Mangarau Stream at flood stage

It has been nearly a year since Cyclone Gabrielle caused mass damage around Hawke’s Bay; a year since residents of Joll Road, Havelock North woke to water lapping around their feet. 

And now it will be another three to six months at least before Hastings District Council develops a business case based on key recommendations mentioned in a recently released Tonkin & Taylor (T+T) land categorisation review report around the Havelock North Mangarau Stream.

The report was commissioned by Hawke’s Bay Regional Council to investigate flood protection measures that would allow the stream to cope with a 1 in 100-year (1% AEP) severe weather event. Itis part of a feasibility review of flood protection options for the Category 2C and 2A areas across the Heretaunga Plains and surrounding areas.

The business case is needed to unlock $10 million of Crown funding to deliver projects.

“The key recommendations include stream widening, improved land access and maintenance, upgrading structures, bank stabilisation, improved monitoring, among others,” said Hastings District Council three waters manager Steve Cave.

“The team is carrying out monitoring and maintenance as we are working on a project plan and design for implementing the larger projects in the report to be ready for construction in late 2024, to coincide with drier weather.”

While the upgrades do not need to be in place for the 2C categorisation to be lifted by Hawke’s Bay Regional Council, a plan is required to be in place.

The plan needs to ensure work can be constructed, is funded, can be consented and have the required property rights.

“This T+T report will also assist with the wider strategy we are developing to manage all five streams and dams,” Cave said.

“The initial part of the strategy – focusing on flooding issues in the short term – will be brought to the community in the first quarter of 2024 for feedback. The second iteration of the strategy focuses on long term objectives and targets for the next 10 years.”

Cyclone Gabrielle resulted in more than 280mm of rainfall in Havelock North catchments, with estimates putting this rainfall in excess of a 1 in 250-year event now, and a 1 in 100 year applying future 2090 climate change factors.

During Cyclone Gabrielle, the Mangarau reservoir dam spillway activated for the first time and flow was released.

The limited capacity in the downstream channel, likely exacerbated by debris, then resulted in flooding to neighbouring properties along Joll Rd. Flooding was potentially worsened by damage to a private road bridge crossing at No 78 Joll Rd. Accordingly, an area of Havelock North, surrounding Joll Rd was then categorised 2C (implying a new scheme will be implemented).

Mangarau Stream bridge trapped debris exacerbating flooding

The T+T report around the Havelock North Mangarau Stream stated the target channel capacity requirement advised by HBRC was about 25 m3/s, which represents the original dam design outflow (understood to be low level outlet flow from a full reservoir head, no culvert blockage, no spillway overflow). By way of comparison, the estimated Cyclone Gabrielle flow release from the dam was 35 m3/s.

However, the assessment confirmed there was insufficient capacity in the lower reaches of the Mangarau stream to convey a dam outflow event of even 25m3/s, in locations generally consistent with observed flooding during the cyclone. The report further stated the conveyance issues appeared to have been exacerbated by localised debris loading onto key structures.

“Maintenance has proved difficult for the local council to manage, made difficult by access limitations. Based on current assumptions, dam release flows in excess of about 25 m3/s will result in flooding even if mitigation works are implemented as currently anticipated,” it states.

“Initial workshopping with key stakeholders noted a variety of options that were considered. Of these, several options were discounted due to technical deficiencies, high cost or impracticality. Following the workshop, it was decided to progress with a review of a combination of stream widening and maintenance/access improvements.”

A HDC spokesperson said the improvements suggested would be captured in the business case upgrade programme.

“At the level required to access this funding business cases are complex, and so will take some time. It is expected that it will take a minimum of three months. Council is working on having this ready for submission to government (Crown Infrastructure Partners) by end June 2024, hopefully sooner,” they said.

“Some project work is expected to commence in summer 2024; continuing over the following years subject to crown funding, consenting and land requirements. The higher level catchment planning and stormwater management is part of a wider management programme. The development of a broader Stormwater Strategy is underway to commence this work programme.”

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