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Marie-Curie IAPP ‘Green Roof Systems’ Project

The Green Roof Research Conference 18-19 March 2013, Sheffield

A green roof runoff detention model Gianni Vesuviano The University of Sheffield, cip09gmv@sheffield.ac.uk

Introduction The limited uptake of green roofs as drainage devices in the UK and other countries may be related to a lack of understanding of their behaviour in response to typical design storm events. Hydrological models for green roofs have been proposed and tested, based on unit hydrograph theory (Villarreal & Bengtsson, 2005), storage routing (Kasmin et al., 2010), Hydrus-1D (Hilten et al., 2008; Palla et al. 2012) and a proprietary combination of methods (She & Pang, 2010). However, these models all depend on test system design, ignore synthetic drainage layers or rely on parameters that are difficult to estimate. Here, a twostage model for runoff detention, consisting of separately-modelled substrate and drainage layers in series, is presented and evaluated on a test system. Separation of hydrological modelling into two stages allows for parameters in each stage to be changed independently to reflect different system designs, while the use of nonlinear storage routing avoids the need for time-consuming substrate tests, which may give inconsistent results.

Overview of Methodology Controlled wetting events were applied to commercial drainage layers, as detailed in Vesuviano & Stovin (2012). High-resolution time-series runoff profiles were recorded in response to each wetting event. Physical configuration (e.g. component choice, roof slope) was varied systematically. For each test, nonlinear storage routing was applied to rainfallrunoff pairs and the routing parameters optimized for best model fit. The variation in parameters was evaluated with respect to test configuration and parameter values were consolidated, where statistically permitted, to simplify the model. The same experimental methodology was then repeated for substrate samples, similarly to Yio et al. (2012). Following these tests, a green roof system was built, consisting of a previously-tested drainage layer configuration under a previously-tested substrate layer configuration. This system was subjected to controlled wetting events, of constant (previously-tested) and design-storm (untested) profiles. The tested wetting profiles were input to the substrate hydrological model, and the output from that model was input to the drainage layer hydrological model. For all tests, the predicted output from the drainage layer model was compared to the recorded runoff profile for the two-layered test system.


Marie-Curie IAPP ‘Green Roof Systems’ Project

The Green Roof Research Conference 18-19 March 2013, Sheffield

Key Findings

Figure 1 – Two-stage modelling results for a constant- and a variable-intensity wetting event.

The two-stage model is able to predict the recorded runoff profile from the two-layered system with high accuracy for both constant- and variable-intensity events (mean Rt2 = 0.971). A slight over-estimate of attenuation is due to the natural variations between different batches of the same substrate. The high accuracy achieved by this model shows that it has the potential to predict the runoff performance of new green roof system designs as a response to arbitrary rainfall profiles.

Further Reading Hilten, R.N., Lawrence T.M. and Tollner E.W., 2008, Modeling stormwater runoff from green roofs with HYDRUS-1D, Journal of Hydrology, 358, 288-293. doi: 10.1016/j.jhydrol.2008.06.010 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 Palla A., Gnecco, I. and Lanza L.G., 2012, Compared performance of a conceptual and a mechanistic hydrologic models of a green roof, Hydrological Processes, 26, 73-84. doi: 10.1002/hyp.8112 She, N. and Pang, J., 2010, Physically based green roof model, Journal of Hydrologic Engineering, 15.6, 458-464. doi: 10.1061/(ASCE)HE.1943-5584.0000138 Vesuviano, G. and Stovin, V., 2012, “A Generic Hydrological Model for a Green Roof Drainage th Layer”, 9 International Conference on Urban Drainage Modelling (9UDM), 4-6 September, Belgrade, Serbia. Villarreal, E.L. and Bengtsson L., 2005, Response of a Sedum green roof to individual rain events, Ecological Engineering, 25, 1-7. doi: 10.1016/j.ecoleng.2004.11.008 Yio, M.H.N., Stovin, V., Werdin, J. and Vesuviano, G., 2012, “Experimental Analysis of Green Roof th Detention Characteristics”, 9 International Conference on Urban Drainage Modelling (9UDM), 4-6 September, Belgrade, Serbia.


A green roof runoff detention model - Gianni Vesuviano