WILLMAR -- There are 27,000 miles of open ditches in Minnesota, and quite possibly, every mile has been built based on a flawed design.
Engineers have always designed ditches to accommodate the volume of water expected at a peak flow event, according to Dr. Bruce Wilson, a professor of engineering at the University of Minnesota.
The design assures the channels can handle large rain events.
The design also makes certain that most ditch channels are too big for low-water periods during much of the year. An oversized channel means slow water flow, and that allows sediment to build up.
The result of what Wilson called a "flawed'' design is a never-ending and costly struggle to keep ditches clean and repaired.
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Wilson spoke recently to people attending the Hawk Creek Watershed Project's annual meeting in Willmar.
New research is suggesting that ditches should really be designed on a "two-stage'' model, Wilson said. A smaller channel within the main channel should be built to keep water flowing during low-water periods. The larger overall ditch channel would continue to be designed to offer the capacity for high-water periods.
Wilson said the goal is to create what are called "self-sustaining'' ditches that do not require ongoing maintenance.
Along with reducing maintenance, research also suggests that this approach can reduce the amount of nitrogen being carried by ditches to waterways and, ultimately, the Gulf of Mexico. Nitrogen can be absorbed by plants in the channels of ditches that are designed in this manner, he said.
The idea for a two-stage ditch comes from the simple observation that over time, manmade ditches attempt to meander and do what natural waterways do. That led to the obvious conclusion: "Maybe we should design it like a natural system in the first place,'' he said.
The two-stage design avoids steep-sloped banks and that also helps eliminate the bank sloughing that occurs from seepage in many conventional ditches.
Wilson is now in the process of conducting research to evaluate the merits of a two-stage design.
While there are obvious advantages, they need to be weighed against their costs. Will the two-stage design require that ditches be wider, and consequently take more land out of production? Will the benefits of allowing vegetation in the lower channel to reduce the nitrogen levels lead to costly maintenance to keep woody plants from taking hold and impeding water flows?
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These are among the questions Wilson hopes to answer. He said Ohio has been a leader in research on two-stage ditches. But he said the information from Ohio cannot be directly applied here, since the precipitation levels and soil types are different.
