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What Is A 1,000-Year Flood?
October 18, 2023 at 07:09 PM EDT
By WeatherBug Meteorologist, Alyssa Robinette
You may have heard the terms “100-year flood,” “500-year flood” or “1,000-year flood.” These can be confusing as these types of floods can happen more frequently than the term suggests. They can also be constantly changing. So, what do these terms means?
A flood happens when water overflows or inundates land that is normally dry. Flooding is a longer-term event, which may last days or weeks. Meanwhile, flash flooding is flooding in a short period of time, generally less than 6 hours.
Flooding can develop in many ways. The most common is when rivers or streams overflow their banks, which can be created due to heavy rain, a broken dam or levee or rapid ice melt in the mountains. However, overflowing water can be caused by coastal storms, storm surge, high astronomical tides or burn scars.
Flood predictions require several types of data, including:
Instead of the term “100-year flood,” hydrologists and other scientists would rather describe this extreme hydrologic event as a flooding having a 100-year recurrence interval. The recurrence interval is a measure of how often an event is expected to occur based on the probability of equaling or exceeding a given threshold.
The term “1,000-year flood” means that, statistically speaking, a flood of a certain magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1 percent chance of happening. Since the term is just a statistic, a certain location can have a “1,000-year flood” multiple years in row, or even multiple times in a year.
Conversely, there is also the frequently used term “500-year” flood. This means there is a 1 in 500 chance, or 0.2 percent, of occurring in any given year.
The values of a 100-year, 500-year and 1,000-year flood can change with every flood that occurs. Statistically, the more years of data available, the more accurate the estimate for the various flood quantiles are. There has also been and will continue to be new installation of flood controls and urban development.
Additionally, as rare heavy rainfall amounts and floods become more frequent due to climate change, the values of a 100-year, 500-year and 1,000-year flood will also need to be adjusted more frequently in the future.
A 1,000-year rainfall does not always correlate to a 1,000-year flood, and vice versa. This comes down to several factors, including the pattern of movement of the rainstorm, the conditions of the soil and plant matter and the timing of the rainstorm.
For example, if the ground is already saturated before a rainstorm due to a wet weather pattern, much of the rain will run off into streams. However, if the ground is dry, it will soak up more of the rain and the runoff will be less significant. Another example could be flooding due to a burn scar from a recent wildfire. This could cause significant and rare flooding, but it does not necessarily correspond to sporadic heavy rainfall amounts.
It is important to prepare ahead of time for flooding. Know your risks for floods where you live and work. Sign up for your community’s warning system or have a way to promptly receive emergency alerts through Emergency Alert System (EAS) or through a NOAA Weather Radio.
Make a plan for your household, including your pets, so that you know what to do, where to go and what you will need to protect yourself from flooding. Gather supplies, including non-perishable foods, cleaning supplies and water for several days.
Purchase or renew a flood insurance policy. Homeowner’s insurance policies typically do not cover flooding. It also usually takes up to 30 days for a policy to go into effect, so make sure you purchase it well in advance.
Keep important documents in a waterproof container and move valuables to higher levels. Also create password-protected digital copies. Make sure to frequently declutter drains and gutters. You can also install check values and purchase a sump pump with a battery.
Source: USGS, NWS, National Geographic, Ready.gov
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Image by Hans from Pixabay
A flood happens when water overflows or inundates land that is normally dry. Flooding is a longer-term event, which may last days or weeks. Meanwhile, flash flooding is flooding in a short period of time, generally less than 6 hours.
Flooding can develop in many ways. The most common is when rivers or streams overflow their banks, which can be created due to heavy rain, a broken dam or levee or rapid ice melt in the mountains. However, overflowing water can be caused by coastal storms, storm surge, high astronomical tides or burn scars.
Flood predictions require several types of data, including:
- The amount of rainfall occurring on a real-time basis.
- The rate of change in river state on a real-time basis, which can help indicate the severity and immediacy of the threat.
- Knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, which can be valuable for determining possible severity of the flooding.
- Knowledge about the characteristics of a river’s drainage basin, such as soil-moisture conditions, ground temperature, snowpack, topography, vegetation cover and impermeable land area, which can help to predict how extensive and damaging a flood might become.
Instead of the term “100-year flood,” hydrologists and other scientists would rather describe this extreme hydrologic event as a flooding having a 100-year recurrence interval. The recurrence interval is a measure of how often an event is expected to occur based on the probability of equaling or exceeding a given threshold.
The term “1,000-year flood” means that, statistically speaking, a flood of a certain magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1 percent chance of happening. Since the term is just a statistic, a certain location can have a “1,000-year flood” multiple years in row, or even multiple times in a year.
Conversely, there is also the frequently used term “500-year” flood. This means there is a 1 in 500 chance, or 0.2 percent, of occurring in any given year.
The values of a 100-year, 500-year and 1,000-year flood can change with every flood that occurs. Statistically, the more years of data available, the more accurate the estimate for the various flood quantiles are. There has also been and will continue to be new installation of flood controls and urban development.
Additionally, as rare heavy rainfall amounts and floods become more frequent due to climate change, the values of a 100-year, 500-year and 1,000-year flood will also need to be adjusted more frequently in the future.
A 1,000-year rainfall does not always correlate to a 1,000-year flood, and vice versa. This comes down to several factors, including the pattern of movement of the rainstorm, the conditions of the soil and plant matter and the timing of the rainstorm.
For example, if the ground is already saturated before a rainstorm due to a wet weather pattern, much of the rain will run off into streams. However, if the ground is dry, it will soak up more of the rain and the runoff will be less significant. Another example could be flooding due to a burn scar from a recent wildfire. This could cause significant and rare flooding, but it does not necessarily correspond to sporadic heavy rainfall amounts.
It is important to prepare ahead of time for flooding. Know your risks for floods where you live and work. Sign up for your community’s warning system or have a way to promptly receive emergency alerts through Emergency Alert System (EAS) or through a NOAA Weather Radio.
Make a plan for your household, including your pets, so that you know what to do, where to go and what you will need to protect yourself from flooding. Gather supplies, including non-perishable foods, cleaning supplies and water for several days.
Purchase or renew a flood insurance policy. Homeowner’s insurance policies typically do not cover flooding. It also usually takes up to 30 days for a policy to go into effect, so make sure you purchase it well in advance.
Keep important documents in a waterproof container and move valuables to higher levels. Also create password-protected digital copies. Make sure to frequently declutter drains and gutters. You can also install check values and purchase a sump pump with a battery.
Source: USGS, NWS, National Geographic, Ready.gov
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Image by Hans from Pixabay