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 – the runoff arising from green roofs in response to a range of design and routine rainfall events. The ultimate goal is to develop a generic modelling tool, which predicts runoff directly from rainfall inputs and selected key physical configuration parameters. We are conducting fundamental research into substrate and drainage layer hydrological performance, using a range of instrumented test plots, full-scale roofs and laboratory experiments. Evapotranspiration (ET) determines the rate at which a roof regains its moisture retention capacity; hence much of our current research effort focuses specifically on the identification of suitable ET values for unmonitored roofs.Please follow the links below for further information, or contact firstname.lastname@example.org
Hydrological Performance Monitoring
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).
Figure 1 Mappin Building Hydrological Monitoring Test Bed
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
We also have ten additional test beds, being monitored to assess the effects of substrate and planting (Figures 4 & 5) (Simon Poë, 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