Post-storm flooding had become the norm for one southeastern Wisconsin neighborhood.
Stormwater would regularly turn streets, yards, and golf course fairways into waterways, damaging property along the way. The storm sewer system once got so overburdened during a 2014 storm, it blew out a manhole and ripped apart the road.
The storms weren’t necessarily big, and the relatively new development should have been able to withstand them. So why did this keep happening?
Getting to the root cause
To get to the bottom of the issue, the community brought on MSA Professional Services, a design engineering firm.
MSA Water Resources Team Leader Eric Thompson and his team dove in, reviewing the storm sewer system’s design and crunching numbers. At first, nothing looked amiss—it appeared the original designer had done everything right.
But when they turned their attention to the area’s three ponds, they discovered the golf course had modified two of them. The golf course had raised the water level of one pond and lowered the overflow of another.
The change made both ponds look more aesthetically pleasing and enhanced golf play but reduced how much stormwater they could hold by 65 percent. This was causing ponds to overflow much more frequently than planned, including onto one resident’s yard and into her basement.
“I thought, ‘There you go, this is where the problem is,’” Thompson explained to attendees of the Minnesota Chapter of the American Public Works Association’s fall 2019 conference. “It wasn’t a problem; it was the problem.”
But moving forward would be trickier than expected. As it turned out, there was nothing the city could do about the modified pond because it legally wasn’t its to maintain. The city owned a storm sewer system that counted on ponds to help capture stormwater, but not the pond itself.
“Developing a plan and putting a stamp on it isn’t the end of the project’s life cycle,” emphasized Thompson, reflecting on what everyone could learn from the experience. “A pond is going to be there for the life of a subdivision…But if someone doesn’t understand what the functionality of it is, that functionality is going to be lost.”
Problems pile on
Undersized ponds weren’t the only issue. Homeowners built houses lower than they should have, with side- and rear-facing doors and windows at or near the ground surface. When water overflowed from the street or ponds, it went right into people’s houses.
The team also discovered the storm sewer that runs beneath the street wasn’t large enough to handle extreme storms.
“It couldn’t save the day in a 100-year storm,” said Thompson, referring to a storm with so much rain that there’s only a 1 percent chance of it happening in any given year.
While the development was designed using the best available data at the time, more recent data about the area’s rainfall projections and soil properties told a more troubling story.
Today, the threshold for a 100-year storm is 6.8 inches over a 24-hour period. Back in 2001, 6 inches of rain would have been considered a 100-year storm (based on data published in 1969). A 0.8-inch increase may not seem like a lot, but translates to nearly 20 percent more runoff in such a storm.
Similarly, a soil map generated before 2018 would have said the development sat atop hydrologic soil group B, a soil type that holds water relatively well. Since then, half that same area was reclassified as soil group C, a more finely textured soil that causes more water to run off elsewhere (e.g., onto the street). Simply having a soil type that absorbs less water can cause 20 percent more water to run off.
“We’ve got 20 percent [more runoff] through more rain and another 20 percent through worse soil. And on top of that, we’ve got a system that really wasn’t up to snuff,” said Thompson. “It just seemed like everything went wrong.”
Working toward a solution
To fix the flooding issues, Thompson and team worked with the golf course to add a small pond in the rough (a less-manicured area of a golf course next to where the game is played). It’s a happy medium: the system can now withstand the older 6-inch, 100-year storms while being discreet enough to not affect golf play.
They also increased the capacity of a portion of the storm sewer system by replacing 1,000 feet of 48-inch pipes with larger ones. This ultimately made the whole system work better.
After experiencing flooding in 2014, 2016, and 2018, the neighborhood now has the proper infrastructure in place. Should the pattern continue with a big storm in 2020 or beyond, they’ll be ready for it.
—Michelle Hoedeman, LTAP freelancer