By Steven Schauer, Deputy Executive Director

In December 2025, King County experienced historic flooding that impacted communities across our region. Rivers swelled beyond their banks, roads were closed, neighborhoods were inundated, and residents were reminded, once again, that flooding is one of the most powerful natural forces we face.

During major flood events like this, it’s common to hear questions such as:

• “Is this a 100-year flood?”
• “Does that mean we won’t see flooding like this again for another century?”

These are understandable questions, but they’re based on a common misconception. Let’s clarify what these terms actually mean and why understanding them matters for you, your family, and your community.

The Origins of “100-Year Flood” and Similar Flood Terminology

The terms like 100-year floodplain and 500-year floodplain originate from federal floodplain management and mapping standards developed by the Federal Emergency Management Agency (FEMA).

FEMA is responsible for:

• Mapping flood hazards nationwide
• Establishing flood insurance requirements
• Setting federal regulatory standards for development in flood-prone areas

Floodplain maps, also called Flood Insurance Rate Maps (FIRMs), identify areas based on the statistical probability of flooding in any given year. Importantly, the term “100-year flood” does not mean flooding happens only once every 100 years. Instead, it refers to the annual chance of flooding.

Now, let’s try to make more sense of what annual chance means.

Annual Chance Flooding: The Key Concept

Flood events are described using annual chance probability. This is the likelihood that a certain level of flooding will occur in any given year.

Think of it like rolling a die. Imagine you roll a 100-sided die. The probability of rolling any specific number on a 100-sided die is 1% (0.01). This applies to a 100-year flood. A 100-year flood equates to a 1%-annual chance flood.

Just because you rolled a specific number the first time doesn’t mean you will roll the same number again next time. Each roll carries the same statistical probability. Similarly, the statistical probability applies to storms just as it applies to rolling die. This is why we can experience a 100-year flood (aka 1%-annual chance flood) multiple years in a row.

If fact, because we’re talking about statistical probability, it’s possible we could experience a 100-year flood (aka 1%-annual chance flood) more than once a year. All the annual chance flood terms described below could occur multiple years (and potentially, multiple storms) in a row.

Here’s how common flood terms translate using annual chance probability.

5-Year Flood

Annual Chance: 20%

A 5-year flood has a 20% chance, or a 1 in 5 probability, of occurring in any given year. Over a 30-year mortgage period, there’s a very high likelihood you could experience this level of flooding. There is a cumulative risk of approximately 99.8% of this level of flooding occurring at least once during a 30-year mortgage.

10-Year Flood

Annual Chance: 10%

A 10-year flood has a 10% chance, or a 1 in 10 probability, of occurring in any given year. Like the 5-year flood, there’s a very high likelihood you could experience this level of flooding. The cumulative risk of this level of flooding occurring at least once during a 30-year mortgage is approximately 96%.

50-Year Flood

Annual Chance: 2%

A 50-year flood has a 2% chance, or a 1 in 50 probability, of occurring in any given year. This level of flooding is more significant and less frequent than a 10-year flood event. However, it is still possible to experience multiple 50-year floods. The cumulative risk of a 50-year flood occurring at least once during a 30-year mortgage is approximately 45%.

100-Year Flood

Annual Chance: 1%

A 100-year flood, as we discussed with the 100-side die metaphor, has a 1% chance, or a 1 in 100 probability, of occurring in any given year. This is the standard FEMA regulatory benchmark for floodplain management nationwide.

If your property is located within the 100-year floodplain:

• There is a 1% chance of flooding each year. As pointed out above, this is a statistical, independent event, meaning it’s possible for it to occur multiple years in a row or even multiple times in a single year.
• Over a 30-year mortgage, the cumulative risk is approximately 26%.
• Federal flood insurance requirements typically apply.

500-Year Flood

Annual Chance: 0.2%

A 500-year flood has a 0.2% chance, or a 1 in 500 probability, of occurring in any given year. While rare, these events do happen. Extreme weather and changing climate conditions can produce these large storms and record-breaking river levels. There is a cumulative risk of approximately 6% of a 500-year flood occurring at least once during a 30-year mortgage.

Why These Terms Matter

Understanding flood terminology can help you:

• Make informed decisions about flood insurance
• Evaluate property risk
• Support responsible development practices
• Prepare for emergencies

It also underscores an important truth: Flood risk is about probability, not a countdown clock! A 100-year flood can happen this year. And it can happen again next year. And it can even happen multiple times in a single year.

Flood Risk Is Dynamic

Since flood risk is about probability, floodplain maps identify areas based on the statistical probability of flooding in any given year. The development of floodplain maps is based on the best available data at the time the maps were created. However, river systems can change, land use changes, storm intensity patterns shift, and climate variability affects rainfall and snowmelt.

That’s why King County Flood Control District continues investing in flood risk reduction projects, such as levee improvements and updated floodplain modeling. We are committed to helping protect communities today and into the future.

Be Flood Ready

We hope this blog post helps you understand flood terminology like 100-year flood (aka 1%-annual chance flood). The misconception associated with this, and similar flood terms runs deep, and the confusion can have significant consequences.

If, for example, following a 100-year flood (aka 1%-annual chance flood), someone mistakenly believes that such a flood won’t happen again for another 100 years, they may fail to take proper precautions to protect themselves and their family from significant flooding, which could happen the very next time a storm arrives in our area!

The December 2025 flood was a powerful reminder that preparedness matters. Despite the historic, generational size of the flooding, no lives were lost in King County. While we don’t want to experience another storm of that magnitude anytime soon, we must prepare for it. Statistically, it can, and will eventually, happen again.

We encourage all residents to Be Flood Ready:

• Visit the King County Flood Control District’s Be Flood Ready page
• Learn your flood risk
• Sign up for flood alerts and download the flood warning app
• Learn about flood insurance options and purchase flood insurance, even if you’re not required to. FEMA reports that between 20% – 40% of flood insurance claims occur outside of the designated 100-year floodplain.
• Create a family emergency plan
• Understand evacuation routes

Flooding can happen with little warning. Being informed and prepared is one of the most effective ways to protect yourself, your loved ones, and your community. Be Flood Ready. Learn your risk. Plan ahead. Stay informed. Together, we can build a more resilient King County.