Identification of Unmapped Special Flood Hazard Areas in Illinois

Philip Graff, Laura Chap, Sally A. McConkey, Michael DePue, Allen Kingsley, Rebecca Bicksler

Research output: Book/Report/Conference proceedingTechnical report


The Federal Emergency Management Agency (FEMA) publishes Flood Insurance Rate Maps (FIRMs) depicting Special Flood Hazard Areas (SFHAs), which have a 1 percent chance of being inundated in any given year. The Flood Insurance Act of 1968 initiated the nationwide identification of SFHAs. The Illinois Administrative Code (1994) defines state oversight of floodways for streams that drain 10 square miles (sq. mi.) or more in rural areas or 1 sq. mi. or more in urban or urbanizing areas. Floodways are the channel, including on-stream lakes, and the portion of the floodplain adjacent to a stream or watercourse, which is needed to store and convey the existing 100-year frequency flood discharge. If the flow is confined to the floodway, there would occur no more than a 0.1 foot increase in stage due to the loss of flood conveyance or storage, and no more than a 10 percent increase in velocities. Floodways are generally depicted on the FEMA FIRMs. This general guidance has been used when developing Illinois FIRMs to identify streams that pose a flood hazard. However, many SFHAs for streams meeting these criteria have not been identified on Illinois FIRMs. There are streams in rural and urban areas meeting the criteria for which SFHAs have never been identified, and there are urban areas where community annexation is expanding into locations mapped according to rural criteria. Unmapped SFHAs pose a flood risk, which has never been communicated to the public, floodplain managers, or elected officials. In order to bridge this gap in Illinois, a screening process was developed, and streams that have a potential flood risk for which SFHAs have not been mapped were identified for each Illinois County. This report contains maps of each of Illinois??? 102 counties illustrating stream reaches with potential flood risk for which SFHAs had not been identified on FEMA FIRMs as of July 1, 2007. Maps are accompanied by tabular data providing stream names and number of unmapped stream miles.
Original languageEnglish (US)
PublisherIllinois State Water Survey
Number of pages22
StatePublished - Jun 2008

Publication series

NameISWS Contract Report


  • ISWS


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