Sumner County staff presents updated floodplain mapping, seeks targeted Army Corps studies for Liberty Creek basin

Sumner County planning staff detailed the county's approach to floodplain mapping and urged targeted basin-level studies, saying existing FEMA maps are limited for local planning and mitigation.

Planning staff framed the issue as one of risk mitigation rather than prediction, saying, “FEMA will tell you, we don't predict flooding. All we do is mitigate risk in these flood zones.” The presentation explained that FEMA's Zone A designations and county floodplain rules are designed to allow residents to obtain flood insurance while guiding development away from high-risk areas.

County staff described how the county establishes an effective floodway in Zone A: surveyors identify the stream's top-of-bank and measure twice the width of the stream from that point to set an effective flow length. Staff also said the county uses a 60-foot stormwater buffer from top-of-bank in some regulated drainages and typically requires finished-floor elevation set at the highest adjacent grade plus three feet for new structures.

Staff showed results from a Corps-funded flood study of Liberty Branch (Liberty Creek). The county presentation said the Corps' study produced a more precise 100-year floodplain and floodway mapping that differed from the broader, estimated FEMA Zone A footprints. Staff said the more-detailed maps show numerous existing houses and parcels located inside the actual floodway that were not previously flagged on generalized maps. “When you see it, it's kind of startling,” the presenter said, describing properties that likely should not have been built inside the newly mapped floodway.

County staff outlined two U.S. Army Corps of Engineers programs for flood studies: a free program with a long queue and variable turnaround, and a cost-share program in which the Corps and local government split study costs (described in the meeting as a 50/50 program). Staff estimated that a standalone basin study for the area near the David Wood bridge on Highway 25 could cost “a couple $100,000,” while acknowledging costs vary by basin.

Staff said the county has set aside $100,000 for flood-study work and that officials intend to submit new study data to FEMA for map revisions once detailed analyses are complete. The presenter described a timeline for some basin work with the expectation that additional flood-study results could be available “by next year,” and emphasized prioritizing basins where people currently live to maximize benefit from Corps resources.

Staff also listed policy and regulatory steps under consideration: (1) completing targeted flood studies to better define floodways and floodplains at basin scale, (2) continuing enforcement and possible strengthening of county floodplain and stormwater regulations, and (3) acquiring and cleaning up repeatedly flooded parcels where acquisition is feasible. The presenter said such buyouts and remediation would require identifying recurring revenue sources beyond the one-time study allocation.

Commissioners and staff discussed examples from rural areas and near Highway 25, noting rural county roads often follow creeks and concentrate housing in low-lying corridors. Staff said past Corps studies, including a Liberty Branch study initiated several years ago, revealed more restrictive floodways than earlier estimated maps and prompted decisions to require elevation changes and other mitigation for some properties.

No formal votes were recorded on study direction during the presentation. Staff said next steps include confirming which basins the county will pursue for Corps or paid studies, coordinating with the Corps and FEMA, and refining county regulatory language to reflect study findings where appropriate.