Managing Stormwater

Why Project Twin Streams?

The idea for Project Twin Streams began when, in 1992, Waitakere became an Eco City. This meant that exploring new ways to manage urban spread, waste and water was paramount.

In 1997, Waitakere City Council commissioned studies to investigate the effects of stormwater on the Oratia and Opanuku Streams.

There were critical issues facing the catchment:

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 in storms

With the added pressure of escalating extreme weather events attributed to climate change, a growing population and demand for water and an aging stormwater infrastructure, the challenges and costs were set to increase.

To combat these stresses, Project Twin Streams was established. In a radical move, it was decided that restoring natural systems to manage stormwater was the right solution. 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. This is now widely recognised as a significant mechanism for improving water quality and enriching the stream ecology, both in New Zealand and internationally.

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How does the Project Twin Streams approach work?

Project Twin Streams has now planted over 800,000 eco-sourced native trees and shrubs along 56 km of streambanks. They provide stream health benefits by:

Providing shading 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. This minimises the entry of contaminants to the streams.
Providing an overspill flood plain for excessive water during storms.

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How do we know it is working?

At the heart of Project Twin Streams is a desire to improve the quality of our streams. To ensure we are making progress, Project Twin Streams carries out two strands of environmental monitoring: Pressure-State-Response (PSR) monitoring and ecological surveys.

The PSR model recognises that human activities such as residential growth put pressure on the environment, affecting the quality and quantity of natural resources. PSR monitoring provides a framework to gauge the cause and effects of environmental problems so that remedial steps can be taken.

The PSR framework for Project Twin Streams uses a variety of indicators that measure performance and indicate how well the objectives of the project are being met. Indicators used include:

Impervious Area: The proportion of the catchment covered by roads, roofs, driveways and other non-natural, impervious surfaces.
Wastewater Overflows: The number of sewer blockages resulting in wastewater overflows into streams for the period of December 2003 to April 2004.
Stormwater Outlets: The number of stormwater outlets greater than 375mm diameter.
Traffic Activity: Calculated by multiplying the road length (km) by the Average Daily Traffic counts, to give Vehicle Kilometres of Travel for roads.
Riparian (stream bank) Width: The percentage of the catchment stream length with riparian zones less than 10m wide.

Ecological surveys monitor biological indicators such as 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. Each of the community contract organisations are also WAICARE groups.
The environmental monitoring campaign, which began in 2003, has noted improvements in certain locations. More information can be found on the PTS Environmental Report Card 1 

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What about the rest of the harbour?

Upper Waitemata Harbour Modelling for the ARC by Dr Malcolm Green from NIWA, demonstrates the impact of contaminants and sediment build up from Henderson Creek on the water quality of the harbour.

It is clear from this work that if Project Twin Streams is to have a significant impact on water quality, it will need to continue to work back from the harbour to the head of the catchment, planting an additional 289 km of riparian margins to complement other treatment train methods.