Cyclone Gabrielle: Havelock North flooding – $10m of Government funding to upgrade dams and streams sought

An additional $10 million of Government funding would allow residents of a flood-smashed Havelock North street to have their land category changed from 2C to 1.

Joll Rd was flooded during Cyclone Gabrielle, due to a huge surge of water flowing through the nearby Mangarau Stream.

A Stantec report into the cyclone performance of the Hastings District Council’s five Havelock North dams, and others at Te Awanga and Clifton, stated all were functioning adequately.

It said satisfactory performance could be expected from them in normal, non-flood, conditions.

The report said the detention dams were tested during Cyclone Gabrielle, which had reinforced areas of vulnerability including that culvert inlets and approaches needed to be maintained clearance of debris before the next flood event, fallen trees cleared, and a review done of trees that are likely to be at risk of falling over.

The report recommended a more frequent recording of water levels – every 15 minutes – with a notification system if water reaches alarm levels.

The Mangarau Dam was built in the late 1970s, following flooding in 1974 in which 127 millimeters of rain fell in a 12-hour period.

During Cyclone Gabrielle 238.5mm of rain was recorded in the Mangarau Dam in a 24-hour period.

Between 1938 and 1974, there were eight recorded “significant rain events” where rainfall in a 24-hour period did not exceed 162mm.

At the height of the cyclone, 35 cubic meters of water a second flowed through the Mangarau Stream. The Stantec report says that, without the dam, an additional 22 cubic meters a second – or 22,000 liters of water – would have been rushing through the stream.

That said, several properties adjacent to the stream still suffered significant flood damage, which is why the Hastings District Council (HDC) is after funding for further resilience work to the dam and stream.

A business case will soon be presented to the government. Should it be approved, it will then be up to the Hawke’s Bay Regional Council to decide if the categorization can change from 2C to 1.

An HDC spokesperson confirmed this was the aim of the strategy.

“I am acutely aware that it has been a very long 10 months for our residents who suffered flooded homes,” Hastings Mayor Sandra Hazlehurst said, in a statement.

“Everything we are doing in this space with Hawke’s Bay Regional Council is focused on finding solutions as soon as possible.

“Our three waters and recovery teams have been keeping those most affected in Havelock North updated. However, I know our wider community is also very interested in what needs to happen across the dams and streams.”

The 2023 report into the HDC’s dams identified seven recommendations relating to the Mangarau Dam. The HDC says all those tasks are complete.

However, there remain outstanding recommendations from 2022 and 2020 reports.

The HDC says these do not meet the threshold where they present dam safety issues.

“The management of the dams and the receiving streams is constantly under review and, given the size and ferocity of the cyclone, even more so now,” HDC Three Waters manager Steve Cave said.

“The dams coped really well during the cyclone given the huge amount of rainfall, but that is not to take away from the stress suffered – and still being suffered – by those whose homes flooded.

“It would have been very traumatic and that is why we are focused on improvements we can make to reduce the chances of it happening again.”

Hamish Bidwell joined Hawke’s Bay Today in 2022 and works out of the Hastings newsroom.