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

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, [email protected]

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. ARC’s 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



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 <15 mm

depth. Peak flow control may be manipulated by altering drainage layer design (roughness

or efficiency) or manipulating roof configuration to extend drainage paths. Hence living roof

size influences performance assessment, as does the duration of monitoring, specifically the

size of the largest storms measured.



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 (<200 mm height) native New Zealand plants can survive

on fully-exposed extensive roofs without summer irrigation, create a dense cover that resists

colonisation by adventive plants, and adequately fulfil the services required for storm water

mitigation. Biodiversity is enhanced by adventive plants, including native orchid species and

native annuals (e.g., Senecio, Pseudognaphaliums and Crassulas) adapted to open ground.

However many adventive plants have potential to be invasive, particularly legumes (all non-

native).

The invertebrate fauna recorded on the two living roofs is typical of open sites, uncommon in

undisturbed New Zealand ecosystems, and limited by low humidity and moisture of both

surface layers and substrates (e.g., wood). However, introduced earthworms and several

native wood-eating native beetle species have persisted on the University roof where wood

rounds created humid refuges. A super-humid ceramic refuge with basal reservoir placed on

the Waitakere roof in 2010 has supported black field crickets (Teleogryllus commodus).

Some moths with restricted plant hosts have colonised, e.g. Magpie moth (Phrissogonus

laticostatus) has established on Senecios and Bedellia psamminella has colonised

Dichondra. There is therefore potential to introduce further species if irrigation can be used

to ensure survival of host plants.



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.