First of its kind map shows scale of 2015 floods in Region

October 23, 2023

How deep did the 2016 flood get in your neighborhood? New online maps will show you.

 See original post on The Advocate.

 A first-of-its-kind interactive map shows the sheer scale of the 2016 floods that devastated the Baton Rouge region.

The online tool, created after years of work by the Amite River Basin Commission, shows how the floods swamped land stretching across more than 67 miles, from as far north as the Mississippi state line to as far south as the swampy fringes of northern Convent, along La. 3125.

Commission officials say the map's real power, however, isn't the big picture overview — it's the comprehensive detail of exactly what places flooded, and how badly.

Once the map goes live in the coming days at the commission website, amitebasin.org, homeowners will be able to zoom in and find flooding levels and flooding depths at the individual house level. That could provide far more powerful information than static federal flood risk maps, commission officials and their consultants said.

Dietmar Rietschier, executive director of the commission, said he hopes the new map, which now marks the region's important flood of record, will be used by the public to understand the risk in their area or in places they may be considering to build or buy a new home.

"One at a time, it opens the eyes of the people," Rietschier said.

The August '16 flood inundated an estimated 93,000 homes and caused $2 billion in damage in the Amite River Basin, according to the commission.

According to a 2017 U.S. Geological Survey analysis, federal researchers concluded many areas likely experienced something greater than the 100-year flood — or a flood that has a 1% chance of happening in any given year — that has been the federal benchmark for flood risk and triggers the requirement for insurance.

With rainfall breaking records for its burst of intensity, researchers found many waterways had flows that were greater than a 500-year flood, which has a lesser chance, a 0.2% chance or less, of happening in any given year.

The basin is the primary drainage route for the Baton Rouge metro area east of the Mississippi River and encompasses 2,200 square miles, covering parts of seven Louisiana parishes and four Mississippi counties.

The August '16 flood also hit areas to the east and west of the Amite basin that were not mapped by the commission, including in eastern Livingston and Tangipahoa parishes and in the Lafayette region.

A data-driven map 

Bob Jacobsen, a local hydrologist and longtime consultant for the commission who was the mapping project's manager, explained that the new flood map builds off a state Department of Transportation and Development computer model created after the flood.

That state model has been used separately to vet post-2016 flood-fighting projects by the U.S. Army Corps of Engineers. But the commission had more data to add to that model to create a precise map and make it available to the general public.

Among the first steps, the basin commission hired surveyors in the first days after the '16 flood to make a sweep of the flooded region and collect high water marks.

Those marks were combined with others separately collected by the Geological Survey, which made its own flood map, and a vast array of other information in the years later to construct the flood map and to adjust areas where the state model was known to be consistently too high or low.

The map's combination of high-resolution detail, increased accuracy and online interactivity represent a major step forward that has only been technically possible in the past few years, Jacobsen said, but that should become a new standard for flood maps. 

"It's what the public can expect of these kinds of maps in the future," he said.

The map comes as the commission is poised for a major overhaul. Legislation passed earlier this year revamped the panel's membership, structure and powers.

The commission, one of the sponsors of the Comite River Diversion Canal now under construction, no longer has a local property tax as a steady revenue source. That tax has expired, though the commission has retained the power to levy new taxes.

In the vision of Rietschier, though, this first map and the methods used to build it can be replicated for future floods in the basin, so residents and builders have an expanding understanding of flood risk.

Rietschier said he is asking the public to visit the commission website, check flood elevations at their homes and neighborhoods, and report back to the commission so it can make needed corrections and sharpen the map's precision.

Richardson said he and others have been impressed with the map's accuracy so far. For example: At West Wendover Drive, in the heavily flooded Monticello neighborhood off Greenwell Springs Road, the new map shows flooding of 3.5 to 5 feet on the street, as residents generally reported five years ago.

But commission officials also know the map won't be on point everywhere. It doesn't have as much ground-level data in St. Helena Parish and the Felicianas, and corrections may be needed there and in other areas.

"We are going to find some discrepancies in some areas where did not have enough information," Rietschier said.

Chokepoints revealed

In a recent presentation on the map at the Goodwood library in Baton Rouge, Stokka Brown, a CSRS engineer and floodplain manager who also worked on the project, demonstrated the power of one of the map's other important new information offerings: peak water surface elevations from the flood.

Brown showed how this elevation data, which is different than water depth, can reveal choke points in the 2016 flood. 

The raw elevation numbers, which are measured from mean sea level, aren't likely to mean much to the average user by themselves. But they can be used to make simple comparisons against one another and show how flood water backed up.

The map broadly shows water elevations through color-coding in the style of a "heat map," but users can also click a pointer on specific locations to find estimated flood elevations.

Brown, for instance, showed Interstate 12 in Livingston Parish, where walls in the interstate median have sparked allegations that they held up water draining south and worsened flooding north of the highway.

The map's color coding shows notable elevation differences between the north and south sides of the highway between Denham Springs and Walker.

Then, the pinpoint elevation readings show that water immediately north of the highway was 3 to 6 feet higher than water immediately south of the highway.

Brown also showed highway choke points in Ascension Parish: at La. 73 between Henry Road and Monticello Drive in southern Prairieville near Airline Highway, along Manchac Road in the Spanish Lake area, and along La. 22 in Acy near the Amite River Diversion Canal.

A previous basin commission analysis of the flood noted the effect some bridges appeared to have in impeding the flood's high waters. The new map also shows that in clear detail.

A bridge on Bayou Manchac appeared to prevent backwater flooding from pushing farther upstream into Ascension and East Baton Rouge parishes, while a bridge over the Comite River appeared to impede the flow of water downstream.

On Manchac, water upstream of the old Airline Highway bridge, which has since been replaced, was more than 1 foot lower than water immediately downstream of the bridge, the map shows.

At the Central city limits, flood water elevations at the Joor Road bridge over the Comite were 2 feet higher on the bridge's upstream side than on the downstream side, the map shows.