31 May 2024

Dam project aims to stop flooding in Christchurch's Early Valley

From Country Life, 7:28 pm on 31 May 2024
Stormwater dams near Tai Tapu

 Putting the final touches to lowest dam. The Rawstrons' old woolshed is in the background. Photo: Cosmo Kentish-Barnes

A series of low dams and basins to capture stormwater as it runs off the Port Hills is being constructed on farmland near Christchurch.

It is the brainchild of Brent and Shirley Rawstron from Rossendale Wines.

They own the land and have teamed up with the Christchurch City Council to prevent flooding in Early Valley.

Brent, a second-generation sheep and beef farmer, said the valley often flooded after heavy rainfall.

"The whole of the valley floor floods every second year and every several years the flooding covers the fences and flows across the Old Tai Tapu Road.

"The valley goes up to the top of the Port Hills and all that water comes down through this farm, so the intention was to hold the water back for five to six days," Brent said.

Stormwater dams near Tai Tapu

 Shirley and Brent above the dam construction site. Photo: Cosmo Kentish-Barnes

The flooding was likely to worsen as the surrounding hills were built on, he said.

"When Dad moved into the valley 70 years ago there were four other residents and now there are over 100. We've just gobbled up land in the hills for housing."

The couple engaged in a public-private partnership (PPP) with the Christchurch City Council to get the project done.

Stormwater dams near Tai Tapu

Stormwater flows into the dams from this steep gully. Photo: Cosmo Kentish-Barnes

The council is paying for the construction work in exchange for ownership of the 12-hectare dam site.

The Rawstrons have stipulated that the land must remain a green space that can never be built on.

Brent said more housing subdivisions were planned on the hills surrounding the valley, which would likely increase the need for a stormwater holding facility.

Stormwater dams near Tai Tapu

A creek filled with rocks leads down to the dams. Photo: Cosmo Kentish-Barnes

"The land to the north of us is zoned residential and there's enough land there for 150 houses and run-off from them will eventually be coming down into the valley floor."

The total water storage capacity of the ponds will be about 128,000 cubic metres.

After the recent fires on the Port Hills highlighted the difficulty of helicopter accessible water, Brent said it would also be feasible to "retro fit" an emergency water source into the project.

"We could dig a hole in the biggest basin three metres deep and however many metres wide and create a permanent water source that helicopters can dip their buckets into."

Stormwater dams near Tai Tapu

The top dam fills with stormwater after heavy rainfall. Photo: Cosmo Kentish-Barnes

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