Housing density in our cities is again being questioned after stormwater systems were overwhelmed by cyclonic rains. Coping with sustained heavy rain is possible will if we design “spongy” neighbourhoods says Robin Allison.
My suburb of Ranui, west Auckland, featured in the news when recent storms caused devastating floods in some areas of the suburb, swamping houses, cars and lives. Everywhere I look as I walk around Ranui I see blocks of six, eight and ten townhouses rising above concrete driveways, on sites that previously held single homes and gardens, and I fear the flooding problem will only get worse.
Just along the road from badly flooded areas, the medium density housing development called Earthsong Eco-Neighbourhood, where I live, fared well in the storms. During the height of the flooding we had water flowing through the property in places we’d never seen before in twenty years, but the water flowed where it was designed to, into the overland stormwater swale system and harmlessly spilling over onto the paths down to the pond. No houses were flooded and the water quickly drained away.
Earthsong was planned 24 years ago to the highest practical standards of environmental sustainability at the time. We were committed to being “good neighbours” to each other and to the wider biosphere which supports us, and that included designing the site layout so that rain water flowed, and was absorbed on our site, much as it would have before we came along and built our houses. Our recent experience shows that these measures work, and they could (and should) be incorporated into other higher density housing as storms and floods become more frequent.
Earthsong has 32 homes and shared community facilities on 1.29 Hectares, a density of 400 m2 per dwelling, including the driveway and carparks. Not as dense as many new townhouse developments, but much denser than the older suburban model of single homes on sections of 600 or 800 m2.
With shared open space, driveway and carparks, the site design is optimised for the benefit of all residents and for effective on-site management of stormwater. The relatively dense neighbourhood still feels spacious and relaxed, with pathways winding between homes and productive gardens, allowing children to play safely and neighbours to have a chat as they come and go from their houses.
Compared to the neighbouring 1960s suburban subdivision over the fence, Earthsong has a similar building footprint but twice as many dwellings. Earthsong carparks and driveway cover less than half the total paved area of the standard subdivision beside us, and our carparks have permeable paving, allowing storm water from the driveway to soak into the ground.
Fittingly for land that was previously a commercial orchard, we have over 200 fruit trees at Earthsong and numerous native trees and shrubs. Vegetable gardens are tucked into small and larger areas around the site, and lawn areas are minimised, all of which contributes to water absorption on site.
We treat water as a gift and a resource rather than a waste problem. Rainwater is collected from roofs into water tanks that supply 60 percent of household water needs, while reducing the amount of rain flowing into the stormwater system.
Water is not channelled into pipes in the ground until it leaves the site. All surface rainwater flows into densely planted swales (wide shallow channels) beside the paths, running down the site to discharge into a pond at the northern end. This overland storm water system acts like a sponge, allowing the water to soak naturally back into the ground, filtering sediments and nourishing plants, increasing biodiversity and reducing water runoff from the site.
All of these measures help to reduce and even out the flow of water into the council stormwater system to what would have flowed off the site in a medium storm when it was still an orchard. With 32 houses and common buildings where only two houses stood before, with associated driveway and carparks, this is a huge reduction in impact. These measures can be designed into other medium density subdivisions, even those much denser than Earthsong.
Re-thinking the design of housing and neighbourhoods is critical to addressing the environmental crises we are increasingly facing, not to mention the social crises of loneliness and isolation. It is possible to design attractive, neighbourly medium density environments, and still absorb much of the stormwater that the site would have coped with pre-development.
