Forecasting future floods

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

by James Pocock

An independent investigation into Wairoa’s flooding after Cyclone Gabrielle has found that future floods should now be easier to predict after the risks were highlighted 30 years ago.

The findings of the report, commissioned by Wairoa District Council and authored by Andrew Newman of Strome Advisory Limited, were released by the council on Tuesday.

Cyclone Gabrielle devastated much of the Wairoa district — North Clyde within the township in particular — when the Wairoa River breached its  banks.

Mr Newman wrote that the review’s most important observation was that extreme soil moisture saturation, coupled with the potential for a major storm event like ex-tropical cyclones, created a high to very high risk scenario for damaging flooding.

“This is not a new view as it was highlighted in an HBRC flood risk analysis conducted in 1994,” Newman wrote.

“The nature, shape and high-volume short-run rivers of and within the catchment means that the Wairoa Flood plain and the township are highly exposed to extreme flooding.”

The report emphasised that forward warnings of flooding that could affect areas such as North Clyde should now be predictable, thanks to improvement over the last 30 years in the dataset of flooding events.

He  acknowledged times for action would still be short and the delivery of timely warning communication systems that enable a rapid community response needs serious attention.

He highlighted large riverbank poplar trees as a primary risk, particularly in the Waiau tributary, and wrote that the spread of rapidly regrowing plant material was another issue post-cyclone.

He wrote that slips due to Wairoa’s highly erodible land should be given attention as it could be a more significant risk issue to infrastructure, such as bridges, than harvest slash.

He stated “there is not one silver bullet” to mitigate flood risk of both urban areas and infrastructure and that was not within the remit of the report, but some issues and actions were clear.

Examples of flood intervention should be considered for people living in North Clyde including flood protection infrastructure like flood spillways or floodbanks which could be coupled with consideration of lifting houses above the inundation level, he said.

He referred to the necessity for an understanding of what the Waikaremoana Power Scheme could and could not do following public claims it was linked to flooding in the Wairoa township.

“In periods of extremely high stress, as was the case throughout and post-Gabrielle, localised issues such as flooding of Lake Waikaremoana and downstream of the Waikaremoana Hydroelectric scheme may be conflated with unrelated issues such as flooding of North Clyde.

“The Genesis team’s interaction with the Mayor of Wairoa and this review has been a positive initiative.”

He wrote that the work was driven by “the sense of isolation that the Wairoa community feels it has from the balance of Hawke’s Bay”, stating that key HBRC technical experts had not been visible in the community.

“Having key technical people present on occasion may help to build confidence and bridge this issue.”

Wairoa Mayor Craig Little said the report highlighted areas that needed to be worked on.

“We look forward to improvements that will help Wairoa be better protected from future flooding events.”