Zone A is a Federal Emergency Management Agency (FEMA) Special Flood Hazard Area (SFHA). An SFHA, as defined by FEMA, is an area that has a 1% of being inundated by flooding during a given year. In other words, it is in the “100-year” flood zone. SFHAs are labeled as Zone A, Zone AO, Zone AH, Zones A1-A30, Zone AE, Zone A99, Zone AR, Zone AR/AE, Zone AR/AO, Zone AR/A1-A30, Zone AR/A, Zone V, Zone VE, and Zones V1-V30. Unlike some other SFHA designations, Zone A is not assigned a base flood elevation (BFE) because Zone A comprises areas not associated with a detailed hydrologic study. However, elevation certificates require the engineer or surveyor to put a BFE on the form. In the following blog post, I will discuss how to determine a BFE in Zone A areas.

Methodology

FEMA outlines the acceptable methodology for determining a BFE in a Zone A area in their FEMA 265 – Guide for Approximate Zone A Areas document. Sometimes, “simplified” methods that do not require hydraulic/HEC-RAS modeling may be acceptable. However, many projects will require that a qualified person (typically a civil engineer) prepare a hydraulic model using HEC-RAS. In other cases, a jurisdiction like Phoenix, Arizona may require that the BFE be associated with the highest bank of an adjacent channel (if applicable).

In areas where high-resolution terrain data is freely available through the United States Geological Survey (USGS), a topographic survey or proprietary LiDAR data may not be required. However, areas that do not have freely available terrain with a spatial resolution of one meter or finer may require ordering LiDAR or a topographic survey for the area of interest. This is to ensure that the topography is accurately reflected in the model. It is worth noting that terrain data will often pick up the water surface of perennial streams as the “bottom” of the channel. In these instances, it may be wise to survey the bottom of the channel/stream of interest to make sure your analysis is not generating an artificially high BFE.

FEMA 265 also outlines some ways to determine the one percent annual exceedance probability (AEP) or 100-year flow rate associated with the upstream drainage area of the project/area of interest. One acceptable method is to use published regression equations. I have found this to be the easiest method, especially for drainage areas larger than one square mile in size.

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Surf City Hydraulic Engineering has prepared multiple Zone A BFE Determination reports for residential and commercial projects in Southern California. If you need help with BFE determination for your project, please feel free to reach out using this form!