Will Lafayette’s largest detention project protect many homes? We don’t know.

The gist: This year, LCG broke ground on the largest detention pond it’s ever built, promising it would turn back the clock on more frequent flooding. The nearly $60 million project’s ability to protect homes isn’t proven. For now, it’s halted in court, with a judge’s decision on whether LCG legally took the private land expected any day now.

What is Homewood? Billed as a regional detention project, it was originally conceived in 2019 as a single 250-acre pond on the Vermilion River near Milton. Today, the project is spread over four ponds, 184 acres combined, on a nearly 400-acre tract and is estimated to cost $60 million over two phases. It’s now part of what LCG calls Bayou Vermilion Flood Control, according to applications requesting state funding.

What does it do? In general, detention ponds are dry ponds that hold water during storms. Holding water is not intended to reduce the amount of water flowing into a nearby channel, in this case the Vermilion River, but to disrupt the timing to ease the burden downstream, says Mike Waldon, a hydrologist. 

“Their purpose isn’t to reduce the amount of water that’s going in the river but to change the timing and lower the peak. It’s like the Covid thing. Lower the peak. Here you’re trying to catch water in the highest flow and release it slowly after that. You’re really working on timing and not volume. And that makes a big difference,” Waldon says. 

Will it stop homes from flooding? We don’t know. LCG has not produced an analysis that translates the impact of the project into homes or structures spared. The U.S. Army Corps of Engineers vets projects with an exhaustive cost-benefit analysis, modeling projects through a wide gamut of storms and calculating projected costs and savings on a per-year basis.

“We don’t ever do that,” Waldon says. “It’s a lot of work. But that’s information that I would love to see for all these projects: What is the annualized cost of the project, and what is the annualized savings of the project?”

Homewood’s primary effect is during smaller storms and is localized to the banks of the Vermilion River. Two reports, one by project engineer Pam Granger of McBade Engineering and another by researchers at UL Lafayette, show that Homewood would lower the level of the river by 5.7 inches or 6.6 inches during a 10-year storm, respectively, but would have a negligible impact during a 100-year rain event like the 2016 storms. 

Granger and UL did not respond to requests for comment.

Study	10-year storm	100-year storm
UL Study	6.6 inches	<1 inch
McBade	5.7 inches	Not Reported

How intense is a 10-year storm? For Lafayette Parish, a 10-year storm translates roughly to 5 to 8 inches of rain in 24 hours. The storm that hit rush-hour traffic in May 2021 and stalled 123 cars, for instance, was in that range. So was the June 2019 downpour that flooded Downtown, the Saint Streets and parts of the Northside. These are also fast storms that are more likely to flood upstream areas like Scott and North Lafayette than longer duration storms that hit the river. 

“Floods are so complicated,” Waldon says, and understanding what kind of storm you’re dealing with is important. “For the river itself, it’s those longer duration storms [August 2016] that are going to cause the river to go up and flood. If you get a two-hour, very intense storm, Scott can flood, but the river isn’t affected at all.”

Historically, few homes flood during a 10-year event. The May 2021 event saw 33 homes take on water. Exactly where and how much water is unknown. A few homes were flooded on the river and from overtopped coulees in the June 2019 event, including in areas of town that had not flooded in 2016, which hit between 4,000 and 5,000 homes. KATC Meteorologist Rob Perillo called that storm a “climate signal,” a noticeable symptom of climate change. 

Is 5 inches of reduction a lot? That depends on where and when the reduction happens. In this case, Homewood mostly affects the Vermilion River with less impact on Coulee Ile Des Cannes, a large channel that hits the river on the other bank. During the May 2021 event, a 10-year storm, the river peaked at 14.7 feet near Homewood — long after street flooding stranded cars. At that height, the river is still well within its banks. (The 2016 flood hit 19 feet or more at the same location.) In other words, Homewood would lower the river when it’s already contained by its banks. 

Most flooding isn’t on the river. Most of the big floods happen along the major channels that hit it — like Coulee Ile Des Cannes or Coulee Mine — and which have seen tremendous development in the last 20 years. LCG has pinpointed a dozen or more detention projects using a heat map of areas that flooded in 2016. Most of the hot spots are along channels that feed the river. 

278 homes flooded along the river in August 2016, according to figures LCG included in a request for state money. To get state dollars, LCG claimed the project would lower the river by 5 inches in an August 2016-scale storm. Both the UL and McBade reports contradict that finding. So it’s unclear this project would protect any of those properties.

Lowering the river is an indirect solution. There is merit to the idea that reducing water levels on the Vermilion could alleviate some upstream flooding. Dredging the Vermilion River, for instance, is an idea sourced in that theory. Waldon argues that Lafayette’s flat slopes of land mean the river level is often higher than the channels that drain to it. Lowering the river level would have impacts upstream.

But modeling completed by UL in 2020 found that dredging would lower the river by 5.5 inches but would not reduce flooding by much on land during a 2016-scale storm. 

“These results indicate that the main river doesn’t necessarily fully control the actual extent of inundation on adjacent floodplains, and further suggest that floodwaters of the river are mostly contained within its own main channel,” UL’s team wrote in a 2021 paper unpacking its finding. 

Detaining water on the coulees themselves is a much more direct way to address flooding on those channels. Indeed UL’s model found that detention ponds located along Coulee Iles Des Cannes lowered water levels on the channel more and for longer distances than Homewood. 

So…why Homewood? LCG still insists the project has “cumulative” merit. It’s now lumped into a group of projects called Bayou Vermilion Flood Control, which includes ponds on Coulee Iles Des Cannes and the spoil banks removal project that put LCG at loggerheads with St. Martin Parish. Still, it’s unproven whether the cumulative effect translates into reduced flood risk for homeowners and lowered premiums for many of the 20,000 or more flood insurance policyholders in the parish. 

How do you fix flooding? Engineers will generally say you can’t. What we can do is make our community more resilient. Flood Factor, a national nonprofit that advocates for climate preparedness, recommends communities dramatically increase development standards, preserve open spaces, use natural detention and overhaul stormwater systems. Lafayette is doing some of that, but it’s unclear how its biggest and most costly detention project to date, Homewood, fits into the bigger picture.