Flood hazard areas
There would be no risk to the community from flooding if floodplains were unoccupied and unused. It is the human interaction with the floodplain and the associated exposure to flood hazard that creates flood risk, and humans are particularly vulnerable to hazardous conditions caused by floods.
The hazard from flooding is not the same in all areas across the region, and ranges from areas where no flood hazard is identified to areas where there are potentially significant hazardous conditions created by flood water depths or the speed at which the water is moving.
Flood 'size' and frequency
Flood Protection Engineers and Hydrologists in New Zealand describe floods using Annual Exceedence Probabilities (AEP) or Return Periods. For example, a 1% AEP or 1 in 100 year return period flood means that there is a 1% or 1 in 100 chance in any given year that a flood of this size or greater will occur. It does not mean that there is exactly one of these floods every 100 years.
It is also important to remember that several big floods could happen in quick succession.
Flood frequencies shown on the interactive map use a combination of modelled flood events from a number of studies. They cover flood hazards for river and lake floods only. For coastal flood risk, see the hazards interactive map.
The table below is an explanation of the flood extents Council has used to create the Flood Awareness Maps.
*AEP Annual Exceedance Probability. The likelihood of a flood of a given size or larger occurring in any year; usually expressed as a percentage.
**ARI Average Recurrence Interval. The average or expected duration of the period between occurrences of a given flood event; generally expressed as a certain number of years (e.g. 50, 100 etc).
***Q100 A flood event that has the probability of occurring once in every 100 years. Some people incorrectly believe that a flood of this size can only happen once every 100 years. However, the probability of a 1 in 100 year flood occurring is 1% in any year and so it is more correctly called a 1% AEP flood.
Floodplain Management PlansGreater Wellington's Flood Protection group works with communities to manage flood risk from the region’s rivers and streams. Our approach is to understand the processes affecting a river/stream and its floodplain within a wider catchment, and to provide a co-ordinated response through our floodplain management plans (in partnership with the community) to reduce the impact of flooding. See Floodplain Management Planning for more information.
Other sources of informationIn addition to flood hazard areas covered by GWRC Floodplain Management Plans, the interactive maps also show some modelled flood hazards for other areas. In these cases, the flood hazard may be adopted as part of a local District Plan. For more information, please contact us.
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Climate changeIt is recognised that climate change will lead to changes in rainfall and a rise in sea level over time. These will affect flooding and coastal processes. The flood hazard includes sea level rise and changing rainfall as a result of ongoing climate change.
Published: 31/05/2017 Last updated: 26/03/2018