MANKATO — It was officially a "mega-rain event," this past weekend, the latest of an increasingly common extreme weather experience in Minnesota that flooded Mankato streets, overwhelmed the city's wastewater treatment plant and created temporary lakes in low-lying areas over.
"Obviously anyone who has a basement is quite aware ...," Deputy City Manager Alison Zelms told the City Council Monday night.
The storm that began late Saturday and intensified into the early hours of Sunday dropped 6 inches of rain over 1,000 square miles with the worst-hit locations topping 8 inches — the definition of a mega-rain event. A total of 8.63 inches fell near Minnesota State University, 7.6 inches was recorded at the wastewater treatment plant and similar amounts were measured elsewhere in Blue Earth and Nicollet counties.
Municipal staff were assigned to keep an eye on the storm late Saturday night and began calling in workers when the accumulated rainfall was approaching 4 inches. By 3:30 a.m., workers were reviewing damage, clearing outlets, barricading flooded streets and dealing with a massive increase in flows to the sewer plant, according to Zelms and City Manager Pat Hentges.
"And they worked very closely with Public Safety, who are our eyes and ears out there," Hentges said.
Multiple streets had to be closed to traffic because of standing water, and a section of Bassett Drive remained closed Monday night. Basements in commercial buildings in some low-elevation areas downtown, as well as residential properties throughout the city, were left soggy or worse.
Fortunately for property owners with ruined carpet or other unsalvageable possessions, Mankato's traditional spring cleanup service — where residents can drop off junk at the Public Works Center — was pushed back from May to August because of the COVID-19 pandemic.
"So we're making sure we can take carpet and other materials," Zelms said.
At the sewer plant, flows reached 13,000 gallons per minute early Sunday morning — forcing the plant to divert wastewater directly to the Minnesota River.
"We did have a couple of bypasses with minimal treatment," said Zelms, adding that the Minnesota Pollution Control Agency has been notified and crediting staff at the plant for keeping the diversions to a minimum.
The diverted wastewater was primarily rainwater which, during torrential rains, cascades into old or leaky sewer pipes or is illegally diverted into the sewer system by sump pumps and foundation drains that are supposed to be hooked up to the stormwater system.
Any sewage bypassed to the river will be further diluted by the tremendous amounts of water now flowing in the Minnesota, which was at about 4 feet on Saturday and is expected to top 15 feet by Thursday morning.
The low river level helped drain the water more quickly, part of the reason the range and severity of problems was not as severe as the last local deluge — a September 2016 storm that dropped just under 7 inches of rain on Mankato.
"Overall, I think we weathered a lot more water in a little better circumstance," Hentges said.
And, yes, mega-rains have become a not-so-unusual event. Minnesota has had 11 mega-rains in the past 20 years after experiencing just three in the previous two decades, according to the Department of Natural Resources.