Floodplains are areas of land prone to inundation by water from a nearby source, such as a river, lake, or ocean. Regulatory bodies map these areas to assess and communicate flood risk. Understanding the terminology used to classify floodplains is important for property owners, potential buyers, and anyone making decisions about insurance coverage.
Deconstructing the “500-Year” Terminology
The term “500-year flood plain” is a statistical concept referring to a flood event with a specific level of severity, not a fixed time interval between occurrences. The technical term is the “0.2-percent annual chance flood,” meaning there is a 0.2% probability that a flood of that magnitude or greater will happen in any single year.
This annual probability remains the same every year, regardless of whether a similar flood happened previously. The common misconception is that a 500-year flood occurs only once every five centuries, but a community could experience multiple 500-year floods within a short period.
For comparison, the 100-year flood plain represents a flood with a 1% chance of occurring annually, also known as the “base flood.” Over a standard 30-year mortgage, a property in a 500-year flood plain has roughly a 6% chance of being flooded, compared to a 26% chance in the 100-year flood plain.
Comparing Different Flood Zone Classifications
The Federal Emergency Management Agency (FEMA) classifies flood zones on official maps to reflect varying levels of risk. Zones are divided into High-Risk Areas, known as Special Flood Hazard Areas (SFHAs), and Moderate-to-Low Risk Areas. The 100-year flood plain determines the boundary of the SFHA, which represents the highest flood risk.
High-Risk Zones, designated by letters like A and V, have a 1% or greater annual chance of flooding. ‘A’ zones are typically inland areas, while ‘V’ zones indicate coastal areas with additional wave action hazards. These zones often provide a Base Flood Elevation (BFE) that specifies the anticipated water level.
The 500-year flood plain falls into the Moderate-to-Low Risk category, often designated as Zone X (shaded). This zone is defined as the area between the limits of the 100-year and 500-year floods. Moderate-risk areas also include locations protected by levees or shallow flooding areas.
Areas outside the 500-year flood plain are considered to have a minimal flood hazard and are generally designated as Zone X (unshaded). However, even these low-risk areas can still experience flooding due to factors like poor local drainage or severe rain events. Approximately one-third of all flood insurance claims originate from properties in these moderate-to-low risk zones.
Mapping, Regulation, and Practical Implications
FEMA creates Flood Insurance Rate Maps (FIRMs) to officially delineate flood zones, which provide the basis for federal flood insurance requirements and building regulations. These maps are developed using statistical estimates and computer models of rainfall and water flow. Property owners may contest their designation if they believe their land is incorrectly mapped.
A Letter of Map Amendment (LOMA) is the official process used to remove a property from the SFHA if it is determined to be on naturally high ground above the Base Flood Elevation. This process requires submitting an application and site-specific elevation data to FEMA. Obtaining a LOMA can eliminate the federal requirement to carry flood insurance.
A primary difference for the 500-year flood plain is that mandatory flood insurance purchase is generally not required by federal law. This contrasts with the high-risk 100-year zones, where federally backed lenders must require flood insurance as a condition of the loan. Despite the lack of a mandate, purchasing flood insurance is highly recommended in the 500-year zone, as the risk remains present.
Even in these moderate-risk areas, local building codes may impose regulations to mitigate flood risk. Some communities require flood-proofing or elevation of structures near the 500-year boundary to enhance resilience. Furthermore, a lender may still require flood insurance as a condition of a mortgage, regardless of FEMA’s designation.