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Collaborative research and development of green roof system technology

Final Report Summary - GREEN ROOF SYSTEMS (Collaborative research and development of green roof system technology)

Marie Curie Industry Academia Partnership Programme
Project full title: Collaborative Research and Development of Green Roof System technologies (Green Roof Systems) Grant agreement no.: (230636)

Green roofs are vegetated roof surfaces. Modern green roof technologies were developed in Germany, and since that time, green roof systems have been exported across the world. Green roofs have received widespread uptake in Germany and other north-European countries, and are a common feature of built development, because of the multiple benefits that they offer to urban environments (including stormwater attenuation, biodiversity support, summer cooling of buildings, and aesthetic and amenity values). The original drivers for the development of green roof technology in Germany are still valid. However, in the last five years the context and reasons for green roof installation have changed somewhat, requiring 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, nutrient cycling, support for faunal diversity. Green roofs must become more sustainable in reducing or eliminating the need for additional water, and in reducing their maintenance requirements, whilst remaining aesthetically attractive and green.

The work within the programme falls into three workpackages:
• Work Package 1 – Plant Selection and Screening Programme
• Workpackage 2 – Product development to optimise hydrological performance
• Workpackge 3 – Integrative Whole System Hydrological Modelling

Programme Objectives
- To develop and implement a rigorous and standardised plant screening programme for green roofs, to test potential plant species for diverse, self-sustaining and low-input green roof vegetation, focused on survival of moisture stress at different depths of growing medium. Work Performed: The resultant screening process identifies optimal growing media depths and the degree of tolerance to differing levels of moisture stress for different plant species. A cross gradient design forms the basis of the method, with one gradient being substrate depth (50, 100 and 150mm), and the other gradient being moisture availability (through applications of different amounts of water at differing irrigation frequencies (below average, average, and above average rainfall amounts). Rain shelters were placed over the green roof test modules to prevent natural rainfall reaching the plants and to allow the specified amounts of water to be given to the plants. Therefore plant species are tested against each combination of substrate depth and water availability. Main results: a large data set was generated giving information on the response of 50 species (some commonly used green roof plants and others that are new to green roof use). As expected, varying depth and irrigation regime produced differing responses between species, according to their growth forms. The cross gradient design enables the response of each species to be shown graphically, using a three dimensional grid that shows all the different treatment combinations from shallow substrate with minimal irrigation through to deeper substrate with regular irrigation, and this enables rapid visual comparisons to be made between species. The methodology developed and tested will enable systematic testing of new species and cultivars for green roof use. The exploration of a wider pool of potential green roof plants beyond the small range that are currently used will have significant benefits for the development of future green roof vegetations to meet climate change and adaptation challenges, to increase the biodiversity value of green roofs, and to enhance the aesthetic and social relevance of vegetated roof surfaces.

- To develop green roof system components that optimise the hydrological performance of green roof systems, with particular focus on water retention and supply, whilst not significantly increasing system weight. Work performed: Substrate formulation to optimise moisture supply and retention has focused on the use of different organic and inorganic amendments that increase plant available water, with particular regard to the early establishment phases of green roof vegetation. Several very promising amendments have been identified, but the work also highlighted the need for proper substrate mixing and handling with these amendments. Part of this work involved the development of a new test for moisture availability on highly granular or mineral substrates, using pressure plate extraction techniques. This new test represents a significant advance on the current industry standard. A new wicking system that delivers water more efficiently from the lower drainage layer of the green roof to the substrate and vegetation has been developed. A new ‘phytometer’ test methodology has been developed that uses indicator plants to test the water delivery efficiency of green roof components and substrate formulations. The development of the hydrological models of work package 3 had to be underpinned by rigorous data collection and understanding of the controlling physical processes for each separate component of the green roof system and therefore much effort during the project focused on completing green roof component specific evaluations. This has included the development of a small scale-rainfall simulator to measure substrate detention effects, completion of a comprehensive programme of drainage layer detention experiments using ZinCo’s large rainfall simulator, and climate chamber-based tests to establish evapotranspiration (ET) rates as a function of vegetation type and substrate composition that have been undertaken for simulated spring and summer conditions. This work will lead to increased water-use efficiency within green roof systems and will therefore contribute to the greater environmental sustainability of green roof technologies, and will also open up the possibility for shallower green roof build-ups that result in lower weight systems, and which will therefore make green roofs more applicable for buildings that have lower load bearing capabilities.

- To characterise the performance of new green roof systems and configurations resulting from the product development phase of the programme. Work Performed: New test rigs that enable full field testing of green roof configurations were set up in summer 2012, allowing the comparison of a small number of new configurations for the European context with examples of currently used configurations and components. The rigs were equipped to measure runoff from the systems, and this could be compared with measures of rainfall amounts and other weather variables collected from the same site. As a result, the test beds have provided a unique continuous record of green roof hydrological performance for nine different systems incorporating three different substrates and three different vegetation treatments. In general the data have confirmed expectations, with commercial brick-based substrates providing better retention and detention characteristics compared with a poorly-sorted LECA-based substrate, and vegetated systems offering marginally enhanced retention due to evapotranspiration (ET). Four of the test beds were instrumented with moisture content probes, and a detailed study of moisture losses during the drying phase has enabled us to understand how the substrate and vegetation effects combine to remove moisture from the substrate. This can only be seen as the start of what will be a longer-term testing regime: long-term field trials will also be required to fully evaluate the new configurations.

- To provide a preliminary life cycle analysis/energy audit of the proposed systems in terms of their contribution to reduction in carbon emissions/sequestration. Work Performed: Preliminary life-cycle analysis has highlighted that significant impacts can be associated with the transport of materials; therefore local sourcing of (bulky) substrates is strongly recommended. Significant impacts are also associated with the use of polymers in the drainage layer components. A simple spreadhseet-based tool has been produced to generate indicative values of carbon footprint for typical systems. This included new data on the CO2 costs associated with pre-grown vegetation components (estimated 2,749 g CO2)

- To develop new, generic hydrological models of green roof performance. Work Performed: both the field monitoring and the controlled climate chamber ET tests (Workpackage 2) have shown that ET is a function of climatic inputs (which determine potential ET), but - critically - is also highly dependent upon moisture content. This understanding has enabled us to propose and validate a robust continuous simulation model of green roof retention. In typical extensive systems, retention can vary from zero to a maximum of around 20 mm per storm event, depending entirely on the antecedent ET conditions; retention efficiency is reduced for larger (design) rainfall events. The separate investigations of detention processes in the substrate and drainage layer components undertaken in WP2 allowed us to propose a two-stage detention model based on reservoir routing concepts. This model has been validated with time-varying laboratory tests and against the test rig field data. Detention processes are relatively insignificant in extensive green roofs, but there is the potential to enhance detention through the use of deeper substrates, increased substrate organic content and/or fibrous matting below the detention layer. Alternative approaches to enhanced detention include the wick-based system and/or refinements to outlet controls; this has been highlighted as a potential area for further development.