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Dry Run Creek study: adding culverts brings limited relief; bridges, levees or buyouts would be needed for major flood reduction
Summary
An engineering study of Dry Run Creek found that adding box culverts at three constricted crossings would lower the 100‑year water surface by about one foot at most locations and would remove only a handful of properties from the floodplain; the firm said larger bridge openings, levees or buyouts would be needed for substantial reductions and estimated costs range from hundreds of thousands per culvert to $10–25 million for bridge replacements.
The Dry Run Creek Drain Study presented to the Mitchell City Council concluded that relatively small fixes — such as adding a fourth box culvert at three street crossings — would offer only modest flood‑reduction benefits in the study area and would not remove most properties from the 100‑year floodplain.
"The improvements to the box culverts or to the channel section did not yield the results that we were looking for to justify the likely impacts and essentially the cost of of a project," Phil Gunvalson, a professional engineer and part owner of Infrastructure Design Group, told the council. Gunvalson said the firm analyzed roughly 12,500 feet of the channel through Mitchell, recreated FEMA models, validated them against historic high‑water survey shots and then ran three scenarios: add culverts, replace culverts with open channel sections, and install much larger bridge openings.
The study found that adding an additional culvert at each of the three constrained crossings (Minnesota, Sanborn and Burr) would lower the 100‑year water surface by roughly 1 foot in plan view at some locations and likely remove only about three structures from the floodplain. For a more meaningful reduction, the engineering firm said longer bridge openings (effectively larger bridges) would be required; those could lower high‑water elevations by as much as about 4 feet at Sanborn in the firm’s modeling.
Gunvalson described estimated costs during the question period: a single new box culvert installation under a normal roadway could run "a half million dollars easily," while a large bridge replacement could range from about $10 million to $25 million.
Residents who have experienced repeated flooding urged the council to act. Bruce Treble, a long‑time Dry Run resident, recounted earlier flood events and asked that the council prioritize work at Minnesota and Burr: "In '84, I got flooded out...My basement was full," he said, describing decades of intermittent flood damage to his home.
City staff noted the study was paid for by a state grant using funds tied to the 2019 FEMA event and reminded residents that the FEMA home‑buyout program exists; staff said they would draft and distribute information on that program to potentially eligible residents.
Gunvalson and staff said other options — including levees/berms (which require FEMA certification if intended to remove properties from the floodplain), property buyouts, and regional detention to store flows upstream — could produce larger benefits but would require far more analysis and significantly larger funding.
Next steps: staff indicated further analysis and permitting conversations would be necessary before any in‑channel work; they also noted crews had performed channel maintenance (regrading and sediment removal) in some culvert areas following prior storms.
