Government of New Brunswick

Flood Predictions Based on Flood Frequency

The flooding predictions shown on many of New Brunswick’s flood maps are based on how often a flood is expected to occur. For example, most of the maps refer to the 1 in 20 year (1:20 year) and the 1 in 100 year (1:100 year) flood. These numbers refer to the anticipated frequency (return period) of the flood shown on the map, based on a statistical analysis for stream flow and precipitation data.

A 1 in 20 year flood does not mean that this flood will occur only once every 20 years; it means that there is a 1 in 20 (5%) chance that this flood could occur in any given year. Similarly there is a 1 in 100 (1%) chance that a 1 in 100 year flood level could be reached in any year. While return periods provide useful guidance for planning for the effects of flooding and related impacts, allowance should be made for extremes that may occur more often than may be expected, due to climate change.  

The flood frequencies used in New Brunswick’s flood maps are based on open water flooding conditions. Frequency projections cannot be estimated for ice-jam flooding, which may occur at any time, given the right conditions.

Predictions Based on Historic Floods

Some of New Brunswick’s  flood maps are based on past (historic) flood events. These maps do not show floods with an expected return period. Instead they indicate the extent of an extreme flood event that occurred at the mapped location in the past.

Determining the Method Used to Create the Flood Map

The legend of each map will indicate which of the two above methods were used to identify the flood area shown on the map.

Finding a flood elevation

The flood elevations are given in metres above sea level and are linked to specific locations that are labelled on the map.