Floods are a natural occurrence that can happen anywhere heavy rainfall overwhelms drainage systems and waterways. While most floods occur frequently, extremely large and rare floods are often referred to as 100-year, 500-year, or even 1000-year floods. But what exactly does this mean and are extremely rare and devastating 1000-year floods real?
What is a 1000 year flood?
A 1000 year flood is an extremely large and rare flood event that on average is expected to occur once every 1000 years. More specifically, a 1000 year flood has a 1 in 1000 chance, or 0.1% chance, of occurring in any given year.
The term 1000 year flood is determined based on statistical flood frequency analysis. Hydrologists examine historical rainfall and river flow data to estimate the probability of different flood magnitudes occurring. The magnitude that is expected to occur on average only once every 1000 years is designated as the 1000 year flood.
Why are they called 100, 500, and 1000 year floods?
Floods are often referred to as 10, 50, 100, 200, 500, or 1000 year floods. This can be misleading, making it seem that if a 100 year flood occurs, another won’t occur for another 100 years. In fact, these terms simply refer to probability.
- A 10 year flood has a 1 in 10 chance of occurring in any given year
- A 50 year flood has a 1 in 50 chance, or 2% chance, of occurring in any given year
- A 100 year flood has a 1% chance of occurring in any given year
- A 500 year flood has a 0.2% chance of occurring in any given year
- A 1000 year flood has a 0.1% chance of occurring in any given year
So it’s possible for two “100 year floods” to occur within a year or even weeks of each other. The term only refers to the statistical probability over long periods of time, not the actual interval between events.
Have 1000 year floods actually happened?
Because a 1000 year flood has a 0.1% chance of happening in any given year, it is extremely rare. But that doesn’t mean 1000 year floods don’t happen.
Looking through historical records, geologic evidence, and flood frequency analysis, there is evidence of rare and extreme floods occurring that match or exceed the magnitude of a 1000 year flood:
- 1844: An enormous flood along the Yellow River in China is estimated to have had a flood magnitude with 0.1% annual chance of occurrence, qualifying it as a 1000 year flood event.
- 1903: A catastrophic flood in Heppner, Oregon caused a 15-foot wall of water to sweep through the town. With a volume of 180,000 cubic feet per second, it matched the estimated magnitude of a 1000 year flood along Willow Creek.
- 1911: Extreme monsoon rains led to exceptional flooding along the Yangtze River in China said to potentially rival or exceed the magnitude of a 1000 year flood.
- 1917: Extreme flooding in Salinas, California was estimated to have had an annual probability of around 0.13%, meaning it statistically matched a 1000 year flood event.
- 1927: Massive flooding of the Mississippi River across multiple states was found to match or exceed 1000 year flood magnitudes at several locations.
- 1978: Flooding in Uttarkashi, India killed hundreds and had an estimated annual probability between 0.1 – 0.5%, potentially reaching 1000 year flood levels.
While none of these historical floods have been confirmed absolutely as exceeding a 1000 year flood threshold, they provide evidence such rare events have occurred.
Where have 1000 year floods occurred in the US?
In the United States, a few rare flood events stand out as potentially reaching 1000 year flood levels based on statistical analysis:
- California Flood of 1862 – Catastrophic flooding statewide with flows said to match or exceed 1000 year flood levels in the Sacramento and San Joaquin River basins.
- Great Dayton Flood of 1913 – Extreme flooding in Ohio that matched 1000 year flood stages along the Miami River.
- St. Louis Flood of 1993 – Prolonged heavy rains led to flooding along the Mississippi and Missouri Rivers with some locations reaching 500+ year flood levels.
- Cedar Rapids Flood of 2008 – Parts of Iowa experienced rainfall statistically as rare as a 1000 year event, leading to exceptional flooding in Cedar Rapids.
Other locations, like Ellicott City, Maryland, Boulder, Colorado, and Deadwood, South Dakota have seen rare flash floods with estimated recurrence intervals matching 1000 year levels though these are difficult to validate.
Are 1000 year floods becoming more common with climate change?
With climate change warming the atmosphere and altering weather patterns, there is concern 1000 year floods could become more frequent. Some studies have found evidence of this:
- A 2017 study found the likelihood of 1000 year floods occuring along the Mississippi River has increased to once every ~500 years.
- Research suggests the Chance of 1000 year floods has doubled in Texas, tripled in the Ohio Valley, and quintupled in the Northeast US.
- Urban flooding may be seeing larger increases in rare flood risk as impervious surfaces and infrastructure amplify rainfall.
However, determining clear trends is difficult with rare events like 1000 year floods which have such little data. Some research argues flood frequency analysis is unreliable for assessing changing risk.
But it is clear that as heavy rainfall events and rainfall variability increase into the future, the magnitude of rare and extreme flooding could increase as well.
Are we due for another 1000 year flood?
It’s impossible to say exactly when another 1000 year flood will occur. By definition, over a long period of time they are expected to happen on average once every 1000 years. But in any given year there is a 0.1% chance, so a 1000 year flood could theoretically occur at any time.
Looking at the paleoclimate record over thousands of years can provide additional insights into past extreme floods beyond our limited historical records. One study found that the Upper Mississippi River has likely experienced multiple floods over the past 2000 years that exceeded 1000 year flood magnitude, occurring about once every 400-500 years on average.
Past clustering of rare paleofloods suggest another 1000 year flood could occur along major US rivers at any time. With climate change, growing urban areas, and land use changes, preparing for and mitigating risk of rare flooding events is becoming increasingly important.
Are we prepared for a 1000 year flood?
Structural flood protections like levees and flood walls are typically only built to the 100-year flood level. Critical infrastructure may be designed to 500-year levels. Almost nothing is built to withstand a 1000 year flood.
Major urban areas also exacerbate flooding, with increased pavement and drainage systems overwhelmed by extreme rainfall. River straightening, wetland losses, dams, and land use changes can also increase large flood risk.
With climate change potentially increasing the magnitude of rare flooding, few places are adequately prepared. Efforts to improve flood readiness could include:
- Updating flood frequency analysis for major rivers using the latest data and techniques.
- Assessing vulnerability and risks to extreme flooding along rivers and urban drainages.
- Improving flood mapping and early warning systems.
- Using green infrastructure to help absorb extreme rainfall.
- Adding freeboard and margin of safety when designing protections like levees.
- Improving development regulations to limit construction in floodplains.
- Protecting or restoring wetlands and floodplains to help absorb floodwaters.
Rare floods like 1000 year events will continue to occur. Only by understanding the risk, preparing vulnerable communities, and pursuing flood-adapted development can we hope to be ready.
Conclusion
In summary, 1000 year floods are extremely large and rare events, but they do happen. Historical records, geological evidence, and flood frequency analysis provide evidence such floods have occurred, though uncertainty remains with such rare events. Climate change may be increasing the risk, but preparation remains limited, making reassessing vulnerabilities and flood protection needs vital to prevent future flooding disasters.