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Marie-Curie IAPP ‘Green Roof Systems’ Project

The Green Roof Research Conference 18-19 March 2013, Sheffield

A Modelling Study of Long Term Green Roof Retention Performance Virginia Stovin University of Sheffield, v.stovin@sheffield.ac.uk

Introduction One of the key outputs from the Green Roof Systems project is a continuous simulation model that predicts the hydrological performance of green roof systems in response to arbitrary rainfall inputs, both continuous time-series and design storm events. The objective of this presentation is to demonstrate the value of the generic modelling approach in understanding the influence of both climate and roof configuration on the long-term hydrological retention performance of a green roof.

Overview of Methodology Four UK locations with contrasting climatic characteristics were selected: NW Scotland, Cornwall, Sheffield and East Anglia. 30-year hourly weather time-series were generated using the UKCP09 Weather Generator. Potential Evapotranspiration (PET) was modelled using the Thornthwaite formula, together with a linear relationship to predict ET from PET and substrate moisture content. The simulated runoff time-series were analysed for overall volumetric retention and for per event & per significant event (> 1 yr return period) retention. 900 Rainfall Modelled Runoff Measured Runoff

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Figure 1 – Model validation against monitored runoff data (2007 full year)


Marie-Curie IAPP ‘Green Roof Systems’ Project

The Green Roof Research Conference 18-19 March 2013, Sheffield The basic configuration considered represented a typical extensive green roof system with an assumed maximum retention capacity of 20 mm rainfall. The occurrence of prolonged periods of low moisture content (drought stress/irrigation) and sensitivity to changes in PET and substrate Moisture Holding Capacity were also examined.

Preliminary Findings Overall volumetric retention ranged from 19% in NW Scotland up to 53% in East Anglia, although the greatest total retention occurred in NW Scotland. If retention per event is considered, average retention increases when all events are considered (many very small events), but – as might be expected – retention is lower in ‘significant’ events, all of which have total depths greater than 20 mm (Figure 2). Drought stress is expected on an annual basis for both East Anglia and Sheffield. The retention model may be further refined through more accurate modelling of PET and better understanding of plant-specific ET response to restricted substrate moisture. 1

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Figure 2 – Per event retention performance comparison

Further Reading Kasmin, H., Stovin, V.R. and Hathway, E.A., 2010, Towards a generic rainfall-runoff model for green roofs, Water Science & Technology, 62.4, 898-905. doi: 10.2166/wst.2010.352 Stovin, V., Vesuviano, G. and Kasmin, H., 2012, The hydrological performance of a green roof test bed under UK climatic conditions, Journal of Hydrology, Vol. 414-415, 148-161. ISSN 0022-1694. DOI: 10.1016/j.jhydrol.2011.10.022 https://sites.google.com/a/sheffield.ac.uk/green-roof-research/ http://greenroof.shef.ac.uk/

A Modelling Study of Long Term Green Roof Retention Performance - Virginia Stovin  

One of the key outputs from the Green Roof Systems project is a continuous simulation model that predicts the hydrological performance of gr...

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