A group of Maplewood Road residents in Riverside and the Frederick Law Olmsted Society have called for a halt in the plans to demolish the Hofmann Dam, just as work has begun in earnest.

Both the residents and the Olmsted Society are citing discrepancies in figures that the U.S. Army Corps of Engineers used to calculate the impact the removal of the Hofmann and Fairbank dams would have on the river channel upstream.

The new numbers indicate that the water levels in the river upstream of the dam will be reduced more than originally anticipated. Army Corps officials say the difference won’t be the great, but residents and village officials are not entirely convinced.

The new information also has village officials scrambling to get to the bottom of the situation, with trustees calling for work to stop pending a complete explanation of what the differences in the numbers truly mean and why they were hidden during a public hearing on the project in September 2011.

“Ideally, I’d like it to stop, but I don’t know if we can do that,” said Trustee Lonnie Sacchi, who, along with Trustee James Reynolds, has been the village board’s liaison for the project.

“It’s paramount and we feel the need for independent verification of the outcomes given these two sets of numbers,” said Sacchi.

Meetings between village officials and the Army Corps and Illinois Department of Natural Resources are in the process of being arranged, said Sacchi, who said that he’d also like to see a public meeting for residents once there are some answers.

“We’re looking for a public meeting after we get things clarified, where we can speak with clarity,” Sacchi said.

Trustee Ben Sells, in an emailed statement, said the Army Corps and IDNR “owe it to our residents to provide objectively verifiable information before further work is done. There is no rush on this project and no going back – it must be done right.”

Meanwhile, the Army Corps is attempting to reassure the village that the figures they have cited in the past are valid and say residents shouldn’t believe the dire scenario some opponents to the project have painted.

“The impacts will not be as great as some people think there will be,” said Jeff Zuercher, project manager for the U.S. Army Corps of Engineers. “The river will be something people will treasure and it will be able to be enjoyed.”

Donald Spatny, the Riverside resident who has been the most vocal critic of the dam removal plan, recently pointed to differences in the numbers the Army Corps has used in determining the flow of the river and believes that the decrease in water levels will be more drastic than the Army Corps is letting on.

Throughout the process of planning for the dam removal, the Army Corps has used the figure of 582 cubic feet per second as the flow rate at the Hofmann Dam. The Army Corps has said that’s the rate the river runs at 80 percent of the time. It was a number determined by an Army Corps contractor about a decade or so ago.

However, the Army Corps apparently also has done some river flow modeling of its own, and has determined that the 80-percent number is between 240 and 250 cubic feet per second,

The numbers have alarmed residents in Riverside who live on the water upstream, particularly on Maplewood Road.

In a letter dated April 20 to the Army Corps, the Illinois Department of Natural Resources and the village of Riverside signed by 23 residents of the street, they state, “The effect of this error is to substantially underestimate the frequency and magnitude of the drop in water levels once the dam is removed.”

The Olmsted Society in a separate letter on April 22 also urged work be suspended until the data can be reanalyzed.

“Of greatest concern … is that the erroneous presentation of river depths and widths presented by [the Army Corps] may greatly underestimate the extent to which riverbank stabilization and restoration will be required upstream of the project work limits after the dam is notched,” wrote Tim Ozga, president of the Olmsted Society.

The lower 80-percent figure was not referenced at the September 2011 public hearing. In an email to Maplewood Road resident Jeff Miller on April 19, which has been obtained by the Landmark, Zuercher explained that the Army Corps discovered the discrepancy just prior to the September 2011 public hearing.

The Army Corps, wrote Zuercher, “decided to stick with the [583 cfs] number for consistency and because the calculations had been completed using that number.

“This was not done to fool anyone it was mostly due to the fact that our modeling used that number and that is how we developed all of those charts depicting depth changes and width changes (sic). Since all of the charts had been created with that number and the error was discovered at the 11th hour there was not time to correct the actual charts or displays.”

In addition, a demolition application by the Army Corps to the Illinois Environmental Protection Agency states that the river’s “low-flow” figure – the 7Q10 rate – is 88 cubic feet per second. That contradicts the 7Q10 rate of 139 cubic feet per second that the Army Corps has used throughout the planning process.

Zuercher on Monday said the low-flow number used in the IEPA application was a mistake and that the low-flow number at Hofmann Dam remains 139 cubic feet per second, despite what it says on that application.

He said the figure may have come from an Illinois Water Survey map, which shows a low-flow of 88 cubic feet per second on the Des Plaines River just south of Cermak Road. The same map shows a flow of 133 cubic feet per second at Hofmann Dam, he said.

“How is that mistake surviving?” asked Sacchi of the apparent map-reading error.

While the two 80-percent numbers differ, said Zuercher, he doesn’t believe the difference in terms of impact to river levels will differ greatly.

“The changes are not going to result in the impact that people are so fearful of,” Zuercher said.

Zuercher said that at Forest Avenue, for example, using the 582 cfs figure, the river is expected to narrow by 3.5 feet on each side. Using the figure of 245 cfs, the reduction would be about seven feet on each side.

“The excess exposed bank is still very small,” said Zuercher.

But Sacchi said he’d like to see visual evidence of the difference, arrived at by an independent source, using both sets of data.

“We’re not experts at analyzing the outcomes of these numbers,” said Sacchi. “When we find out they didn’t tell us [about the lower figures], it certainly gives us pause about how forthcoming they are about everything.”

Letter from Maplewood Residents to Army Corps of Engineers FLOS Hofmann Dam Letter