Leander City Council heard an update Thursday on a multi-year effort to revise the city's official Federal Emergency Management Agency flood insurance maps through a Letter of Map Revision (LOMR).

Consultants and city engineers said updated rainfall estimates and more recent lidar topography have produced revised models that expand the mapped 100-year floodplain and identify potentially affected buildings.

"This is a letter from FEMA officially revising the National Flood Insurance Program maps," said Tatum Lawton, a consultant with Lochner, describing a LOMR application submitted to FEMA. Lawton and City Assistant Engineer Sarvesh Takala presented the phase-1 modeling work and options for phase 2.

Why it matters: the city has been regulating to NOAAAtlas 14 rainfall and newer lidar for several years, but FEMA's official flood insurance rate maps remain based on older data. Updating the FEMA models would align the regulatory maps used for insurance and broader planning with the city's current technical basis.

What the models show: Lawton and Takala said the citywide model revisions incorporate Atlas 14 rainfall depths and updated topography. Leander's effective FEMA maps currently show about 1,430 acres in the 100-year (special flood hazard) area. The revised models show about a 638-acre increase in some places and a small 34-acre decrease in others, for a net modeled increase of roughly 2,034 acres (about 40% more area modeled as special flood hazard). Structure counts change as well: the FEMA-effective maps list 329 structures in the floodplain; the updated draft models add 414 structures and remove 19, for a net modeled total near 724 structures that intersect the revised floodplain (the consultant cautioned the intersect analysis does not use first-floor elevations).

"These two impacts, the increased rainfall and the urbanization, kinda have this combined effect, resulting in changes in runoff characteristics, conveyance, and ultimately the shape of the flood plain," Lawton said.

Paths forward: staff and the consultant presented two main options. Option 1 is to finalize remaining technical updates (including adopting the April 2024 lidar) and submit a LOMR application to FEMA, which requires concurrence letters from affected entities such as the Upper Brushy Creek Water Control and Improvement District and a technical comment/response cycle with FEMA. Council staff estimated FEMA review and concurrence could take on the order of about a year. Option 2 is to adopt and use the updated models and maps internally ("local best available data") but not take the maps through FEMA; under option 2 the city would still gain planning value from the models and could do homeowner outreach but would not change the federal regulatory maps used for mandatory flood insurance determinations.

Council directions and questions: council members pressed on downstream/upstream contributions to flow, the difference between the citylevel modeling and countywide mapping, and how new lidar would be incorporated. Assistant City Engineer Sarvesh Takala said Leander sits in headwaters of Brushy and Blockhouse creeks and the modeling accounts for the contributing drainage areas within the modeled reaches, while noting model limits and the tradeoffs in the LOMR scope.

Council also asked staff and consultants for follow-up materials before a final decision, including:
- Close-up exhibits that show which houses and blocks would be newly included in the modeled floodplain and a public-safety oriented map set to identify flood-risk hotspots, travel routes and potential entrapment areas;
- A cost/benefit or high-level financial estimate comparing option 1 (submit to FEMA) vs. option 2 (local adoption only), including likely homeowner insurance impacts and potential grant eligibility differences;
- An estimate and timeline to incorporate the April 2024 lidar into the phase-2 models and to finish a FEMA-ready LOMR package; and
- Coordination plans and draft letters of concurrence for affected partners including Upper Brushy Creek WCID and Williamson County.

The presenters recommended incorporating the 2024 lidar in phase 2 if the council elects to pursue the FEMA route, to keep the application current and lengthen the useful life of the new FIRMs.

What comes next: council asked staff to return with the requested exhibits, cost information and a recommended procedural path. Staff also noted parallel county mapping efforts and that if Leander proceeds first, those models could feed into future countywide submittals.

Ending note: Lawton stressed the application process is iterative with FEMA and not automatic: "FEMA would come back and make comments. We would have to address those comments," she said. The council did not vote on a course of action Thursday and directed staff to provide the additional materials before making a formal decision.