The debate about whether or not to build on floodplains will always follow flooding disasters. LEED and other sustainable design certification systems encourage teams to locate development above certain floodplains. However, referencing this information is not something architects and engineers do everyday. Do you know how to determine if your project is in a flood hazard area? In the digital age, it is very easy to do.
A floodplain can be defined as a flood hazard area shown on a legally adopted flood hazard map or otherwise legally designated by the local jurisdiction or the state. For projects in locations without legally adopted flood hazard maps or legal designations, it is good practice to locate development on a site that is entirely outside any floodplain subject to a 1% or greater chance of flooding in any given year. Best practice is for a development to avoid the flood hazard area shown on a legally adopted flood hazard map instead of specifically referencing the 100-year floodplain.
In order to ascertain the proximity of flood hazard areas, design teams should consult legally adopted flood hazard maps of the project site area.
In the U.S., most local governments, flood management agencies, or other local entities maintain flood hazard maps, which may include flood hazard areas designated by both the Federal Emergency Management Agency (FEMA) and a local agency.
How to Generate a FEMA Flood Hazard Map
FEMA provides flood hazard maps through their Map Service Center (MSC) at msc.fema.gov
Step 1: Search by address and then select the flood map boundary in your area of interest using the Locator Map.
Step 2: Click the "View Web map" button above to access the flood insurance rate map data on the FEMA GeoPlatform.
Step 3: Note the legend and graphic depictions.
The Flood Hazard Zones are fairly self-explanatory:
- The 1% Annual Chance Flood Hazard zone is commonly equated to the 100-year floodplain.
- The 0.2% Annual Chance Flood Hazard zone is equated to the 500-year floodplain.
- If the project’s zone begins with A or V (e.g., A, AE, AO, A11, V1), it is below the 100-year floodplain.
- If the project’s zone begins with B, C, or X, it is above the 100-year floodplain.
- Zone D identifies areas of undetermined flood risk. The team must find additional information to determine whether additional flood-resistant design will be necessary.
LOMR and LOMA are perhaps less familiar. As defined by FEMA (hyperlinks added):
A Letter of Map Revision (LOMR) is FEMA's modification to an effective Flood Insurance Rate Map (FIRM), or Flood Boundary and Floodway Map (FBFM), or both. LOMRs are generally based on the implementation of physical measures that affect the hydrologic or hydraulic characteristics of a flooding source and thus result in the modification of the existing regulatory floodway, the effective Base Flood Elevations (BFEs), or the Special Flood Hazard Area (SFHA).
A Letter of Map Amendment (LOMA) is an official amendment, by letter, to an effective National Flood Insurance Program (NFIP) map. A LOMA establishes a property's location in relation to the Special Flood Hazard Area (SFHA). LOMAs are usually issued because a property has been inadvertently mapped as being in the floodplain, but is actually on natural high ground above the base flood elevation.
Example Flood Insurance Rate Map (FIRM) from FEMA.
Know your Flood Hazard Area
The debate over development within flood-prone locations will continue. However, every design professional should know the flood hazard area for any prospective development. FEMA's Flood Insurance Rate Maps are a tremendous resource. Design teams should always consult legally adopted flood hazard maps within the jurisdiction in which the project is located.