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Hydrology and stormwater publications

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:

Stovin, V., Vesuviano, G., Kasmin, H., 2011. The Hydrological Performance of a Green Roof Test Bed under UK Climatic Conditions. Journal of Hydrology. 10.1016/j.jhydrol.2011.10.022

Abstract: This paper presents new rainfall and runoff data from a UK green roof test bed which has been collected almost-continuously over a 29-month period from 01/01/2007 to 31/05/2009. Overall, the monitoring period was fairly typical of the location’s long-term climatic averages, although the data set includes some extreme events in June 2007, which were associated with serious flooding locally. To focus on the system’s performance under rainfall events likely to be of interest from an urban drainage/stormwater management perspective, return period analysis has been applied to identify those storm events with a rainfall depth in excess of 5 mm and a return period greater than one year. According to these criteria, 22 significant events have been identified, of which 21 have reliable runoff records. Overall the roof provided 50.2% cumulative annual rainfall retention, with a total volumetric retention equivalent to 30% during the significant events. The annual performance figures are towards the lower end of a range of international data, probably reflecting the fact that rainfall depths may be higher and evapotranspiration rates lower than in some more continental climatic settings. The roof’s finite retention depth means that the maximum possible retention percentage declines as storm depth increases, and retention varied from between 0 and 20 mm, or 0 to 100%. Although some attenuation and delay of peak runoff is generally observed (mean peak flow reduction of 60% for the 21 significant events), the irregularity of natural rainfall patterns, combined with the variable influence of detention storage in specific events, makes the identification of peak-to-peak lag times difficult and arguably meaningless. Regression analyses have been undertaken to explore the potential to predict the roof’s hydrological performance as a function of storm characteristics. However, these are shown to have poor predictive capability, even for the system from which they were derived. Through a detailed examination of data from three contrasting events, it is argued that the inter-event processes are too complex to be captured by this type of modelling approach. Instead, an understanding of the hydrological processes affecting the flux of moisture into and out of the substrate is required to explain the observed runoff response. Locally-derived evapotranspiration rates and the roof’s observed maximum retention capacity are utilised to provide pragmatic guidance on the retention performance to be expected in response to selected design events.
                         
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.

Berretta, C. Poe, S., Stovin, V.R., 2014. Moisture content behaviour in extensive green roofs during dry periods: The influence of vegetation and substrate characteristics. Journal of Hydrology, Volume 511, 16 April 2014, Pages 374-386

Abstract: Evapotranspiration (ET) is a key parameter that influences the stormwater retention capacity, and thus the hydrological performance, of green roofs. This paper investigates how the moisture content in extensive green roofs varies during dry periods due to evapotranspiration. The study is supported by 29 months continuous field monitoring of the moisture content within four green roof test beds. The beds incorporated three different substrates, with three being vegetated with sedum and one left unvegetated. Water content reflectometers were located at three different soil depths to measure the soil moisture profile and to record temporal changes in moisture content at a five-minute resolution. The moisture content vertical profiles varied consistently, with slightly elevated moisture content levels being recorded at the deepest substrate layer in the vegetated systems. Daily moisture loss rates were influenced by both temperature and moisture content, with reduced moisture loss/evapotranspiration when the soil moisture was restricted. The presence of vegetation resulted in higher daily moisture loss. Finally, it is demonstrated that the observed moisture content data can be accurately simulated using a hydrologic model based on water balance and two conventional Potential ET models (Hargreaves and FAO56 Penman–Monteith) combined with a soil moisture extraction function. Configuration-specific correction factors have been proposed to account for differences between green roof systems and standard reference crops.