Towards an Enhanced Green Roof System

Towards an Enhanced Green Roof System Christian Berretta, University of Sheffield, (c.berretta@sheffield.ac.uk), United Kingdom Tobias Emilsson, ZinCo GmbH, (tobias.emilsson@zinco‐greenroof.com), Germany Nigel Dunnett, University of Sheffield, (n.dunnett@sheffield.ac.uk), United Kingdom Virginia Stovin, University of Sheffield, (v.stovin@sheffield.ac.uk), United Kingdom Ralf Walker, ZinCo GmbH, (tobias.emilsson@zinco‐greenroof.com), Germany

Abstract Green roof research has generally been developed as single stranded projects investigating either plant, stormwater attenuation or aesthetic performance. There are few examples of integrated research projects linking plant performance and substrate design to stormwater management either on a local roof or drainage basin scale.

The University of Sheffield Green Roof Centre, together with ZinCo GmbH, is involved in the project “Green Roof Systems” within the EU FP7 Marie Curie (IAPP). The main aim of the project is to enhance traditional intensive and extensive green roof systems by revisiting the fundamental basis of green roof system design. The aim is to optimize both the stormwater management and the plant performance with a renewed focus on the aesthetics.

The project is divided into three Work Packages (WP). In WP1 a standardized plant screening protocol has been developed and used to investigate plant performance for a range of species in relation to growing media depth and moisture availability.

The protocol has been tested in two climatic contexts: continental (Stuttgart, DE) and maritime climate (Sheffield, UK). WP2 is focused on the detention effect in the substrate and drainage layer, water transfer between components and physical characterization of substrates optimised for retention and plant survival during drought. Evapotranspiration rates have been studied as well as vertical fluxes from drainage layer to substrate. WP3 is focused on studying the complete green roof system by using the knowledge developed in the previous parts of the project. In WP3 we are implementing a physically-based hydrological model specific for green roofs, validated on experimental data acquired through test beds characterized by traditional and innovative green roof systems and evapotranspiration tests from climate chamber.

The paper will explain how key findings from WP1 and WP2 have informed the development of the enhanced systems that will be trialled and modelled as part of WP3.

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Authors’ Biographies Dr Christian Berretta’s research focuses on hydrological and environmental processes monitoring and modelling in urban environment, Sustainable Drainage Systems (SuDS) to restore pre‐development hydrological condition while controlling targeted pollutants, and the assessment of source area runoff impact on aquatic ecosystems and human health. He developed his research activity at the University of Genoa, Italy (2001‐2007) and at the University of Florida, US (2007‐2011). He is currently working as Marie Curie senior research fellow at the University of Sheffield, UK.

Dr Tobias Emilsson is working as Marie Curie senior research fellow at Zinco GmbH. His main work is currently focused on substrate design and water relations of extensive green roof substrates. Tobias Emilsson has a background in Plant ecology and a PhD in technology focused on Extensive green roofs. His previous work has involved vegetation development and nutrient runoff from extensive green roofs.

Dr Nigel Dunnett is Director of The Green Roof Centre, Sheffield, UK, and Professor of Planting Design and Vegetation Technology in the Department of Landscape, University of Sheffield. He has a background in botany, horticulture and ecology. His work revolves around innovative approaches to planting design, and the integration of ecology and horticulture to achieve low‐input, dynamic, diverse, ecologically‐tuned designed landscapes, at small and large scale.

Dr Virginia Stovin is a Senior Lecturer in the Department of Civil and Structural Engineering at The University of Sheffield. Her work focuses on urban drainage structures and processes, most recently on the hydrological performance of SuDS. She is an enthusiastic proponent of SuDS retrofitting, and is a co‐author of the recently‐published CIRIA retrofitting guidance. As part of the Sheffield based Green Roof Centre, she has constructed 11 green roof test beds. The long term records from these beds are being combined with laboratory studies to underpin the development of green roof hydrological performance modelling tools suitable for urban stormwater management planning.

Ralf Walker is the Head of R+D at ZinCo GmbH in Germany and has undertaken lots of developments in the field of Green Roofs. His background is in horticultural engineering, soil science and plant nutrition as well as in plants.

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Background Industrial Context Modern green roof technologies were developed in Germany and guidelines for roof greening have been published by the German organisation for landscaping research (FLL) since 1984.

Green roofs have become a common feature of built environment because of their multiple benefits, including stormwater attenuation, biodiversity support, cooling effect of buildings and aesthetic value (Oberndorfen et al., 2007).

In the last five years the context and reasons for green roof installation have changed somewhat. The demand for extensive green roofs that are less resource and maintenance‐ intensive systems and which have high aesthetic value is increasing. This ecological view demands that extensive green roofs become biologically more diverse whilst also offering improvements in delivery of ‘ecosystem services’ such as stormwater retention, carbon sequestration, energy conservation and nutrient cycling. The model light‐weight sedum extensive roof does not appear to deliver, in the long‐term, these desired objectives (Compton and Whirlow, 2006; Brenneisen, 2006).

The key drivers for the evolution and development of commercial green roof products for extensive green roofs include developing a longer water supply out of the systems in order for the vegetation to survive drought periods, whilst ensuring that under wetter conditions there shall not be an over‐supply of water, which may promote changes in vegetation, such as establishment of grasses dominating over drought resistant species. A new focus on evapotranspiration and inter‐event moisture balance and transfer within components of the system is required. This relates to a better understanding of the plant physiology and growing medium physical properties. There is an increasing pressure to include native species and to widen the plant diversity of green roofs in the contest of an increasing interest in biodiversity potential. Furthermore a new focus on stormwater attenuation represents another development driver.

For intensive green roof there is also the need to minimize or eliminate irrigation to increase sustainability. This will require the investigation of suitable plant species alternative to current planting regimes as well as the development of green roof system components, substrate and drainage layers, that optimize moisture fluxes. Particularly, there is a specific need to research alternatives to lawn or turf grass that can give a uniform, evergreen surface, without the need for intensive irrigation.

Problem Plant selection for extensive green roofs at an international level is still largely dependent on lists produced from research in Germany in the last 20 years and, for intensive green roofs, on the use of standard lawn mixtures and landscape plants. This assemblage of plants was developed under one climatic regime. There is a need to further extend the range of plants that are used and that are suitable under different climatic regimes through the development of rigorous and standardized methodologies that enable the characterization of plant species World Green Roof Congress, 19-20 September 2012, Copenhagen Page 3

according to their optimal growth requirements and their tolerance limits to environmental stress. Often plant testing and screening are performed on a small scale, and not undertaken on a rigorous basis that gives a full and detailed characterization of the plant’s requirements.

Much of the existing knowledge about the hydrological performance of green roofs is derived from field or laboratory experiments in which observations of rainfall and runoff have been used to derive empirical ‘black‐box’ performance functions. The predictive value of these relationships is, however, restricted to each study’s specific system configuration and climatic influences. This means they cannot be utilised with confidence in other contexts. At the same time, as the individual influence of each system component (plant, substrate, drainage layer) is lost in the overall system performance, it is difficult to optimise the design of either individual components or complete systems to meet specific performance objectives. There is the need for single component based understanding of performance as well as, linked to fundamental physical properties of the system, to enable modelling and system design.

By having a better understanding of how each component influences the processes occurring in green roof systems, it is possible to adapt the configurations or combinations of components to meet specific criteria, such as aesthetic, stormwater management, lower maintenance. If structural limitations provide the boundaries for the design, climatic characteristics dictate the most effective combination of substrate and vegetation. Vapour pressure gradient, radiation, wind, temperature, hydrological regime in terms of rainfall intensity, depth, antecedent dry weather periods, internal intermittency causing frequent wetting‐drying cycles are all factors to take into consideration in the design. In green roofs, especially extensive ones, design criteria can be conflicting. One example is the tension between optimizing the system for stormwater management or for inter‐event plant survival. Substrates with higher maximum water holding capacity are usually characterized by higher organic matter content and a larger number of small pores. If these properties favour plant survival during drought, they are also likely to induce greater matric pressure and increased resistance to water balance changes, thus decreasing the stormwater management benefit due to a slower regeneration of the available water capacity within the system. Other examples are choosing native plant species or species with higher aesthetic value over drought resistance species or favouring retention over detention through the design.

In this framework, the University of Sheffield Green Roof Centre, together with ZinCo GmbH, is involved in the project “Collaborative Research and Development of Green Roof Systems Technologies” that aims at enhancing traditional intensive and extensive green roof systems by revisiting the fundamental basis of green roof system design. The objective is to provide a profound understanding of green roof system performance and of the potential for optimization to meet the new challenges described previously. The project is funded under the European Union People programme as a Marie Curie Industry Academia Partnerships and Pathways (IAPP) project. It is the largest international green roof project to date, involving 11 researchers from an academic institution and a commercial partner and has a long time span, running over 4 years. This paper presents the research methodology developed during this project.

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Learning Objectives: •

A deeper understanding of the processes occurring in green roof system through a multidisciplinary integrated approach

•

The influence of single components in the performance of the complete systems and potential for their optimization

Approach The project is divided into three Work Packages (WP). In WP1 a standardized plant screening protocol has been developed and used to investigate plant performance for a range of species in relation to growing media depth and moisture availability. The protocol has been tested in two climatic contexts: continental (Stuttgart, DE) and maritime climate (Sheffield, UK). The experimental work is conducted in genuine roof environments in urban areas (Figure 1). A cross factorial experimental design is used, which involves 3 different levels of water availability obtained by different irrigation regimes (low, moderate and abundant) and three different depth of the growing medium (5, 10 and 15 cm). A growing medium composed of 55% crushed brick, 30% pumice, 10% coir fibre and 5% composted bark was specifically adopted for the project as a reference substrate not containing peat. 46 plant species (29 forbs, 10 grasses, 7 succulents) have been tested. Spaces of sowing was 10 cm to permit plant interaction at an early stage. The arrangement of plant species in each module was determined randomly. At this stage of the project data have been collected for two growing seasons (2010 and 2011) in the University of Sheffield site and 1 growing season (2011) at ZinCo in Germany. The following data were collected: percentage germination, shoot extension, maximum height of flowering stem, mean diameter, number of inflorescence or flowering stem, species survival and percentage dieback of vegetative growth.

Univ. of Sheffield, UK

ZinCo , DE

Figure 1 Experimental sites showing the plant trials at the University of Sheffield, UK and at ZinCo, DE.

WP2 focused on investigating the hydrological processes occurring in a green roof system. Prior to a rainfall event the substrate is characterized by an initial moisture content MC0. The maximum moisture content that a substrate can hold is referred to as its field capacity, WCmax.

During a rainfall event the substrate will retain moisture up until field capacity is reached. The amount of rainfall retained therefore equates to the difference between MC0 and WCmax. As a World Green Roof Congress, 19-20 September 2012, Copenhagen Page 5

result of plant evapotranspiration, the moisture content will tend to decrease during dry periods, but it is approximately close to WCmax soon after a storm event that generated runoff.

Any excess rainwater is temporarily stored within large air pores, but will typically drain from the roof under gravity within two hours. This temporary storage effect is referred to as detention, and it provides an important stormwater management function through delaying and reducing the impacts of storm peaks on sewer systems or watercourses. The rate at which temporarily stored moisture exits from the detention storage depends on substrate physical characteristics (‘vertical’ detention) and drainage layer characteristics (‘horizontal’ detention).

After the storm event has ceased, the roof will continue to drain until the transient detention storage is empty and the substrate is at field capacity (Kasmin et al, 2010). Over subsequent days, the substrate will then lose moisture gradually as a result of plant evapotranspiration.

The rate at which moisture content decreases depends on the plant physiology, the substrate characteristics and the climatic conditions. During dry periods the moisture level in the substrate can decrease to the level that that plants experience drought‐stress. The permanent wilting point represents the condition in which moisture is no longer available to plants. The soil structure and the pore size distribution characterize its moisture release behaviour or pF curve.

WP2 aims to understand and quantify each of these processes and has therefore focused on the establishment of experimental methodologies to assess the following: ‐

Evapotranspiration rates;

‐

pF curve quantification for substrates;

‐

Detention in the drainage layer;

‐

Detention in the substrate;

‐

moisture vertical flux (drainage layer to substrate);

‐

Substrate amendments to enhance runoff detention and plant soil moisture availability.

All aspects of the investigation have required either new methodologies to be developed or for existing approaches to be significantly modified to account for the specific features of green roof substrates. At the same time, physical characteristics of substrate were investigated through FLL tests (FLL 2008) and pore space distribution. Table 1 provides a list of the tests performed for each green roof element.

Although WP2 focuses primarily on the performance of individual elements within the green roof system, the work is placed into context through the continuous monitoring of 9 external test beds installed at the University of Sheffield. The test beds have been established to assess the extent to which substrate type and vegetation treatment affect long‐term runoff retention and detention performance. In particular, three vegetation options (Sedum, meadow flower mixture, no vegetation) and three substrates were selected for investigation. Two commercial substrates manufactured by Alumasc, namely Heather with Lavender (HwL) Substrate and Sedum Carpet (SC) Substrate, were considered alongside a Lightweight Expanded Clay Aggregate (LECA) based substrate. The Heather with Lavender and Sedum Carpet substrates contain crushed brick and selected mineral aggregates, enriched with a small amount of mature compost (Alumasc, 2011). The LECA‐based substrate contains 80% of LECA, 10% of loam (John Innes No. 1) and 10% of compost (Poë et al, 2010). The field installations include weather World Green Roof Congress, 19-20 September 2012, Copenhagen Page 6

stations, and selected beds incorporate water content reflectometers for moisture content vertical gradient measurement.

Table 1 Tests performed during the project for each component of the green roof system

Complete system

Drainage layer

Substrate ‐ Drainage layer

Substrate

Vegetation‐ Substrate

Vegetation

Component

Test

Methodology

Plant Screening Programme

46 plant species (29 forbs, 10 grasses, 7 succulents) tested in 2 climatic conditions (maritime, Sheffield, UK and continental, Stuttgart, DE); 3 irrigation regimes; 3 depths of growing medium

Evapotranspiration

Phytometer (substrate test using plants as bio‐ indicators)

3 substrates and 3 vegetation options tested in a climate chamber to simulate spring (5.01‐9.76 °C, 12 hours sunlight) and summer conditions (13.76÷19.84 °C, 17 hours sunlight) 8 synthetic/inorganic and 6 organic amendments tested vs 3 plant species at greenhouse condition set at 22 °C and relative humidity 70% (OECD 2006)

Pressure plate extraction (pF curves)

Substrates and amended substrate tested from ‐0.35 to ‐ 15 bar pressure (specific method)

FLL tests (FLL 2008)

Granulometric distribution, apparent density (dry condition and at max water capacity), total pore volume, max. water holding capacity, permeability, organic content, pH, nutrients.

Pore space distribution

Image analysis of sections of substrate cores solidified in resin ‐ 4 substrate options

Detention

Small‐scale laboratory rainfall simulator for detention process vs substrate depth, organic content, rainfall intensity, presence of moisture mat

Moisture vertical flux from drainage layer to substrate

Moisture balance observation in controlled climatic condition (35 °C and 20% relative humidity) through specifically designed trays

Detention

5×1 m rainfall simulator to test 4 drainage layer components vs rainfall intensities, roof length, roof slope.

Retention/detention of novel component

Hydraulic tests of different detention enhancing devices

Field test beds

Rainfall – runoff – climatic conditions and moisture content vertical gradient monitored in 9 test bed configurations (3 substrates and 3 vegetation options)

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WP3 is focused on studying the complete green roof system by using the knowledge developed in the previous phases of the project. To achieve this objective the combination of green roof elements that provided the most promising performances as well as innovative elements will undergo a second field monitoring program at the University of Sheffield. Furthermore, in WP3 we are implementing a physically�based hydrological model specific for green roofs, validated on experimental data acquired through the field tests. The model will also be used for simulating the impact of green roofs on a catchment scale.

Analysis The integrated multidisciplinary method The adopted research approach is represented in Figure 2.

Measuring Components Physical characteristics

Complete system

Processes

Test beds Hadfield roof

ET test Pore space distribution

Pressure plate extraction

Interpreting

Enhancing June 2010 Hadfield roof

0

0.5

0.4

20

0.4

0.3

40 Top WCR Mid WCR Low WCR

0.2

60

0.1

80

0.0

100 Mon 07

Mon 14

Moisture Content in test beds

Mon 21

Mon 28

Time

Figure 2 Scheme of the integrated research approach adopted in the project.

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0.3 0.2 0.1 0.0

Volumetric Water Content [m3/m3]

14/5/2012 21/5/2012 28/5/2012

Rainfall Intensity [mm/h]

Flowrate [l/min]

7/5/2012 0.5

A deeper knowledge of each element’s performance, as well as of the combination of elements (complete system), allows interpretation and understanding that can result in enhancing the system by new product development or more effective combination of traditional elements.

The described process restarts from the testing of the new or more effective solutions. The adopted approach requires a combination of expertise including horticulture, plant ecology, plant physiology, hydrology, civil engineering and the collaborative partnership between academia and the industry. Also the duration of the project allows the possibility to collect representative data and when needed to repeat series of tests after more promising solutions have been determined.

Selected outcomes from the project are briefly highlighted below.

Plant screening programme. The main result of WP1 is the rigorous characterization of plant species and their performance. Preliminary analysis of the data collected during the first growing season in the UK showed that leaf extension growth response is influenced by depth of the rooting medium (Figure 3). However there is clear differentiation at this growth stage between horticultural plant groups, with grasses showing a greater relative increase in growth with increasing substrate depth if compared to succulents behaviour. For many species under high water availability a significant drop in shoot extension was observed due to a restriction of resources by competition from neighbour plants.

Figure 3 Leaf length classified by plant categories (forbs, grasses and succulents) and growing medium depths measured after the first growing season at the University of Sheffield site Substrate detention tests. Preliminary tests (Yio et al., 2012) have identified the effects that substrate depth and composition (type and percentage of organic matter) have on runoff detention. In Figure 4 the laboratory small scale rainfall simulator is shown and the measured cumulative runoff of 6 substrate options. Laboratory data has shown that the detention in green roof substrates increases as a function of depth and organic matter content. The latter is associated with a reduction in permeability. A modified reservoir routing model was used to simulate the detention process. The model parameters are largely independent of rainfall intensity, and it appears feasible to predict them from known physical characteristics of the substrate, specifically its depth and permeability.

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Figure 4 Small scale rainfall simulator for vertical detention tests and measured runoff of different substrates characterized by different organic material (coir and composted bark) and organic content at 0.10 mm/min inflow rate (Yio et al., 2012) Drainage layer detention tests. Different drainage layers have been tested in a 5 × 1 m rainfall simulator (Figure 5). Preliminary results showed a moderate detention effect with no significance difference between different drainage layers, while an increased detention effect was observed with the use of a moisture mat combined with the drainage layer. A runoff model based on storage routing and a power�law relationship between storage and runoff and incorporating a delay parameter were created. A sensitivity analysis showed the influence of roof slope and drainage material. More details are reported elsewhere (Vesuviano and Stovin, 2012).

Figure 5 Rainfall simulator for horizontal detention tests and hydrographs of five drainage component configurations at a roof slope of 1.15o, drainage length 5 m and inflow rate 0.6 mm/min (Vesuviano and Stovin, 2012).

Phytometer tests. Preliminary results of the Phytometer tests support the choice of substrates containing 15% of organic matter (bark or peat) and inert material (bricks and pumice) to increase the maximum water holding capacity while maintaining the permeability above the FLL target. Pressure plate extraction results (pF curves) showed a large remaining water reservoir at �15 bar extraction pressure (wilting point). Results showed that no correlation was found between the FLL water holding capacity values and the pressure plate extraction World Green Roof Congress, 19-20 September 2012, Copenhagen Page 10

measurements. Also a higher maximum water holding capacity was not reflected in prolonged plant survival or increased aesthetics (Emilsson et al., 2012) Evapotranspiration test. Poë and Stovin (2012) quantified the net contribution to ET losses during typical UK spring and summer conditions for the two plant species Sedum and Meadow Flower. The latter showed a higher contribution in restoring the available water content during spring conditions and in the first days of summer. In summer after circa 14 days a change in behaviour was observed due to drought stress condition. Results are explained in relation to climatic condition and substrate characteristics, thus providing information on the impact of drought‐resistant species over more aesthetic ones on the hydrological process in an extensive green roof.

Field test of green roof systems. Clear differences in runoff retention have been observed dependent upon substrate characteristics and plant species choices as shown in Figure 2 where cumulative retention for the month of June 2010 is reported for the 9 green roof system configurations tested at the University of Sheffield (Poë et al, 2010).

The enhanced system The described results obtained in WP1 and WP2 are used to define the green roof configurations to undergo a second field test and to develop a hydrological model specific for green roofs (WP3). As for the field test, different vegetation mixes with higher aesthetic value will be compared to traditional ones. A moisture mat will be included in the tested systems combined with drainage layers. A novel drainage layer that favours moisture flux to the substrate, thus functioning as reservoir for plants during drought, has been developed and will be tested. A novel ‘slow draining’ system specifically designed to enhance the detention effect will be tested as alternative to traditional drainage layer. The reference substrate will be compared with the two substrates that provided the best performances in the retention and phytometer test. Also, the new configurations will provide novel data for rainfall‐runoff model validation.

Results and Business Impacts Key Findings This paper presents a research approach that aims to investigate each element of traditional green roof system to quantify their impact in the hydrological processes occurring on a green roof. From a deeper knowledge of the performance of traditional element this project explore potentials of enhancing these systems by developing novel elements but also by testing combination of elements in the field as well as in the laboratory. A rigorous characterization of plant species was conducted that will provide essential information on the design of a green roof to meet specific criteria. Different tests highlighted the need for more detailed ways of measuring substrate characteristics such as permeability and maximum water holding capacity.

Other studies (Fassman and Simcock, 2012) confirm the growing interest in using alternative test methods to FLL ones. One of the objectives of this project is to develop a hydrological model specific for green roof. Model approaches were proposed within the project for the simulation of the detention effect of the drainage layer and the substrate. In developing a model for long term simulation of a green roof system a key challenge will be to describe the World Green Roof Congress, 19-20 September 2012, Copenhagen Page 11

changes in the system over time. Green roof systems are living elements and changes in vegetation as well as in the substrate due to organic matter decomposition and vegetation establishment (especially for plant species supporting the increase biodiversity demand) are expected. The first phase of the monitoring program investigating complete systems at the University of Sheffield (Poë et al, 2010) will end in October 2012 after more than 2 years of continuous data collection. There will be then the opportunity to conduct a series of test that aim at investigating these changes, especially in the substrate physical characteristics.

Business Impacts This research intends to provide a better understanding of the processes occurring in green roof systems and on the influence of each specific element. A rigorous characterization of plant species performance as well as substrates and drainage layers can lead to the design of more efficient systems or to the development of new products. Novel design of drainage layers and combination of elements will be presented at the end of the project as well as a hydrological model specific for green roof systems.

Conclusions The University of Sheffield Green Roof Centre, together with ZinCo GmbH, is involved in the project “Collaborative Research and Development of Green Roof Systems Technologies” that aims at enhancing traditional intensive and extensive green roof systems by revisiting the fundamental basis of green roof system design. This paper presents the integrated multidisciplinary approach adopted in this research and highlights preliminary findings of this project.

Key Lessons Learned: •

A multidisciplinary research approach is the key to address the new challenges of green roof system design

•

Integrated physical property measurement of each element and laboratory and field tests of single elements and complete systems can lead to hydrological modelling specific for green roofs

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Emillsson, T., Berretta, C., Walker, R., Stovin, V., and Dunnett, N. (2012) Water in Green Roof Substrates – Linking Physical Measurements to Plant Performance. World Green Roof Conference 2012, Copenhagen, 19 ‐20 September, 2012.

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Yio, M.H.N., Stovin, V., and Werdin, J. (2012) Experimental Analysis of Green Roof Detention Characteristics, Proceedings of the 9th International Conference on Urban Drainage Modelling (9UDM), Belgrade, Serbia, 3‐7 September, 2012.

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