Another round of heavy rains forecast for this week has again prompted warnings about potential flooding in the area, highlighting a persistent issue that has affected northeast Johnson County in recent years. But as cities and the county look at mitigating the dangers of damaging floods, questions linger about what standards engineers should use to measure those threats.
Johnson County taxpayers may fund millions of dollars in stormwater management projects in coming years after municipalities like Prairie Village, Fairway and Leawood faced intense flooding in 2017 that caused serious damage. But in some cases, the project plans are using stormwater standards established with data from more than 40 years ago — even though new data from 2014 shows higher rainfall depths for rarer, longer-lasting storms.
The rainfall depth — an estimate of how much water falls within a period of time — for 100-year-storms that last for 24 hours increased by about one inch in Johnson County, from 7.80 inches to 8.62 inches, said Dr. Bruce McEnroe, a Professor Emeritus in the School of Engineering at KU and author of current and past rainfall depth data. One-hundred year flood events are typically the largest event stormwater projects are built to withstand.
The KC metro chapter of the American Public Works Association (APWA) sets stormwater standards that are typically implemented in cities in Johnson County. They have yet to adopt the four-year-old data, though they are currently considering doing so. Adoption of the data would likely add cost because the size of stormwater detention facilities and other pieces of infrastructure would need to be increased, McEnroe said.
Three stormwater projects in Prairie Village are all using standards provided by the APWA, including the study that suggested Mission Road be raised by 4.5 feet, said Cliff Speegle, a Stormwater Engineer for the City of Prairie Village. None are under construction yet. Speegle is not concerned that these are planned using lower rainfall depths than those published in 2014.
“There might be a little bit of change but it’s not (as if) the systems we build now can completely not function,” Speegle said. “They will still function as intended.”
Fairway Public Works Director Bill Stogsdill sits on the APWA board, so he’s been closely tuned into the consideration of changing data sets.
“If we had a large scale project that we were looking at being compliant with [the APWA’s stormwater standards], I would request that we use the newest rainfall,” he said. “But fortunately we don’t have any large scale projects in design that that would impact.”
Burns & McDonnell, contracted by the City of Fairway, produced a study in December investigating potential solutions to alleviate flash flooding along Rock Creek. It referenced the Standard Specifications & Design Criteria by the APWA. Potential solutions included stormwater storage upstream in Mission and modifications to the creek channel; Burns & McDonnell recommended the second strategy.
Leawood is also undergoing stormwater projects, the majority of which are built to the APWA standards and are relatively small, said David Roberts, a stormwater engineer for the City of Leawood. These include obtaining easements to put new stormwater pipes underground and replacing rusted metal storm sewers. Bigger projects like designing a bridge may use the new data, but none of these are underway, he said.
Land use factors important in assessing stormwater threats
The rainfall depth increases cannot necessarily be attributed to a change in weather or extreme precipitation characteristics, McEnroe said, attributing them instead to changes in statistical analysis. However, he recommends that the APWA adopt the new data.
Nevertheless, a new study found that the land use factor, which is used to understand how much rainfall either sinks into the ground or becomes stormwater runoff, was lower than the APWA had been assuming. This may offset the increase in rainwater, making the adoption of the new data unnecessary, said Chad Johnson, Chair of the APWA Water Resource Committee. Adopting the new data would likely raise infrastructure costs, he said.
“We want to look at rainfall and land use factor together to make sure that we’re not spending too much money on infrastructure,” Johnson said. He noted that recently built infrastructure — which doesn’t use the 2014 data — was not experiencing flooding problems. “I look in the field, and I see new infrastructure that’s carrying water as we would expect it to,” he added. “Do I really want to rush into a decision when what we have now for the most part seems to work?”
Johnson said the decision will require more conversations amongst the chapter, though McEnroe expects the data will be adopted.
Still, some residents say they have seen a change in the amount of water that comes down during storms, or the frequency of intense storms.
When Lauren Conderman moved into her home in north Leawood in 2008, water that overtopped the creek behind her house during heavy storms remained 40 to 50 feet away from her back door. Now, she says, the creek comes within ten feet of her house during the heaviest storms.
“I’ve only been here ten years, and I’ve seen a significant change,” Conderman said. “The water runs faster, there’s more water. It’s coming up in everyone’s yards much higher than it used to.”
Phil Hofstra, a 35-year Fairway resident whose home sits in the flood plain near Rock Creek, has lived through multiple historic floods: ’93, ’98 and 2016. But the floods in 2017 came with a frequency he had never experienced.
“In the ’98 event we had an excess of three feet of water in our first floor,” Hofstra said. “In August of 2017…there was overtopping that was nearly that deep three weeks in a row. In 35 years here, I’ve never seen that kind of sequence of events before.”