greenfields STormwater system
The flow through planter acts as a rocky stream and the detention area acts as the surrounding flood plain to mimic NATURE
ACH Consulting were engaged to refine the civil engineering design for a 6ha Greenfields development in a West Auckland Commercial Hub. The stormwater system in the area was known to be under capacity, and significant rain was known to cause flooding issues.
The original design engineers had proposed including surface ponding, a wetland that would typically remain dry, and detention tanks providing 200m3 of storage. ACH Engineers determined that these three separate systems would be difficult to maintain and not the best fit solution to the site.
The site itself required 500m3 meters of storage to be provided in order to maintain pre-development flows and not exacerbate downstream flooding during a 1 in 100 year rainfall event. Treatment for over 19,000m2 of parking needed to be able to remove at least 75% of Total Suspended Solids (TSS).
To best meet the needs of the development ACH designed a blue-green infrastructure option in the style of a flow-through planter to both treat and detain stormwater runoff from the 6ha site. The design parameters assume no infiltration of stormwater into the ground, as the subject property is underlain by clay material.
Pre-treatment was provided using 120m2 of 100–300 mm rounded river stone, planted out with native species tolerant of both extreme wet and dry conditions. The flow through planter reduces velocity and removes dust sediment and macro-contaminates, while creating an aesthetic landscape element and microhabitat within an urban industrial environment. The removal of dust and macro contaminants increases the life of the downstream Stormwater360 Stormfilter™.
The requisite 500m3 of storage, so as to maintain pre-development flows, is provided in an area surrounding the flow through plant filter. This area has gentle slopes planted with native self-sustaining plants appropriate for extreme environments. The planted area serves as available water storage during the 1 in 100 year rainfall event as well as serving to filter any sheet flow off the parking area.
As the detention of water in the area surrounding the flow through plant filter will be limited to rainfalls greater than the 1 in 10 year events, and filling limited to the 1 in 100 year event, it was deemed more practical to design a detention device analogous to a natural system. The flow through planter acts as a rocky stream and the detention area will perform the function of a surrounding flood plain, to mimic our natural stormwater management systems and to provide amenity value to the area.
Additionally, pre-treatment of the parking area in this way decreases the required maintenance frequency of the Stormfilter™ device which was designed to provide treatment of the 13.5 ha catchment. This means less work for the Council maintenance team to undertake in future.