Back to Takanini – Pipes? Where we’re going, we don’t need pipes

Auckland Council, in partnership with multiple developers and landowners, is in the process of constructing the innovative Takanini Strategic Stormwater Corridor (“TSSC”, McLennan Park wetlands upgrade and Artillery Drive tunnel. This radical stormwater management intervention by Auckland Council is facilitating the development of over 100 Ha of flood prone Special Housing Area (SHA), the Takanini SHA. The TSCC is a man-made stream of 25-50m width that provides attenuation and conveyance for the 1% AEP event with additional attenuation and treatment being provided at the McLennan Wetlands. Attenuated and treated flows are then discharged to the Pahurehure Inlet via the Artillery Drive Tunnel.

In 2015, the Takanini SHA area was extended to include an additional 16 Ha at Mill Road to provide 264 residential lots. The Mill Road site straddles two stormwater catchments; the Papakura Stream catchment and the Pahurehure Inlet catchment. Successful development of the site relies on the ability to pass post development flows downstream. This is not possible for the Papakura Stream portion of the site due to a lack of downstream infrastructure and downstream flood risk. Through close collaboration with Auckland Council, AR & Associates designed the site such that post development flows from the entire site pass to the Pahurehure Inlet catchment via the TSSC. The predevelopment flows to Papakura Stream remain.

The entire site was raised by approximately 750mm and the orientation changed to facilitate the catchment boundary adjustment and make the land less vulnerable to flooding by providing resilience and freeboard. The loss of floodplain storage due to infilling was not an issue as the TSSC channels and associated downstream infrastructure was designed to cater for floodplain storage and conveyance for the 100-year ARI.

The volume of imported material was kept to a minimum due to the costs and availability of fill. The resulting site profile was very flat, and this together with the need to provide groundwater recharge and SMAF mitigation, lent itself well to a water sensitive urban design (“WSUD”) approach. Local stormwater runoff is managed through swales, recharge pits and infiltration, resulting in a largely pipe-less, integrated solution. This approach also responded to other project drivers such as urban design, landscaping and roading. The WSUD approach also facilitates the critical requirements for ground water recharge of the underlying peat soils to prevent settlement.

As the downstream infrastructure is not yet operational, the site had to consider the implementation of temporary drainage solutions. This included the incorporation of a greenway corridor that will operate as a temporary stormwater attenuation area and will connect to and mimic the TSSC concept once it comes online. The greenway provides amenity, improved ecological habitat and a public park that includes a playground and cycleway for the wider community.

The Takanini Strategic SHA Extension Development at Mill Road exemplifies effective adaptation of a water sensitive design approach and ‘outside the box’ thinking to suit local topographical, geotechnical and flooding constraints, while responding to specific project urban design drivers.

This paper discusses the design process, the implementation and lessons learned as well as customer feedback from key stakeholders including the developer and Auckland Council, who will ultimately be the asset owners of these WSUD features within the development. The paper also touches on a disconnect between the resource consent phase undertaken by the developer and the building consent phase undertaken by the future lot owners. All too often the best intentions and the innovation of the resource consent stage is lost in translation to the building consent phase. This particular development has attempted to try and bridge this gap with some success.