Building on Nokomis swampland will haunt Minneapolis in wetter future, report says

Residents of a section of south Minneapolis were alarmed when, starting nearly a decade ago, sinkholes suddenly opened in their streets, trees toppled over in Solomon Park, basements flooded and waterlogged backyards devolved into swampy stands of cattails. Broken sewer pipes cost residents thousands to repair.

Federal, state and local officials set out to find out what was happening in the Nokomis community. On Tuesday, they released a report saying the problem goes all the way back to when the area's natural marshes were erased for development. Decisions of more than a century ago are rippling back to Minneapolis after the record precipitation of recent years.

It's a "classic example of humans trying to out-engineer Mother Nature," said James Wisker, Minnehaha Creek Watershed District administrator. "And so, as we see precipitation patterns shift … we're going to continue to increase the frequency and severity of the issues in the future."

The neighborhood's water problems were set in motion by the larger-than-life landscape architect Theodore Wirth, who was the city's parks superintendent from 1906 to 1935.

Back then, bogs and swamps were considered "useless, unsanitary" impediments to development instead of the biodiverse, carbon-sequestering ecosystems they're acknowledged to be today. So, Wirth embarked on massive dredging and filling projects to radically transform the city's shallow wetlands into the clear lakes and parkland for which Minneapolis is famous.

The area near Lake Nokomis, according to an 1853 federal survey, once contained more than 1,500 acres of wetlands with extensive peat deposits left after glaciers 11,000 years ago. From 1914 to 1918, the Park Board dug 2.5 million cubic yards of that peaty soil out of present-day Lake Nokomis, which was then reused to fill adjacent wetlands. The result: 100 acres of man-made, buildable land.

Homes went up in the early half of the 20th century despite water interference with construction even then. New city sewer lines needed immediate and ongoing repair because of wet soils. Enormous peat bogs up to 16 feet deep disrupted road construction and drowned a 5-year-old boy who fell into an excavation in 1941. Still, development persisted, partially because the long-lasting effects of the Dust Bowl drought misled city planners into thinking the water issues were manageable.

Wetter climate's impact

Fast forward: The climate changed. The last decade was the Twin Cities area's wettest on record, according to the Minnesota Department of Natural Resources' Climatology Office. From 2010 to 2019, the Twin Cities area received nearly 100 inches more precipitation than the decade when the Nokomis community's homes were built.

Residents noticed serious high-water problems, and a multiagency team from the city of Minneapolis, Park Board, Minnehaha Creek Watershed District, Hennepin County, DNR and U.S. Geological Survey assembled to determine their cause. From 2017 to 2021, they spent more than $200,000 to install six groundwater wells, gather data, review historic records and host affected residents in a series of public forums.

"Record rainfall, combined with peat soils that were excavated and buried during one of the largest lake shaping efforts in the city's history, are driving water issues in the area," according to the paper.

Peat, a spongy material that can hold up to 10 times its weight in water, became a barrier preventing that rainfall from draining into deeper layers of the soil, causing the water to accumulate near its surface and become "perched groundwater."

"This may be why property owners near Lake Nokomis continued to experience water issues during drought conditions in 2020 and 2021, even though water levels were lower than normal," the report concluded.

Water problems detailed

There are three key areas of residential water problems on the west and south sides of Lake Nokomis — Lake Nokomis Parkway, West Nokomis and Solomon Park. Each one responded differently to record rains, based on the location of peat and their respective elevations.

The report found that wet basements off Lake Nokomis Parkway right above E. 54th Street. were due to homes having been built over former swampland, below the current water level of Lake Nokomis. Record rains and high groundwater levels are likely to blame.

Wet backyards and basements in West Nokomis homes between 54th and 57th streets, as well as south of Solomon Park between 59th and 61st streets, are the result of building on wetlands and over peat deposits, leading to perched groundwater.

What can be done?

The multigovernmental agency team is recommending that property owners implement their own mitigation measures to protect their property from flooding. The steps include installing sump pumps in basements and gutters to discharge rainwater away from homes, said Angie Craft, Minneapolis Public Works director of surface water and sewers.

More mitigation information is available on the city's website for Nokomis water issues. An upcoming town hall for residents to ask questions will be held at a time and location to be determined.

Next, the University of Minnesota and the Minnesota Geological Survey will try to map the location of the area's underground peat deposits and perched groundwater. That modeling is the first step to coming up with additional solutions, Wisker said.