Olsson Associates

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Missouri town looks to Olsson to resolve flood plain problem

Monday, July 17, 2017

Linda Van Hoosen, Communications

It had been a while since the City of Lebanon, Missouri, took a hard look at their Federal Emergency Management Agency (FEMA) flood insurance rate maps (FIRM). After recent floods, the city had a suspicion the maps were unrealistic and the flood elevations were set too high. There was another issue. The city’s flood model was more than 30 years old and created with a dated program, which made the model difficult to find and difficult to modify. A hydraulic model uses cross sections of the stream, the 100-year discharge rate, and other design parameters to calculate the flood elevations and flooding extents along the stream. The city hired Olsson Associates to study the old model and FIRM and identify ways to reduce the floodplain extents and flood elevations.

“The biggest challenge was finding their old model and analyzing it,” said Eric Shelton, Olsson project manager. “Having the existing model was critical to understanding why the current flood elevations were so high from the start and identifying errors.”

From that point, we did a zoomed-out scale watershed study of the area, updated the model geometry, and developed a plan of potential projects that will help reduce actual physical flood elevations.”

Eric says at least two projects can be done to improve the water surface elevations in Lebanon. These include culvert replacement and raising roads in certain areas. Additionally, Olsson identified two errors in the existing map that had caused the effective flood elevations to be artificially high.

He said the city liked the revised map. Olsson will submit a revision request to FEMA to update the map.

“We were able to take a complicated issue the city was not able to resolve, sort it out, and fix it,” Eric said.

Olsson has an on-call agreement with the City of Lebanon for survey, water resources, and general civil engineering services.

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