Holistic Catchment Management for Healthier Waters

Recent Articles in the Herald highlighted the pollution caused by the 122 active sewage overflow pipes spread throughout Auckland. There are hundreds of beaches in close proximity to the city there are plenty of opportunities for people to interact with the water. Whether swimming, sailing or surfing, Auckland is a city of water lovers, and water quality is directly correlated to the quality of living here: the news that sewage is regularly being pumped into our aquatic environment has therefore alarmed many.

Over 16000 properties in some of the most affluent neighbourhoods of Auckland are serviced by a combined sewage system which intercepts foul water and surface water. The impervious nature of urban catchments creates a distinct hydrology with high volumes of surface water and short drainage times so that during storm events water flows often exceed the hydraulic capacity of the combined system causing sewage overflows to nearby watercourses.  

Watercare plan to overcome this problem through the construction of an interceptor pipe network for combined sewage overflow (CSO) capture and storage. The scheme is not expected to be completed by 2026 following its completion 190,000 cubic metres is still expected to be discharged from CSOs annually, which produces the question of what else can be done to reduce this?

There are measures which capture and temporarily store surface runoff closer to source preventing excessive loading of the sewer system, these techniques are particularly helpful in short ‘flashy’ catchments such as those covering Auckland city.

In the UK and Europe, sustainable urban drainage (SUDs) offers an integrated approach to dealing with rainfall, it uses the landscape to protect from flooding and prevent pollution, delivering a controlled flow of clean water that can be used for amenity and wildlife benefits. Water sensitive urban design (WSUD) is a similar approach popularised in the Middle East and Australia where other aspects of water sustainability are considered, such as treatment and recycling of wastewater which can improve water security.

WSUD and SUDs are an efficient investment for water companies to reduce CSO events whilst avoiding high-cost sewerage upgrades and maintenance. These techniques mimic nature and typically manage rainfall close to where it falls. They can be designed to transport (convey) surface water, slow runoff down (attenuate) before it enters watercourses, they provide areas to store water and can be used to allow water to soak (infiltrate) into the ground or evaporated from surface water and lost or transpired from vegetation (known as evapotranspiration). SUDs can be highly engineered with pipes and hard materials reminiscent of traditional drainage networks or they can include soft engineered solutions which utilise greened open channels and depressions to store water. These techniques tend to be cheaper than traditional drainage systems and provide additional benefits, enhancing air quality and biodiversity creating better places to live.

WSUD has been integral in Auckland’s continuing development, however incorporating stormwater management systems into new projects alone will not have much impact on the sewage overflows. There is a need for a strategy which includes historic installations and retrofitting into existing urban areas throughout the city.

Many examples from around the world show that reducing surface water from entering an existing drainage system, through retrofitting, is more cost effective than increasing drainage capacity (for example the Green City, Clean Waters in Philadelphia).

A flood alleviation scheme in Cheltenham, UK retrofitted swales and rain gardens into a 1960s housing development, to maximise surface water interception, attenuation and infiltration to reduce flooding and improve water quality in the nearby river. The scheme which was delivered in partnership with the Environment Agency and the council encouraged Local residents to be involved with decisions on design and future management of the features which included rain gardens and swales.

 Similarly, a greening streets project in Nottingham retrofitted 21 linear rain gardens on grass verges in residential areas. Installation of WSUDs has been proven to be a cost-effective way of improving water quality as well as reducing flooding, there is an abundance of unused green space in Auckland suited to WSUD.

Rainwater harvesting systems are another means of capturing and attenuating which would buffer the system during wet weather and therefore be part of the solution to CSOs. New technology means tanks can now be plumbed into existing pipework and the rainwater used to flush toilets and wash clothes. Capable of filtering and storing up to 6,500 litres of clean water it is possible to reduce a household’s water consumption by as much as 40%, (Rainwater Harvesting Association) which will in lower your water bills as well as reducing the impact of stormwater events. While such systems are frequently used in rural areas they are not often used in the urban environment.

Reducing the frequency and magnitude of CSO events will be key in restoring the amenity of Auckland’s waterways and beaches. To achieve this a combination of strategies will need to be employed which will holistically manage water in the urban catchment. It must cover the whole system including old and new neighbourhoods. Such a pervasive problem will only be mitigated with a combination of measures, the interceptor infrastructure upgrades will provide CSO storage, retrofitting and creating new sustainable drainage features into our streets and homes and uptake of rainwater harvesting systems will increase rainwater attenuation thereby reducing pressure on the combined system.  The final important mechanism will be to educate Auckland’s population about the cities water issues to enable customers to change behaviour, enhancing incentives for customers to reduce surface flowing to sewers and innovative permitting arrangements across drainage networks and sewage treatment works.

By Rhianna Drury