One legacy of last year’s devastating floods in Duluth will be a storm sewer system that catches up with more than a century of development, climate change and even some finicky fish.
The overhaul, much of which was underway even before the June 2012 deluge, is intended to meet the needs of a city that has more pavement — and thus more runoff — than it did 130 years ago, when some of the current sewers were installed, said Chris Kleist, Duluth’s stormwater program coordinator.
But bigger pipes also are needed to handle more stormwater that’s resulting from increased rainfall, a documented trend that many researchers link to a warming climate. Kleist said the rainfall may be the bigger factor, since Duluth’s population is smaller than it was in 1920.
The storms last June 19-20 dumped up to 10 inches of rain in some parts of the long, narrow, steep-sided city along Lake Superior. According to a new U.S. rainfall atlas, that kind of rain can be expected once every 1,000 years.
Kleist said the city is targeting more likely extremes, building sewers to handle a storm with a chance of occurring once every 50 years, which would bring about 5.5 inches of rain in 24 hours. Culverts and bridges are being designed to handle a once-in-a-century storm — 6.3 inches in 24 hours.
Replacement costs x 5
But rebuilding a stormwater system that comprises 431 miles of conduits is a long-term and expensive process. Duluth spent $30 million in the past three years building underground stormwater holding tanks designed to reduce the volume of runoff from the city into Lake Superior, according to City Auditor Wayne Parson. Repairing flood damage to the stormwater system — sewers, culverts and catch basins as well as stream banks — is expected to cost at least $15 million.
While the Federal Emergency Management Agency will pay for 75 percent of replacing what was damaged in the 2012 storm, it won’t cover work on streams, and the cost of infrastructure upgrades will also fall to local and state agencies. How much could that be? Consider a 60-foot-long, 36-inch-diameter corrugated metal pipe that carries water under a road. Simply replacing it might cost $20,000, Kleist said; replacing it with a wide, reinforced concrete “box” culvert could cost three to five times that. One pipe.
“Long term, there would probably be some cost savings,” Kleist said. “It would last longer and blow out less. But if dozens or hundreds have to be replaced. … We can’t do them all.”
Some of the improvements the city does make to store, slow down or otherwise manage stormwater, such as grassy swales along streams, will be in the public realm. Private landowners are also being encouraged to install green roofs, rain gardens and pervious pavements to better absorb rains.
The repairs will also offer improvements for fish.
Fourteen streams that tumble through the city are home to naturally reproducing brown, brook and rainbow trout. The fish could be overwhelmed by high flows through the former, smaller culverts, said Deserae Hendrickson, manager of the Duluth regional office of the state Department of Natural Resources.
So new, wider concrete box culverts will be designed as fish “resting areas” — flat spots in otherwise steep streams where fish can recover from fighting the forces of high water. Kleist said many will have rocks bolted and glued to the bottoms and sides to mimic natural stream bottoms, where fish lay eggs and find food.
“In almost every case, it’s an improvement over what the previous condition was,” Hendrickson said.