How we forecast floods, issue warnings and assess flood risk

We have monitoring stations across Wales that measure river levels. This data is analysed along with weather, storm surge and wave forecasts from the Met Office to help us forecast when flooding might happen.

Flood warnings and alerts

We use our flood forecasts alongside local knowledge, to make decisions about issuing warnings to those at risk.

Flood Alert. Be prepared.

We issue Flood Alerts when the risk of flooding is lower but it is possible.

Expect flooding of low-lying land, minor roads, car parks, recreational land and farmland. There may be spray or wave overtopping at the coast.

What to do:

  • prepare a flood kit of essential items
  • check flood warnings
  • farmers should consider moving livestock and equipment away from areas likely to flood
  • monitor local river levels
  • check the 5 day flood forecast

Changes to flood alert information

We have improved how we issue flood alerts so we can issue them earlier if we have confidence in the forecasts. Giving people more time to prepare. Flood alert messages will also include a link to check the latest river Level, rainfall and sea data to view the water levels as they change.

These changes have been made following the February floods review. To help make sure everyone across Wales can get the most up to date information, quickly and easily.

The flood team have written a blog about these improvements and why they have happened

Flood Warning. Immediate action required.

We issue Flood Warnings when flooding is expected and we think properties could flood. We may also expect:

  • travel on road and rail may also be impacted
  • significant waves and spray on coast
  • extensive flood plain inundation
  • flooding of caravan parks or campsites

What to do:

  • move family, pets and valuables to a safe place
  • turn off gas, electricity and water supplies if it is safe to do so
  • put flood protection equipment in place

Severe Flood Warning. Danger to life.

 

We issue Severe Flood Warnings when there is a significant risk to life or significant disruption to the community.

They are the highest warning level and are issued when dangerous deep and fast flowing floods are expected or already happening. 

With a Severe Flood Warning we may expect:

  • deep, fast flowing water
  • debris in the water adding to the danger
  • damage or collapse of buildings and other structures
  • communities isolated by floodwaters
  • critical infrastructure disabled
  • communities evacuated 

What to do:

  • stay in a safe place with a means of escape
  • be ready to leave your home
  • cooperate with the emergency services
  • call 999 if you are in immediate danger

What we can't forecast

All catchments in Wales are covered by our Flood Alert service. Our Flood Warning service covers around 60% of the properties at risk of flooding from main rivers or the sea in Wales, and we are continually working to extend this service.

There is also a risk of flooding from other sources which we don’t warn for:

  • surface water
  • small watercourses

These types of flooding can occur very quickly and are often very localised, meaning there isn’t time to issue advance warning messages.

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.

How we assess ‘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.

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:

  • High: 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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