One planted flow-through system replaced three hard-to-maintain devices, holding back 500 m³ of stormwater and treating runoff from 19,000 m² of parking.
- Client
- Private Client
- Services
- Three waters · Civil design
- Location
- Henderson, West Auckland
- Scale
- 6 ha · 500 m³ storage · 19,000 m² parking treated
- Status
- Complete 2020
01 / Project overview
The brief
A new 6 ha commercial hub in West Auckland was being developed on a site where the existing stormwater system was already under capacity, and heavy rain regularly caused nuisance flooding. We were engaged to refine the civil engineering design.
The original design used three separate systems: surface ponding, a normally dry wetland and 200 m³ of detention tanks. Each would have worked, but together they would have been hard and costly to maintain. Our task was to meet the same performance targets with something simpler, on a clay site where infiltration couldn't be relied on.
02 / Our approach
How we delivered it
One system instead of three
We replaced all three with a single blue-green flow-through planter. Because the site is underlain by clay, we designed it on the basis of no infiltration, so its performance doesn't depend on ground conditions.
Storage and flood protection
The development needed 500 m³ of storage to hold flows at pre-development levels and avoid worsening downstream flooding in a 1 in 100-year rainfall event. The detention area is gently sloped and planted with native species. It begins storing water above the 1 in 10-year event and fills only in a 1 in 100-year event, while filtering sheet flow from the car park the rest of the time.
Treatment at source
Runoff from more than 19,000 m² of parking had to be treated to remove at least 75% of total suspended solids. At the inlet, 120 m² of 100–300 mm rounded river stone, planted with natives that tolerate both wet and dry conditions, slows the flow and captures dust, sediment and larger contaminants. The planter works like a rocky stream and adds habitat to the site.
Less maintenance downstream
Pre-treatment protects the downstream Stormfilter™ unit, extending its life and reducing how often it needs servicing. That means less maintenance for the council over the long term.
03 / The outcome
“One planted system now does the work of three: holding back a 1 in 100-year storm, treating car park runoff and giving the site a living stream feature instead of tanks and ponds.”
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