Journal of Hydrology 490 (2013) 11–20

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Journal of Hydrology

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4 Living roofs in 3 locations: Does configuration affect runoff mitigation?

0022-1694/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jhydrol.2013.03.004

⇑ Corresponding author. Tel.: +64 9 373 7599; fax: +64 9 373 7462.E-mail addresses: e.fassman@auckland.ac.nz (E. Fassman-Beck), simcockr@land-

careresearch.co.nz (R. Simcock).1 Tel.: +64 9 373 7599; fax: +64 9 373 7462.

Elizabeth Fassman-Beck a,⇑, Emily Voyde a,1, Robyn Simcock b, Yit Sing Hong a,1

a University of Auckland, Department of Civil and Environmental Engineering, Private Bag 92109, Auckland Mail Centre, Auckland 1142, New Zealandb Landcare Research, Private Bag 92170, Auckland Mail Centre, Auckland 1142, New Zealand

a r t i c l e i n f o

Article history:Received 3 September 2012Received in revised form 4 March 2013Accepted 4 March 2013Available online 25 March 2013This manuscript was handled byKonstantine P. Georgakakos, Editor-in-Chief,with the assistance of Michael Bruen,Associate Editor

Keywords:Living roofGreen roofHydrologyStormwater runoffFlow attenuationUrban runoff

s u m m a r y

Four extensive living roofs and three conventional (control) roofs in Auckland, New Zealand have beenevaluated over periods of 8 months to over 2 yrs for stormwater runoff mitigation. Up to 56% cumulativeretention was measured from living roofs with 50–150 mm depth substrates installed over syntheticdrainage layers, and with >80% plant coverage. Variation in cumulative %-retention amongst sites isattributed to different durations of monitoring, rather than actual performance. At all sites, runoff rarelyoccurred at all from storms with less than 25 mm of precipitation, from the combined effects of sub-strates designed to maximize moisture storage and because >90% of individual events were less than25 mm. Living roof runoff depth per event is predicted well by a 2nd order polynomial model(R2 = 0.81), again demonstrating that small storms are well managed. Peak flow per event from the livingroofs was 62–90% less than a corresponding conventional roof’s runoff. Seasonal retention performancedecreased slightly in winter, but was nonetheless substantial, maintaining 66% retention at one site com-pared to 45–93% in spring-autumn at two sites. Peak flow mitigation did not vary seasonally. During a 4-month period of concurrent monitoring at all sites, varied substrate depth did not influence runoff depth(volume), %-retention, or %-peak flow mitigation compared to a control roof at the same site. The mag-nitude of peak flow was greater from garden shed-scale living roofs compared to the full-scale livingroofs. Two design aspects that could be manipulated to increase peak flow mitigation include lengtheningthe flow path through the drainage layer to vertical gutters and use of flow-retarding drainage layermaterials.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction Department of Defence, 2010) requires on-site retention of the

Stormwater runoff water quality treatment, peak flow mitiga-tion, or on-site retention (volume control) is increasingly beingregulated for the ‘‘frequently occurring’’ events. For design, this of-ten translates to stormwater control measures (SCMs) that aresized to capture up to 80–90% of the annual runoff volume (Califor-nia EPA, 2009; City of Santa Monica, 2012; MARC and APWA, 2008;Maryland Dept. of Environment, 2009; City of Portland, 2008; Phil-adelphia Water Department 2009; Metro Water Services 2012;Government of Western Australia, 2009; Vermont Agency of Natu-ral Resources, 2002; Washington Dept. of Ecology, 2011), the 1-yr,24-h design storm (Atlanta Regional Commission, 2001; Philadel-phia Water Department, 2009; Virginia Dept. of Conservation andRecreation, 2004), or runoff generated from 12.5–25.4 mm of rain-fall (Kane County, 2001; City of Chicago, 2012; New York Depart-ment of Conservation, 2012;City of Pittsburgh, 2007, 2010;Virginia Dept. of Conservation and Recreation, 2004; MMSD,2012). The 2007 Energy Independence and Securities Act (USA

95th percentile event for USA federal facilities undergoing newor re-development. In Brooklyn, NY, the majority of combined sew-er overflows (CSOs) occurred from rainfalls less than 25 mm(Mayor’s Office of Long-Term Planning and Sustainability, 2008).Space restrictions, subsurface infrastructure, and poor infiltrationpotential (e.g. from compaction resulting from construction activ-ities or high water tables) renders difficult on-site retentionthrough ground-level SCMs in dense urban centers.

Living roofs provide a unique opportunity to manage these fre-quently occurring rainfall events since rooftops comprise a signif-icant proportion of the total impervious area in urban settings.Considerable opportunity exists to reduce runoff volume and peakflow from new construction or retrofit of existing building stock,particularly low-pitched roofs comprising gravel ballast over mem-branes. Living roofs offer two advantages for urban stormwatermanagement: they act as at-source control to prevent runoff gen-eration from an otherwise impervious area, and they provide astormwater management opportunity in otherwise usually unusedspace (rather than valuable ground space). In Auckland, New Zea-land, the term ‘living roof’ has been adopted to acknowledge the vi-tal role of plants in providing environmental and aesthetic benefits.An extensive living roof (6150 mm substrate [growing media]

12 E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20

depth, usually designed for function rather than form) is also com-monly referred to as green roof, eco-roof, or vegetated roof, while adeeper (>150 mm substrate depth) intensive system is known asroof garden or landscape over structure.

Hydrologic performance monitoring studies of full scale fieldinstallations of extensive living roofs report cumulative retentionof 50–78% (Berghage et al., 2009; Carter and Rasmussen, 2006; Car-penter and Kaluvakolanu, 2011; Hutchinson et al., 2003; Mentenset al., 2006; Moran et al., 2005; Villarreal and Bengtsson, 2005;Voyde et al., 2010), and average or median event-based peak flowmitigation of 50–93% (Hutchinson et al., 2003; Moran et al., 2005;Bergage et al., 2007; Carpenter and Kaluvakolanu, 2011; Voydeet al., 2010). Comparisons are often made against direct rainfallor runoff from a conventional (control) roof. Climate factors suchas rainfall depth and intensity, antecedent dry period, solar radia-tion, temperature, and evapotranspiration influence performance(Carpenter and Kaluvakolanu, 2011; Kasmin et al., 2010; Voydeet al., 2010), as do substrate moisture storage capacity and depth(Fassman and Simcock, 2012).

Widespread variation in living roof design across studies com-plicates understanding of performance. Likewise, the duration ofstudy periods and climate differences preclude robust comparisonsamongst systems. Since 2006, several living roofs have been de-signed, constructed, and monitored across Auckland through sup-port from the former Auckland Regional Council, formerWaitakere City Council (WCC), and the Foundation for Research,Science, and Technology, with on-going support from AucklandCouncil. In this paper, monitoring of four living roofs and threecontrol roofs across the same city is used to link extensive livingroof design to stormwater management performance. Field resultsare interpreted with respect to physical properties of the substrateas well as the configuration (geometry) of the roof surface.

2. Methodology

Auckland, New Zealand is a sub-tropical climate with approxi-mately 1200 mm average annual rainfall, distributed relativelyevenly throughout the year over 137 ‘‘wet’’ days (defined asP1 mm of precipitation) (NIWA Science, 2007). The studied livingroofs are located within 20 km of each other. As a living roof ‘‘re-places’’ a conventional roof whose runoff would normally be man-aged by an at- or below-grade SCM, the performance of each livingroof is assessed against a conventional (control) roof monitored atthe same location, or modeled from measured rainfall.

The studied living roofs are all classified as extensive livingroofs, with substrate 50–150 mm deep. The substrates are 80% byvolume (v/v) light-weight aggregate and 20% v/v organic matteras recommended minimum and maximum values, respectively,for extensive living roofs (FLL, 2008). Each substrate was installedover a synthetic drainage layer with geotextile providing separa-tion. Details on design and construction are provided Fassmanet al. (2010a,b) while detailed investigation of substrate character-istics are found in Fassman and Simcock (2012). Each living roofhad >80% plant coverage during the monitoring periods. Detailsof control roofs are provided in each sites’ description.

2.1. University of Auckland living roof

The University of Auckland (UoA) living roof was constructed inSeptember and October 2006 on level 13 of the Faculty of Engi-neering in Auckland’s central business district. The retrofit projectreplaced a thin gravel ballast roof with a 217 m2 living roof (94% oftotal roof area), including a gravel edge (200–800 mm width).Three substrate recipes were developed in the laboratory for fieldtesting (Fassman and Simcock, 2008, 2012; Fassman et al., 2010).

Each substrate consists of volcanic pumice (40–80%), and 20% com-posted pine bark fines. Where the pumice was <80% v/v, either nat-ural zeolite (30% v/v) or manufactured expanded clay (40%)completed the mixture. Hydraulically isolated but adjacent plotsallow comparison between substrate types and two finished sub-strate depths: 50 mm and 70 mm, as per Voyde et al. (2010). Indi-vidual plot surface area ranges 17–54 m2. Average roof slope is1.2% towards vertical drainage points. A combination of New Zea-land- native and non-native (Sedum species) plants was initiallyplanted at 18 plugs/m2. Plant cover reached at least 80% by thetime monitoring commenced in 2008.

Rainfall data were collected on site using a Sigma 2149 0.25 mmtipping bucket rain gauge. Runoff was measured from each of thehydraulically isolated plots using a Global Water WL16USB pres-sure transducer rated for 0–0.91 m depth and custom-designedorifice restricted device (ORD). If the ORD fills to its maximumcapacity (1 L/s outflow at 751 mm maximum water depth) thenthe maximum storage provided by the ORD is 722 mL, which isequivalent to 0.02–0.06 mm across each plot (depending on plotsize), and thus is not considered to interfere with measurement(Voyde et al., 2010). All data were logged continuously at 5-minintervals for 28 months. A regularly-cleaned wire mesh screen pro-tected the ORDs from clogging.

Site limitations prevented direct measurement of a conven-tional roof surface (control roof) at the UoA site. Runoff volumefrom a control roof was modeled as 75% of total rainfall received,as per a traditional roof surface with 50 mm gravel ballast (Men-tens et al., 2006). Peak flow was modeled using the Rational For-mula, with an assumed runoff coefficient of 0.75. The effect ofthe gravel ballast led to the adoption of a conservative valueamongst literature reports for conventional roof runoff coefficients(0.75–0.95) (Bedient and Huber, 2002; American Society of CivilEngineers, 1992; Viessman and Lewis, 2003).

2.2. Tamaki mini-roofs

A 70% v/v pumice, 10% v/v natural zeolite, and 20% v/v organicmatter substrate blend was installed in May 2008 on four 4 m2

‘‘mini’’ living roofs at the Landcare Research office in East Tamaki,Auckland. The mini living roofs are built on reinforced gardensheds with Color Steel roofs. One additional shed was constructedwithout a living roof (a control roof) for concurrent monitoring.

Duplicate mini-roofs are covered with either 100 mm or150 mm depth of substrate. Approximately 20 New Zealand-nativeplant species and 18 non-native plant species were established asplugs, root trainers, pots or plants salvaged from rock walls at�22 plants/m2. Monitoring commenced approximately 1.5 yr afterconstruction and occurred over 14 months.

Runoff from each mini-roof was collected in a gutter draining toa down- pipe. Wire mesh over the opening of each down-pipe pre-vents clogging of the orifice. In the down-pipe, runoff was mea-sured using a similar ORD and Global Water WL16USB pressuretransducer arrangement as on the UoA living roof. Rainfall wasmeasured at 5-min intervals using a HOBO 0.2 mm tipping bucketrain gauge (December 2009–May 2010) or a Sigma 2149 0.25 mmtipping bucket rain gauge (October 2010–March 2011).

2.3. Waitakere civic center living roof

The Waitakere Civic Center (WCC) living roof was constructedin mid-2006 and modified in 2009. This 500 m2 nearly-flat roofhas a substrate blend of 60% v/v pumice, 20% v/v expanded clayand 20% v/v compost-based garden mix. Additional expanded claywas spread as a 4–8 mm deep mulch to provide an aestheticallypleasing cover until plants covered the surface. Only New Zealandnative plants were used on this roof.

E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20 13

In winter 2009, two areas of the roof with less than 100 mmdepth were amended with a 70% v/v pumice, 10% v/v natural zeolite,20% v/v compost blend based on specifications of the Tamaki mini-roofs. After supplementation, the majority of the living roof had aminimum 100 mm depth. Three isolated mounds with 150 mmaverage depth were also constructed. The mounds were intendedto support dense vegetation up to 500 mm height as refuges for na-tive skinks. The two amended areas were replanted with relocatedand new native species and lightly irrigated every 1–3 days through-out the subsequent two summers using a basal irrigation mat to pro-mote rapid growth of native vegetation. Specific irrigation recordswere not kept. Early monitoring indicated small, regular runoffhydrographs from irrigation. Consultation with the maintenancemanager resulted in substantially reducing the amount of water ap-plied through irrigation, eliminating its runoff.

Adjacent to the living roof, but one story higher is a conven-tional (control) roof surface with a bituminous waterproofing sheetmembrane system with a plain sand finish (Soprema Flam 180/Soprema Jardine 2 layer torch-on membrane, according to designdrawings). Runoff monitoring from the living and control roofscommenced approximately 10 months after the site’s modification.

Approximately 171 m2 of the WCC extensive living roof drainsto a PVC pipe which channels runoff to a small weir box with a90� v-notch weir. Runoff from the 79 m2 control roof drains intoa downpipe and separate weir box. A rating curve was developedin the laboratory for each weir box using Global Water WL16USBpressure transducers (Fassman et al., 2010b). Water depth was re-corded by each logger at 5 min intervals over 8 months. Rainfallwas recorded on-site or was obtained from the National Instituteof Water and Atmosphere’s (NIWA) Waitakere Domain rain gauge(http://cliflo.niwa.co.nz/).

2.4. Statistical analysis

All results were statistically analyzed using SPSS to detect dif-ferences in means or distributions. Non-parametric Mann–Whit-ney U tests with Bonferroni correction and/or Kruskall Wallistests were used for all hydrologic analysis as data transforms failedto yield normal distributions according to Shapiro–Wilks and Kol-mogorov–Smirnov tests.

3. Results and discussion

Data are analyzed in terms of cumulative and event-based per-formance. Cumulative effects and frequency analyses includes alldata. The event-based analysis considers data for storms of at least2 mm. Smaller rainfall events (<2 mm) rarely generate meaningfulrunoff, but the predominance of occurrence creates significantskew in the data which might be considered as exaggerating thesummary performance. A conservative event-based analysis is pre-

Table 1Summary rainfall characteristics at monitoring sites.

UoA living roof Tamaki mini-roofs

Rainfall (mm) Frequency(# ofevents)

Totalrainfall(mm)

Cumulativeproportion oftotal rainfall(%)

Cumulativeproportion bynumber ofevents (%)

Frequency(# ofevents)

Totalrainfall(mm)

<2.0 198 126 5 50 87 502.0–5.0 71 234 15 68 25 815.2–10.0 53 414 32 81 17 12710.2–15.0 26 322 46 88 13 15815.2–25.0 30 562 69 95 9 17225.2–40.0 10 321 83 98 6 184>40.0 8 417 100 100 9 723Sum 396 2396 166 1494

sented by including only events with rainfall of at least 2 mm.Coincidentally, there are no meaningful differences in the event-based statistical assessments whether the analysis considers allstorms, or only those P2 mm rainfall depth. The hydrologic resultscompare performance for a full range of extensive living roofdepths (by definition up to 150 mm).

Rainfall events (Table 1) were defined by an inter-event dry per-iod P6 h (Shamseldin, 2010). The range of rainfall measured at eachsite (Table 1) was relatively consistent with long-term statistics forthe Auckland Region. Auckland’s average 80th–95th percentilestorm events range from 21.8 mm to 42.4 mm (Shamseldin, 2010).The majority of events occurred over up to 12 h, but several stormsat each site lasted for approximately two days.

When runoff from the previous rainfall event was still discharg-ing from a living roof at the start of the next rainfall event, eventswere combined as one larger event. From the UoA living roof, 28 ofthe 396 rainfall events analyzed were generated by combining twoor more individual events with extended runoff durations, whilecombinations were used for 8 of the 166 rainfall events from theTamaki mini-roofs, and 8 of the 79 rainfall events from the WCCliving roof.

Because the UoA living roof was constructed as isolated butadjacent plots, an initial statistical analysis was performed to as-sess the effects of substrate type and depth. Each of the plotsshowed a statistically significant difference (p < 0.001) betweenrunoff volume or peak flow and the modeled control roof. Amongstthe five living roof plots considered over 28 months of continuousmonitoring, a statistically significant difference was not detected interms of runoff volume or %-retention (p = 0.05), confirming find-ings in Voyde et al. (2010) which were based on one full year ofmonitoring at the same site. Statistically significant differences inpeak flow were observed between most plots (p < 0.05 in all cases);however, the analysis is likely confounded by the very small mag-nitude of actual peak flow. Factors affecting peak flow are dis-cussed in Section 3.3. Data from all plots were combined torepresent a single weighted average (by plot area) 217.4 m2 livingroof for subsequent analysis.

At the Tamaki mini-roofs, Kruskal–Wallis tests confirmed con-trol roof runoff was significantly different from living roof runofffor all sheds when either total volume or peak flow rates were ana-lysed (p < 0.001 in all cases). However, there were no statisticallysignificant differences among the living roof sheds (p = 0.05). Theconclusion applies to both total volume and peak flow analysis.For subsequent analyses, data from the living roof sheds were aver-aged to present runoff response from living roofs of either 100 mmor 150 mm substrate depth.

3.1. Event-based performance

Runoff depth and peak flow rate from all living roofs was signif-icantly lower than that measured or modeled from control roofs

WCC living and control roofs

Cumulativeproportionof rainfall(%)

Cumulativeproportion bynumber ofevents (%)

Frequency(# ofevents)

Totalrainfall(mm)

Cumulativeproportionof rainfall(%)

Cumulativeproportion bynumber ofevents (%)

3 52 33 20 3 429 67 10 34 7 54

17 78 14 106 21 7228 86 8 100 35 8239 91 8 166 57 9252 95 2 66 66 95

100 100 4 254 100 10079 747

Table 2Median (standard deviation) performance per rainfall category: living roof vs. control roof.

Performance measure Precipitation depth (mm) Monitoring site

UoA Tamaki 100 mm Tamaki 150 mm WCC

% Difference Peak Flow All events >2 90 (14) 62 (34) 74 (29) 84 (20)% Retention All events >2 76 (26) 56 (29) 66 (25) 72 (21)

2–25 84 (18) 60 (28) 71 (24) 73 (20)26–69 49 (14) 31 (25) 48 (19) 58 (16)>70a 22 (27) 39 (29) 32

a No events in this category at UoA; 5 events at Tamaki; 1 event at WCC precludes standard deviation calculation.

14 E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20

(p < 0.001 for all sites). Large performance data variability wasmeasured amongst events, as evidenced by relatively large stan-dard deviations (Table 2). For storms with greater than 2 mm ofrainfall, median %- peak flow mitigation ranged 62–90%, whilemedian %-retention ranged 56–76%, depending on the site. Therewas no statistically significant difference (p = 0.05) in either runoffdepth or peak flow among the Tamaki mini-roofs of differing sub-strate depths (100 mm vs. 150 mm).

Performance measures expressed as a %-difference have limiteduse for a designer or regulator, as each storm provides a differentinput against which to compare. Analysis of actual runoff depthsprovides greater insight into the utility of living roofs to mitigaterunoff over a range of storm events, and is presented in Section 3.4.

3.1.1. Frequency analysisSignificant variability in hydrologic response is seen at an indi-

vidual event scale. However, depth- and peak flow- frequencycurves provide a general picture of expected flows, inherently

Fig. 1. Runoff depth-frequency analysis based on all events monitored.

incorporating antecedent climate conditions, plant condition, andrainfall characteristics. Fig. 1 shows significantly (p < 0.001, allsites) lower distributions of runoff volume are generated fromthe living roofs than from the control roofs. Likewise, the livingroof peak runoff flow rate distributions were significantly(p < 0.001) less than peak runoff flow rates from the control roofs(Fig. 2). Although there were no statistically significant differencesamong the four Tamaki mini-roof sheds on a per event basis, thefrequency curves demonstrate that the 150 mm depth living roofconsistently showed slightly improved performance over the100 mm depth living roof. Since control roof and living roof curvesdo not overlap in any case, it is concluded that the living roofs al-ways produce less runoff than a conventional roof surface at thesame location.

3.1.2. Seasonal analysisSeasonal variation in living roof performance was originally

anticipated due to the change in temperature and rainfall

Fig. 2. Peak flow-frequency analysis based on all events monitored.

E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20 15

experienced with the change of season (Fig. 3). Seasons were de-fined based on the calendar year whereby the Southern Hemi-sphere spring is September–November, summer is December–February, autumn is March–May, and winter is June–August(NIWA Science, 2010). The 28-month dataset from the UoA livingroof was used for seasonal analysis, considering storms with atleast 2 mm of rainfall. For the Tamaki mini-roofs, gaps in the14 month monitoring period limited seasonal analysis to spring,summer and autumn datasets. The relatively short WCC monitor-ing period precluded seasonal analysis.

Median peak flow reduction does not show significant seasonalvariation at either site (Table 3). Performance declines slightly inautumn and winter, with the only statistically significant differ-ences measured between winter and summer and/or spring (eachp 6 0.006) for the UoA roof.

Median retention per event was 81–85% in spring and, 83–92%in summer, which decreased to 45–75% in autumn and 66% in win-ter (UoA only, insufficient winter data were available from theother two roofs) (Table 3). Winter performance compared to sum-mer and/or spring showed the only statistically significant %-reten-tion performance difference (p 6 0.006), which is attributed tostatistically greater winter living roof runoff depth (p = 0.024 [win-ter vs. spring]; p = 0.012 [winter vs summer], Fig. 4). Kruskal–Wal-lis tests indicated there were no statistically significant differencesbetween the seasons for rainfall depth (p P 0.029) or either controlroof runoff depth (p P 0.087). Lower winter performance is antic-ipated due to lower evapotranspiration and thus a reduced abilityof a living roof to dry out between storm events (Voyde et al., 2010;Voyde, 2011). These results differ somewhat from the analysis inVoyde et al. (2010) which did not find any seasonal variation inperformance based on 1 year of data at the same site. The change

Table 3Median living roof performance when compared to control roof runoff for rainfall events

Performance measure Living roof site Spring Summer Autumn

% Retention UoA 83 92 75Tamaki 100 85 83 45Tamaki 150 81 88 48

% Peak Reduction UoA 95 98 91Tamaki 100 66 66 63Tamaki 150 71 70 82

Event Count UoA 63 38 32Tamaki 22 16 12

a Modeled for UoA living roof from on-site rainfall, measured for Tamaki mini-roofs.b The statistical test assesses differences in distributions rather than mean or median va

leading to lower Autumn %-retention, yet similar distributions to Spring and Summer.c Insufficient data for Winter performance comparison.

Fig. 3. Auckland’s climate by season.

in results demonstrates the importance of long-term monitoring.Research in Rock Springs, Pennsylvania (USA) showed a larger var-iation in seasonal living roof performance (Berghage et al., 2009).Pennsylvania, USA experiences >10 �C cooler and drier winterscompared to Auckland, in which plant ET will be negligible in theformer (Sanderson et al., 2004).

3.2. Cumulative runoff retention

Cumulative retention by the living roofs over individual moni-toring periods ranged from 39% to 57% (Table 4), demonstratingthe considerable benefit of living roofs at the site scale. From theUoA living roof, there was no meaningful difference (�1%) in per-formance when the total monitoring period was broken down intoindividual years, despite minor differences in annual precipitation(982 mm for 2008–2009 vs. 1233 mm for 2009–2010).

The largest storm monitored on the UoA living roof was lessthan the 2-y 24-h return period event, which is 75 mm in centralAuckland (Auckland Regional Council, 1999). Although the UoA liv-ing roof demonstrated significant annual retention for the moni-tored period, the overall volume of rainfall retained would likelydecrease somewhat as the frequency of events increased (pro-longed very wet conditions) or if a large storm event occurs, aswas the case at the Tamaki mini-roofs with five events larger than70 mm rainfall. Large storm events typically occur infrequently.Back-of-the-envelope estimation suggests that even if the totalamount of living roof runoff increased by over 200 mm (for exam-ple, if there were two 100 mm storms in each year of monitoring),and if the UoA living roof provided no retention for either event (allrainfall becomes runoff), cumulative retention compared to rainfallwould still be approximately 60%. This estimation does not takeinto account any effects on subsequent storms, but it is useful todemonstrate that on a cumulative, long-term basis, the living roofcould provide substantial runoff mitigation even if infrequent,large storms were poorly retained.

The volume of rainfall captured in an event depends on the sub-strate’s ability to store rainfall against gravity drainage (Fassmanand Simcock, 2012) for subsequent evapotranspiration. Table 5provides the water retention potential of each substrate, basedon characteristics of the specific media combined with installeddepth (Fassman and Simcock, 2012). The Tamaki mini-roofs havethe greatest moisture storage capacity and UoA roof the least stor-age capacity. However, these differences in storage are not re-flected in their long-term rainfall attenuation; the shallow UoAroof appears to perform as effectively as the deeper roofs. The addi-tional potential storage from deeper substrates does not provideadditional measurable hydrologic mitigation because of the pre-dominance of events with rainfall depths substantially less thanthe total storage capacity; more than 95% of individual rainfallevents were less than 40 mm depth, more than 90% of individual

P2 mm depth.a

b Winter Statistically significant differences

66 Winter vs. spring (p = 0.006); winter vs. summer (p < 0.001)None amongst summer, spring, autumnc

None amongst summer, spring, autumnc

87 Winter vs. spring (p = 0.006); winter vs. summer (p < 0.001)None amongst summer, spring, autumnc

None amongst summer, spring, autumnc

65

lues. 5 Of 12 monitored events had rainfall depth >20 mm at the Tamaki mini-roofs,

Fig. 4. Rainfall, living roof and control roof runoff depths based on rainfall events with at least 2 mm depth: (a) UoA and (b) Tamaki.

16 E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20

rainfall events were less than 25 mm depth, and more than 80% ofindividual events delivered less than 15 mm precipitation.

While stormwater mitigation may not be substantially im-proved by increasing depth, it will improve plant viability. In-creased depth increases plant-available moisture, hence reducing

potential irrigation requirements (Fassman et al., 2010; VanWoertet al., 2005), and allows for taller plants and a wider range of spe-cies to be grown (Fassman and Simcock, 2012). Greater water stor-age extends the duration plants can freely transpire (cooling the airabove a living roof) before becoming stressed and reducing

Table 4Summary of runoff response from the UoA, Tamaki, and WCC living roofs for the full monitored period for each site.

Location Substrate depth(mm)

Monitoring length Cumulative rainfall(mm)

Number of stormevents P 75 mm

Cumulative living roof retention vs.control (%)

UoA 50–70 28 Months 22 September 08–27 December 10 2395.6 0a 56Tamaki 100 14 Months December 09–May 10 and August

10–March 111494.4 5 39

150 53WCC 100 8 Months August 10–March 11 746.6 1 57

a Maximum storm size 74.4 mm.

Table 5Living roof event-based water storage potential according to installed depth.a

Measure of event-based water storage potential UoA Tamaki WCC

50 mm 70 mm 100 mm 150 mm 100 mm

Plant-available waterb 11.3–12.0 14.3–16.2 28.9 35.8 20.2Maximum water holding capacityc 24.2–24.5 28.8–33.3 63.0 94.5 n/a

a After Fassman and Simcock (2012).b Tension test (Gradwell and Birrell, 1979) over 10–1500 kPa (Hillel, 1971).c Moisture content to oven dry at 105 �C after 24 h water-bath soak followed by 2-h draining (FLL, 2008).

E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20 17

transpiration (Voyde, 2011). It also may provide resilience againstclimate change in areas where rainfall is predicted to decline.

Whether the apparent difference in long-term retentionamongst sites is attributed to the differences in climate (i.e. differ-ence in size, duration and frequency of rainfall events) during non-concurrent monitoring periods, or is due to system design, is fur-ther investigated in the Section 3.3.

3.3. Hydrologic performance across a range of living roof designs

Substrates’ water holding capacity and installed depth havesubstantial impact on runoff retention (Fassman and Simcock,2012). Peak flow is influenced by substrate permeability and thetravel time through the drainage layer to vertical drainage points(assuming adequate rainfall occurs to exceed storage capacityand percolate through the substrate, with no surface flow acrossthe vegetation). Relative properties of each living roof design arepresented in Fig. 5 (Fassman et al., 2010; Simcock et al., 2006).All four living roofs monitored had P80% plant coverage indicativeof a fully established roof. The control roofs also varied in rough-ness and retention/detention capabilities (Section 2).

The four living roofs’ and three control roofs’ performance wascompared over a four month period (August–December 2010) dur-ing which all sites were monitored concurrently. Although eachsite exhibited localized minor variation in rainfall patterns duringthe four months, the sites experienced very similar rainfall charac-

Fig. 5. Qualitative comparison of living roof design components.

teristics per event; rainfall depth (p = 0.965) and peak rainfallintensity (p = 0.233) were not significantly different, nor was con-trol roof runoff depth (p = 0.148, coincidentally supporting the ap-proach of modeling a control roof at UoA). Control roof runoff peakflow rates were significantly greater at Tamaki than WCC(p = 0.015).

The UoA and WCC living roofs demonstrated the same 4-monthretention efficiency (66%, Table 6), despite greater substrate depthand maximum water retention capacity on the WCC living roof(Table 5). The frequency of small events entirely captured by allof the roofs and frequent irrigation during summer of the WCC liv-ing roof are likely contributing factors. Since the WCC’s water stor-age potential is almost double that of UoA, there appears to becapacity for low irrigation amounts during hot weather while stillcapturing the majority of rainfall events. Alternatively, additionalstorage capacity might be used to manage run-on from nearby con-ventional roof surfaces.

Despite greater maximum water retention capacity, the re-duced cumulative retention at Tamaki (48–57% vs 66% at UoA orWCC) is likely influenced by the high efficiency of runoff fromthe control roof. The Color Steel roof provides minimal retentionor peak flow reduction potential compared to the flatter, rougherWCC and modeled UoA control roofs.

The four living roofs were equally effective and efficient com-pared to a control roof, as measured by % runoff retention (Table 6).Runoff depth within control roofs and within living roofs (Fig. 6)were not significantly different when analyzed on a per event basis(p = 0.148, and p = 0.061 amongst control roofs and living roofs,respectively).

When comparing UoA and WCC performance, statistically sig-nificant differences were not found between living roof peak flows(p = 5.766) or % peak flow reduction compared to its control roof

Table 6Living roof vs. control roof performance comparison August–December 2010: all sitesmonitored concurrently.

Living roof Cumulative retention(%)

Event-based median

Retention(%)

Peak flow reduction(%)

UoA 66 75 89Tamaki 100 48 55 73

150 57 66 74WCC 66 72 86

Fig. 6. Living roof runoff depth and peak flow rates per event for rainfall events P2 mm: August–December 2010.

Fig. 7. Runoff depth per rainfall event, combining data from all sites for each fullmonitoring period. Q = runoff; P = precipitation.

18 E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20

(p = 3.786). Each of the Tamaki mini-roofs demonstrated statisti-cally higher peak flow than either the WCC or UoA living roofs(p < 0.02) (Fig. 6), and less peak flow reduction compared to itscontrol roof (p < 0.04). Four factors probably influence this resultat Tamaki: (i) a short flow path to the outlet (maximum 2 m);(ii) higher substrate permeability; (iii) faster and greater runofffrom the hydraulically efficient control roof; and (iv) a greater pro-portion of edge gravels.

Substrates designed to maximize water holding capacity (Fass-man and Simcock, 2012) clearly benefits retention, as substantialcontrol is realized on all roofs. Results suggest the physical config-uration of a proposed living roof may influence the extent of peakflow control provided. To maximize peak flow mitigation, designshould extend the length of the horizontal flow path to the verticalgutters. Increased physical resistance within the drainage layercould also be used to increase peak flow reduction, for example,using a granular drainage media rather than the egg-crate typesynthetic drainage layers used at the testing sites. Insufficient datahave been collected to date to identify specific design thresholdsfor peak flow control, and it is recognized that peak flow controlat Tamaki was still significant. However, future research on peakflow control should identify scaling influences such as flow pathlength.

3.4. The influence of storm size on event-based retention: fullmonitoring period at all sites

Since a statistically significant difference was not found amongsites for runoff depth or %-retention (Section 3.3), event-based datafrom all sites were combined from each site’s full monitoring per-iod for further analysis. Fig. 7 demonstrates a non-linear increasein runoff depth as storm size increases, with relatively good fit ofthe trendline (R2 = 0.81). Interestingly, analysis of the first year ofdata from the UoA living roof did not identify a single factor that

Table 7Living roof runoff statistics by rainfall category (mm).

Rainfall range 2–5 mm 6–10 mm 11–15 m

n 148 96 51Median (mm) 0.3 2.8 5.1Mode (mm) 0.03 0.3a 0.2a

Std. deviation 1.0 2.9 3.5Variance 1.1 8.6 12.2

a Multiple modes exist. The smallest value is shown.

was strongly correlated to runoff depth (Voyde et al., 2010). It ispresumed that the ability of a living roof to retain precipitationper event depends on multiple factors including the substrate’sability to store moisture (discussed further in Section 3.2) for sub-sequent evapotranspiration, rainfall characteristics, and evapo-transpiration potential (Carpenter and Kaluvakolanu, 2011;Fassman and Simcock, 2012; Kasmin et al., 2010; Voyde et al.,2010). In the current analysis, the ability to combine data frommultiple sites overcomes data variability induced by these factors,and identifies the dominant influence of rainfall depth on runoff.

Despite design and composition differences, the living roofsmonitored empirically demonstrate that substantial control of upto Auckland’s 85th percentile event (regional average 25.6 mm)is provided by the living roofs. The median runoff depth was1.9 mm while the most frequently measured runoff depth (themode) was 0.03 mm for 2–25 mm rainfall events (Table 7). Asthe majority of the 347 events in this range delivered less than

m 16–20 mm 21–25 mm 2–25 mm

32 20 3476.7 18.2 1.96.7 6.4a 0.034.7 6.8 5.222.2 45.7 26.7

Fig. 8. Runoff depth (mm) per 5-mm rainfall event increments for up to the �85thpercentile rainfall event in Auckland.

E. Fassman-Beck et al. / Journal of Hydrology 490 (2013) 11–20 19

15 mm of rainfall, the predominance of these data may skewinterpretation. Data were thus investigated according to �5-mmincrements of storm depth (the smallest category covers 2–5 mmrainfall) (Table 7 and Fig. 8). For events 2–15 mm, while the med-ian runoff depth per rainfall category increases 0.3–5.1 mm, themode only ranges 0.03–0.3 mm. For 16–20 and 20–25 mm catego-ries, median runoff depth again increases, but the mode onlyslightly increases to 6.7, and 6.4 mm, respectively. Inspection ofperformance according to incremental rainfall depths confirmsthat little meaningful runoff is generated for up to a 25 mm event.The field results are also consistent with the substrates’ ability tostore water for subsequent evapotranspiration as per laboratoryanalysis (Table 5).

4. Conclusions

Significant and consistent runoff control was measured fromfour extensive living roofs with different design configurationsand in three locations across Auckland, New Zealand.

All monitored living roofs effectively mitigated peak flow, withmedian living roof runoff peak flow measured at 62–90% less thana corresponding control roof per storm event. Peak flow controlwas not influenced by season at any site. Living roof configuration,namely horizontal flow path length through the drainage layer tovertical gutters and drainage layer roughness or material, mayinfluence effectiveness and could be manipulated to increasemitigation.

The duration of the monitoring period for living roof perfor-mance appears to influence data interpretation. The authors donot intend to suggest that shorter duration studies are not useful;rather, it helps to explain one reason why studies from the litera-ture show varying performance.

Living roofs in Auckland have the potential to retain and evap-otranspire up to 56% of cumulative runoff compared to a conven-tional roof surface on a long-term basis. Mitigation wasmaintained across seasons, with summer event-based retentionat 83–92% (depending on living roof site) decreasing to 66% in win-ter (one site). Among established living roofs of 50 mm, 70 mm,100 mm, and 150 mm substrate depth and >80% plant coverage,sites were equally effective in reducing runoff volume comparedto a control roof, despite design differences in substrate composi-tion and installed depth.

On an event-basis, runoff depth is strongly influenced byrainfall depth. Runoff depth increases non-linearly, particularly as

rainfall exceeds 25 mm. The predominance of small storm events(<25 mm) in Auckland means that shallow substrates with highwater holding capacity rarely generate runoff because the system’sstorage capacity is infrequently exceeded. For larger events, reten-tion is still meaningful.

Increased living roof depth beyond minimum requirements forstormwater mitigation may provide multiple benefits in a sub-tropical climate. Although a 50 mm depth living roof is likely lessexpensive to construct, a minimum 100 mm substrate depth is rec-ommended whenever structural loading is adequate to promoteplant resilience and diversity, particularly in the absence of irriga-tion and/or shade. Should irrigation be applied, careful monitoringof irrigation amounts is necessary to ensure that stormwater cap-ture is not compromised. Where irrigation is not regularly applied,additional water storage capacity could be used to mitigate run-onfrom other conventional roof surfaces.

Acknowledgements

This study was funded by the former Auckland Regional Councilthrough the Stormwater Action Plan, the former Waitakere CityCouncil, and the Foundation for Research Science and Technology.Continued work is supported through the Auckland Council Storm-water Technical Services Team. Viewpoints expressed in this paperare those of the authors and do not reflect policy or otherwise ofthe Auckland Council or former Councils. The authors would liketo thank technician Chris Winks from Landcare Research for hisassistance with this research.

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