HB Regional Councillors received a workshop presentation last week from consultants Tonkin + Taylor previewing their diagnosis of weaknesses in the current Heretaunga and Upper Tukituki flood protection schemes and the kinds of interventions that could be considered.
Council and consultants alike were emphatic that the advice presented was early stage, requiring more technical review, Council direction and community engagement.
Tonkin + Taylor has so far examined the present infrastructure in detail, reviewed the actual breaches and flooding that occurred, identified a range of potential system improvements and modelled the effectiveness of those in various flood scenarios.
A few takeaways from the presentation …
Changes must be made in a holistic framework, meaning that a ‘fix’ at one location (such as raising a stopbank or installing an additional one) will change the flooding behaviour and effectiveness of the system elsewhere. Planning must constantly be looking at the big picture.
Yes, bridges were a major obstacle exacerbating flooding, and their design must be addressed, but modeling shows that even without bridges the intensity of rainfall would have caused major overtopping. [As many other observers have noted, the issue is woody debris + bridges.]
Throughout the schemes, the vulnerability of the ‘backside’ of the stopbanks was exposed (i.e., severe erosion of the landward side of the banks where overtopping occurred, causing further stopbank failure).
The blocking of waters at the converged mouth of the Tutaekuri and Ngaruroro Rivers requires major rethinking of river mouth management. Likewise the railroad and highway bridges at Awatoto represent huge flow barriers at that location. It was emphasized that in this event both these rivers peaked at the same time, exacerbating not only these choke points but obviously every structural protection in the system.
Gravel build-up was deemed not a significant cause of flooding in either scheme, given the magnitude of the cyclone rainfall. However build-ups must be monitored as this could be a factor in lesser flood events.
Beyond such broad strokes, Tonkin + Taylor picked apart the each of rivers section by section, indicating the types of specific interventions that could be considered and their various impacts – raising stopbanks, new and/or secondary stopbanks, spillways, ponding areas, connections to drainage systems, stopbank design and surface treatment … and in the case of the Ngaruroro, perhaps widening of the river course.
Plenty of food for thought emerging.
My own reaction was that I was listening to engineers with a natural affinity to structural interventions, which are certainly needed. The discussion noted but didn’t really dig into widening of river channels, and the consultants were, I thought, overly dismissive of the importance of land use (i.e., land cover) changes in upper reaches of the catchment.
One hopes those matters will be more fully vetted as this work proceeds.
Most disturbing to me is the expected pace of the planning to be done. A very brief overview of the process from here on involved:
Rest of 2024 devoted to further analysis and sorting out how to engage both the community at large and its many stakeholders (i.e., interest groups)
2025 working toward community understanding and stakeholder buy-in for various options.
2026 devoted to governance and funding issues, decision-making around these, and translating all that into LTPs due in 2027.
That sounds like a heap of ‘talk’ for two-and-a-half years before protective improvements begin. Even keeping in mind the caution about ensuring a holistic approach given the interdependencies of changes in the system, surely earlier mitigation measures can be identified and implemented – e.g., making the backside of stopbanks more resilient.
The politicians in this process must keep up the pace and ensure that the widest range of options – including land use change and ‘natural’ measures – are given due consideration.
As one of the councillors observed, this planning is not just about responding to another Cyclone Gabrielle-like ‘design event’. Measures that better manage our water in more ‘normal’ scenarios could have collateral benefits and should be on the table.
So it sounds like tinkering around the edges but seems like no significant emphasis on “making room for rivers” as outlined by Tom Kay from Forest and Bird in his community presentations after Cyclone Gabrielle.
Agreed. Rivers have always flooded and are supposed to flood, it’s an important cycle of soil replenishing and species migration. Ideally we should build our infrastructure around that necessary and inevitable element.
The report of the independent review into the HB flooding covers a wider range of issues in some depth – recommended reading for everyone concerned, affected, involved or responsible: https://www.hbrc.govt.nz/assets/Document-Library/Cyclone-Gabrielle/Report-of-the-Hawkes-Bay-Independent-Flood-Review-Digital-Version.pdf In particular, ‘managed withdrawal’ from increasingly flood-prone areas (despite the civil works) has significant human, cultural and economic implications. Yes, time is pressing and action is needed … but better to plan and do it properly in conjunction with the community which takes time. Thank you Bay Buzz for keeping us informed of progress throughout.
Anyone interested in this topic might enjoy this video explaining the four factors affecting river erosion/deposition and why it is hard to control rivers with engineering (sediment load, sediment size, slope, water flow).
https://m.youtube.com/watch?v=vLZElIYHmAI
You can see what happens at 5 minutes 40 when a channel is straightened (as we have done with our rivers).
Taking out sediment (gravel extraction), straightening the river or increasing the flow of water will increase erosion somewhere (could be up or downstream) according to Lane’s model https://geofaculty.uwyo.edu/neil/teaching/geologypics/GravelMiningGraphic.jpg
It is beyond belief that we pay council engineers but still have to employ consultants to tell them how to do their work. Time is of the essence we need work started now not 2 years talking .then maybe start. Two years from now we will be told it is unaffordable to complete the work. The CEO of the council should do the job he is paid to do, make a decision, and start the work now
Instead of wasting money that could be spent on this work.
Just build bridge wider for types of transportation including trains even as freight in business needs to get from port to Hastings or waipawa & beyond. By building wider bridges for traffic should been thought of now that Brookfield bridge is gone. Should there b a bridge built there again as it was a useful crossing before cyclone. Meenae is a area of what for the future.