We receive a lot of queries about the probability of properties/locations at risk of flooding.

Our reports provide a ratio indication, for example as a 1 in 75 chance. This is the probability rating that the searched location with flood in any one year. A figure 1 in 75 chance can also be expressed as a 1.3% or as having odds as 74 to 1.

It is important to note that this is the probability rating for the location flooding in any given year; it does not mean if the location floods in one year it won’t happen again for the next 74 years, nor does it mean if it hasn’t flooded for 74 years it will flood in the 75th year.

The RoFRaS Return period is divided into 4 probability bands:

  • Less than 1 in 1000 (0.1%) chance in any given year Very Low
  • Less than 1 in 100 (1%) but greater than or equal to 1 in 1000 (0.1%) chance in any given year Low
  • Less than 1 in 30 (3.3%) but greater than or equal to 1 in 100 (1%) chance in any given year Medium
  • Greater than or equal to 1 in 30 (3.3%) chance in any given year High

Surface water flooding is divided into 8 probability bands:

  • Less than 0.1m in a 1 in 1000 year rainfall event – Negligible
  • Greater than 0.1m in a 1 in 1000 year rainfall event – Low
  • Between 0.1m and 0.3m in a 1 in 200 year rainfall event – Low to Moderate
  • Between 0.3m and 1m in a 1 in 200 year rainfall event – Moderate
  • Greater than 1m in a 1 in 200 year rainfall event – Moderate to High
  • Between 0.1m and 0.3m in a 1 in 75 year rainfall event – High
  • Between 0.3m to 1m in a 1 in 75 year rainfall event – Significant
  • Greater than 1m in a 1 in 75 year rainfall event – Highly Significant

We’ve also produced a handy flood guide which contains a lot of useful information.

 


FLOOD INFOGRAPHIC_05-15_FA

Understanding flood risk

Different types of flood risk

Probability of flooding

What’s included in our reports?