How we forecast floods, issue warnings and assess flood risk

Warning and informing the public

We forecast floods and warn the public. We also raise awareness of flooding and help people make preparations in advance. Our staff are on standby 24 hours a day, seven days a week and are ready to warn of and respond to flooding events.

Monitoring and forecasting

We use the latest technology to monitor rainfall, river levels and sea conditions. Using these observations, along with weather, storm surge and wave forecast data from the Met Office, we use computer models to provide local, community-level forecasts of the possibility of floods and their likely severity.

Issuing warnings

Our flood warning duty officers use these forecasts, along with their local knowledge of community flood risk, to make decisions about issuing warnings to those at risk. We issue three different kinds of flood warning: Flood Alerts, Flood Warnings and Severe Flood Warnings. We ensure that warnings are broadcast on local radio and television. This information can also be obtained through our dedicated Floodline information service (0345 988 1188) and on our website, which is updated every 15 minutes.

Operating our defences, pumping stations and other assets

Our operational teams are out on the ground before, during and after flooding to make sure that our flood defences, pumping stations and other assets (eg trash screens) operate as they should, to protect communities from flooding. At some locations, our staff install temporary defences which can then be lowered again once the flooding has passed.

Warning and informing emergency responders

We work with the Joint Flood Forecasting Centre, based at the Met Office in Exeter, to provide a five-day assessment of flood risk from all sources (river, sea, surface water and groundwater) on a county scale.

This risk assessment is shared on a daily basis with local authorities and emergency responders, helping them take earlier and more effective action when there is a heightened risk of flooding.

Specialist advice

The Joint Flood Forecasting Centre also provides specialist advice to Natural Resources Wales's staff about upcoming weather conditions. This helps us make local predictions about the likelihood of flooding and informs our decisions about issuing warnings.

Assessing ‘flood risk'

'Flood risk' is not just the likelihood of flooding; it includes also the possible damage a flood could cause.

When we talk about 'flood risk', we are referring to a combination of two elements:

  • The likelihood of a particular flood happening. This is expressed as an annual chance or probability. For example, 'In this location, there is a 1 in 100 chance of flooding in any given year'
  • The impact or consequences that will result if flooding occurs

Calculating probability and severity

We need to know both the probability of a flood occurring and the severity of its impact (which may change depending on how extreme the flood is). With this information, we can describe the risks facing a particular location.

Modelling and flood maps

We carry out modelling to enable us to produce maps showing the extent of flooding for a range of probabilities. Our flood modelling studies consider the following factors:

  • The source of a flood, eg river, tidal or coastal water
  • The paths that the water will take during floods, and how the severity of a flood might affect its path
  • The impact on people, land and property affected by flooding
  • Historical data from past floods
  • Present water levels

This work is used to inform planning developments, emergency operations and the building of flood defences to protect communities.

The likelihood or probability of flooding

The probability or likelihood of flooding is defined as 'the chance that a location will flood in any one year'.

If a location has a 1.3% chance of flooding each year, this can also be expressed as a 1 in 75 chance of flooding occurring in that location in any given year.

Interpreting the probability of flooding

Such figures, however, do not mean that if a location floods one year, it will definitely not flood for the next 74 years. Nor do they mean that if the location in question has not flooded for 74 years there will definitely be a flood this year.

Categories of probability

When we describe the chance of flooding, we place that description into one of three categories:

  • Significant: the chance of flooding in any one year is greater than 1.3% (1 in 75)
  • Moderate: the chance of flooding in any one year is 1.3% (1 in 75) or less, but greater than 0.5% (1 in 200)
  • Low: the chance of flooding in any one year is 0.5% (1 in 200) or less
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