Hydrological Performance of Green Roofs
Introduction
This research area is led by Dr Virginia Stovin, of the Department of Civil and Structural Engineering. Virginia is an urban drainage engineer, with a long-standing interest in the role that SUDS – and retrofit SUDS – need to play in stormwater management. Green roofs provide source control for rainfall, retaining some of the rainfall and attenuating the remaining runoff.
Urban drainage engineers need to quantify these processes in order to accurately model flood and pollution control in urban catchments, and it is clear that a roof’s performance will vary in response to local climatic inputs and to roof configuration variables, such as the type and depth of substrate, slope and vegetation.
The main objective of our green roof research is to quantify – and develop models to predict
Please follow the links below for further information, or contact v.stovin@sheffield.ac.uk
• Hydrological Performance Monitoring
• PhD students
- Hartini Kasmin
- Simon Poë
- Gianni Vesuviano
• Publications
• Marie-Curie Industry Academia Partnership with Zinco [link?]
• Retrofit SUDS Research Group
Hydrological Performance Monitoring
Introduction
In 2006 we established our first instrumented test plot, located on the Mappin Building roof, University of Sheffield. This test bed has yielded high quality continuous record of rainfall and runoff and forms the basis of most of our published outcomes to date (Hartini Kasmin, PhD student).
This test bed has subsequently (2009) been duplicated 10 times as part of a Green Roof Demonstration project [link?] on the roof of the Hadfield Building. These beds will feed into a comparative study of how vegetation and substrate affect runoff retention and attenuation (Simon Poe, PhD student).
Real-time monitoring data from these test plots can be accessed from here.
We have also established systems to monitor the performance of a full-scale green roof on the University’s Jessops West Building.
Mappin Building Test Bed Set-up
The test bed (3 x 1m) (Figure 1) uses a standard commercial (Alumasc/Zinco) extensive green roof system, comprising a sedum vegetation layer growing in 80 mm of substrate. The base of the rig is laid at a slope of 1.5 °. The substrate is composed of a mixture of crushed brick and fines. A fine particle filter membrane separates the substrate from the underlying FloraDrain FD25 ‘egg box’ drainage layer. The drainage layer alone has a nominal retention capacity of 3 l/m 2 (i.e. 3 mm rainfall).
Runoff from the roof is collected in a tank via a gutter at its downstream end. The collection tank is tapered to provide greatest sensitivity for low levels of runoff. Depth in the collection tank is monitored using a pressure transducer to provide a continuous record of runoff. Rainfall is monitored using a standard rain gauge sited adjacent to the test bed. Data from the pressure transducer and the rain gauge is logged using a Campbell Scientific data logger (CR1000) at 1-min interval.
Figure 1 Mappin Building Hydrological Monitoring Test Bed
Selected Observations
Preliminary data on the test bed’s hydrological performance was presented by Stovin et al. (2007). Figure 2 presents sample monitored data from a storm that occurred on 14–15 February 2006. In this event 9.2 mm rain fell, resulting in only 3.55 mm runoff, i.e. 61% stormwater volume retention. The peak reduction was 61%. Over the whole of the (wet English) spring 2006, 11 events were monitored, and the average volume retention was found to be 34%, with an average peak reduction of 57% compared with the monitored rainfall. These figures are comparable with other European data reviewed by Mentens et al. (2003). Based on an average volume retention of 34%, it may be inferred that the roof could reduce annual runoff in many parts of the United Kingdom by 300 mm when compared with a conventional roof.
Figure 2 Rainfall–runoff response of the green roof test plot 14–15 February 2006
Data from summer 2007
Figure 3 shows the daily rainfall and runoff totals for the Sheffield test bed during June and July 2007. This period includes the period 24–25 June, when Sheffield was affected by serious flooding, largely as a result of the River Don bursting its banks in the city centre. One contributory factor in the flooding event was the heavy rainfall that occurred in the previous fortnight, leaving the ground saturated, and with little capacity to absorb further rainfall. The local records presented in Figure 3 show that nearly twice as much rain fell during 13-15 June (115.8 mm) compared with 24-25 June (61.8 mm).
The green roof temporal runoff response was as might be expected. The event on 11 June occurred after a long dry spell and all of the 12.8 mm rainfall was retained within the roof. At the start of the 13-15 June event some retention was observed, with 16.1 of 24.8mm rainfall retained (65%) on 13 June. The following day the roof was evidently saturated, with only 5% retention (of 74.4 mm) being observed. On 15 June, slightly negative retention was observed as some of the accumulated rainfall drained out of the roof. The roof had limited opportunity to regain its moisture-holding capacity over the following days, and only 27% of the 24.8 mm rainfall on 24 June was retained. The following day the roof was unable to offer any retention, with virtually 100% of the 37 mm rainfall becoming runoff.
Overall retention in June still amounted to 33%, whereas the less extreme rainfall conditions in July enabled the roof to achieve 45% retention. In May and August 2007 the roof retention was 79 and 100%, based on 80.2 and 23.6 mm rainfall, respectively.
The performance data presented above suggests that green roofs alone cannot provide complete protection against very large (extreme) events. However, this does not diminish their value as source control elements within more comprehensive SUDS treatment train approaches. The data provided here does demonstrate that they can play a significant role in reducing the total volume of runoff, with potential benefits to runoff quality. The CIRIA SUDS Manual (C697, 2007) highlights the importance of using source controls that mimic natural interception storage to retain small events (< 5 mm) to reduce the flashiness of urban runoff and the pollutant loads conveyed to urban watercourses.
Figure 3 Rainfall and runoff totals for June and July 2007
Hadfield Roofs
We also have ten additional test beds, being monitored to assess the effects of substrate and planting (Figures 4 & 5) (Simon Po ë, PhD student).
Figure 4 Aerial photograph of the demonstration roof
Figure 5 Close-up of the test beds
Jessop West roof
The recently-constructed Jessop West University Building provides an opportunity to monitor a full-scale biodiversity roof.
Figure 6 Green Roof on the University of Sheffield's Jessop West Building
Publications
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.
Kasmin, H, Stovin, V and Hathway, A, (2009) “Towards a generic rainfall-runoff model for green roofs”, 8 th Int. Conf. On Urban Drainage Modelling, 7-11 Sept, Tokyo, Japan.
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.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.
