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.