Hidden Flood Risks

Floods do occur outside of the mapped “one-hundred year floodplain”, also known as the Base Flood or Special Flood Hazard Area (SFHA). The Base Flood is by no means the worst flood possible. Flooding events greater than the Base Flood will inundate areas outside of the mapped SFHA. For flooding along the Willamette River, the five-hundred year flood is expected to reach four feet higher than the Base Flood in some locations. Some areas may also flood because of poor local stormwater drainage conditions. Such flood prone areas may have little or nothing to do with the mapped SFHA.

Our currently adopted floodplain maps were created by FEMA and have an effective date of 06/02/99. These maps were largely based on the original flood hazard boundary maps that were created in 1985. These maps are essentially a snapshot in time and cannot account for development in our watershed since then. New homes, streets and other development remove vegetation and often direct rainwater directly to the public stormwater system. The removal of vegetation and the piping of natural drainage systems reduce the time it takes water to reach our rivers and floodplains, and can result in greater flooding levels. Additional information regarding the Effects of Urban Development on Floods is available from the U.S. Geological Survey.

The National Flood Insurance Act of 1968 gave FEMA the huge task of creating flood hazard maps for the nation’s flood prone areas. Some areas such as parts of the River Road/Santa Clara area did not have sufficient population to warrant the detailed hydrologic and hydraulic analysis of the floodplain. In fact only the Willamette and Amazon Rivers received detailed studies. For other areas approximate data was used to determine the floodplain boundaries on the flood maps.

The City of Eugene requires that the lowest floor elevation for new construction and substantial improvements be located at least one foot above the base flood elevation. This extra level of protection or ‘freeboard’ is required partly due to mitigate these unknown factors. Some interesting facts about the likelihood of being flooded according to FEMA:
  • Between 25 to 30% of flood losses occur in areas outside of high-flood-risk areas.
  • The average home has a 5% chance of having a home fire during the life of a 30 year mortgage. For that same time period a house located within the Special Flood Hazard Area (SFHA) has a 26% chance that it will be inundated by a flood.
Flood Hazard Information Services 
The City of Eugene provides free information regarding the flood hazard for specific sites within our community and related issues at the Permit & Information Center. We can be reached by phone at 682-8400 or in person at 99 W. 10 th Avenue. We are open to the public from 9 AM to 5 PM Monday through Friday. We strive to return all calls within one business day.