Council defends flood dams, investigating whether stream channels properly maintained

One of the lesser known casualties of Cyclone Gabrielle was a group of more than 30 properties in Havelock North that bore the brunt of torrents that gushed down the western face of Te Mata Peak.

The properties, situated on a low-lying section of Joll Road and other streets nearby, suffered massive damage as the swollen Mangarau Stream, which runs adjacent to the road, burst out of its channel.

One of those flooded was the home of Marg and Don McDonald. Water was gushing into their home when Marg, 82, was able to walk off their property, but by the time Don, 90, came to leave, the floodwater had risen so high he needed to be carried on the back of a rescuer.

“We only just got out in time,” said Don.

The couple, who had lived in their house for about 12 years, have since moved into a retirement home in the village.

They, like their neighbours, lost most of their possessions and their car to the flood.

Now, as investigations into various councils’ preparedness and responses to the cyclone continue, questions are being asked around the Hastings District Council’s maintenance of streams and dams, designed to protect the village from flooding.

The Mangarau dam is the largest of five ‘flood detention’ dams on the western face of the peak. Built in the early 1980s and intended to reduce peak flood flows, they are operated and maintained by the council.

Stantec, which undertakes annual inspections of the dams for the council, has been saying for at least seven years that the dams were under capacity and noted in 2021 that the Mangarau and two other dams “have populations downstream and failure of any one of these could lead to widespread inundation, destruction of property and, more likely, cause fatalities”.

The council has come under fire for poor maintenance of the dams in the past, and in 2010 was told to fix the issues or face prosecution.

In the latest inspection of the Mangarau, carried out in August last year, Stantec found fences, trees and vegetation in the dam’s spillway that needed to be cleared.

A month after the inspection, residents living upstream of Joll Road advised the council of concerns around trees and debris in the channel of the Mangarau stream beneath the dam. It wasn’t the first time they had done so, with other similar concerns passed on in the preceding two years.

But the recommended clearance of material couldn’t be carried out by the council, which said this was due to “continued wet weather conditions” that meant the streams and dam faces were unsafe and unsuitable for heavy machinery.

The work had been delayed until early 2023, but the cyclone came first.

During the cyclone, the Mangarau dam filled to about 300,000 cubic metres of water and, as it was designed to, its spillway began pouring water into the stream below.

In response to questions from Stuff, the council’s Three Waters manager Steve Cave, said the dams performed as designed. The spillway was designed to activate in a one-in-one hundred year rainfall event and Gabrielle was a 1:250 year rainfall event.

The dams were not considered to have contributed to the flooding of homes, Cave said.

But the capacity of the streams, and the material that was in them – both before the cyclone and because of the cyclone – as well as the huge amount of water that fell in a short time, did contribute to the flooding, he said.

“Issues include that stream maintenance has been primarily managed reactively rather than proactively, and that landowners have been responsible for managing maintenance/stream clearing along their stream-frontages. (The property boundaries extend into the waterway, which means council has no automatic right of entry to carry out maintenance),” Cave said.

Hawke’s Bay Regional Council has contracted Tonkin and Taylor, to investigate a list of potential options that could provide a 1:100-year flood protection level of service for affected properties in Havelock North, and “as well as looking at level of service, the list of potential options is also factoring in things such as buildability (including land access), engineering feasibility, cultural impact, ecological/environmental impact, consenting constraints and opportunities as well as cost,” he said.

Information from that report as well as information from affected residents, technical information on dam and stream design and capacity, and dam inspection reports, will be used to create a “more stringent management plan”, which will be considered by the council.

Regardless of any of this, Cave said the amount of rain that fell over a short timeframe during Cyclone Gabrielle “will always be challenging for an event greater than what the system is designed to manage”.

“Moving forward there will need to be consideration by the councils and community as to what level of flood risk protection is achievable, what is desired and which ratepayers should pay for this, but this is a longer-term discussion,” he said.