Localised Cyclone Gabrielle report identifies triggers for flooding in Wairoa’s North Clyde township
Flood risk levels will dramatically increase for the Wairoa Catchment in La Niña type years, and an awareness of soil moisture conditions ahead of storm events should be a critical indicator of the need for flood preparedness, a new report has found.
The report, which was commissioned by the Wairoa District Council to establish why the North Clyde township of Wairoa flooded so badly during Cyclone Gabrielle, says that there is very good local knowledge” of regularly reoccurring flood risk issues and that this expertise should be actively utilised and trusted.
The report has identified a combination of triggers contributing to the extreme event last year.
The report was prepared by Andrew Newman, a former CEO of the Hawke’s Bay Regional Council (HBRC). Newman was the driving force behind the HBRC’s attempt to build the Ruataniwha Dam in Central Hawke’s Bay.
Assisting Newman, of Strome Advisory Limited, in compiling the report was Mike Adye, a Civil Engineer and ex Group Manager, Asset Management of the Hawke’s Bay Regional Council; Peter Manson – AgFirst – ex Land Management, HBRC in Wairoa; and Brett Gilmore of Brett Gilmore Consulting. The report was peer reviewed by Graeme Hansen, a former Group Manager, Asset Management, River Engineer with HBRC.
The report says that while there is no ‘one silver bullet’ to mitigate the future flood risk, a series of insights have been identified to resolve some of Wairoa’s enduring uncertainties.
A focus of the report was on improving the community, businesses, and governing institutions' understanding of what contributed to the flooding and putting attention and energy into actions that will further aid recovery and reduce risks for future flooding events.
The report described the nature, shape, river gradients, geology, lack of soil moisture retention capacity, the convergence of all tributaries into a single stem, and the condition and position of the bar at the river mouth as all contributing to the Wairoa catchment’s vulnerability to floods. This, combined with extreme soil moisture saturation and the intensity of the ex-tropical Cyclone Gabrielle, resulted in a high to very high-risk scenario for damaging flooding.
“Flood warning, utilisation of local knowledge, whole-of-catchment riverbank management, acknowledgment of Wairoa’s highly erodible land, and more than one flood mitigation intervention will contribute to improved community resilience.”
Newman said in a statement that the findings of the post-Cyclone Gabrielle report are not new and were highlighted in a Hawke’s Bay Regional Council flood risk analysis conducted in 1994.
The report states that forward warnings of flooding that could affect areas such as North Clyde should now be predictable, acknowledging times for action will be very short, and the delivery of timely warning communication systems that enable a rapid community response needs serious attention.
The report says that there is “very good local knowledge” of regularly reoccurring flood risk issues, such as the closure of the Wairoa River mouth, and that this expertise should be actively utilised and trusted.
It also suggested that given the strong cultural affiliation tangata whenua has with living in North Clyde, more than one flood intervention should be considered – i.e. flood protection infrastructure – flood spillways/flood banks could be coupled with consideration of lifting houses above the inundation level.
“Looking forward, large riverbank poplar trees are a primary risk, particularly in the Waiau tributary. A further issue post-Cyclone Gabrielle is the spread of plant material, which is regrowing extremely rapidly.”
The report recommended dedicated attention be paid to Wairoa’s highly erodible land, where slipping affects whole trees and is a more significant risk issue to infrastructure, such as bridges, than harvest slash.
Newman described the Wairoa community as feeling a sense of isolation from the rest of Hawke’s Bay and also wanting to learn from the events surrounding Cyclone Gabrielle.
He says that a collective response is ultimately the way forward, acknowledging that
future communications and interactions and a visible presence and sign of attention in the community around flood risk response will build confidence.
Wairoa Mayor Craig Little thanked Mr Newman for his thorough research.
“This report has highlighted areas that need to be worked on, and we look forward to improvements that will help Wairoa be better protected from future flooding events.”
