Determining 1% Annual Chance Base Flood Elevation

In many communities across the Nation, including the unincorporated Ventura County, there are remote rural areas that have yet to receive flood mapping from the Federal Emergency Management Agency. Although FEMA recognizes that there is a level of flood risk in these areas, due to limited budget and other resource constraints, it is not possible for FEMA at this time to undertake the necessary technical and expensive engineering studies, including hydrological and hydraulic modeling and topographic mapping, to generate accurate flood maps specifically for these areas. However, if a community decides that it intends to expand major development into such an area, FEMA will consider partnering with the community to create DFIRMs.

It is FEMA’s policy that for lands that are, in their technical opinion, located in a potential (unmapped) floodplain, they will be shown on the community’s digital Flood Insurance Rate Maps (DFIRMs) as an ‘Approximate A Zone’ floodplain.  Approximate A Zones do not show the boundaries of the floodway and floodplain nor is the estimated height of the floodwaters illustrated (called the base flood elevation).

Property owners wishing to construct a single family dwelling or other structure, or develop a subdivision or manufactured home park, or grade land, within an Approximate A Zone are required to retain the services of a California-licensed Civil Engineer or Land Surveyor to generate an engineering study that determines floodway and floodplain boundaries and estimates the base flood elevation. The accepted engineering methodologies are presented in FEMA publication 265 entitled “Managing Floodplain Development in Approximate Zone A Areas: A Guide for Obtaining and Developing Base (100-Year) Flood Elevations” (April 1995). The study will need to be stamped/ sealed and signed by the engineer consultant or land surveyor and included as part of a Floodplain Development Permit application.

More Information:

Zone A Presentation


Managing Floodplain Development in Approximate A Zones: AGuide for Obtaining.. FEMA 265