Faculty of Engineering


Living roofs for stormwater management

We have set out to determine how to build living roofs suitable for the Auckland climate (substrate, plants, etc.), to measure what impact a living roof has on runoff, and how the design influences those impacts.

 

Our cities are covered in impervious surfaces such as roads, paving and buildings. When it rains, water runs off these surfaces and the runoff directly enters streams, lakes and harbours via the stormwater drains. Runoff results in pollutants washing into the water ways and increases the potential for stream channels to erode. Amount and speed of runoff are critical factors.

b-iande-livingroofs1

Living or green roofs are planted roof top areas that act to mitigate the effects of runoff. Vegetation reduces the amount of runoff as well as slowing it down. The substrate that the plants are anchored in absorbs and retains water which is used by the plants during dry periods.

cl-iande-living-roof-compnts1

 The components of a living roof are:

  • Vegetation (intercepts rainfall, releases moisture to the atmosphere).
  • Substrate (plant growth, absorbs rainfall).
  • Non-woven geotextile filter.
  • Drainage layer (drains to gutter system).
  • Waterproof layer.
 

Key focus areas/issues


This project aimed to quantify how well a living roof controls stormwater and included measurement of:

  • Three different substrates.
  • Two different substrate depths.
  • Individual and cumulative rainfall events.
  • Plant establishment and influence on performance.

This project has been made possible with the support of the Auckland Council.

Current major developments


The roof of the Faculty of Engineering Building in central Auckland has had a living roof for four years. The 235m2 area, shown at the top of the page, contains a mixture of succulent plants (Sedum species) and New Zealand natives adapted to survive when there is little rain.

Three different substrate types (Pumice, Zeolite and Expanded Clay, all pumice based but named for their distinguishing components) at depths of 50 and 70m have been compared side by side.

We have also built living roofs at several other sites, with two at the Auckland Botanic Gardens shown below. We hope to monitor these for impact on runoff as well as carrying out water quality testing.

cl-iande-living-roof-bt-gdn1

Key achievements


Field monitoring of the living roof at The University of Auckland has been carried out over about a 24 month period (see the first two references below for detail). 

No statistically significant differences in runoff response were found between the three substrate types tested or the two different depths. The cumulative retention efficiency of the living roof was 72%. The implication is that locally sourced, naturally occurring materials can provide an excellent basis for stormwater design, even at shallow depths.

On an event basis, the living roof demonstrated reductions in both volume and peak flow rates regardless of the rainfall and climatic characteristics. The living roof retained a median of 82% of rainfall received per rainfall event, with a median peak flow reduction of 93% compared to the peak flow that might be generated from a conventional rooftop. The hydrologic response of a living roof is controlled by multiple parameters such as rain depth, rain intensity, climatic variables and antecedent dry days. Detailed analysis indicates that antecedent dry days have the greatest influence on retention. Seasonal differences do not influence runoff response; living roofs will effectively moderate runoff hydrology year round in Auckland’s sub-tropical climate.

A technical report documenting the extensive testing and recommendations for living roof design and construction has been prepared. The report focuses on using readily-available, locally sourced materials to build living roofs for stormwater management in New Zealand (see the third reference below for detail).

Key people


  • Elizabeth Fassman
    Civil and Environmental Engineering
  • Robyn Simcock
    Landcare Research
  • Emily Voyde
    PhD Candidate, Civil and Environmental Engineering
  • Hayden Easton
    Auckland Council

Contact


Elizabeth Fassman
Email: e.fassman@auckland.ac.nz
Phone: +64 9 373 7599 extn 84540

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Related publications


Voyde E., Fassman E., Simcock R., 2010. Hydrology of an extensive living roof under sub-tropical climate conditions in Auckland, New Zealand, Journal of Hydrology, 394(2010):384-395. 

Voyde, E.A., Fassman, E.A., Simcock, R., Wells, J. 2010. Quantifying Evapotranspiration Rates for New Zealand Green Roofs. Journal of Hydrologic Engineering 15(6):395-403.

Fassman, E.A., Simcock, R., Voyde, E.A., 2010. Extensive Green Roofs for Stormwater Management. Part 1: Design and Construction. Auckland UniServices Technical Report to Auckland Regional Council. Auckland Regional Council TR17/2010.

Fassman, E.A., Simcock, R., Voyde, E.A., 2010. Extensive Green Roofs for Stormwater Management. Part 2: Performance Monitoring. Auckland UniServices Technical Report to Auckland Regional Council. Auckland Regional Council TR18/2010 (in preparation).

Fassman, E., Simcock, R., and Voyde, E., 2009. ‘Achieving Multiple Outcomes with a Single Technology: Stormwater and Energy Benefits of Green Roofs’ EWRI Thailand 2009; An International Perspective on Environmental & Water Resources Conference, Bangkok, Thailand 5-9 Jan.

Fassman, E. and Simcock, R., 2008. ‘Development and Implementation of a Locally-Sourced Extensive Green Roof Substrate in New Zealand’ World Green Roof Congress, London 16-17 Sept.

Simcock, R., Smale, M., Ross, C., Whitelaw, L., Dando, J., Lamert, R., and Trowsdale, S., 2006. "Developing an Inigenous Greenroof for Waitakere City Council." Stormwater '06, Rotorua, NZ, 7.

Simcock, R., Dando, J., Ross, C., and Smale, M., 2005. "Waitakere Indigenous Green Roof: Substrate Selection and Plant Species." LC0405/148, Landcare Research.