TURNING THE RED ZONE GREEN – REGENERATIVE STORMWATER DESIGN IN THE ŌTĀKARO AVON RIVER CORRIDOR

Hamish Cotter (Beca Ltd), Vicki Clarke (Beca Ltd), Kate Purton (Beca Ltd)

ABSTRACT

Christchurch’s Ōtākaro Avon River flows for approximately 26 kilometres from its springfed source to its mouth at Te Ihutai / Avon Heathcote Estuary. The lower reaches of the river are located within the red zone, land that was previously residential but deemed unsuitable to rebuild on following the 2010/11 Canterbury earthquakes and was acquired by the Crown. This land is now owned by Christchurch City Council (Council). The river corridor has historically been an area of cultural and ecological significance; however,ongoing development in addition to earthquake damage has reduced the overall ecological habitat, water quality, vegetation cover and hydraulic capacity.

The Ōtākaro Avon River Corridor (ŌARC) Regeneration Plan was developed by Regenerate Christchurch in consultation with mana whenua, the Crown, Council and the wider community to support the regeneration of ŌARC area within the red zone. It sets out the vision, objectives and green print for the 602-hectare ŌARC and was approved by the

Minister for Greater Christchurch Regeneration in August 2019. The objectives are:

• Create a restored native habitat with good quality water so there is an abundant source of mahinga kai, birdlife and native species

• Support safe, strong and healthy communities that are well connected with each other and with the wider city

• Provide opportunities for enhanced community participation, recreation and leisure

• Create opportunities for sustainable economic activity and connections that enhance our well-being and prosperity now and into the future.

In 2022, Council engaged Beca to undertake stormwater investigations and design for the ŌARC Wainoni and Bexley areas, with a combined area of approximately 80 hectares.

The purpose of this project is to provide flood resilience and stormwater management in these areas, consistent with the Regeneration Plan, the Iwi Management Plan and Council’s six values approach to surface water management (the six values are: ecology, drainage, culture, heritage, landscape and recreation) (Christchurch City Council, 2003).  Stopbanks and flood walls will be used to provide flood resilience, while wetlands and swales will treat and convey stormwater runoff before it is discharged to the Ōtākaro /Avon River. The sites present several challenges:

• The ground is low lying with high groundwater and is susceptible to liquefaction and lateral spreading in seismic events

• Areas of contaminated land are present

• The proximity to the estuary will result in increased river levels which will increase flood risk and groundwater salinity

• The existing stormwater networks are low lying relative to water levels in the river

• There remain some residential properties within the red zone

• Space is constrained in some areas by the river and State Highway 74 (SH74)

• New excavations for basins or wetlands in liquefaction-prone land, if unmitigated, may increase the land instability and risks to SH74, private property and the proposed stopbank.

This paper discusses how the project has balanced these challenges during the concept design phase to achieve a successful outcome using innovative design and collaborative efforts across a wide range of disciplines and several organisations (including Council, Beca, Rough Milne Mitchell, Aurecon, WSP and GHD). The design is subject to ongoing development informed by Council discussions with mana whenua and other stakeholders.