Floods and cyclone gabrielle
What is a 1 in 100-year flood?
A 1 in 100-year flood event is sometimes referred to as an event with a 1% Annual Exceedance Probability (AEP). This means that a flood of this magnitude will statistically occur once every 100 years, or that there is a 1% chance of an event of this magnitude occurring in any one year.
For more information on event probability, you can watch the animated video on our Video Resources page.
A 1 in 100-year flood event is sometimes referred to as an event with a 1% Annual Exceedance Probability (AEP). This means that a flood of this magnitude will statistically occur once every 100 years, or that there is a 1% chance of an event of this magnitude occurring in any one year.
For more information on event probability, you can watch the animated video on our Video Resources page.
What size was Cyclone Gabrielle?
Water scientists estimate that during Cyclone Gabrielle, the Wairoa River flows were slightly less than that predicted for a 1 in 100-year flood event for the Wairoa River.
Water scientists estimate that during Cyclone Gabrielle, the Wairoa River flows were slightly less than that predicted for a 1 in 100-year flood event for the Wairoa River.
What caused the flooding in North Clyde during Cyclone Gabrielle?
The Wairoa River catchment (the area of land that flows to the river) is large.
Engineers and scientists believe that the flooding was primarily caused by the significant rainfall that fell across the catchment combined with the wet weather that occurred during the 2022- 2023 period.
The sudden onset of the flooding through North Clyde was due to the area acting as a secondary flow path of the river. With or without woody debris, North Clyde would still have flooded.
The Wairoa River catchment (the area of land that flows to the river) is large.
Engineers and scientists believe that the flooding was primarily caused by the significant rainfall that fell across the catchment combined with the wet weather that occurred during the 2022- 2023 period.
The sudden onset of the flooding through North Clyde was due to the area acting as a secondary flow path of the river. With or without woody debris, North Clyde would still have flooded.
When a flood mitigation solution is chosen, will it protect Wairoa from a Cyclone Gabrielle-level event?
The preferred flood mitigation solution will be designed to help protect Wairoa from a 1-in- 100-year flooding event. For Wairoa, Cyclone Gabrielle did not reach this level of flooding. It should be noted that with any flood scheme there is always a risk of a large event exceeding the level of mitigation, and no solution will provide absolute, foolproof flood protection.
The preferred flood mitigation solution will be designed to help protect Wairoa from a 1-in- 100-year flooding event. For Wairoa, Cyclone Gabrielle did not reach this level of flooding. It should be noted that with any flood scheme there is always a risk of a large event exceeding the level of mitigation, and no solution will provide absolute, foolproof flood protection.
