The "Evil Zone A"

The "Quick-2" software mentioned in this article requires a basic knowledge of stormwater modeling, and your own topographic data to be plugged into the models of various shaped culverts. This is not a universally useful program, but is helpful for certain situations. However, if you aren't familiar with stormwater modeling and can't tell whether or not the numbers you are getting at the end seem reasonable, don't use it! You will create more problems for yourself than you solve.
The software can be downloaded from FEMA's Website at:
www.fema.gov/mit/tsd/FRM_soft.htm

Any of us who use Flood Insurance Rate Maps (FIRMs) from the Federal Emergency Management Agency (FEMA) encounter dark shaded areas depicting flood hazard areas that are subject to federal, state, and local regulations, but that yield no details about depth of flooding. These dark areas are "unnumbered A zones," and on the Technical Mapping Advisory Council, one member has referred to them as "Evil Zone A" due to the difficulties in locating and regulating them. On the maps, these black holes of the flood data world are merely identified as "A" with no suffix number or letter to indicate type of flooding, and no Base Flood Elevation or depth of flooding identified.
Unnumbered A zones are 1 percent annual chance flood hazard areas (also referred to as "100-year flood hazard areas," a term that FEMA is trying to phase out due to public misunderstanding about its frequency or severity). This means that there are zoning, planning, and construction restrictions that apply to use of land in these areas. However, because they have no further identification other than "A," it is difficult to determine the true extent and depth of this flood risk on the ground in real life. The Flood Insurance Study reports that are prepared for many FIRMs to document the details of how the study was performed, usually contain little or no discussion of unnumbered A zones for maps that contain both numbered and unnumbered flood hazard areas. The study reports do not, therefore, contain profiles that might prove helpful in identifying flood levels. Further, maps that contain only unnumbered A zones are not documented in any Flood Insurance Study report. Seekers of this data are left to dig for it on their own.

Many Parties Face Problems
Unnumbered A zones present problems to communities that must enforce floodplain management ordinances, because of the approximation of the location and depth of the floodplain. They also present a problem to surveyors and others who must determine Base Flood Elevations (BFEs) in order to complete Elevation Certificates or enforce construction codes. Unnumbered A zones do not have published BFEs due to lack of sufficient study detail beyond basic identification of the mere existence of a 1 percent annual chance floodplain.
This leads to inconsistency in determination of BFEs, based upon the various approximate methods that are available and acceptable. Methods vary from:

• Establishing the location of the gray shaded hazard area on the ground and measuring the field elevation at those limits; to

• Overlaying the FIRM onto a USGS topographic quadrangle (often the original basis for a FIRM) and interpolating the nearest contour followed by the gray shaded, mapped Special Flood Hazard Area on the FIRM; to

• Applying present day ground elevations to software such as "Quick 2" to calculate a BFE.

The results from these varying methods yield a range of BFE values, and any of them may be more or less reliable or accurate than another in relation to the true risk of flooding. A more approximate method that attempts to duplicate original calculation of the floodplain limits may in fact be more accurate than a more technologically advanced software calculation, based on solely modern measurements.

A Zone Inconsistencies
Such inconsistencies make it difficult to equitably determine whether or not a Letter of Map Amendment, a Certificate of Occupancy, or an application for construction should be issued or denied. And it falls to us as technical map experts to help our clients solve such problems. But where is the edge of the A zone on the ground? Can we know how the line on the map was created to help in our identification? Basically, the presence of an unnumbered A zone on a FIRM serves as a red flag, letting us know that we will have to figure out a "work-around" to accommodate the true flood hazard.
FEMA has been struggling with the repercussions of unnumbered A zones for years. While recognizing their shortcomings, at least such designations do alert the public to possible hazards. In a community where funding for a full detailed study just isn't available, perhaps any old map with a splotch of dark gray on it is better than nothing, in order to allow local officials to regulate what they know to be risky areas for development.
But that doesn't help us as the ones in need of a solution for establishing a base flood elevation that has some relevance to the mapped flood hazard area. Discussion on the resolution to this problem has been heated, and at the same time, cautious.

The Debate Continues…
Version One of the debate goes like this: Should FEMA or the local governments re-run the models from the original 25-year-old studies (if they can be identified) using newer hydrologic and hydraulic analytical programs than were available at the time of original mapping? Because of lack of funding, this would have to be done without new fieldwork, but could provide approximate BFEs. Should this information then be provided to those who request it, but with a caveat as to its accuracy? Such data might prove useful in better defining the BFE on the FIRM (although not very rigorously), but probably should not be published for general use.
Version Two of the debate suggests community development of "advisory" BFEs. This would require the community to assess historic flood levels to reconcile them with the mapped dark shaded areas on their FIRMs. Is it proper for communities to use such approximately derived elevations to restrict and control construction? In rapid growth environments, everyone wants something better than an unsubstantiated approximate BFE, but is an advisory elevation subject to too much community discretion? On the other hand, to assure cumulative effects of watershed development are accommodated, should an advisory BFE be based upon the 0.2 percent annual chance flood (500 year) instead of the 1 percent annual chance flood?
In the absence of immediate answers to these questions or of funding to improve flood mapping nationwide, the best thing we as professionals can do is to become as informed as possible about how Base Flood Elevations will be put to use. To help our clients and protect ourselves from liability, we must be familiar with the National Flood Insurance Program regulations, local enforcement, proper use of FIRMs, and appropriate determination and use of Base Flood Elevations.

Wendy Lathrop is the ACSM representative to FEMA's Technical Mapping Advisory Council, and a Contributing Editor for the magazine.

» Back to our October 2000 Issue