Manage water to optimise ecosystem services of living roofs - Robyn Simcock

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NZ findings in storm water mitigation, aesthetics and biodiversity Auckland, New Zealand has one of the most favourable climates in the world for effective bioretention, receiving around 1100 mm/year as frequent small rain events totalling and long growing season with mean daily temperatures rarely higher than 25 C or below 5 C. Bioretention has been promoted in Auckland since the early 2000s by Auckland Regional Council (ARC, Mr Earl Shaver) primarily for storm water mitigation. ARCs Technical Publication 10 (TP10) Storm Water Management Devices: Design Guidelines Manual (ARC 2003) included green or living roofs but in the absence of local living roofs, lacked detailed design guidance and performance evidence relative to more common bioretention devices such as rain gardens and swales.

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  • Marie-Curie IAPP Green Roof Systems Project

    The Green Roof Research Conference 18-19 March 2013, Sheffield

    Manage water to optimise ecosystem services of living roofs: NZ findings in storm water mitigation, aesthetics and biodiversity

    Robyn Simcock1, Elizabeth Fassman2, Emily Voyde 2, Renee Davies3

    and Yit Sit Hong2

    1Landcare Research NZ Ltd, 2 Department of Civil and Environmental Engineering,

    University of Auckland, 3 Department of Landscape Architecture, Unitech Institute of

    Technology, SimcockR@landcareresearch.co.nz

    Introduction

    Auckland, New Zealand has one of the most favourable climates in the world for effective

    bioretention, receiving around 1100 mm/year as frequent small rain events totalling and long

    growing season with mean daily temperatures rarely higher than 25 C or below 5 C.

    Bioretention has been promoted in Auckland since the early 2000s by Auckland Regional

    Council (ARC, Mr Earl Shaver) primarily for storm water mitigation. ARCs Technical

    Publication 10 (TP10) Storm Water Management Devices: Design Guidelines Manual (ARC

    2003) included green or living roofs but in the absence of local living roofs, lacked detailed

    design guidance and performance evidence relative to more common bioretention devices

    such as rain gardens and swales. In 2007 two Auckland Councils sponsored design,

    construction and monitoring of two full-scale extensive living roofs to quantify their ability to

    mitigate storm water and support native ecosystems. Ecosystem services such as surface

    temperature moderation, provision of native plant and invertebrate biodiversity, and

    aesthetics were also of interest as stacked benefits.

    Figure 1 Waitakere Civic Centre Living roof, December 2012 aged 6.5 years, 100 mm media

    depth (with full irrigation since 2010) is visually dominated by tussock (Festuca coxii), NZ iris

    (Libertia peregrinans) and Astelia banksii

  • Marie-Curie IAPP Green Roof Systems Project

    The Green Roof Research Conference 18-19 March 2013, Sheffield

    Overview of Methodology

    Growing media were developed using locally abundant volcanic aggregates and imported

    expanded clay, as no proprietary media were available. Media were a blend of 1-10 mm

    pumice and zeolite with 15 to 20% v/v organic material, largely based on composted bark.

    In 2007 3 media were installed on two roofs at 50 to 120 mm depth and planted with both

    New Zealand native, largely endemic plants and/or non-native sedums. Five small sheds

    were constructed in 2009 to performance-test a more resilient medium that applied lessons

    gained from specification, blending and installation. This allowed more plant species to be

    trialled under conditions of minimal irrigation. A hydrological balance was developed using

    continuous monitoring of runoff and rainfall for 8 to over 24 months, supported by short-term,

    in-situ and glasshouse evapotranspiration measurement. Comparing runoff from the

    different roofs demonstrated the sensitivity of storm water performance to rainfall event size,

    season, media depth (retention volume) and scale. Plant mortality, cover, and diversity of

    planted and adventive species, were used as indicators of aesthetic values. Invertebrate

    fauna was monitored for one month in most summers using pitfall and emergence traps.

    Figure 2 University of Auckland Roof plot 2, established with sedum mat on 50 mm media in

    April 2008, about 1 year after placement (left), October 2011 (centre) and December 2012 (right)

    Key Findings

    The extensive living roofs achieved up to 56% cumulative storm water retention, with

    relatively consistent seasonal performance. Rainfall depth, rather than media depth, had the

    greatest influence on runoff retention where moisture retention of all media, even at 50 mm

    depth, was greater than common storm depths; 80% of Auckland storms are

  • Marie-Curie IAPP Green Roof Systems Project

    The Green Roof Research Conference 18-19 March 2013, Sheffield

    Figure 3 Runoff of the studied living roofs (Fassman-Beck et al. in press)

    Healthy plant cover is the key determinant of living roof aesthetics, and a transpiring plant

    cover provides effective storm water mitigation, particularly for successive rain events.

    Transpiration removes stored water, renewing pore space for rainfall storage. Interception

    probably has a very minor role. In summer, sedum transpiration reduces markedly within 2 to

    4 days of an event that restores nominal field capacity. Regular, strategic irrigation and/or a

    combination of sedums and plants lacking a strong response to decreased water availability

    may enhance both extensive living roof aesthetics and stormwater performance. Plant

    moisture needs, not storm water mitigation, determine media depth in the Auckland climate.

    This creates excess storm water volume capacity and potential to use on-flow from adjacent

    roofs.

    A very narrow range of low-stature (

  • Marie-Curie IAPP Green Roof Systems Project

    The Green Roof Research Conference 18-19 March 2013, Sheffield

    Further Reading

    Davies R, Toft R, Simcock R. 2012: Biodiversity opportunities for a NZ indigenous living roof. World

    Green Roof Congress, Copenhagen. 18-21 September 2012.

    Fassman-Beck E, Simcock R, Voyde E, Hong Y (in press). 4 living roofs in 3 locations: does

    configuration affect runoff mitigation? Journal of Hydrology

    Fassman E, Simcock R 2012: Moisture Measurements as Performance Criteria for Extensive Living

    Roof Substrates. Journal of Environmental Engineering.

    Fassman E, Simcock R, Voyde E. 2011: Extensive Living Roofs for Stormwater Management. Part 1:

    Design and Construction Auckland UniServices Technical Report to Auckland Regional Council.

    Auckland Regional Council TR2010/017.

    http://www.aucklandcouncil.govt.nz/SiteCollectionDocuments/aboutcouncil/planspoliciespublications/t

    echnicalpublications/tr2010017greenroofsstormwatermitigation.pdf

    Lewis M, Simcock R, Davidson G. 2010:Landscape and ecology values within stormwater

    management. Auckland Regional Council Technical Report TR2009/083.

    http://www.aucklandcity.govt.nz/council/documents/technicalpublications/TR2009083.pdf

    Voyde E, Fassman E, Simcock R. 2010: Hydrology of an extensive living roof under sub-tropical

    climate conditions in Auckland, NZ. Journal of Hydrology 394:384-395.

    Voyde E, Fassman E, Simcock R, Wells J. 2010: Quantifying Evapotranspiration Rates for New

    Zealand Green Roofs. Journal of Hydrologic Engineering 15(6):395-403.

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