When Waitakere City became an Eco City in 1992 finding new ways to manage urban spread, waste and water were important and the idea for Project Twin Streams was formed. In 1997 Waitakere City Council commissioned studies to investigate the effects of stormwater on the Oratia and Opanuku Streams.
There were critical issues:
- high levels of pollution in the discharge from Henderson Creek to the Waitemata Harbour
- sporadic flooding, erosion and sedimentation of stream beds
- unacceptable levels of sewer overflows during storms
Extreme weather events attributed to climate change, a growing population and demand for water and an aging stormwater infrastructure were set to increase challenges and costs.
To combat these stresses Project Twin Streams was established in 2003. With the aim of reducing pollution, flooding, erosion and sedimentation in our waterways, the project set out to restore the riparian margins of our streams. Back then it was a radical move – deciding to restore natural systems to manage stormwater. This is now widely recognised as a significant way to improve water quality and enrich stream ecology, both in New Zealand and internationally.
How does the Project Twin Streams approach work?
Project Twin Streams has planted more than 800,000 eco-sourced native trees and shrubs along 56 km of streambanks. They provide benefits by:
- providing shade to the stream which reduces light and stream temperatures, thus reducing excessive algal and plant growth
- stabilising stream banks through tree and root structures holding bank material together and reducing the impact of sedimentation
- providing leaf litter and woody debris forming in-stream habitat and cover
- providing a buffer and filtration zone between the streams and local land uses
- providing an overspill flood plain for excessive water during storms
How do we know it’s working?
To ensure we are making progress we carry out two strands of environmental monitoring: Pressure-State-Response (PSR) Monitoring and Ecological Surveys
PSR Monitoring recognises that human activities like residential growth put pressure on the environment affecting the quality and quantity of natural resources. It gives us a framework to measure the cause and effects of environmental problems so remedial steps can be taken.
Ecological Surveys use biological indictors like the presence of birds, bats, lizards, geckos and insects together with fruiting and flowering phenology, vegetation structure and composition and animal pests to measure environmental conditions.
In addition, a number of community groups carry out Wai Care monitoring which means that they monitor and report stream water quality on a regular basis. Some improvements have been noted in certain locations.
Snapshots of Environmental Monitoring:
Project Twin Streams has been the focus of a number of external university papers and research reports:
- Hall, K.L. and Helsel, C.M. (2009) The Waitakere Guidelines for Riparian Restoration: Lessons from Project Twin Streams, proceedings from Stormwater 2009: 6th South Pacific Stormwater Conference , Auckland, New Zealand, 29 April to 01 May 2009 [CH1]. (Size 150K)
- Brierley G.J., Gregory, C.E.,and Reid, H.E.(2008) Measures of Physical Heterogeneity in Appraisal of Geomorphic River Condition for Urban Streams:Twin Streams Catchment, Auckland, New Zealand, Physical Geography, 2008, 29, 3, pp. 247-274. (Size 746K)
- Brierley G.J., Gregory, C.E.,and Reid, H.E.(2008) River Recovery in an Urban Catchment: Twin Streams Catchment, Auckland, New Zealand, Physical Geography, 29, 3, pp. 222-246.