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We see rainwater as a wonderful resource, not as a nuisance. We have avoided concentrating rainwater into pipes under the ground and then conveniently forgetting the impact it has on downstream properties, streams and harbours. Instead, we've kept the building footprints and hard paving surfaces to a minimum, allowing as much land as possible for gardens and soft landscaping where rainwater can soak back into the ground and nourish life. We've worked with the natural contours of the land to direct the water into shallow channels running beside each path, allowing the water to run overland and more closely follow the natural system. We've created a pond which allows any sediments to settle, reduces floods downstream in a storm, provides habitat for wildlife and is a wonderful place for our annual boatrace!
Overland Stormwater System
The fact that we collect roof water into our tank system for use within the houses substantially reduces the amount of rainwater that would otherwise be piped off the site. We've also reduced the runoff into our overland system by using semi-permeable paving for all carparks and a large area of the driveway, and limiting the area of impermeable or semi-permeable paving around each house. The channels beside each path are planted with water-loving plants such as native carex, flaxes and cabbage trees, ornamental bog sage, taro and irises, and edible plants such as banana palms, berries and watercress. Two "marsh gardens" along the route provide extra storage for water in a storm, reducing the peak runoff and releasing the water through throttled outlets into the channels over the next day or two after a big rain.
Our stormwater channels are based on "swales", known in permaculture terms as shallow vegetated channels that collect rainwater and follow the contours to retain as much water on-site as possible. However due to our small site area and high rainfall, our channels are a hybrid between swales and more conventional watercourses, designed to slow the passage of water, but running down the contours to the pond so that in a storm the water can still flow away from houses.
The channels and marshes not only slow the flow of water, filtering sediments and absorbing some of the flow, but they are also beautiful, productive, and a habitat for wildlife. That looks like win, win, win, win, and win to us!! The only downside, if you are not a gardener, is the occasional maintenance required to weed the channels and thin the plants sufficiently so that in a big storm the water can flow safely to the pond and off site.
Any water that has not been absorbed on its journey down the channels eventually flows into a large pond at the north end of the site, firstly into a sediment forebay which allows further settling of sediment, and thence into the main pond. The pond is designed to hold enough water that the runoff from the site is equal to pre-development levels for up to a 1-in-5 year flood.
Beyond the boundary, the natural overland watercourse was piped underground in the 1960s when housing was built on the land downstream of the Earthsong site. In very heavy rainfall, rainwater fills the pond until it reaches the "high-tide" level formed by the lip of a large manhole leading directly into the council stormwater sewer. Once at this level it pours over the lip into the sewer. Smaller outlets built into the middle of the pond release the water into the stormwater sewer over 24 hours to lower the pond level back to the "low-tide" mark, ready to give storage capacity for the next storm.
The pond, because it is shaped to mimic a natural water basin with gently sloping sides, is not considered a "pool" which would require fencing for the safety of children. A child who stumbled into the water would get very muddy, but would not be in deep water at the edges and could therefore clamber out. For our own peace of mind however, with very young children living nearby, we have built a temporary fence to keep children out until the flaxes and other planting around the edges become thick enough to form a useful barrier to curious children.
The overland system, allowing the water time to soak back into the ground and nourish plants, reduces and evens out the flow of water into the council stormwater system to what would have flowed off the site when it was still an orchard, for storms up to the severity expected only once in 5 years. With 32 houses and common buildings where only 2 stood before, and with the associated driveway and carpark areas, this is a significant reduction in impact, and as the trees and landscaping mature this will only improve.
The first major cleanout of accumulated silt from the pond was accomplished with spade and wheelbarrow over the course of last summer. While the engineer predicted the main pond would need cleaning out only once every 12 years, the pond silted up in half that time due to the large areas of clay exposed through the protracted construction period, and the channels being unable to be completed and fully vegetated until all other construction had ceased. We are expecting the amount of sedimentation to reduce significantly now that construction is complete and as the vegetation matures.
The pond sediment was tested for metals and other contaminants in 2005. It was found that the metal results were within the background levels for the Auckland region, and no pesticides were detected. The Auckland Regional Public Health Service assessed the results and determined that they had no concerns from a public health point of view with spreading the sediment on land at Earthsong, so it was barrowed up the site and used mainly to create the channel beside the top end of the drive.
Infrastructure Auckland grant
In 2000, as plans were being finalized and the project was gearing up to begin construction, we applied for a grant from the regional body Infrastructure Auckland which was funding projects at that time relating to either stormwater or transport. As a demonstration project, Earthsong aimed to implement an integrated approach to addressing problems of stormwater management, and demonstrate the viability of environmentally sensitive medium density housing with particular regard to stormwater. Earthsong was committed to publicise, to as wide an audience as possible, information on the range of stormwater options available in an urban environment, a key element of the regional stormwater strategy.
In particular, from an engineering point of view, the vegetated channels, pond, rainwater tanks, and permeable paving give stormwater treatment over and above that required by the existing code of practice, and a significant increase of peak storm flow attenuation over that required, relieving pressure on downstream pipework known to be at-capacity and stressed.
A total of $93,471 was granted to assist with the implementation of these items. Due to the protracted construction period, it was 8 years later that we were able to uplift this grant, though it was very welcome nevertheless.