As discussed in a previous post, the climate is the long-term average condition of the atmosphere in a specific region. Rare extreme weather events, like intense floods, are a normal part of any area’s climate. Natural variation in weather patterns will generate a certain number of floods of varying magnitudes over time. Understanding the risk of how often large floods occur in an area is vital to protecting infrastructure, resources, and lives.
Texas’ rivers and creeks range from scorched dry beads to calm flows to raging torrents in any given year. During heavy rainfall, water can run over the bank of a river channel to flood the surrounding land. This area of overflow is referred to as the floodplain. Floodplains serve important ecological functions such as improving water quality, recharging groundwater, and providing habitat for riparian wildlife. They also act as areas of cultural and recreational value. The National Flood Insurance Program, run by the Federal Emergency Management Agency and local governments across the United States, relies on the 100-year floodplain to determine insurance premiums and development regulations, respectively.
The term “100-year floodplain” is easily misleading. A 100-year flood is not a flooding event that happens only once every 100 years; instead, it is the probability that a flood with a 1% chance of happening will occur in any given year. The phrase is a way to describe a flood’s return period. A 100-year event has a 1% chance of occurring in any year. While statistically less likely, two 100-year flood events can happen within a year or even a few months of each other. The 100-year floodplain is similarly the land with a 1% chance of flooding during any single year. This term is merely a shorthand way to summarize this concept of flood risk but can often be more complicated and less intuitive than it should be.
While this post is focused on flooding, 100-year events can be calculated for any form of extreme weather events or natural hazards such as storms, hurricanes, droughts, wildfires, earthquakes, or volcanic eruptions. An important distinction is that a 100-year storm will not always cause a 100-year flood, as a watershed’s size and characteristics independently influence rainfall and streamflow dynamics. The watershed’s size, the storm’s length, how wet the soil is before the storm, and the concentration of rainfall over the watershed all affect whether a stream’s channel is exceeded.
The 100-year flood is based on large amounts of data collected by scientists related to when and how often floods of different sizes occur in an area. This data is used to calculate the probability that a future event of that size will occur or be exceeded during any given year. The statistical technique applied is called frequency analysis. A flood is classified based on how often it occurs and its depth, so 10-year, 25-year, 50-year, 100-year, and 500-year floods are possible. A 500-year flood occurs less often than a 10-year flood but is much deeper and significantly more destructive. The longer the return period (i.e., 100 years vs. 10 years), the less probable it will occur in a single year.
Another way to think of this type of probability is with a coin toss. Like a flood, a coin toss is based on statistical chance and independent of the previous toss’s result. Just because you flip a coin and it lands on tails doesn’t mean that your next flip can’t be tails again, or the one after that too. The chance of landing on tails for any toss is always 50%.
In the United States, buildings in 100-year floodplains must purchase flood insurance to qualify for federally backed mortgage or home equity loans. As most mortgages are repaid over 30 years, any structure within a 100-year floodplain has a 26% chance of experiencing a flood during that period. That’s certainly a noteworthy risk. Additionally, living in a 100-year floodplain carries a 64% risk of experiencing at least one major flood in 100 years. Over the payback period of a 30-year mortgage, flood risks based on a home’s flood level classification is as follows:
- 25-year flood: 71% chance of being flooded
- 50-year flood: 45% chance of being flooded
- 100-year flood: 26% chance of being flooded
- 500-year flood: 6% chance of being flooded
Flood event classification is based on the observed data in an area. As time passes and more floods occur and more data is collected, that classification can change. Flood reoccurrence should be regularly re-computed to accommodate natural- and human-caused changes.
Floodplains are not static, as changes in land use (i.e., buildings, culverts, grading, roads, and bridges) alter how floodwaters travel over the surface. Depending on how infrastructure is built, these changes can expand or reduce the floodplain. Imagine 3 inches of rainfall in a meadow. Much of the rain will be caught by vegetation or soaked up by soils. Compare that to a street, where nothing can intercept or absorb the rainfall, only running off the land into diverted pathways and drainage systems. If the rainfall exceeds the capacity for the built system to handle, a flood will occur. Also, especially significant flood events can change a river’s course through sediment transport, altering the extent and characteristics of the floodplain itself. This can expose areas to floods that were previously unaffected.
Weather is highly variable, with larger storms happening inconsistently. In Texas, you can experience multiple years in a row with regular, intense rainfall followed by a decade of drought with only intermittent flooding during the rare rain period. Climate change affects these trends, increasing the risk of heavy floods due to more intense and frequent rainfall events. The National Weather Service’s Atlas 14 study looked at rain patterns and found that Austin is currently at a 1% risk of receiving 13 inches of rainfall in 24 hours in a given year. This is significantly more than the previous definition of a 100-year storm: a 1% chance of receiving 10.2 inches over 24 hours. This increased rainfall frequency is a considerable concern for elevating flood risks in growing urban areas like Austin. The city is updating its floodplain maps to accommodate these changes, with areas previously designated in the 500-year floodplain are now in the 100-year floodplain. Houston has endured three 500-year floods three years in a row, one of them caused by Hurricane Harvey. Rising sea levels increase floodplains in low-lying coastal regions and elevate the risk of flooding in these communities. The city’s zoning regulations have also been revised to account for changes to the flood drainage basins in the area. Central Texas has seen three 100-year flood events over five years between 2013 and 2018.
As cities expand, more people are moving to floodplain zones. Since 2000, there’s been a 14% increase in US floodplain residents. The rapid sprawl of urban areas in Texas reduces the natural land available to absorb floodwater and, combined with increasing rainfall intensity and occurrence, puts a large proportion of the most populated cities at risk. Texas recently passed a law that requires landlords to inform new tenants of the floodplain status of the property. You can check the floodplain status of your current address anywhere in the United States here. The Texas Water Development Board’s Floodplain Management Guide details how the state prepares for flood events. They also produce a map tracking active floods in Texas, and more information is found on their website. Tenants can purchase federally backed floodplain insurance on their possessions.
Removing the phrase “100-year flood” from our collective vocabulary will improve communication between scientists, policymakers, and the public. The key is to focus on conveying risk in terms of relatable timeframes. In recent years the Federal Emergency Management Agency has made strides in clarifying what the phrase means by describing it as the base flood and relabeling 100-year floodplains as Special Flood Hazard Areas. News articles are also regularly being published that highlight this issue. While shifting the focus to the risk of flooding expressed as a percentage, another consideration is that most people struggle with risk probabilities. If we see a small percentage, we tend to assume safety even when that’s not the case. Putting things in perspective with more relatable comparisons—like a 30-year mortgage—is more helpful. Of course, there are also issues here, as the risk of a home being flooded isn’t always in line with flood maps due to natural floodplain elevational variations. Experts are still developing better methods to communicate risk to the public. Sponsored by the Federal Emergency Management Agency and the Texas General Land Office, Texas A&M University is currently conducting a study on better mapping, managing, and reporting flood risk to Texans.
Conveying the dangers of living in at-risk areas is essential to building sustainable infrastructure, preparing adequate emergency response plans, and adapting to a changing climate. Not understanding or taking these numbers seriously can easily be fatal.
Additional reading and resources:
- Return Period Calculator (GFDRR)
- The ‘100-year flood’ fallacy: Return periods misleading in communication of flood risk (Earth Magazine)
- The 100-Year Flood (USGS)
- What’s a 100-year flood? A hydrologist explains (The Conversation)