Heavier rain estimates offer a warning, opportunity to reassess drainage needs

New research suggests heavy rains pose a more frequent threat in Lafayette, but more info is needed before overhauling local flood prevention efforts. Photo courtesy of The Acadiana Advocate

Experts say heavier rainfall estimates should put Lafayette on notice, but caution that better local data is needed before making changes to any community’s rules and infrastructure in response to the heightened storm risk.

Lafayette and many surrounding parishes in Acadiana are at a much greater risk of experiencing 100-year storms than official estimates suggest, according to a new precipitation model published last week by the First Street Foundation, a New York-based nonprofit research group. 

What the National Oceanic and Atmospheric Administration considers a 100-year storm in Lafayette — 4.5 inches of rain in an hour — has a far greater likelihood of occurring, according to First Street’s report, which found that such a storm could hit Lafayette once every 22 years. 

It’s not an unexpected finding, as rainfall data around Acadiana supports the trend, says Dr. Robert Miller, assistant director of UL Lafayette’s Louisiana Watershed Flood Center, though he notes the scale of the disparity is worse than he anticipated. 

“I’m not surprised by what they’re finding. When we look at just the rain gauges around here in Acadiana, there is an increasing trend in the precipitation patterns,” he says. “What was a little bit unexpected for me was the degree to which, the magnitude to which, it is currently being underestimated.”

Southwest Louisiana in particular is an area of concentrated disparity between NOAA’s figures and the First Street model, as 100-year storms are at least twice as likely to occur, according to FSF, in every parish in the region. 

The increased precipitation estimates generated by the First Street model, which accounts for more rainfall nationally caused by climate change, should be a “wake up call” for communities and developers, Association of State Floodplain Managers Executive Director Chad Berginnis told the New York Times last week

The implications are myriad, as precipitation estimates dictate standards that local infrastructure and even neighborhoods are built to, particularly in areas like Southwest Louisiana where regular flooding is a concern. 

Lafayette’s Development Code, for example, relies on NOAA’s rainfall estimates to dictate detention pond standards for new subdivisions, pegging the standard for larger developments at a 25-year storm, which NOAA gives a 4% chance of occurring in Lafayette each year. 

But the First Street model estimates roughly the same probability for storms that historically called 100-year events.

By NOAA’s standards, which the code relies on, a 25-year storm would mean about 3.7 inches of rain in an hour for Lafayette. But the First Street Foundation’s model suggests that Lafayette is likely to see much more rain at a slightly more frequent interval, as the foundation’s estimates give a return period of just 22 years — that’s 4.5% a year — to a system that could dump 4.5 inches in an hour, what NOAA currently categorizes as a 100-year storm in Lafayette. 

Storm IntensityAnnual ProbabilityNOAA 1-hour rainfall estimate for Lafayette
10 year10%3.21 inches
25 year4%3.73 inches
100 year1%4.53 inches
500 year0.2%5.47 inches

The disparity suggests the parish’s drainage infrastructure requirements are inadequate for increasingly frequent occurrences of substantially greater rainfall.

But even the foundation’s leadership warned that the national-scale findings don’t warrant immediate changes to local infrastructure plans and regulations and should instead be a launch pad for localized efforts to better understand climate change’s impact on growing storm severity in recent years. 

“If our data is showing something that is dramatically different than what the [NOAA] data is showing, then you need to invest in more local research. There’s clearly a signal there that something could be dramatically different,” First Street Foundation CEO Matthew Eby said during a video press conference last week.

It’s the same approach endorsed by Miller, who worked as a professional engineer in the private sector for more than a decade before returning to UL. He says the scale of FSF’s continental model makes it like a “rough cut” for localized estimates, which “you really have to drill down and make sure you use the best available information we have on the ground” to improve for widespread use. 

Beyond that, the threat of greater rainfall should prompt a review of existing drainage infrastructure and consideration for strategic retrofits based on priorities that should be informed by updated rainfall data, Miller suggests. As for changing local regulations to reflect the greater frequency of stronger storms, that’s a challenge communities will have to navigate while weighing the scale of the threat and the impact on their growth.

“It’s a balancing act,” Miller says. “On one hand, we want to protect the future and we want to protect the current. But how do we do it in a way that’s reasonable with the models we have and the situation we’re dealing with? In my opinion, it would have to be a dialogue, and it would have to be a mutual solution.”