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FEMA Region 9 flood remapping update: Gila County told to review preliminary maps and prepare for public appeals
Summary
FEMA Region 9 provided Gila County with preliminary floodplain mapping using regional regression hydrology and HEC‑RAS hydraulics; county flood control staff urged careful review citing some flow‑rate differences versus previous studies and outlined outreach during the appeal period.
Jeff Despain briefed the Gila County Flood Control District Board of Directors on FEMA Region 9’s preliminary floodplain remapping, saying the project began in 2023 and that FEMA used regional regression (USGS Bulletin 17c) and HEC‑RAS hydraulics to develop new baseline engineering (BLE) and detailed mapping across county watersheds.
Despain said FEMA completed roughly 80 miles of detailed mapping and more than 1,000 miles of baseline engineering in parts of the county, with about 158 panels updated. He cautioned that regional regression hydrology can produce substantial differences compared with earlier, more detailed hydrology studies—he pointed to instances where flow‑rate estimates differed by as much as about 80 percent—and recommended county review comments on hydrology and hydraulics during the preliminary phase.
The mapping work includes one‑ and two‑dimensional modeling; two‑dimensional models produce different representations of depth and extent and are newer in FEMA practice. In some areas FEMA modeled the landscape without unpermitted berms and other built features, which can expand floodplain widths compared with prior maps. Despain noted that some previously accepted Letter of Map Revisions (LOMRs) may be superseded by the new mapping.
Next steps, Despain said, include FEMA issuing preliminary maps, a public appeal period (timeline currently TBD), post‑preliminary coordination and eventual letter of final determination and map effective dates. County staff said they will post outreach materials and invite public comment; Despain said the county supplied extensive review comments and will seek involvement in post‑preliminary coordination.
Supervisors discussed whether the maps could be used as “best information” prior to final adoption; Despain said FEMA sometimes allows early use but not uniformly. Supervisors also credited a FEMA‑funded University of Arizona post‑fire study (now extended) that models watershed impacts of wildfire and said such studies can inform post‑fire map revisions.
Despain urged property owners and local officials to review proposed changes during the appeal period; county staff will notify the public when the appeal window opens.
