Making sense of flood study

Sept. 03--The agreement with the Department of the Army for the Lebanon Flood Risk Management Study will diagnose and relay information about the two areas to the Federal Emergency Management Agency.

The city's total estimated share of the study is $700,000 and would be appropriated in $175,000 increments for the 2015-2019 fiscal years, totaling $525,000 for the three-year period.

The council also asked Wilson County to participate in the study.

"This is huge. This is a monumental task for Lebanon to take on," Lebanon Mayor Philip Craighead said Wednesday.

"Using new models and design and techniques to determine where our 100-year and 500-year floodways are. They will use that to take to FEMA to update the maps," he said.

The federal government received a letter from the City of Lebanon dated Aug. 20, 2014, in which the city expressed its desire to participate in a study for flood risk management.

The watershed has experienced more flooding than the major flood of 2010, which is described in the Bartons Creek Watershed Management Plan.

Major flooding with widespread impact occurred in 1928, 1939, 1962, 1963, 1989 and 2010, thus occurring every 10 years. Less widespread, more localized flooding also occurs throughout the basin at greater frequencies. Flooding can be the result of widespread major rain events or small intense storms and thunderstorms.

History of the Bartons and Sinking Creeks includes studies and modeling by the Corps of Engineers, Nashville District (USACE LRN), U.S. Geologic Survey and others over the last about 50 years.

Following the 1939 flood, a review report prepared by the Corps of Engineers described four types of flood protection that were considered for Sinking Creek --channel modification, channel diversion levees and floodwalls and detention reservoir.

The only determined feasible solution at the time was a flood-retarding reservoir, according to the report, however it had an unfavorable economic ratio and it was not recommended that the federal government participate at that time.

In 1971, a Floodplain Information Report was produced by the Nashville District Corps of Engineers, which presented and explained the flood situations on Bartons and Sinking Creek.

Six years later, another flood study was prepared under contract by Berger Associates of Columbus, Ohio. The study completed for the Tennessee Department of Conservation and the info was used to update the USACE flood damage data.

Following 1979 flood, the City of Lebanon requested USACE conduct a study of possible flood damage reduction measures under Metro Region of Nashville authority.

Initial studies indicated that a rock fill headwater dam with a permanent pool was not a feasible alternative for the Sinking Creek basin due to increased development.

The recommended NED plan (1986) consisted of a dry-bed dam in the headwaters of Sinking Creek. The non-regulated roller-compacted concrete structure would have been 1,900 feet long and 19 feet high at the steam bed, and would have a low level outlet at its base to pass normal flow.

The dry bed reservoir would have only reached its 135-acre capacity during flood events greater than 100-year frequency. At the maximum capacity, floodwaters would subside and the ponded area would drain in less than two days.

The dam would have impounded stormwater more than 40 to 50 acres once a year and would have drained in less than one day.

Recreation facilities such as practice fields, hiking and bicycle trails were planned for the less frequently flooded areas of the dry reservoir.

Following the 2010 flood, the Nashville District received supplemental funding to produce flood preparedness products for communities that were greatly impacted.

"This will enable the development of projects to move much quicker because they know they have updated maps of the area," said Craighead, who said the study would take about four years to complete. "This is really huge, so we're excited about it."

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