, and deposits tons of sediment on a third farmer’s land. At first, Backman thought he could take care of the problem on his own, but he soon realized that he’d need cooperation from his neighbors to stop the erosion. “We got everybody together and it’s been a good experience.”
The $105,000 project includes 17 water and sediment-control dams on the two upland farms. The downslope landowner is providing access to an outlet through a new 18-in. tile line. “They recognize the benefit they will get, too,” Backman says. “They get all the silt on their land, and it smashes the crop.”
Backman has experience farming around WASCOBs on some of his rented land. In terms of field operations, the dams are “a bit of a disadvantage,” he says. “But the advantages outweigh the disadvantages. When the topsoil is gone, there’s no fix.”