The late July storm that dumped more than six inches of rain on west Wilmette caused overland flooding and some basement sewer backups west of Ridge Road, but would have caused far worse damage had the village’s $18 million West Park sewer storage reservoir not gone into service only a few weeks before, a village official told board members in a post-storm report.
However, Wilmette must still improve its stormwater control efforts, Engineering and Public Works Director Brigitte Berger said July 26.
Berger said Aug. 9 that her department has gotten feedback from residents who experienced either overland flooding or sewer backup from the July storm – something she told trustees July 26 she understands is a frustrating task for residents who have experienced multiple floods and backups.
Berger said she hopes to have all that information collected by Aug. 12 and sent to Christopher B. Burke Engineering and RJN Group, the village’s stormwater consultants.
In her report to the board, Berger invited residents to attend a committee of the whole session that will take place in September, at which trustees will discuss potential projects to control overland stormwater flooding in west Wilmette.
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Those projects won’t be easy or inexpensive, she said, adding that early cost estimates for replacing large portions of west Wilmette’s sewer system are as high as $77 million, would require years to build, and would only reduce street flooding for storms of a so-called 10-year magnitude.
The July 23 storms deposited the equivalent of a 100-year storm in west Wilmette, compared with just under three inches of rain that fell on Wilmette east of Ridge Road, which represented what would fall during a five-year storm, Berger said.
Without the West Park sewer reservoir storing roughly 5 million gallons of sanitary sewer water that the storms pushed into west Wilmette’s separate sanitary system, Berger estimated that between 600 and 1,200 west Wilmette homes would have experienced some sewer backup.
“I know that some homes still experienced sewer backup” in west Wilmette, Berger said. “But the basement flooding and resulting property loss would have been significantly worse had this improvement not been built.”
Those that did occur were largely a function of the small diameter of sanitary sewer system pipes, Berger said, because they couldn’t carry enough of the sewer flow into the detention reservoir fast enough. In addition, flow monitoring of that system shows “that 70 percent of the flow going through the sanitary system during rain events is stormwater. It’s water that doesn’t belong there,” she said.
Berger told trustees that Wilmette has tackled such “inflow and infiltration” problems by lining sewers and manholes to prevent non-sewer water getting into that system.
“The next big challenge will be to greatly reduce (inflow and infiltration) sources on private property,” she said.
She told trustees Julie Wolf and Cameron Krueger that property owners should check the sanitary lateral lines that connect their homes to Wilmette sewers for cracks caused by things such as tree root incursions, because those breaks allow groundwater and stormwater into the sanitary systems: “If you took every sewer lateral and put one right next to each other, there’s close to a million feet of unprotected pipe that’s allowing all this ground water to infiltrate.”
In February 2015, Wilmette board members got their first look at overland flood prevention proposals from Christopher B. Burke Engineering. At the time, the price tags for three proposals ranged between $44 and $75 million, with the highest cost being to install 42,000 new feet of trunk and lateral sewers to hold and deliver stormwater to the north branch of the Chicago River.
At the time, consultants said the existing west Wilmette storm sewer system, as opposed to the sanitary sewer system, could only handle a so-called two-year storm.
In March 2015, the board directed staff and consultants to provide less expensive alternative versions of the three options – between $39.1 and $65.8 million – and those were presented to the board’s municipal services committee last September.
The village board has not yet made any decisions.
Twitter: @pioneer_kathy