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Cochrane Goes Over Stormwater Options



David Van Leer and Nick Wehner of Cochrane Engineering went over options to improve stormwater drainage with the Ste. Genevieve Board of Aldermen during a work session last Thursday night.

Cochrane has been studying the problem in recent months, and brought a couple of options to consider.

They looked into three areas in town that have had issues dealing with stormwater following heavy rains. Two other drainage areas will be examined later.

Van Leer explained that their presentation was based on various levels of storms. The lowest is a two-year storm, which has a 50% probability of happening each year. A 25-year storm has a 4% probability of happening each year. A 100-year storm, meanwhile, has just a 1% likelihood of happening each year.

The 100-year storm features 4-5 inches of rain per hour.

Drainage Area 1 included Market Street and Highway 61 and Market and Ninth Street – areas that have had flooding issues.

While going over Area 1, Van Leer and Wehner unveiled their options for improving draining.

One option is to improve infrastructure all along the area, replacing pipes and culverts that are too small to handle the bigger rains.

The other option is to construct a retention or detention basin above the flooding areas.

Van Leer explained the difference between a detention basin and a retention basin. A detention basin would remain dry except following heavy rains, when it would largely fill up. A retention basin would essentially be a lake, with extra space to allow water levels to rise, accommodating flooding events, without topping the banks.

Van Leer initially favored the retention pond idea, saying it could also be “an amenity for the city.”

Ward 2 Alderman Bob Donovan asked how it would be accessed and who would maintain it. He also expressed concerns about liability issues.

The three-acre lake would have four feet of storage, on top of its normal depth, to accommodate heavy rain.

Van Leer showed figures that demonstrated how the flow of water would be reduced below the basins.

He property where the lake would be on is owned by Mississippi Lime Company.

As the session went on, a preference emerged for a dry basin as apposed to a lake.

They later went over some specific culverts on Market street, showing slides of a handful of them.

“I think that’s what we need us a dry retention basin,” Mayor Paul Hassler said, after much of the presentation gad been given. I don’t think you want the liability of a lake.”

Donovan agreed,

“It would allow you to get this as a first phase, the basin, then if it ever didn’t correct the problem, then you could l further down the line. But at least you started at the beginning.”