Base Flood Elevations, Base Flood Depths, and Design Flood Elevations

Understanding Base Flood Elevation (BFE), Design Flood Elevation (DFE), and Base Flood Depth (BFD)

The Base Flood Elevation (BFE) is the projected height that floodwaters are expected to reach during a base flood (1% annual chance flood, or “100-year flood”). BFEs are essential to FEMA’s flood risk assessments, guiding insurance, building codes, and local planning for high-risk flood zones.

In addition, FEMA uses the Base Flood Depth (BFD) in areas where BFE data is unavailable. BFD reflects the expected depth of floodwaters above the ground level during a base flood, often used in zones subject to shallow flooding.

Many communities also establish a Design Flood Elevation (DFE), typically set above the BFE, to account for additional safety and future flood risks. DFEs often include “freeboard” (extra elevation above the BFE) to provide a greater margin of protection.

Importance of BFEs, DFEs, and BFDs

1. Flood Insurance Requirements: BFEs and BFDs are integral to determining flood insurance rates for properties in Special Flood Hazard Areas (SFHAs). Properties in SFHAs with federally backed mortgages must carry flood insurance. Elevating structures to or above the DFE can reduce premiums by decreasing flood risk.

   Building and Construction Standards:

   • BFEs are the minimum elevation requirements for new constructions and substantial renovations in high-risk zones. Building to the BFE reduces the likelihood of flood damage.
   • DFEs provide an added safety margin beyond the BFE, offering greater protection for new or substantially improved structures in flood-prone areas.
   • BFDs apply where BFEs are unavailable, particularly in shallow flooding zones, indicating the necessary elevation above ground level for safe construction.
2. Community Planning and Zoning: BFEs are fundamental to zoning, floodplain management, and emergency planning, but many communities adopt DFEs to address local needs and future risks. Building to the DFE often lowers long-term risk and can improve a community’s standing within FEMA’s National Flood Insurance Program (NFIP) Community Rating System, potentially reducing flood insurance rates for residents.
3. Enhanced Flood Risk Management: BFEs set a baseline for flood safety, while DFEs promote resilience against anticipated increases in flood events. BFDs, where applicable, offer insight into flood depth in shallow flooding areas, helping communities enforce effective flood risk management.