FEMA Floodplain Mapping System

FEMA has been creating flood hazard maps since the 1970s. Flood Insurance Rate Maps (FIRMs) are the primary tool for state and local governments to mitigate the effects of flooding in their communities. The flood risk information in the maps is based on meteorological, hydrologic (study of rainfall) and hydraulic (study of how water flows through creeks) data, as well as open-space conditions, flood control infrastructure and urbanization.

The National Flood Insurance Program (NFIP) is a federal program that makes flood insurance available to property owners in flood-prone areas identified in the FIRMS, if their community agrees to adopt and enforce floodplain management ordinances. The Alameda County Flood Control District has put such a program in place.

The FIRMs show where flooding could occur during a storm that has a 1% chance of occurring during a given year (sometimes called a 100-year floodplain). The maps shows Special Flood Hazard Zones, ranked by the severity or type of flooding that might occur. In Zone 2, FEMA maps show several types of Special Flood Hazard Zones:

• Zone A, AE, AH, AO: Areas that corresponds to 100-year flooding; flood insurance is mandatory.
• Zone V, VE: Coastal areas that corresponds to the 100-year floodplain with the additional hazard of storm waves; flood insurance is mandatory.
• Zone D: Areas where there is possible but undetermined risk of flooding; flood insurance is available but not mandatory.
• Zone X: Areas outside the 1% floodplain or where flooding would be less than 1 foot; flood insurance is available but not mandatory.

In the future, FEMA may be able to change Zones A, AE, AH and AO to Zone AR, if the District is able to proceed with flood protection plans.

• Zone AR: The flood insurance rate zone that results from de-certification of a previously accepted flood protection system that is being restored to provide protection from the 1% storm or greater event. FEMA will consider using the Zone AR designation for a community—with lower flood insurance rates—if the flood protection system has been deemed restorable by a Federal agency; a minimum level of flood protection is still provided to the community by the system; and restoration of the flood protection system is scheduled to begin within a designated time period and in accordance with a progress plan negotiated between the community and FEMA. The Zone AR designation will be removed and the restored flood control system shown as providing protection from the 1% annual chance flood on the NFIP map upon completion of the restoration project.