Green Roof Systems Project Overview

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COLLABORATIVE RESEARCH TO ENHANCE GREEN ROOF SYSTEMS Christian Berretta1, Tobias Emilsson2, Zoë Dunsiger1, Ralf Walker2, Nigel Dunnett1, Virginia Stovin1 1University of Sheffield, Green Roof Centre, United Kingdom 2 ZinCo GmbH, Germany Marie Curie Industry Academia Partnership Programme (IAPP)




Plant Screening Programme

ü  2 climatic conditions (maritime, Sheffield, UK and continental, Stuttgart, DE) ü  46 plant species in UK (29 forbs, 10 grasses, 7 succulents) and 144 species in Germany (90 forbs 10 grasses, 44 succulents) - 32 common with UK ü  3 irrigation regimes ü  3 depths of growing medium

Systems Technologies” that aims at enhancing traditional intensive and extensive green roof systems by revisiting the fundamental basis of green roof system plant performance during drought, with a renewed focus on aesthetics. 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.


design. The objective is to provide a profound understanding of green roof system performance that will enable the optimization of stormwater attenuation and




As for the hydrological performance of green roofs, the existing knowledge 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. 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


thickness. There is an increasing pressure to include native species and to investigate alternative plant species according to their optimal growth


Sedum on HLS


Sedum on SCS




MF on L ECA Non-­‐vegetated HLS Non-­‐vegetated SCS


Non-­‐vegetated L ECA


Test Bed Characteristics Size: 3 m x 1 m Slope: 1.5° Drainage Layer: ZinCo FD25 Filter Sheet: ZinCo SF Substrate Depth: 80 mm

3 substrates: •  Heather with Lavender (HwL) •  Sedum Carpet manufactured by Alumasc - crushed brick and selected mineral aggregates, enriched with a small amount of mature compost •  Lightweight Expanded Clay Aggregate (LECA) - 80% of LECA, 10% of loam (John Innes No. 1) and 10% of compost

Data Collected continuously since January 2010: •  Rainfall: tipping bucket rain gauges (ARG100, 0.2 mm resolution) •  Runoff: collected in tanks - measured with pressure transducers PDCR1830; •  Climatic conditions (wind speed, temperature, solar radiation, relative humidity, barometric presure): Campbell Scientific; •  Soil moisture content: CS616 Water Content Reflectometers installed in 4 test beds at 3 different substrate depths

(Poë et al., 2011)



(Emilsson et al., 2012)

The project is divided into three Work Packages (WP).

Moisture retention/ Pressure plate extraction (pF curves)

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). 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%

Physical/Chemical Characteristics (FLL 2008)

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 one growing season (2011) at ZinCo in Germany. The following data were collected:

ZinCo , DE

•  Amended substrates with 10 to 15% organic material (peat, composted bark or biochar) increased WHCmax, but no statistical difference in between the three amendment rates •  The permeability was significantly effected by amendment type and all substrate mixes with biochar had significantly higher permeability than all the other substrates •  No correlation was found between the FLL WHCmax values and the pressure plate extraction measurements •  Dramatic loss of water from the substrates during the first week. Plant aesthetics decreased in a similar fashion and all non‐succulent plants were dead after 35 days following onset of the drought treatment •  A higher FLL WHCmax was not reflected in prolonged plant survival or increased aesthetics

ü  8 synthetic/inorganic and 6 organic amendments ü  3 plant species at greenhouse condition set at 22 °C and relative humidity 70% (OECD 2006)

properties of the system, to enable modelling and system design.

survival and percentage dieback of vegetative growth.

(Poë et al., 2012)

ADWP (Days)

meet specific performance objectives. There is the need for single component based understanding of performance linked to fundamental physical

percentage germination, shoot extension, maximum height of flowering stem, mean diameter, number of inflorescence or flowering stem, species

•  Summer ET losses are greater than spring •  ET rates decay exponentially with respect to time / moisture content •  Initial daily ET rates of 2 and 3.5 mm (spring and summer respectively); •  After 10 days, ET losses total 14 and 22 mm (spring and summer respectively) •  Substrate composition and permeability affects ET; variably influenced by climate •  Plant cover initially limits ET rates, but after time, ET losses are greater

Sedum on L ECA


Climate Chamber

Phytometer Test



ET Losses (mm)

In this context, there is the need to broaden the available plant palette that can be used under different climatic regimes and growing medium

10 Depth (cm)

3 plant strategies: •  Non-vegetated; •  Meadow Flower; •  Sedum Mat.


ü  3 substrates and 3 vegetation options ü  Saturated samples drained to field capacity ü  Weight of each microcosm recorded at 1 hour intervals using calibrated load cells ü  Temperature, Humidity & Light controlled to replicate spring (5.01-9.76 °C, 12 hours sunlight) & summer (13.76-19.84 °C, 17 hours sunlight) conditions ü  2 iterations of 14 days & 1 of 28 days for each condition

ET L osses (mm)


services’ such as stormwater retention, carbon sequestration, energy conservation and nutrient cycling.

components of the system is required. This relates to a better understanding of the plant physiology and growing medium physical properties.

The main result 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. However there is clear differentiation at this growth stage between horticultural plant groups.


This ecological view demands that extensive green roofs become biologically more diverse whilst also offering improvements in delivery of ‘ecosystem

requirements and their tolerance limits to environmental stress. A new focus on evapotranspiration and inter-event moisture balance and transfer within

Forbs Grass Succulent


INTRODUCTION The demand for extensive green roofs that are less resource and maintenance-intensive systems and which have high aesthetic value is increasing.


Preliminary results 200

Leaf length (mm)

The University of Sheffield Green Roof Centre, together with ZinCo GmbH, is involved in the project “Collaborative Research and Development of Green Roof

Pore space distribution

ü  Substrates and amended substrate tested from -0.35 to -15 bar pressure (specific method)

ü  ü  ü  ü  ü  ü  ü

Granulometric distribution apparent density total pore volume Max. water holding capacity Permeability Organic content pH, nutrients

ü  Image analysis of sections of substrate cores solidified in resin ü  4 substrate options


LECA Sedum Substrate

Univ. of Sheffield, UK Detention

ü  Small-scale laboratory rainfall simulator •  Detention in green roof substrates increases as a function of depth and organic matter content. •  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.

ü  detention process vs substrate depth, organic content, rainfall intensity, presence of moisture mat

referred to as its field capacity, WHCmax. 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. It depends on substrate physical characteristics (‘vertical’ detention) and drainage layer characteristics (‘horizontal’ detention). During dry periods, 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

Substrate Drainage layer

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

Moisture vertical flux from drainage layer to substrate

climatic conditions. The moisture level in the substrate can decrease to the level that that plants experience drought-stress. The soil structure and the


pore size distribution characterize its moisture release behaviour or pF curve (Kasmin et al., 2010; Stovin et al., 2012). WP2 aimed at understanding and quantifying the influence of each component of green roof systems on the described hydrological processes. For this

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

The rate of moisture flux from the synthetic layers to the substrate was observed to be approximately 0.12-0.14 mm/ day, in the absence of plants. A greatly improved experimental design was developed to provide definitive, repeatable results.

-  Moderate detention effect with no significance difference between different drainage layers -  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 was developed. A sensitivity analysis showed the influence of roof slope and drainage material.

mixture, no vegetation) and three substrates were selected for investigation. The field installations include weather stations, and selected beds incorporate water content reflectometers for moisture content vertical gradient measurement.

Drainage layer

These methodologies are described on the right.

continuous monitoring of 9 external test beds installed at the University of Sheffield. In particular, three vegetation options (Sedum, meadow flower

Retention/detention of novel component

Enhanced detention device

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. Finally, novel drainage layers that enhance retention and detention and novel substrate enhanced with amendments have been tested in this phase of the project.

(Vesuviano and Stovin, 2012) ü  Hydraulic tests of different detention enhancing devices

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.


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

purpose specific experimental methodologies aimed at characterizing and enhancing green roofs components have been developed in this study.

WP3 focused on studying the complete green roof system by using the knowledge developed in the previous phases of the project and data from the

(Yio et al., 2012)

WP2 focused on investigating the hydrological processes occurring in a green roof system.



Two components have been tested to optimize detention in green roof systems. -  The proposed system is alternative to traditional drainage systems and consists of a shallow tank under the substrate layer that collect runoff through a perforated top surface and store temporarily runoff with a slow release to the drainage network regulated by hydraulic devices. -  A novel drainage layer designed to enhance temporary storage and detention is at the moment under test.

Berretta, C., Emilsson, T., Dunnett, N., Stovin V. and Walker R. (2012). Towards an Enhanced Green Roof System. Proceedings of the World Green Roof Congress. 18-21 September 2012, Copenhagen, Denmark Emilsson, T., Berretta, C., Walker, R., Stovin, V., and Dunnett, N. (2012) Water in Green Roof Substrates – Linking Physical Measurements to Plant Performance. Proceedings of the World Green Roof Congress. 18-21 September 2012, Copenhagen, Denmark Kasmin, H., Stovin, V. and Hathway, E. (2010) Towards a generic rainfall-runoff model for green roofs. Water Science & Technology, 62, 4, pp. 898-905. Poë, S., Stovin, V., and Dunsiger, Z. (2011) The Impact of Green Roof Configuration on Hydrological Performance, Proceedings of the 12th International Conference on Urban Drainage, Porto Alegre/Brazil, 11-16 September, 2011 Poë, S., and Stovin, V. (2012) Advocating a Physically-based Hydrological Model for Green Roofs: Evapotranspiration during the Drying Cycle. Proceedings of the World Green Roof Congress. 18-21 September 2012, Copenhagen, Denmark Stovin, V., Vesuviano, G. and Kasmin, H., (2012) "The hydrological performance of a green roof test bed under UK climatic conditions", Journal of Hydrology, 414-415, pp. 148-161. Vesuviano G and Stovin V. (2012) A Generic Hydrological Model for a Green Roof Drainage Layer, Proceedings of the 9th International Conference on Urban Drainage Modelling (9UDM), Belgrade, Serbia, 3-7 September, 2012. 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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