DENHAM SPRINGS — More than 80 highway and railroad bridges crisscrossing the Amite River Basin exacerbated high water levels in the August 2016 flood but this worsened flooding was also to be expected under the historic inundation of the Baton Rouge region, a new preliminary report has found.

The Amite River Basin Commission’s recently released analysis of high water marks across the basin found many bridges boosted peak water levels by more than a foot, with the worst case at the Interstate 12 bridge over Gray’s Creek in Livingston Parish.

That bridge and a related median wall boosted peak flooding on the bridge’s upstream, or north side, by 4 feet, the report says. The bridge is located just east of the Pete’s Highway overpass and at the corporate limits of Denham Springs, which was hard hit by the August flood.

“Bridges had a widespread impact on peak flood levels throughout the ARB (Amite River Basin),” the Aug. 21 report found.

Much of the high water effects occurred on the upstream side of bridges as water draining downstream backed up. But, due to the complicated nature of flooding last year, some bridges forced water to back up on their downstream sides, as backwater tried to flow in reverse, or upstream, mostly notably along Bayou Manchac, according to the report’s preliminary findings.

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A combination of bridges over Bayou Manchac between La. 73 and Interstate 10, for instance, worked together to push up water levels on the downstream side of all those bridges by more than five feet, according to the report's preliminary findings .

The 145-page analysis underscores the impact of man-made activity on the 2,200-square mile basin over the past century, as well as the changing climate and sea levels. The report noted not only the effects of bridges but also other man-made structures that restrict river flow, increased urbanization, and past alterations to river courses and stream beds of the Amite River and its upstream tributaries.

The report concluded, for instance, that the Marvin J. Braud Pumping Station flood gate in McElroy Swamp and the section of Manchac Road along the upper portion of Bayou Manchac and near the Spanish Lake basin also affected high water levels.

The independent report comes as Livingston Parish government, the city of Denham Springs and the town of Walker have sued the state Department of Transportation and Development and a host of engineering firms over the impact of I-12 median wall on upstream flooding last year. That wall extends through the I-12 bridge over Gray’s Creek.

Separate class action suits have also been filed in state and federal court over flooding in Tangipahoa Parish last year, allegedly due to I-12 bridges in the Robert area, though that area is in a different river basin.

Denham Springs Mayor H. Gerard Landry and Walker Mayor Jimmy Watson declined to comment Friday on the commission’s report and referred calls to the attorney handling the lawsuit, who could not be reached by deadline.

But Erick Nowak, one of the plaintiff’s attorneys for the lawsuits over the Tangipahoa flooding, said the report findings are what he expected.

The Amite River Basin, which is the Baton Rouge region’s primary drainage sink, stretches from rural counties in southwestern Mississippi to Lake Maurepas and extends through populous parts of East Baton Rouge, Livingston and Ascension parishes.

The commission report provides a picture of peak water levels throughout 1,060 miles of streams and rivers, though most thoroughly in the middle and lower portions of the basin.

Teams of surveyors working for the commission found 250 high water marks, adding to the findings of 34 existing river and stream gauges and another 198 high water marks the U.S. Geological Survey found earlier this year for another analysis of the flood. A total of 482 high water marks were used to generate stream and river “profiles.”

At the same time, the report also points to the difficulties in simply undoing the long-standing man-made impacts because, the report’s author said, actions that would change the floodplain for the benefit of some could result in new impacts on others.

In fact, the commission’s preliminary report doesn’t provide an opinion on whether bridges or other man-made impacts should be removed or changed in some fashion. The report does say the bridge impacts were an anticipated consequence of a flood that, in some areas, came close to a 0.2 percent chance flood, or a 500-year flood. That’s more severe than the benchmark 1 percent chance flood, or 100-year flood, around which federal flood insurance and local construction standards are set.

Bob Jacobsen, the report’s author and a consulting hydrologist for the commission, noted that the bridges across the basin are of varying ages, some decades old, and were built to varying standards. Some, such as the La. 73 bridge over Bayou Manchac, which was built in 1931, were established well before the National Flood Insurance Program even mapped the region for the predicted extent of severe flooding.

“Nobody is going to suggest going around the state and rebuilding every bridge opening because then, all of sudden, you change the flooding downstream and you change what the floodplain looks like downstream,” Jacobsen said. “Yeah, I mean somebody upstream might say, ‘I’d love for you to make the bridge opening wider,’ but then, hey, over the course of decades, people have adapted to the way the floodplain has been modified.”

The report was instead aimed at creating a baseline data set to aid in the creation of a broad computer model of the basin’s hydrology. That model, which the state is developing and will be used for planning, must take into account the effect of bridges and other man-made impacts, the report says.

Current DOTD standards say roads and bridges should be built at least a foot higher than the expected height of a 25-year or 50-year flood. A 50-year flood is less extreme and more likely to happen than a 100-year or 500-year flood. A 50-year flood has a 2 percent chance of happening in any given year, the USGS says.

Rodney Mallett, DOTD spokesman, said Friday the agency has been focused on its emergency response to Hurricane Harvey, so the commission’s report has not been fully reviewed.

“We appreciate the Amite River Basin Drainage and Water Conservation District’s interest in this issue and will fully review the insights contained in the report,” Mallett said. “We anticipate the information in the report will contribute to the state's efforts to manage water.”

Follow David J. Mitchell on Twitter, @NewsieDave.