Reading a floodplain mapthe zone, the floodway and the base flood on one panel

Open FEMA's Flood Map Service Center and type the parcel's street address into the search. FEMA calls the Service Center the official public source for flood hazard information produced for the National Flood Insurance Program, and the search takes an address, a place name, or longitude and latitude coordinates. What comes back is the flood map panel covering that spot.

Carroll County's floodplain development permit application makes a good checklist for reading the floodplain map, because it asks for exactly five things from the map, one line each, and every other step in this guide follows that order:

1. Map panel number: which sheet of the map covers the site.
2. Panel date: which edition of that sheet.
3. Flood zone: the letter the map gives the ground.
4. Floodway: yes or no.
5. Base flood elevation: a height, where the map supplies one.

Virginia's Department of Conservation and Recreation runs a second door to the first question. Its Virginia Flood Risk Information System, VFRIS, lets a user locate a property and see whether it falls inside the Special Flood Hazard Area. That is a quick first look. The county form asks for a FEMA panel and its date, so the Service Center is where those two lines get copied from.

A rural tract with no street address can be searched by coordinates instead. If the tract itself is the uncertain part, find it on paper first: locating a parcel by its recorded plat and its tax map number, and why GPIN is a borrowed name in these counties, is covered in plats, parcel numbers and GPINs.

The zone letter says which side of one line the ground falls on. Under the federal NFIP definitions at 44 CFR 59.1, the area of special flood hazard is the land in the flood plain within a community subject to a 1 percent or greater chance of flooding in any given year, and the special flood hazard area is the same thing under a second name.

Base flood means the flood having a one percent chance of being equalled or exceeded in any given year.

44 CFR 59.1, National Flood Insurance Program definitions

A 1 percent chance in any given year is an annual odds figure, not a timetable. It does not mean the ground floods once a century, and a flood this year does not use up the next ninety-nine.

The letters themselves appear in the federal floodplain management criteria at 44 CFR 60.3, which set standards for Zones A, A1-30, AE, AH and AO on a community's Flood Insurance Rate Map. Those are the letters this guide can name from the regulation. A map legend carries others, the X zones among them, and what those letters mean is FEMA's to explain; nothing here defines them.

For the word itself, what a floodplain is in Virginia statute and the federal rules is set out in the glossary. On the panel the useful habit is simpler: copy the letter exactly as printed, because the elevation standards in the next steps are written zone by zone.

The last line asks for a number of feet. The base flood elevation is not the depth of water on the lot and not the height of the ground. It is the level of the base flood, and the federal rules compare a building's lowest floor to that level.

Once FEMA has provided final flood elevations on a participating community's map, the federal minimum standard for the lowest floor of new homes and substantial improvements in Zones A1-30, AE and AH, set in 44 CFR 60.3, is written against this level. What that standard asks of a house belongs to the build question in step four.

Zone A reads differently. Where base flood elevation data are utilized within Zone A, the same criteria direct the community to obtain the elevation, in relation to mean sea level, of the lowest floor of new and substantially improved structures. A Zone A panel without a printed elevation still leaves an elevation question, and the floodplain office for the parcel is where to ask how it gets answered.

This is where a bottomland hayfield and a house site on the slope above it part ways. Two spots on one tract can share a zone letter and sit at very different heights relative to the base flood level. The letter alone does not say how high the ground is, and the next document is where a building's height gets written down.

A panel is a regulatory drawing. It marks the ground the flood rules are written for. It is not a record of every place water has stood.

Flooding can occur on any property, not just within high or moderate flood risk areas.

Virginia DCR, Virginia Flood Risk Information System

A site outside the special flood hazard area on the panel is outside the mapped 1 percent line, and nothing more. A low corner of a field or a road culvert that backs up after a hard rain does not need a zone letter to hold water.

River frontage raises the question in its sharpest form, and whether New River frontage floods is answered on its own page.

The flood panel is also one record among several. Access, utilities, grade, soil and zoning each have a record and an office of their own, and a flood line joins that list rather than replacing it; the order those passes go in is set out in how land is read, factor by factor.