What Return Periods and Annual Probabilities Really Mean

As a Flood Risk Consultant, I work with probabilities every day – using them to help people understand something that feels anything but predictable. But to many, the way flood risk is quantified – with decimals, technical jargon, and colour-coded tables – can feel like a foreign language. It’s no wonder messaging, alerts and warnings can get lost.

I remember watching a BBC News interview with a flood victim who’d been hit in 2007. They were told it was a “1 in 100-year event” and understandably assumed they wouldn’t see flooding again in their lifetime. But they were flooded again in 2020, then 2023, and again in 2024. It’s heartbreaking, and it highlights something we see time and time again in our consultations: the language used around flood risk often creates false reassurance, when it should be helping people prepare.

I believe this confusion comes from two main causes:

Firstly, terms like “1-in-100-year flood” are often misunderstood to mean flooding happens only once every 100 years.

Secondly, the numerical expressions of flood event return periods and a property’s flood risk can look deceptively similar. For example, a “1-in-100-year event” sounds similar to a “1 in 100 chances”, which leads people to mix them up, even though they refer to different things. One term relates to the severity, and the other the risk.

Magnitude (Severity):

The “1-in-100-year event” refers to the magnitude of a flood, in other words, a large flood that has a 1% chance of happening in any given year.

Likelihood (Risk):

Meanwhile, “1 in 100 chances” could describe the likelihood of a specific location being affected in any given year, which does not depend on just the size of the flood, but also on aspects such as flood defences, elevation, and local topography.

What is a Return Period?

A return period describes how likely a flood of a certain size is to occur in any year. It’s based on historical data and helps us understand how rare or extreme a flood event is.

For example:

Saying a storm was a “1-in-100-year event” means that flood size has been recorded on average once every 100 years.

This corresponds to a 1% annual chance of that flood size happening in any given year.

Importantly, this is a statistical average, not a schedule, you could see two such floods in a decade or none for over a century.

Return periods focus on the flood event itself, how big the flood is or how far the water spreads.

Flood Zone 3 maps show the area a 1-in-100-year flood would cover. This is flood-centric data based on the flood’s size. If your property lies within Flood Zone 3, it’s a strong indication it could flood in that event, but the actual chance your home floods depends on many factors.

Here’s an analogy to help visualize return periods:

Return Period	Annual Exceedance Probability	Analogy
1 in 30 year	3.3%	Rolling a 30-sided die once a year — if it lands on “1,” your house floods that year.
1 in 100 year	1%	Rolling a 100-sided die once a year — if it lands on “1,” your house floods that year.
1 in 1000 year	0.1%	Rolling a 1000-sided die once a year — if it lands on “1,” your house floods that year.

Communicating the Annual Probability of Flooding at a Property

When the Environment Agency states your property has a “1% annual chance of flooding,” it means:

“There’s a 1-in-100 chance this location will flood in any given year.”

This is about the chance floodwater will actually reach your specific property in any year. It’s a property-centric risk.

Although the probability output “1 in 100 chance” sounds is similar to “1 in 100 year event” they are not the same. Tools like the EA’s Long Term Flood Risk checker use property-centric data that combines flood modelling with local conditions, such as flood defences and topography. This gives a tailored estimate of your property’s flood risk than broad flood extent maps.

What is Annual Exceedance Probability (AEP)?

AEP is the chance that a flood of a certain size or larger will occur in any year.

For example:

A 0.1% AEP means there is a 1 in 1000 chance a flood that size or bigger will happen this year.

When the Environment Agency states:

“Your property has less than a 0.1% chance of flooding each year,”

they mean:

“Your flood risk is rarer than a 1-in-1000-year flood event.”

Key Differences to Remember

Return periods describe how often a flood event of a certain size happens (flood-centric).
Annual probability of flooding describes how likely it is that floodwaters affect a specific property in any year (property-centric).
Both use similar statistical expressions but refer to different things:
one is about a flood scenario itself, the other about its potential impact on your property.

Flood reports often reference both:

The return period of a flood event, e.g., “the property was affected by a 1-in-100-year storm event.”

And the probability of flooding at the property, e.g., “the property has over a 1 in 100 chance of flooding,” using datasets like the EA’s Long Term Flood Risk checker.

Both are based on the same idea — probability — but one talks about the flood event itself (how big or rare it is), and the other about the chance of that flood reaching a specific location. These are not the same: a 1-in-100-year flood event might occur multiple times at a river, but depending on the flow path, flood defences, or ground levels, it might not flood the same property each time.

Final Thoughts

By clearly separating event-based risk (return periods) from location-based risk (annual flood probability), we can better communicate what these numbers mean in practice — and help clients make sense of a complex but critical subject.

Sebastian Henshaw BSc (Hons) – Flood Risk Consultant

Date: 9th June 2025