The Twelve-Lot Subdivision in Unnumbered Zone A

The study is a requirement, not a courtesy. In approximate Zone A the community must obtain, review, and reasonably use the best available flood data - and must require the applicant to supply BFE and floodway data for subdivisions of 50 lots or 5 acres or more (44 CFR 60.3(b)(3), (b)(4)). At 22 acres the threshold is met.

Which data governs. The effective FIRM remains the regulatory map; the developer's model does not change it. The community may, however, permit using more restrictive, technically supported data if it documents the basis. Because the model delineates a floodway and shifts the boundary 150 ft, the map itself must be changed through a CLOMR before construction and a LOMR afterward - paid for by the applicant, not the taxpayers.

Datum. "Negligible" is not a datum statement. Require a written conversion from NGVD 29 to NAVD 88 with the benchmark and conversion factor used, and require that every permit, elevation certificate, and plat carry one stated datum. A 0.8–1.5 ft slip is enough to put a lowest floor under the BFE on paper while looking compliant.

Model credibility. A 150 ft boundary difference from the effective study signals that the model may not reproduce effective conditions. Require calibration to the upstream gage, documented culvert and bridge geometry, and current land use, plus independent peer review before staff relies on it. An unreviewed model is not "best available" data.

Floodway. If the floodway is delineated as the model shows, lots 4–6 cannot be filled as drawn. Encroachment in a floodway requires a no-rise certification supported by a registered engineer's analysis showing no increase in BFE, and staff should have that analysis reviewed, not accepted on the applicant's signature. Realistically, the design should be reconfigured to keep fill and structures out of the delineated floodway.

Flood fringe. Require compensating storage on at least a 1:1 basis - on-site cut to offset every yard of fill, with the storage graded to drain and protected from future filling by plat note or easement. Require the pad elevations to be set from the adopted BFE plus freeboard and shown on the plat and on each lot's building envelope.

Cumulative impact. No-Adverse-Impact analysis: this project must be evaluated together with other approved and pending fills in the reach, not lot by lot. Require a cumulative storage-loss accounting for the reach.

Construction-phase conditions. Erosion and sediment control, no stockpiling in the floodway, a prohibition on raising grades after final plat without a new permit, and disclosure of the flood hazard on each lot's deed and on the plat.

The absence of a mapped floodway is not permission. In approximate Zone A the floodway has simply never been delineated. The community has authority - and, under its duty to reasonably use best available data, an obligation - to require the applicant's study to delineate the floodway and then apply a no-rise standard to that delineation. A developer cannot manufacture a permissive result by pointing to a 1985 map that predates the analysis.

Interim permitting approach:

• Permit only against the higher of the effective FIRM BFE or the peer-reviewed model result, documented in writing as the community's best available data.
• Require an elevation certificate on each lot foundation, tied to the stated datum, before framing.
• Do not allow any fill until the storage compensation and floodway questions are resolved; approve in phases if necessary.
• Notify the objecting owners in writing of the data relied on and of their right to participate in the map revision process. When FEMA issues a preliminary map, there is a 90-day appeal period; the developer pays for the revision, but the appeal rights belong to the public.
• A later LOMR may remove lots from the SFHA, but that does not retroactively excuse fill placed without a permit. Track the conditions on the plat so future buyers and builders are bound.