Piha floods: Evacuation plans and $45m tunnel among options to prevent further flooding

The speed with which Piha can flood makes it hard for city-based emergency crews to help. Photo / Jason Oxenham

Piha residents who had their homes damaged in a major flooding event last year will be asked to develop their own evacuation plans to deal with future emergency events.

The plans could potentially work alongside more drastic large-scale engineering options - such as a $45 million tunnel to take the Piha stream straight out to sea, or the building of two dams - which have been presented to Auckland Council this morning.

In a media briefing on the two independent reports by consultants Tonkin+Taylor into three floods at the west Auckland beach last year, councillors said "community resilience" will be a key part to ensuring lives are saved if and when future flooding occurs.

Glenesk Rd in Piha flooded three times last year. April saw the worst flooding, with 80.5mm of rain falling in four hours, resulting in a catastrophic surge in the stream that swallowed roads and houses. Flooding reached a depth of 2m in some parts of Glenesk Rd, causing residents to flee, and extensively damaging 24 properties.

Craig McIlroy, head of Healthy Waters, said the area around Glenesk Rd stream was unique in that the terrain was "so steep the water gets into the stream very, very quickly, and that creates a situation were in a heavy rain event, we can go from nothing to a flood situation in 40 minutes".

In a situation like that, it would be almost impossible for anyone from council's emergency management teams to get from the city to Piha in time, so "community resilience", including personalised evacuation plans, would be key to mitigating disasters.

"Piha is one of the areas that has been chosen by the council to trial community resilience," McIlroy said. "Piha needs to become quite self-sufficient….if you're talking about a 40-minute return period on a flood, that's a really good example of why we need to build that community resilience."

He added: "Because we're dealing with a significant risk to life, one of the key recommendations is having individual evacuation plans for all of the buildings that are in 100-year flood plain."

Authorities would work with property owners to develop personalised evacuation plans, he said.

Waitakere councillor Linda Cooper said: "We've been working alongside the local board and emergency management to develop individual emergency plans for each home, because everybody's in a different situation in their home.

"It's really important that in that really short window of time people need to know exactly what to do and they need to do it quickly. They can't wait for the emergency services to get there...people need to know how to keep themselves safe until people can come and help."

A number of large-scale engineering options have been suggested as part of the review, including widening the Piha stream, clearing the sandbar to improve the flow of the stream out to sea, damming it, or diverting it by tunnel out to sea.

However, west Auckland councillors Penny Hulse and Linda Cooper both emphasised community options as being key, as opposed to potentially controversial construction work inside the Waitakere Ranges Heritage Area.

"We have to be open to looking at all the solutions that are in front of us, but we need to remember that the engineering solutions are complex, challenging and …. would need to be discussed in depth with the community," Hulse said, describing Piha residents as environmentally-minded.

With 8000 Auckland properties currently classed as flood-risk, any option chosen for Piha needs to be able to be rolled out across the city, Hulse said.

"The bigger context for climate change and the increased intensity of rainfall in these areas is something that Auckland is going to have to grapple with, and not just Auckland, New Zealand," she said. "So what we do here needs to have some thought to the implications to the wider Auckland areas as well as other councils in the same position as us."

Hulse, who described last year's flooding as the worst she'd witnessed in her 27 years as a councillor for the area, said she hoped to be able to present council's options to the environment and community committee at next month's meeting, but "we aren't going to rush this".

"For the homeowners the key thing is we want to come up with some options that we can consider with them, we want those options to be robust and do-able, we don't want to promise a whole bunch of solutions that we can't deliver, and we want to take the time to do it properly."

POTENTIAL OPTIONS:

• Stream widening: $16m-$19m
• Clearing the sandbar from, Seaview Rd Bridge to the sea: $3.5m-$4.5m
• Flood retention dam - single: $17m-$21m
• Double: $39m-$49m
• Flood flow diversion by tunnel: >$45m
• Raise habitable buildings out of the 100 year floodplain: $1.3m-$1.5m