Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Stormwater Management Modelling for Green Roof Systems Virginia Stovin University of Sheffield, v.stovin@sheffield.ac.uk
Overview It is widely reported that green roofs provide both retention and detention of rainfall runoff. From a stormwater management perspective, it is important to quantify this hydrological performance, and to be able to predict performance in response to unseen, arbitrary, rainfall inputs, including high return period design events and/or continuous rainfall time series. Numerous individual empirical studies, on both full-scale roofs and test plots, have provided useful data. However, without proper understanding of the controlling physical processes, findings cannot be transferred to different roof configurations or climates. Hydrological performance observations, based on a 3 x 1 m extensive green roof test bed in Sheffield (Figure 1), have been reported by Stovin et al. (2012). Based on 29 months continuous records of rainfall and runoff, 50.2% cumulative annual retention was observed. However, retention in 21 significant events (> 1 year return period) was reduced to 30%. The roof’s finite retention capacity means that the maximum possible retention percentage decreases as storm depth increases, varying from 0 to 20 mm or 0 to 100%.
Figure 1 – Mappin Test Bed
Figure 2 – Conceptual Hydrological Model
Regression analysis showed that it was not possible to relate storm retention (depth or percentage) to either the ADWP (Antecedent Dry Weather Period) or the rainfall depth, or to more complex multi-parameter formulae. Stovin et al. (2012) argued that the inter-event processes are too complex to be captured by regression-based modelling. Instead, an understanding of the hydrological processes affecting the flux of moisture into and out of the substrate (and drainage layer) is required. With reference to Figure 2, the substrate’s maximum water-holding capacity defines the condition when the substrate can hold no more moisture under gravity (i.e. its field capacity).
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield The moisture content at any given time will lie somewhere between field capacity and a minimum practical moisture content. In a rainfall event, the substrate will absorb moisture until field capacity is reached. If further moisture is added, excess runoff will pass through the substrate and leave the roof via the underlying drainage layer, where it will be temporarily detained before becoming runoff. Detention effects can be modelled using reservoir routing. Between rainfall events the roof’s storage capacity will be restored via evapotranspiration. Kasmin et al. (2010) demonstrated that this basic conceptual model could be calibrated to replicate the observed data from the Sheffield test bed (Figure 3). 2.5
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Figure 3 – Model results for 13-16 June 2007
Generalisation of the model requires appropriate input parameters and process models to describe: i) potential and actual evapotranspiration rates as a function of substrate, plant and climatic characteristics; and ii) detention due to both the substrate and the drainage layer. Research activities in WP2 and WP3 have focused on enhancing our understanding of these key processes and have led to the development of new models and the identification of relevant input parameter values.
Further Reading 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. Stovin, V., Vesuviano, G. and Kasmin, H., 2012, The hydrological performance of a green roof test bed under UK climatic conditions, Journal of Hydrology, Vol. 414-415, 148-161 https://sites.google.com/a/sheffield.ac.uk/green-roof-research/ http://greenroof.shef.ac.uk/