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Hydraulic Neutrality – Effective policies benefit us all

NZ Hydraulic Neutrality

Stormwater Detention Pond

Hydraulic neutrality is a tool that is applied around the world to reduce the impact of urban development on our waterways.

Hydraulic neutrality seeks to address increased runoff rates generated in developed areas, compared to undeveloped areas. In a number of New Zealand and international jurisdictions, stormwater volume is managed in addition to peak flow. Urban development typically results in increased stormwater runoff volumes and faster runoff. This can have negative impacts on our waterways such as increasing the likelihood or magnitude of downstream flooding and changing the streams' morphology due to increased erosion. Stormwater runoff also impacts the quality of our water bodies as runoff collects and conveys various pollutants to our streams.

Stormwater network owners (often District and City Councils) are responsible for ensuring their networks aren’t impacting the environments that they discharge into. The requirement for all new developments to be hydraulically neutral is a common approach used by many stormwater network owners and environmental agencies within New Zealand and internationally to manage these impacts.

"When stormwater management, including hydraulic neutrality, is approached in a holistic way, the benefits are diverse and include improvements to the mauri of watercourses and habitat of mahinga kai species. We can also improve amenity and recreational value where stormwater management is integrated with public open space."

The intention is logical – if we provide sufficient storage on-site, such that there is no change in peak flows leaving new development sites, then it stands to reason that there should be no impact on our receiving environments. Unfortunately, this is not always the case. In practice, the superposition of attenuated peaks can cause increased flooding downstream, and sustained flows at the pre-development peak flow rate can increase the rates of erosion in our waterways.

Still – hydraulic neutrality and variations of it remain our primary tool for the management of stormwater quantity. As the installation of stormwater attenuation devices is often seen as an 'additional' cost to development, it is essential that policies for hydraulic neutrality work, and that they achieve the intended outcomes for our receiving environments.

The best approaches to stormwater management take into consideration any known issues or opportunities within the catchments and seek to address them. Volume management such as retention of frequent events can assist with reducing the erosive power of streams, which maintains groundwater recharge and stream baseflows; as well as reducing the likelihood of superposition of attenuated peaks downstream. This is in addition to water quality benefits.

Stormwater network owners should also consider the tools that are available for developers to determine their attenuation requirements as this may assist in ensuring policy is applied in an effective way. A recent review of a number of newly developed sites found that the rational method is frequently applied incorrectly when calculating peak flows and that various approaches are applied when calculating attenuation volumes.

For more information on hydraulic neutrality, or potential policy or tools to assist in managing the impacts of stormwater runoff, please contact Cardno’s Senior Environmental Scientist, Carrie Hopkirk.

Carrie Hopkirk

Senior Environmental Scientist

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Carrie is a Senior Environmental Scientist based in Cardno’s Wellington office. Carrie has expertise in a broad range of hydrological studies, including studies in stormwater management, Water Sensitive Urban Design, flood management, water quality, water supply scoping, source vulnerability and hydropower.