By Lynn Betts
A mile-long stretch of re-designed drainage ditch in southeastern Minnesota is now partially filtering the tile water it carries away from crop fields. Vegetated, flat benches on each side of the ditch channel catch the tile water before it runs into the channel below, slowing the flow to the channel and allowing some infiltration.
The stretch of what's known as the Mullenbach Ditch has 10 major side inlets as well as numerous field tile inlets emptying into it. Riprap and erosion control fabric at each inlet dissipate the water's energy and promote water spreading over the bench area.
"We estimate the 2-stage design is removing about 20% of the nitrates in the water from 3,500 acres of tile draining into that part of the ditch," says Rich Biske of The Nature Conservancy (TNC). "It's doing that with no negative effects on the twenty-some farmers in the watershed, including the three with land bordering the ditch."
Narrow stream of water makes sense
Forcing water into a narrower stream in the center of the ditch to scour the bottom makes sense to Randy Smith, who has a corn, hay, and hog operation along the stream in partnership with brothers Rick and Joe. "The narrow stream and benches keep the water cleaner. I like the idea of the benches to filter the tile water, too," says Smith, whose land borders the ditch.
"We lost a little ground -- maybe ten feet on each side of the ditch with the wider design, but that's no big deal," says Smith, the president of the landowner ditch group that maintains the private ditch. He says water has been up in the ditch at least four times since it was completed about three years ago, and it's holding up well.
The University of Minnesota is researching the long-term stability of the ditch as well as its ability to remove nitrates. The monitoring includes finding how much more nitrate is removed from two areas graded into linear wetlands within the bench, effectively stacking N-removal practices.
Dozens in Indiana
While the Mullenbach Ditch is one of only a few 2-stage ditches in Minnesota, more can be found in Indiana, Ohio and Michigan. "I became interested in the practice after talking to colleagues in Indiana. I found researchers at the U of M with a similar interest in 2-stage ditches." Biske says.
"They won't work everywhere," Biske says. "You need stable soils and the right hydrology. But they have dozens of them in Indiana -- it's an EQIP practice of NRCS there. The environmental benefits haven't been fully assessed in Minnesota yet, but when they're better proven, I expect the 2-stage ditch to be an NRCS practice in more states."
An ideal candidate for a 2-stage makeover would be a drainage ditch with high nitrate loading and stable soils that requires cleanout, in a watershed of a priority stream. Landowners should consult with a qualified drainage engineer, Biske says.
Conservation innovation grant funds
TNC secured funds from Cargill and General Mills to match funds from a USDA Natural Resources Conservation Service Conservation Innovation Grant for the $200,000 project. The grants are intended to demonstrate and promote wider use of proven conservation practices. Partners in the project include TNC, the Mower County Soil and Water Conservation District, Minnesota Pollution Control Agency, and the Minnesota Board of Soil and Water Resources.
"This is one of only a few 2-stage ditches in the state, but we hope to see more," Biske says. So would county drainage inspector Cody Fox. "I like what I see so far," Fox says. "I think we'll have less sediment in the channel with flatter side slopes and the benches below. This should be good to go for years to come.
"There's been a lot of effort to get buffers planted at the field edge above the ditch, and that's good," Biske says. "But you can treat so much more area for nitrate reductions when you treat the tile water, too."
Slow runoff with side inlet filters
The Mullenbach Ditch has one side inlet that's designed as a filter itself. Rather than using a large culvert that delivers surface runoff directly to the ditch, a vertical inlet was installed in a filter strip that first funnels water through a 20-foot wide bed of small rocks.
"Joel Peterson, an engineer with the Minnesota Board of Water and Soil Resources at the time, designed the filter," Biske says. "The purpose was to slow the runoff down, to hold it in the filter strip for a few hours.
So many of the water impairments we have are from flushing so much water off the land so quickly," Biske says. "If we can reduce peak flow and allow the soil to do some filtering, we can do a lot of good."
A filter system like this could apply anywhere a side inlet empties into a drainage ditch.
Betts writes from Johnston, Iowa.