UGA Researchers Redefine the 100-Year-Flood

Researcher contacts: Dr. Brian Bledsoe (bbledsoe@uga.edu) and Tim Stephens (tas48127@uga.edu).

The term “100-year flood,” which has long been used as a way for homeowners and city planners to visualize potential risk to property, is misleading—suggesting that only one flood in every 100 years will pose a risk to homes and public. Additionally, the maps that are used to define areas that might be impacted are often outdated, and fail to account for changes caused by urbanization and increases in heavy rainfall that may worsen flooding. 

Institute for Resilient Infrastructure Systems and RBC researchers Dr. Brian Bledsoe, Georgia Athletic Association Distinguished Professor in the College of Engineering, and Tim Stephens, graduate student in the College of Engineering, are tackling these inaccurate terms and creating tools that accurately and clearly represent the risk of flooding to homeowners and decision makers. 

Instead of using the term “100-year flood,” the researchers portrayed flood risk by using percentages, or chance over a time frame that people can more easily relate to. For example, a 100-year flood has a 1% chance of happening any given year, but that same risk can be expressed as a 26% (or greater than one in four) chance of the same flood happening in the next 30 years. 

The researchers chose the timeframe of 30 years purposefully; it is the average length of a homeowner’s mortgage. 

“When homeowners picture a 26% or more than one in four chance of a flood happening within the timeline of their mortgage, it changes their perception of risk,” Bledsoe said. 

This relatable representation of flood risk will help homeowners make informed decisions about how to protect their assets through flood insurance, elevating their homes, or choosing to buy in another location. It also will help city planners fully understand the risks of developing in areas that are prone to flooding. 

On top of clarifying the language used around flood-risk, Stephens and Bledsoe also created flood maps for Charlotte, North Carolina that incorporate the ever changing nature of flow, hydraulic roughness and river geometry—all of which are impacted over time by land use, development around and within rivers, and severe rain events—and will help to more accurately show which areas are at risk of flooding. 

“Flood hazard estimates are inherently uncertain due to changes in land use, river channels, and precipitation. Yet, land use planning and infrastructure development depend on these estimates. By accounting for uncertainty, IRIS researchers are developing innovative approaches that provide more realistic and socially relatable depictions of flood hazards for informed decision making,” Stephens said. 

Traditionally, flood hazard maps were created based on the past flood history of an area. But as development increases and climate change causes new patterns of precipitation, solely looking to the past isn’t enough to keep people and homes safe. 

“These maps address the uncertainty in the changing landscape and communicate risk in terms that people can relate to,” Bledsoe said. “We hope that by combining accuracy with relatability, we’re taking a big step forward in how we communicate about flood risks.”

This research was recently published in the scientific journal, Anthropocene. To read the study, visit this link: https://doi.org/10.1016/j.ancene.2019.100231