What are Flood Return Periods?

Return Period

What are Flood Return Periods?

According to the most recent Climate Change Risk Assessment (CCRA) report, an estimated 1.8 million people in the U.K. are living in areas that are at significant risk of flooding. The population of people living in such areas is projected to rise to 2.6 million by the 2050s.

The Environment Agency now state that 1 in 4 homes are at some risk of flooding.

The U.K. is facing frequent record-breaking flooding, often due to more intense rainfall events brought on by climate change. Rainfall intensity in the U.K. is expected to increase by up to 50% by the end of this century.

It is now a regular occurrence to see ‘record breaking flooding’ affecting communities across the world. In different places, these flooding events are often described as a ‘one in thirty’ or ‘one in a hundred’ year event. These expressions are known as Return Periods, which are a standard metric when assessing the occurrence and extent of a flood event. But what exactly do they mean?

Return Periods:

Commonly referenced return periods when assessing flood risk include:

  • 1 in 30 year (3.3% annual exceedance probability)- High risk;
  • 1 in 100 year (1% annual exceedance probability)- Medium risk; and
  • 1 in 1000 year (0.1% annual exceedance probability) Low risk.

Unfortunately, many people are under the impression if they experience a 1 in 100 year flood, they will not experience another one like it for the next 99 years. However, this is not the case! In reality the chance of being flooded by a 1 in 100 year flood event is the same in any given year- 1%.

For meteorologists, a 1 in 100 year event is an event of a size that will be equalled or exceeded on average once every 100 years. This means that over a period of 1,000 years you would expect the 1 in 100 year event would be equalled or exceeded ten times.

But several of those ten times might happen within a few years of each other, and then none for a long time afterwards. Therefore, the chance of a 1 in 100 year flood event occurring in any given year remains at 1%.

The same applies for a 1 in 30 year flood event, its probability of occurring in any given year is always 3.3%; not once every thirty years.

Why Are There So Many 1 in 100 Flood Events Now?

Firstly, Return Periods are a statistic derived from using historical records of flooding frequency. Using historical data assumes the underlying conditions haven’t changed since the historic data was collected. But in many parts of the world, we know rainfall and streamflow are changing, as well as other changes in the vegetation and land use of an area. Climate change and higher global temperatures are resulting in higher frequency, duration, and intensity of extreme storm events.

Therefore, Return Periods are constantly evolving due to the effects of climate change, flood events that were once considered ‘rare’ or ‘once in a lifetime’ are becoming increasingly common and widespread. In the near future these events will become the new norm.


The Cumulative Effect:

It is highly important to determine a property’s risk to flooding, especially if you plan to live at the property for a longer duration of time.

The annual exceedance probability looks at the risk of flooding annually in a given year. However, it is also important to know your chances of experiencing flooding at your property across a longer time period, such as 10, 20 or even 30 years.

For example, if a property has a 1 in 1000 risk of flooding, over a one year period the flood risk will be 0.1%. However, over a 20 year period, the chance of flooding within this time increases to 2%.

In contrast, if a property has a 1% AEP risk of flooding, then the chances of experiencing a flood over a 20 year period would be 20%. Would you still buy the property if you knew the risks over a longer length of time?

This recent article published by insurance firm Zurich explains how and why flood risk is so poorly understood:


The Environment Agency currently reports flood risk based on the chances of an area flooding in any one year.  It rates homes with a so-called 1 in 100-year to a 1 in 30-year chance of flooding as being at “medium” risk.  This means there is a 1% to 3.3% chance of a flood occurring in any single year.

But Zurich has warned that when viewed over a typical 30-year mortgage, this equates to between a 26% and a 63% likelihood of being flooded once in that period – a far greater risk than the current terminology suggests.

Zurich said this could leave homebuyers with a misleading impression of their flood risk and lull them into a false sense of security.

In Practice:

Flood Risk terminology can be confusing and can often downplay the actual risks.

It is vital that flood risk is correctly understood. It is highly beneficial to have a society which is more resilient to flooding. A property’s risk to flooding must be considered over an extended period of time, taking into account the cumulative effect of return periods, not just the chance of flooding within a single year.

Just because a property has no history of flooding, it does not mean the property is not at risk. Taking climate change and extreme weather into account, more and more properties that were previously not considered at risk, now find themselves experiencing flooding.

It is now crucial to know the flood risk at your property, and give yourself time to implement and protect your home with flood mitigation measures if necessary, before it’s too late.

How Can We Help?

FPS Environmental offers a range of consultancy services throughout the U.K. specialising in flood mitigation.

Our qualified Flood Risk Consultants can undertake a Flood Risk Survey of your property, investigating the risk along with potential routes of ingress.

We use data from our site visit alongside external data from the Environment Agency and Lead Local Flood Authority, to construct an in-depth report about your property’s risk to flooding. This will include predicted flood depths, the types of flooding which pose a risk, as well as our recommended flood mitigation options.

If you’re concerned about the flood risk at your property, get in touch with us on enquiries@fpsenvironmental.co.uk or call 020 8050 3194.

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