Green Roof Research at the University of Sheffield

Abstract: Urban trees play an important role in the urban hydrological cycle, yet little consideration has been given in the UK either to the increasing pressures that act to reduce urban tree cover or the opportunities that might be provided by land-use planning policies to increase tree cover.

Research in North America, particularly by American Forests, suggests that urban tree cover may be directly equated to stormwater volumes and, therefore, to the costs of providing engineered structures for stormwater management. Tree planting policies have been justified on the financial benefits associated with their stormwater management function alone, notwithstanding the broader spectrum of benefits they provide within the urban environment.

This paper presents preliminary research aimed at transferring these findings into a UK context. Two residential morphology units (RMUs) have been defined within the city of Sheffield, for which current levels of tree cover have been accurately quantified. Current tree cover levels are relatively low, but approaches to integrating more trees into these two landscape types are outlined.

		Green roof hydrology and stormwater management


Back		One of the many benefits of green and vegetated roofs is associated with the role they play in smart water management, particularly in urban areas. The following papers relate to hydrology and stormwater management. Papers:

		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
		Abstract: A simple conceptual model for green roof hydrological processes is shown to reproduce monitored data, both during a storm event, and over a longer continuous simulation period. The model comprises a substrate moisture storage component and a transient storage component. Storage within the substrate represents the roof’s overall stormwater retention capacity (or initial losses). Following a storm event the retention capacity is restored by evapotranspiration (ET). However, standard methods for quantifying ET do not exist. Monthly ET values are identified using four different approaches: analysis of storm event antecedent dry weather period and initial losses data; calibration of the ET parameter in a continuous simulation model; use of the Thornthwaite ET formula; and direct laboratory measurement of evaporation. There appears to be potential to adapt the Thornthwaite ET formula to provide monthly ET estimates from local temperature data. The development of a standardized laboratory test for ET will enable differences resulting from substrate characteristics to be quantified.

		Stovin, V, 2009, “The use of green roofs to manage urban stormwater”, Water and Environment Journal. Published Online: May 12 2009, DOI: 10.1111/j.1747-6593.2009.00174.x
		Abstract: Green roofs have considerable potential for stormwater source control, both for new developments and as a retrofit option. In the United Kingdom the lack of local quantitative performance data and modelling tools, together with more general barriers to sustainable drainage systems (SUDS) implementation, may explain their limited uptake to date. This paper presents preliminary findings from a small-scale instrumented green roof test plot located in Sheffield, UK. During spring 2006 the average volume retention was 34% and the average peak reduction was 57%. The key hydrological determinants were the antecedent dry weather period (ADWP), mean rainfall intensity and rainfall depth. Detailed examination of rainfall–runoff relationships in summer 2007 demonstrates the dependency of performance on antecedent moisture conditions. Structural appraisal of a range of flat roof types suggests that retrofitting a green roof will be a feasible option in many cases, particularly for concrete slab roofs.

		Stovin, V.R., Jorgensen, A., and Clayden, A., 2008, Street trees and stormwater management, Arboricultural Journal. Vol. 30, No. 4, 297-310, ISSN 0307-1375.
		Abstract: Urban trees play an important role in the urban hydrological cycle, yet little consideration has been given in the UK either to the increasing pressures that act to reduce urban tree cover or the opportunities that might be provided by land-use planning policies to increase tree cover. Research in North America, particularly by American Forests, suggests that urban tree cover may be directly equated to stormwater volumes and, therefore, to the costs of providing engineered structures for stormwater management. Tree planting policies have been justified on the financial benefits associated with their stormwater management function alone, notwithstanding the broader spectrum of benefits they provide within the urban environment. This paper presents preliminary research aimed at transferring these findings into a UK context. Two residential morphology units (RMUs) have been defined within the city of Sheffield, for which current levels of tree cover have been accurately quantified. Current tree cover levels are relatively low, but approaches to integrating more trees into these two landscape types are outlined.

		Stovin, VR , Dunnett, N, Hallam, A, 2007, Green Roofs – getting sustainable drainage off the ground, 6 th International Conference of Sustainable Techniques and Strategies in Urban Water Management (Novatech 2007), Lyon, France, 25-28 June, pp 11-18. ISBN 2-9509337-7-7.
		Abstract: Green roofs have considerable potential for stormwater source control, both for new developments and as a retrofit option. In the UK the lack of local quantitative performance data and modelling tools may explain their limited uptake to date. This paper presents preliminary findings from a small-scale instrumented green roof test plot located in Sheffield, UK. During Spring 2006 the average volume retention was 34% and the average peak reduction 57%. The key hydrological determinants were the antecedent dry weather period, mean rainfall intensity and rainfall depth. Structural appraisal of a range of flat roof types suggests that retrofitting a green roof will be a feasible option in many cases, particularly for concrete slab roofs.