Meeting SMAF Requirements

28 November 2016

Part 1: ChamberMaxx™ - Retain and Detain with one system


The Auckland Unitary Plan has recently introduced new stormwater management regulations to the Auckland region including new Retention and Detention flow controls for sites located in a SMAF (Stormwater Management Areas: Flow) area. These controls seek to protect and enhance Auckland’s river, streams and aquatic biodiversity in urban areas by managing the small frequent stormwater flows of up to the 1 in 1 to 2-year ARI event.

 The SMAF areas are mapped catchment areas that drain to streams that have been identified as being particularly sensitive to changes in stormwater flows, have high natural values, and are at potential risk from an increase in impervious area associated with future development. Two separate SMAF areas have been identified;

  • Stormwater Management Area control – Flow 1 (SMAF1) are those catchments which discharge to sensitive of high value streams that have relatively low levels of existing impervious area.
  • Stormwater Management Area control – Flow 2 (SMAF2) areas typically discharge to streams with moderate to high values and sensitive to stormwater, but generally with higher levels of existing impervious area within the catchment.

With the new regulations surrounding this, there is now a mandatory requirement for sites within these SMAF areas to provide;

  • Retention (volume reduction) of at least 5mm runoff depth for the impervious area for which hydrology mitigation is required; and
  • Detention (temporary storage) and a drain down period of 24 hours for the difference between the pre-development and post-development runoff volumes from the 90-95th percentile, 24 hour rainfall event minus the 5 mm retention volume or any greater retention volume that is achieved over the impervious area for which hydrology mitigation is required.

It is hoped that by retaining the first 5mm of runoff and allowing it to infiltrate at a rate of 1mm/hr, a 30% volume reduction could be achieved which will maintain base flow in nearby streams. Further by infiltrating the first 5 mm, a high percentage of the dissolved containments will be removed from the runoff.

There are a number of ways we can approach this;

  • Reduce the proposed impervious area on your site
  • Reduce runoff volumes, and hence peak flow rates, by water reuse and harvesting schemes (Not because we need the water in Auckland but to protect our fresh waterways)
  • Retain and detain on the same site


Stormwater360 has a number of products that can assist with meeting SMAF requirements.

One of our key detention and retention products is ChamberMaxx™ - a corrugated, open-bottom plastic arch underground chamber system designed to collect, detain, retain and infiltrate stormwater runoff. The uniqueness of this product is that it can provide retention and detention in the same space.

SMAF areas will have mandatory retention/ detention requirements, and installing a ChamberMaxx™ system can meet both objectives. Retention and detention when using chambers can be determined by where the chamber outlet pipe is positioned. Water stored below the outlet will infiltrate between rainfall events and water above will be detained.


ABC Chamber Design

Exceeding New Zealand Heavy Traffic loading (HN-HO-72) standards, this underground system is ideal for installing under car parks and roads when land is at a premium. Stormwater360’s largest detention project using ChamberMaxx™ was a project in Christchurch where 677 chambers were installed under the roads in a residential development, providing 2200m3 of detention and delivering an innovative solution for water storage.

With a low profile of 770mm, Chambermaxx™  has the advantage of being able to store a large volume of water in a shallow install and the chambers can be designed as a modular system allowing the system to work around services and other site restraints. 

For more information on ChamberMaxx ™ and meeting SMAF requirements please feel free to contact a stormwater consultant via [email protected] or 0800STORMWATER.



Auckland Unitary Plan Operative in part (15 November 2016)

Kettle, D.,  Mayhew, I., Irvine, J. & Young, D., (2013). The Methodology For Developing Stormwater Management Areas For Flow Control For The Auckland Unitary Plan. Presented at Presented at Water New Zealand's 8th South Pacific Stormwater Conference, Auckland.

Kettle, D. & Kumar, P., (2013). Auckland Unitary Plan stormwater management provisions: cost and benefit assessment. Auckland Council technical report, TR2013/043