TenHaken issues emergency declaration as flood forces untreated wastewater into Big Sioux River

In the wake of a deluge, the next 48 hours will be crucial for not only Sioux Falls, but also the health of the Big Sioux River.

Mayor Paul TenHaken signed an executive order to issue an emergency declaration Saturday after days of persistent precipitation inundated the city, along with southeastern South Dakota as a whole, with significant amounts of rainfall.

This will allow the city to skip the bid process to hire contractors and procure equipment to more quickly respond to the flood situation.

In a 7 a.m. virtual press conference Saturday hosted by Gov. Kristi Noem, TenHaken told reporters the city’s retention ponds, used specifically in flood events, were all at capacity.

As the city’s wastewater system undergoes the “most strain the system has ever had,” as TenHaken described during the presser, the mayor urged residents to limit water usage as the city’s wastewater system undergoes the 

“We have thousands of people depending on this system,” TenHaken said. “We need people to limit their wastewater use right now, and that means limiting doing loads of laundry. If you can avoid a shower today, that will be great. If you can avoid anything that puts capacity or strain on the system, it will be an incredible benefit to what we’re seeing right now.”

Hours later, Marc Cotter, the city’s director of public works, elaborated on the capacity issues during a 10:30 a.m. press conference held at the city’s Public Safety Campus. He told reporters the Sioux Falls Wastewater Treatment Plant ran out of storage for excess wastewater in the system.

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As a result, plant operators were forced to discharge untreated wastewater into the Big Sioux River at 7:15 a.m. Saturday.

This decision was a “last resort,” Cotter emphasized. The wastewater plant’s inflow was measured at about 45 million gallons per day Saturday, Cotter said, while about 34 million gallons per day were diverted into storage basins.

Collectively, that’s about four times the normal amount of water the plant typically sees, the publics work director estimated.

A “significant amount” of the excess water is comprised of rainwater and groundwater, Cotter explained.

“The state also knows that, so when we test the river, upstream and downstream, we expect to see, you know, a significant amount of dilution that occurs with that,” Cotter said. “But it is always our last resort. We’ll only do it for the minimum amount of time that’s required until the system catches up.”

Cotter said the last time the city had to discharge raw wastewater was in 2010. He called it a “very uncommon” event.

There’s a safety rationale behind the decision, since releasing the excess water quickly was meant to protect property and keep wastewater out of neighborhoods, Cotter explained.

“The large storage systems that we’ve built for days like this, you know, those have also been great assets for us up until this point, and they’ve worked really good for this event, but just the peak of this just wasn’t enough,” Cotter said.

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Asked how officials will respond if the city’s flood contingencies continue to stay at or surpass capacity, the public works head told the Argus Leader they’re reliant on a current National Weather Service that’s favoring sunshine and drier weather later Saturday through early next week.

In any event, they’ll “adapt accordingly,” Cotter said.

“If things were to change, our systems are, we’ve been managing this on an hour by hour basis …,” he said. “But I think we all are looking forward to the sun coming out and starting to dry this place out.”

The flooding has also given rise to rescue events, City Emergency Management Director Regan Smith told reporters.

Since Friday morning, Smith said there have been nine water rescues in Sioux Falls and Minnehaha County, all of which were successful.

In the same region, Smith said there have also been five calls for stranded motorists, 30 for vehicles stalled in water, 10 regarding water problems and 75 traffic accidents.