Flood risk is often thought of as probability; a chance that an event may happen with increased chance resulting in increased risk. Under National Planning Policy Guidance, the Environment Agency uses the probability of flood events to assess whether an area is located within a specific Flood Zone. Under this guidance, a site that lies within a Flood Zone 3 is defined as having a 1 in 100 or greater annual probability of river flooding or a 1 in in 200 year or greater annual probability of flooding from the sea, ignoring the presence of defences. A 1 in 100-year flood event means that each year we have a 1 in 100 chance of that flood event occurring (1% chance each year).

However, with climate change bringing increased rainfall and an increase in the frequency and severity of storms and flood events, will this impact how we see the probability of these events? And how will this affect our current understanding of flood risk?  

If your site is located within a Flood Zone 3, it does not necessarily mean that a flood event will occur every 100 years. However, it can be confusing as to why some areas experience these extreme flood events repeatedly over a few years.

When probability is calculated, it is assessed singularly for a given catchment and for a given risk over a given time. Therefore, for areas that may be at multiple risks of flooding i.e. river, coastal, surface water, then these events may be experienced as a combined risk, and therefore, they can occur more often.

Additionally the pre-conditions of an area, such as whether an area has been subject to heavy rainfall, the permeability of the ground, and the local drainage conditions can increase the impacts of a flood event and in turn the probability of these events occurring.   

As stated in the Environment Agency Climate Change Impacts and Adaption Report, climate change is anticipated to lead to ‘hotter drier summers, milder wetter winters, rising sea levels and more extreme weather events’. As extreme weather events become more frequent and intense, the number and severity of these flood events will increase. Perhaps what was once a 1 in 100 year event will soon become a 1 in 30 year event as we alter the probability science through climate change?

The increased flood risk has been estimated to increase the number of residential properties exposed to flooding within a 1 in 75 year flood event from 860,000 today to 1.2 million by the 2080s (Sayers et al, 2016). This increased risk brought about by climate change may mean that areas described as having a probability of a 1 in 100 year risk might become more common than 1% chance per year. Many people therefore argue that a new baseline for flood probability is necessary as we move towards a ‘new normal’.

Correlating short-term extreme weather events with climate change and how this may impact future probability is difficult. However, science increasingly shows that events have intensified due to climate change. Therefore, it is important to understand flood risk holistically to identify not only current and potential future risk, but also provide solutions to dealing with this risk and identifying how to mitigate where appropriate.

Tamsin Jones BSc MSc
Environmental Consultant

You may also be interested in

How are we doing?

We aim to deliver the best service possible, so please take a moment to click on our feedback link and let us know how you think we're doing.