Nanako Stream: Developing a comprehensive stormwater solution

Robert Kelly (Aurecon), Josh Hodson (Tonkin + Taylor), Richard Mocke (Tauranga City Council / CAPE Consultants)

Pyes Pa West (commonly referred to as “The Lakes”) has been one of the fastest growing and most popular residential subdivisions in the Tauranga area. Its location close to the city centre, Tauriko Business Estate for employment opportunities, Tauranga Crossing shopping precinct, easy access to SH36 (to Rotorua) and SH29 (to Hamilton/Auckland), as well as the diversity of housing choice and affordability has proved to be exceptionally popular and an attractive place to live and work.

The development area is located on the lower Nanako Stream catchment, which feeds into the Kopurererua Stream which then discharges into the Tauranga Harbour near Judea. Managing stormwater and flooding within this catchment has been very complicated due to several factors that have made reaching a suitable and appropriate solution difficult to achieve, these include: 

• Several historical inappropriate development areas along and adjacent to the floodplain, including residential developments along Pengary Lane along the Nanako Stream and Judea Industrial area at the confluence of the Kopurererua Stream and Tauranga Harbour estuary;

• Historical and ongoing infilling of floodplain areas, both consented and non-consented, which has reduced the capacity of the floodplain to accept flood events;

• Steep escarpments either side of the stream with roading and residential developments either side subject to geotechnical risk and potential failure during flood events; and

• Several flood management and flow control devices (dams, ponds, culverts, treatment devices, etc) throughout the catchment creating flooding risks (e.g. due to dam break scenarios).

Tauranga City Council (TCC) has been developing a comprehensive stormwater solution, to effectively and sustainably manage stormwater and flooding in the catchment of the Nanako Stream.

In order to understand the flood attenuation and stormwater treatment needs of the catchment, hydraulic modelling has been undertaken to determine the volume of water that needs to be attenuated so as to protect downstream properties from flooding effects, as well as the ecological values of the stream.

Through the evolution of this work, a proposed solution has been developed. With these works in place, there is predicted to be no increase in peak flood levels or an increase in maximum flow rates.

Additionally, the works will improve the stability and resilience of existing embankments, increasing safety for downstream properties and the public. All existing and new dams will be designed and constructed in accordance with the New Zealand Dam Safety Guidelines.