When roads become rivers - the invisible flow paths on our streets

Friday, 24 April 2026

Wellingtonians were shocked by the flash flooding caused by Monday’s deluge. But should we have been? Nikki Macdonald investigates the overland flow paths that turn our streets into rivers.

“Shops in The Parade flooded, water levels reach bonnet-levels on some cars, manhole covers removed by force of water, bowling club underwater,” the newspaper graphic reads.

It sounds like a description of Monday’s deluge in Island Bay and Berhampore. In fact, it’s a report of a remarkably similar flash flood 31 years earlier, on 11 April 1995, when a localised downpour delivered more than 70mm of rain to Wellington’s southern suburbs over three hours. Just 6km away, the city centre received only half that.

Cars floated on the rush of runoff using Island Bay’s main shopping street as a streambed. A resident in nearby Derwent St was woken by her children’s dinghy banging against the house, lifted by a metre of water. And everywhere was shock and disbelief.

“It was like a river,” Derwent St resident Brian Stempa told The Evening Post. “I’ve never seen it like this before.”

Yet here we are again, three decades later. Because while Monday's deluge wasn’t easily predictable, its escape path was.

The rivers of last resort

Unless you live on flat land close enough to eyeball a stream or river, you probably thought you were safe from flooding.

But when you concrete over nature’s stormwater systems, the water dumping from the sky and sheeting off roofs still has to find somewhere to go.

Engineers call those makeshift streambeds overland flow paths.

“They’re basically just the path of least resistance, that the water will flow down when it’s got nowhere else to go,” says Morphum Environmental water sensitive design lead, Stu Farrant.

And the capital’s famously lumpy topography signposts where they are. Farrant spots them as he drives the city.

“It’s very predictable, because water follows gravity…You can clearly see where the sag points in roads are and you sort of gasp and go, ‘Gosh, those houses are within the overland flow path’. And unfortunately, those are the houses that have now flooded. So the landscape tells us what is going to happen during a rainfall event.”

For the non-expert eye, they’ve also been mapped.

Dial up Wellington City Council’s overland flow path maps and you’ll see a confronting criss-cross of silent streams netting the city centre. Zoom out to Berhampore, and there’s a slash of blue down Emerson St, where on Monday a car was left teetering on a fence and houses were swamped with water above head height.

Many of the lines mirror the paths of old streams now paved over and shoved into pipes, Farrant says. Aro Valley’s Waimāpihi Stream was culverted during the 1880s and now tracks under the streets to the harbour. So if rainfall outstrips what the pipe can carry, its overland escape route follows the same path.

The Basin Reserve to the harbour was once marshland. So was Island Bay’s racecourse, which was turned into a greenfield subdivision in 1905.

“Flooding is in places that are directly linked to natural systems, be they wetlands, be they streams,” Farrant says. “It’s very unfortunate, but not unforeseen.

“When you haven't had a rainfall event for a long, long time, people look at flood maps and think it's over-dramatised. After the fact, we find that the flood maps are generally very, very accurate. There'll be no flooding in Wellington, in these catchments, that's completely out of whack with the flood maps. It all lines up.”

The battle to contain city stormwater is a fight against history, says John Tookey, professor of construction at Auckland University of Technology. Around the world, early arrivals tended to settle on high points above rivers - access to water, but safe from floods.

But as populations grew, a patchwork of developments popped up in between.

“It’s a global thing…the historic town was built above the flood zone, and then you get the peripheral development taking place in the flood zone. It happens again and again.

“Every time you cover real estate with concrete and impermeable cover, you increase the risk downstream for these overland flow zones.”

And the situation is only going to get worse, says Earth Sciences NZ chief scientist for flooding and extreme weather Emily Lane. That’s because dramatic deluges are likely to spike with climate change, with one-hour, 1-in-100 year rainfalls expected to increase 14% for every degree of warming.

“Unfortunately, short and relatively localised but very intense events like those we’ve seen over the past few days are the type of event expected to increase the most with climate change.”

Is this another pipes problem?

If stormwater pipes are made to cope with a 1-in-10 year rainfall event, surely we just need bigger drains?

“It’s not a case of the pipes being under capacity,” Farrant says. “It’s a case of there being too much rainfall to fit into a pipe. There’s nowhere that would have pipes big enough to accommodate all of this water.”

And Wellington City Council says its streets are already designed as flow paths, with kerbs and channels to help direct run-off when underground stormwater pipes are overwhelmed.

So if you can’t build pipes big enough to contain the flow, there are really only two options, Farrant says. Either you clear the flow path so water can drain off with minimal damage. Or you try to reduce the volume of water running off. As well as the blindingly obvious - “not building in places where flooding is going to occur”.

“It’s trying to accommodate flow paths so that they can move through a catchment in a way that might cause disruption, but that won’t cause lasting loss of property, and all the associated social and economic hardship.”

For homeowners, that means checking if you’re on a flow path and, if you are, looking at how water moves through the property and making any changes that might help it drain out.

“Things like fences and retaining walls and modifications to your back garden can actually have quite an influence on where water can go,” Farrant says.

If you’re repiling, consider lifting the house above flood level. Or if there’s a heavy rain warning, park the car out of the danger zone.

New developments are more straightforward, Farrant says. Houses should be built with rainwater detention tanks to capture and hold rainfall, reducing the amount entering stormwater systems.

Some areas have built-in massive detention basins to hold stormwater, such as the lowered sports field at Greenslade Reserve in the Auckland suburb of Northcote, which is lined with free-draining fill and can hold 12 million litres of stormwater.

“We’re living with the legacy of the past,” Farrant says. “It’s very hard to fix that, but every time we do new developments, and new works, is a chance to get that back.”

Tookey reckons the single most effective fix to retrofit our concrete jungles to better soak up water would be to fit every existing property with a rainwater detention tank.

That’s detention, not retention - an underground tank that holds water temporarily in a downpour, then drains out to flush the toilet or water plants. Unlike a retention tank, it’s supposed to sit empty, not full.

If you can catch 10mm of rain on a 200sqm roof, that’s two tonnes of stormwater you're keeping out of the system.

“That's quite a lot,” Tookey says. “You multiply that up by every road that you see, every concrete casting that you see that’s impermeable. The amount of water that is shed by a built-up area is huge. The only thing you're going to be able to do realistically is stormwater detention systems that stop the stormwater surge that overwhelms the system.”

But installing a 10,000-litre tank might cost north of $10,000, and that won’t help you unless everyone else does it too, Tookey says. So there would need to be either a financial incentive or penalty - or a legal requirement - for everyone to install the tanks to make any real difference.

“People who have property in the existing environment, and they're not getting flooded out on a regular basis, frankly, they don't care. That's just human nature. They don't care what's happening elsewhere, and if they're not going to do it for the principles of civic duty, then you have to incentivise them to do it.”

Letting the light in

There’s a global movement to bring long-buried streams out from their dark city culverts back above ground. Engineers call it daylighting.

The water reclaims its natural course, eels get to see the sun, and city slickers reconnect with the lifeblood of nature running under their feet.

But that’s not necessarily a solution for flooding. Ōwhiro Stream is one of the city’s few remaining daylighted streams, but it still caused damage on Monday.

That, says Farrant, is because it’s hard against a road so has no room to flood naturally.

“It’s there, but it’s not there as it used to be…Should we be daylighting streams? Yes, if you have space. That’s the kicker - you need to have space. You’re bringing biodiversity into the city and human connection. But you can also do it in a way that you increase the capacity of what used to be a little pipe below ground. It’s much bigger.

“In Wellington that’s difficult, but we maybe have to have some of those long, hard discussions about what the next 100 years is going to look like.”