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Conveying 100-Year Flood Flows

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Resilient St. Vrain: Restore & Revitalize


The projects that comprise Resilient St. Vrain are being designed to safely convey the estimated 100-year flood flows.100_year_flood

The term 100-year flood refers to an event that has a 1 percent chance (or 1 in 100 probability) of occurring each year; however, 100-year floods can happen much more often than once every 100 years, as flooding events are irregular and depend on many factors. The 100-year floodplain is the area covered by water during a 100-year flood. The Federal Emergency Management Agency (FEMA) refers to the 100-year floodplain as the Special Hazard Flood Area (SHFA).

View a U.S. Geological Survey poster that explains what "100-year flood" means (1 MB PDF)

Design plans for Resilient St. Vrain use the estimated 100-year flood flows and the 100-year floodplain as benchmarks. FEMA, state and local agencies use the 100-year floodplain in regulatory processes related to floodplain development, building permits and environmental regulations, as well as for setting flood insurance requirements and costs.

Resilient St. Vrain designs include a wider creek channel and bridges to safely convey the estimated 100-year flood flows, which in turn will reduce the size of the 100-year floodplain. This reduction means that fewer properties ultimately will be located within the floodplain.

Flood insurance is only required for properties in the 100-year floodplain that have federally backed loans. Flood insurance is available to any property owner in Longmont, regardless of whether the property is located in the 100-year floodplain. However, flood insurance is much less expensive for properties mapped outside the 100-year floodplain.

View maps and information about proposed revisions to the floodplain >

(For comparison, a 500-year flood event has a 0.2% chance of occurring each year. Resilient St. Vrain does not use the estimated 500-year flood figures as a standard because neither FEMA nor the state regulate the 500-year floodplain.)

Learn more about Resilient St. Vrain project goals and design >

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