Modelling of green roof hydrological performance for urban drainage applications Luca Locatelli a,⇑ , Ole Mark b , Peter Steen Mikkelsen a , Karsten Arnbjerg-Nielsen a , Marina Bergen Jensen c , Philip John Binning a a Technical University of Denmark, Dept. of Environmental Engineering (DTU Environment), Miljoevej, Building 113, 2800 Kgs Lyngby, Denmark b DHI, Agern Allé 5, 2970 Hørsholm, Denmark c University of Copenhagen, Rolighedsvej 23, 1958 Frederiksberg, Denmark article info Article history: Received 20 December 2013 Received in revised form 1 October 2014 Accepted 11 October 2014 Available online xxxx This manuscript was handled by Konstantine P. Georgakakos, Editor-in-Chief, with the assistance of Michael Bruen, Associate Editor Keywords: Green roofs Water Sensitive Urban Design Modelling Rainfall–runoff Retention Detention summary Green roofs are being widely implemented for stormwater management and their impact on the urban hydrological cycle can be evaluated by incorporating them into urban drainage models. This paper pre- sents a model of green roof long term and single event hydrological performance. The model includes sur- face and subsurface storage components representing the overall retention capacity of the green roof which is continuously re-established by evapotranspiration. The runoff from the model is described through a non-linear reservoir approach. The model was calibrated and validated using measurement data from 3 different extensive sedum roofs in Denmark. These data consist of high-resolution measure- ments of runoff, precipitation and atmospheric variables in the period 2010–2012. The hydrological response of green roofs was quantified based on statistical analysis of the results of a 22-year (1989–2010) continuous simulation with Danish climate data. The results show that during single events, the 10 min runoff intensities were reduced by 10–36% for 5–10 years return period and 40–78% for 0.1–1 year return period; the runoff volumes were reduced by 2–5% for 5–10 years return period and 18–28% for 0.1–1 year return period. Annual runoff volumes were estimated to be 43–68% of the total precipitation. The peak time delay was found to greatly vary from 0 to more than 40 min depending on the type of event, and a general decrease in the time delay was observed for increasing rainfall inten- sities. Furthermore, the model was used to evaluate the variation of the average annual runoff from green roofs as a function of the total available storage and vegetation type. The results show that even a few millimeters of storage can reduce the mean annual runoff by up to 20% when compared to a traditional roof and that the mean annual runoff is not linearly related to the storage. Green roofs have therefore the potential to be important parts of future urban stormwater management plans. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Urbanization significantly affects the natural landscape turning green areas into built environment. This process creates new impervious surfaces such as roofs, roads, cycling lanes, sidewalks, public squares and parking areas which modify the natural water cycle. Impervious areas increase stormwater runoff peaks and vol- umes and reduce the time delay between peak rainfall and peak runoff when compared to natural areas (Bengtsson, 2005). Current urban drainage systems have limited capacity to deal with flooding and climate change will increase the risk of flooding from sewers in urban areas (Larsen et al., 2009; Madsen et al., 2009). Green roofs are one of the many WSUD (Water Sensitive Urban Design; the concept of water sensitive cities was presented by Wong and Brown, 2009), LID (Low Impact Development), SUDS (Sustainable Urban Drainage Systems), LIUDD (Low Impact Urban Design and Development) techniques aimed to improve storm- water management and address future climatic challenges. Green roofs have the great advantage of not using new spaces; in fact they can in some cases be retrofitted onto existing traditional rooftops. The roof area in urban residential areas can be as high as 40–50% of the total impervious area (Palla et al., 2009; Lindblom et al., 2011; Vezzaro et al., 2012). Green roofs are claimed to reduce the risk of urban flooding and to re-establish a more natural water balance (Bengtsson et al., 2005; VanWoert et al., 2005) and to reduce water and contaminant loads to sewer systems (Buccola and Spolek, 2011). http://dx.doi.org/10.1016/j.jhydrol.2014.10.030 0022-1694/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +45 4525 1432. E-mail address: lulo@env.dtu.dk (L. Locatelli). Journal of Hydrology xxx (2014) xxx–xxx Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol Please cite this article in press as: Locatelli, L., et al. Modelling of green roof hydrological performance for urban drainage applications. J. Hydrol. (2014), http://dx.doi.org/10.1016/j.jhydrol.2014.10.030