Floodproofing, FEMA and flood maps
The following is an interview between Randy Lewis, VP of Loss Prevention Education, Design Professional and Adam Reeder, PE, CFM, Senior Project Manager and a principal at CDM Smith, a full-service architectural and engineering firm headquartered in Boston, Massachusetts. Since 2007, Reeder has been working with the U.S. Federal Emergency Management Agency (FEMA) on dry floodproofing and other disaster issues. He also recently completed the flood-related portion of an economic analysis report for the National Institute of Building Sciences entitled Mitigation Saves 2.0.
Randy asked Reeder to help enlighten designers about the dangers of overreliance on FEMA flood maps when working in areas with flooding potential and the risks associated with signing FEMA floodproofing certificates. He spoke to us from his office in Raleigh, North Carolina.
Randy Lewis: You’ve expressed serious concern about the tendency of design professionals to rely too heavily on FEMA’s flood insurance rate maps, or FIRMs, when determining the elevation of a structure’s foundation. What’s wrong with these FIRMs?
Adam Reeder: There’s nothing wrong with them as long as they’re used for their intended purpose, which is insurance rating, and not used as the sole source for design information. FIRMs indicate the Base Flood Elevation (BFE) and are the maps referenced by local ordinances, building codes, and design standards as the basis for calculating the minimum required elevation. Think of them as a starting point for designers.
RL: What’s their insurance-related purpose?
AR: FEMA initially created the maps so the National Flood Insurance Program (NFIP) could rate buildings within a floodplain. The maps are used to compare a reference elevation, such as the elevation of the top of the lowest finished floor, to the elevations (BFEs) presented on the maps, which are based on the one-percent annual chance of a flood, also referred to as the chance of a 100-year flood.
RL: Why have the FIRMs been adopted for use by designers?
AR: More than 20,000 communities participate in the NFIP. Since participation requires a community to adopt the FIRMs in its floodplain ordinances, local building codes eventually adopted the maps as the minimum standard that builders must meet. The FIRMs are the most widely available source for flooding information.
RL: Why shouldn’t designers rely so much on the FIRMs?
AR: The maps represent the probable flood patterns at the time FEMA did the hydraulic and hydrology modeling. If a map has been updated, a designer must determine whether the map reflects new modeling or whether there was some other change that FEMA recognized (e.g., incorrect elevations on the map that may not change the flooding dynamics) and revised the map without re-running the flood modeling. A map with a 2009 date could represent modeling done in the 1980s. Overall, the maps represent the best available data at the time the modeling was done. A lot can change in a community in 20 or 30 years—roads are paved, new developments arise, forests are cleared and so on. All of these factors could increase flood heights. We should also remember that the maps are intended to represent current conditions— designers should think about changing flood conditions over the life of a building and design with these changes in mind.
RL: When an area is hit with major flooding, does FEMA take any immediate action to upgrade the maps?
AR: Whenever there’s major flooding, such as in Houston with Hurricane Harvey, FEMA will study the affected area. They’ll create advisory maps that are conservative and probably set elevations higher than reality just to be safe. They’ll tell communities to use those until the agency can do more detailed maps, which usually takes about three years. The communities then need to formally adopt the maps in order for them to be considered effective.