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Deer stands dot the marsh around the Laurel Ridge Levee, Thursday, February 15, 2018, in Ascension Parish near French Settlement, La.

GONZALES — A little publicized draft report completed for Ascension Parish officials last summer raises fresh questions about flooding impacts that controversial levee projects in southeastern Ascension may have on neighboring Livingston Parish.

The August 2017 report by engineering firm HNTB found two interconnected and long-sought improvements to the Laurel Ridge Levee could result in combined flooding impacts in Livingston far higher and far wider than earlier known.

Ascension and the Pontchartrain Levee District are trying to proceed with construction of one of the projects examined in the study after receiving permits from regulatory agencies: a 4.5-mile, $24 million extension of the Laurel Ridge Levee in the St. Amant and Lake areas.

That project had already provoked alarm in parts of Livingston. But the discovery of the HNTB report's projections earlier this month prompted a top Livingston Parish official to say it's time for the U.S. Army Corps of Engineers to revoke its permit for the levee extension.

“You can’t do that. Morally, you can’t do that," Mark Harrell, Livingston's homeland security chief, said of the possibility of worse flooding in his parish.

The other Ascension levee project, raising the height of the existing Laurel Ridge Levee to match the 100-year flood protection the future extension would provide, is unfunded and not permitted. But that improvement has long been envisioned by Ascension Parish leaders as part of the overall work needed to be done to protect the southern part of the parish. Both projects are part of the still incomplete ring levee around eastern Ascension that parish leaders first proposed when the area's half-cent drainage tax was adopted in the early 1980s.

While the combined levee projects would lower flood levels inside the Ascension levee system by more than 3 feet in a 100-year flood, they would also raise water levels outside the levee in Livingston, the HNTB study found. More than half a foot of water could be sent to the French Settlement area and as much as 2.5 inches to 3.5 inches as far east as the Whitehall and Maurepas areas.

The report additionally found a flood similar to the August 2016 event, which inundated wide swaths of Ascension and Livingston, would bring even higher flood levels outside the proposed levee.

If the Laurel Ridge Levee were both extended and raised, some areas like Lake Martin in Ascension and French Settlement in Livingston would see more than 9 inches. Swamps just west of homes along the Amite River Diversion Canal would see more than a foot. The Maurepas area would see half a foot and Port Vincent would see 3.5 inches in additional water.

"The Laurel Ridge levee (raising) and extension does provide significant reductions in water surface elevations in the interior of the system," the study summarizes. "However, it increases the water surface elevations upstream in the Amite River and into Livingston Parish."

Two studies, different conclusions

Despite the alarms raised in Livingston about the levee extension, the levee district and Ascension officials have downplayed flood concerns. They have pointed to an earlier study by GSA Consulting Engineers and GEC Inc. showing a much smaller impact from that particular project. In contrast, the HNTB study looks at potential flooding if both the levee raising and extension are completed.

Widely cited with regulators for the past several years and more recently with the public, the GEC report estimates a three-quarters of an inch rise in water nearest the levee during a 100-year flood. Earlier this month, GEC revised its findings further to say the increase is closer to 1.3 inches at the highest point.

In defense of the extension, parish and levee officials have not mentioned the HNTB report and the parish only shared it with Livingston in early April as part of promised data sharing, Ascension officials said. The Advocate had filed a public records request for the document a month earlier.

William Daniel, Ascension interim public works director, said the two studies are "apples and oranges" because one considers the effect of the extension alone and the other considers the extension and levee raising together. He suggested the GEC study — the one widely cited in recent months — shows a lesser effect because it is looking only at the levee extension, the project now slated for construction.

The HNTB report, which shows the greater flooding effects, does not separate the effects from the levee extension and from the levee raising.

Daniel said that after the Corps permitted the levee extension Jan. 31, he asked both engineering firms to try to reconcile the two studies' differences, though that has been complicated by the different models each study uses.

“We asked them to go back and iron out the differences to have an apples and apples, and that’s what they’re doing today,” he said.

Harrell, the Livingston official, noted he had been told the GEC study didn't consider the levee raising project and so shows lesser impacts, but that doesn't change his position. For him, even three-quarters of an inch is too much extra flooding.

“It depends on which modeling they do. And they’re having different people do the modeling and different people are taking different things into account when they do the modeling," Harrell said. "But regardless, everything they’ve sent to us shows an increase, so we can’t support it.”

Both studies have been done by reputable firms using widely recognized modeling methods. The GEC study that shows a lesser effect uses the same kind of one-dimensional model the U.S. Army Corps of Engineers uses to design its civil works projects, an agency spokesman said.

Zhi-Qiang Deng, an LSU civil and environmental engineering professor, said hydrological models can produce different results based on the assumptions used to set them up. 

Based on documents provided to him for both studies, Deng said he was unable to pick out all the assumptions, but said the addition of the levee raising project to the HNTB study could affect what flood impacts are projected.

Still Deng noted the type of model HNTB used is better designed to consider flooding across the land. The model used by GEC is better for simulating elevation changes inside rivers, he said.

He added that HNTB seemed to have more detailed data and was able to calibrate its model against the August 2016 flood, saying the study should be factored into officials' decision making.

At the same time, he noted that both studies found similar benefits inside the Ascension levee system and said the differences in flooding impact outside the levee system could fall within the margin of error of both studies. He said maybe a panel of experts should review the studies to understand why there is a difference.

“You need to do more analysis of the data,” Deng said.

No public presentation

Authorized by Ascension in early 2017, the HNTB analysis is a broad look at a variety of flood control projects, using high water marks from the 2016 flood, as well as new field survey and other data. 

The report built on a new two-dimensional model HNTB had created a year and a half earlier at a cost of $1.7 million for Ascension, encompassing information from 400 square miles of Ascension and Livingston.

The model was built in a separate project for Ascension to analyze the parish's levees and see if they could be accredited by the Federal Emergency Management Agency to reduce flood insurance costs. The existing Laurel Ridge Levee is not accredited and was overtopped in the August 2016 flood.

Parish payment records indicate HNTB finished its work Aug. 25, and the parish paid the small remainder of the $392,053 bill in September. At the time, the levee extension's Corps and Louisiana Department of Natural Resources permits were still pending, parish payment and state permit records show.

Regulatory files show neither agency had been supplied HNTB's findings. Agency spokesmen added agencies had not been told verbally or by email about HNTB's findings until contacted by The Advocate last week.

Though HNTB has turned over the study to Ascension administrative staff, the engineering firm has not delivered a public presentation about the report's findings to the East Ascension drainage board, said John Basilica Jr., a firm vice president.

The drainage board members are the elected Ascension Parish Council members who ultimately authorized the study.

Daniel, the Ascension public works official, said he didn't really learn about and understand HNTB's findings and the differences with the GEC report until early January when he took over for the now retired public works director Bill Roux. But Daniel also defended not sharing the HNTB report with regulators, saying there was no attempt to hide it.

He said the levee district is the permit applicant for the levee extension, not the parish, and the levee district hired GSA and GEC and so shared the results of their study with regulators.

The HNTB study is still a conceptual one that looked at a number of ideas while the GEC study zeroed in on the sole project before regulators, Daniel said. If the parish ever gets funding for the levee raising project, he said, Ascension would have then shared HNTB's findings with regulators and others.

He noted the HNTB study also suggests looking further at the proposed Comite River Diversion Canal's ability to mitigate flooding and also did consider other improvements in Ascension that could have mitigated the effects of the two levee projects, though the cost is in the hundreds of millions of dollars.

It’s not clear when the levee district became aware of the HNTB report’s findings, but levee district engineers displayed at least some knowledge of the study in a meeting April 4 at GEC’s offices when they explained the GEC study’s latest results.

At the same time, responses to public records requests issued before that meeting also show the levee district did not have a copy of Ascension’s HNTB report then. Levee district officials did not respond by deadline to provided questions about the HNTB study.

Provided copies of the HNTB study, a spokesman for DNR said it was too soon to say whether its findings would or could even legally affect the extension’s permit.

Ricky Boyett, spokesman for the Corps, said all Corps permits say they can be suspended, reviewed, revoked or modified based on new information, but he wasn’t sure if the levee district is required to share the report with the agency for review.

He added that outside entities can petition the Corps to revoke a permit but said the agency considers only final documents, not drafts.

Though Ascension’s HNTB study has been fully paid for since Sept. 21, Daniel said it remains in draft status while engineers continue to review the report.

“We just didn’t feel like the report was finished,” Daniel said.

He said the results of the comparison of the two studies will be shared publicly once finished.

Follow David J. Mitchell on Twitter, @NewsieDave.