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Published April 14 2012

Diversion Discussion: Corps says dredging not feasible for permanent flood protection

FARGO - Among debates over the proposed Red River diversion, critics continue to suggest one potential solution that they say would only affect land in and near the river channel: dredging.

Dredging would entail digging a deeper and wider riverbed, so the Red River would have more room to flow naturally through Fargo-Moorhead.

Theoretically, dredging might sound like a simple alternative to diminish the river’s chances of flooding, but engineers say dredging is a lot more complicated and risky than expanding a riverbed.

Early on in their study of Fargo-Moorhead flood protection, the U.S. Army Corps of Engineers looked at dredging as one of a few dozen possible solutions to the metro area’s persistent flood problems.

But within the first months of the three-year feasibility study, dredging was one of several options scratched off the list.

The corps’ initial look offered anything but optimism that dredging would be the ideal solution for the metro region.

According to the corps’ feasibility study – which details the alternatives the corps looked at before deciding on a diversion – these were the main reasons dredging was deemed an unacceptable option:

• Limited hydraulic effectiveness, since dredging would make the riverbanks unstable and lead to further erosion, which could affect nearby roads and bridges.

• Constant upkeep, since the build-up of sediments in the river would negate any benefits of the project and require repeated dredging to maintain any of the protection gained.

• Extreme environmental consequences, such as displacement of mussel populations, habitat loss, impact to aquatic life and potential mortality issues for wildlife during construction.

“This proposal would probably have the greatest environmental impact of any of the alternatives considered,” corps project manager Aaron Snyder said recently.

Snyder added that the sheer size of the channel that would be needed to protect against the revised definition of a 100-year flood would also make dredging prohibitive.

The current channel can pass about 9,600 cubic feet per second of water. In comparison, a 100-year flood passes 34,700 cfs and a 500-year flood passes 61,700 cfs, Snyder said.

“It would not be reasonable to make the channel wide enough through town to handle three to six times as much flow as it currently does,” he said.

The corps’ initial look at dredging also yielded the possibility of necessary buyouts along the river through the heart of Fargo-Moorhead.

Snyder said the corps didn’t estimate how many properties might be affected in the process, since that figure would depend on the final width of the dredged channel.

Regardless, Snyder said properties would need to be bought out to accommodate both the dredging itself and to make room for the soil that would be removed from the riverbed.

During various public meetings on the proposed Red River diversion project, critics have suggested various options – like dredging – that corps engineers looked at but didn’t study as in depth as they did the chosen diversion plan.

Such critics have said the corps should have studied all possible solutions more fully so engineers would know for certain the potential benefits and consequences of each option.

Snyder said ruling out some options while pursuing more viable ones is part of the corps’ process.

“As stewards of public funds, the corps uses professional judgment in our studies,” Snyder said. “It would be unwise to spend large sums of money developing details for alternatives we can reasonably dismiss on the basis of our experience with similar projects.”

On dredging specifically, Snyder said, “We can say for certain that this plan would not be implementable.”

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Have a question about the proposed Red River diversion, or want to recommend a topic you’d like to see addressed in an upcoming column? Send an email to Forum reporter Kristen Daum at kdaum@forumcomm.com (Subject: Diversion Discussion) or write to Kristen Daum c/o The Forum, P.O. Box 2020, Fargo, ND 58107.