A master planned Mt Wellington terrace house development provides a model for stormwater design that can cope with extreme rainfall as experienced recently.

The newly constructed Richmond community, which is built on the site of a former quarry in Mt Wellington, Auckland, came through the extreme weather events of the past month unscathed.

The Richmond development is being built in five stages with the fourth stage of 111 terraced homes starting to complete this month. Stages One to Three are complete, and 230 homes are occupied. Once the development is complete it will hold over 480 homes. A fifth stage is also now underway.

Historically the site, Te Apunga-o-Tainui or McLennan Hills, was a series of scoria cones that had been quarried down to a quarry floor and escarpment 60 to 70 years ago. The 10.5 ha site then became a plant nursery before the Auckland housing boom brought it to the attention of property developers.

However, it took several years for the site to gain Council confidence around the application of stormwater for housing development as it sits around 20m above an aquifer. The aquifer creates additional challenges to build on, but it also offered an advantage as an aquifer acts like a giant rock sponge, taking water from above, storing it and releasing it gradually into springs, rivers and harbours.

Simone Horrobin, General Manager Property Development, Founders Developments (formerly Wilshire), says there was initial regulatory resistance to building hundreds of townhouses on the old quarry. She says much modelling was done to prove houses could be built there and not flood in a significant weather event.

“The stormwater design was a comprehensive and complex part of the overall development Master Plan. But it was good to see the system working and handle large volumes of water with no flooding,” she says.

The stormwater system is designed to cope with and dispose of 1.4m3/s onsite stormwater within rock bore chambers.

Brendon Verhoeff, Director of Maven – the civil engineering consultant on the development – says the stormwater design for Richmond was unique as instead of using one large soak hole, it has a diversified network of over 50 soak hole devices.

“When you have a system of pipes and drainage designed for a one in 10-year event, you are bound to get blockages, so we built in redundancy with an interconnected system. When this system is exceeded then overland flood paths have been designed to direct water east towards watercourses and ultimately the harbour.”

As it turned out, the Auckland extreme rain event on Friday 27 January broke rainfall records with 265mm recorded in nearby Mangere in a day. NIWA reported that an entire summer's worth of rain fell within one day in what it described as a 1-in-200-year event.

“We typically design for an event that is manageable. In this case we had 30% more water than we had designed for, and the system performed very well,” Brendon says.

Building on scoria, quarry floors and buried lava flows is common in Auckland as the city is the site of 53 former volcanoes and much of the central isthmus and South Auckland sits on volcanic fields.

As a large brownfields, site the Master Plan including the stormwater strategy that was approved by Auckland Council’s Healthy Waters and local iwi. The design included creating flood paths to direct water onto the newly formed streets and into soak holes.

A series of scoria soak holes and filters capture the rainwater, filter it to remove road debris, and then gradually release the excess water into the subterranean aquifer. From there, it flows through the porous rock aquifer to the sea.

Richmond’s Residents Association is tasked to maintain the stormwater network to ensure that maintenance of the catchpits and filters happen regularly.  This Association is run by a committee of elected owners who employ property managers to ensure that the assets, including the stormwater system, is regularly checked and maintained.   

Extensive landscaping and greenspaces of eight pocket parks also help absorb stormwater. All dwellings within the site are to be a minimum finished floor level which will have adequate freeboard above the flood level in accordance with Auckland Council standards.