Plea to let excess water drain naturally

Kritzo Venter explains the workings of water to Te Kura Kaupapa Maori o Te Atihaunui a Paparangi pupils. Photo/Supplied

Drain pipes have a lot of negatives, says engineer

A keen young Wanganui engineer wants to see more of the city's water wending its way through streams and sinking into the ground rather than running through pipes.

Kritzo Venter is Wanganui's senior stormwater engineer. He said this new and more natural way of dealing with excess water was international best practice and "the way the industry is going".

It's also a return to what happened to water in pre-industrial times.

He wants to improve the streams that run through town, and there's a Clean Streams Whanganui logo for that.

Stormwater is what runs off roofs, roads and paving.

The new idea is for unwanted water to sink into the ground or be used by plants rather than run quickly over hard surfaces into the Whanganui River.

One way to do that is to have "rain gardens" as well as concrete gutters on roadsides. Water flowing into them is slowed and absorbed by plant roots.

If there's rubbish floating in the water, it will get caught in the foliage.

"People will be shocked and outraged and asking council to clean it up, but I say let them see the results of our litter bug culture," Mr Venter said.

Downpipes from buildings can empty onto grass, rather than into more pipes.

Water can move along swales and floodways - low, vegetated points in the ground. The plants can remove excess nutrients and heavy metals, keeping them out of rivers and streams.

"Wetlands are by far the most effective filterers and purifiers of water."

Wanganui's stormwater system is only designed to cope with the once-in-10 years rainstorm. In anything bigger it has to run over the ground - anywhere as long as it's below habitable floor level.

Mr Venter has some pilot projects started and planned.

One is returning Putiki's Awarua Stream, which is prone to flooding, to its natural state. That way it will cope better with flooding and also support fish and insect life.

Putting a stream in a culvert does irreversible damage, he said.

"They're basically just about lost forever. You end up with flooding issues, because of the capacity being smaller than open channels, and there are hard surfaces, which speeds the water up."

Creatures like eels and whitebait can't live in pipes. Churton Creek, which drains Westmere Lake and enters the Whanganui River near BP Taupo Quay, is completely in pipes. There are no eels in the lake as a result.

In contrast, the Matarawa Stream, which runs through Wanganui East and empties into the Whanganui River at Kowhai Park, is not in pipes and has koura, eels and even freshwater mussels.

Mr Venter wants property owners to take ownership of streams on their land, and keep them free of rubbish and obstructions.

In a pilot scheme at Aramoho Cemetery, a central roundabout will be used to store excess water during rain events and rain gardens will be installed.

Most people think these new ideas are great when they hear about them, but they don't always put them into action. Maori have been quick to take them up, and they fit with the aspirations behind the Whanganui River Claim.

Schools and Horizons Regional Council are likely to be allies, and he'd like developers to get on board.

All those new measures would clean water before it enters the river - a logical next step now that the city's sewage is treated elsewhere.