Reading a Floodplain Map

Reading a Floodplain Map

August 7, 2018

Floodplain mapping terminology is used frequently and just as often misunderstood. For instance, a 100-year flood has a 26% chance of occurring during a 30-year period (the length of most home mortgages).

Lizzy Bache answers the question, "What does a floodplain map tell you?"

Since the 1970s, FEMA has been creating a database of floodplain maps to help communities mitigate their flood risk through participation in the National Flood Insurance Program (NFIP). These floodplain maps, referred to as Flood Insurance Rate Maps (FIRMs), provide an abundance of information to the user, such as: map land marks, highway information, base flood elevations, flood zones, and floodplain boundaries.

The 100-year flood has a 26% chance of occurring during a 30-year period (the length of most home mortgages).

Property owners can utilize FIRMs to determine the flood zone in which their property lies. However, it is important to consider that maps are frequently updated due to changes in geography, new development in the area, or significant meteorological events. Community flood maps are typically available at local planning offices, as well as on FEMA’s website.

What is a floodplain?

In order to accurately read a floodplain map and extract worthwhile information, it is useful to know what a floodplain actually is. Although the term “floodplain” is commonly used to describe the area that will flood during a 100-year/24-hour storm, a floodplain is defined as any area subject to frequent overbank flooding and inundation from a riverine or coastal source.

While used as the regulatory benchmark, the nomenclature “100-year flood” does not refer to a flood that occurs once every 100 years. In fact, the 100-year flood has a 26% chance of occurring during a 30-year period (the length of most home mortgages).

This text was originally published in Walter P Moore's 2018 Stewardship Report.