Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Field trials to establish the impact of substrate and vegetation on long-term rainfall-runoff response Christian Berretta University of Sheffield, c.berretta@sheffield.ac.uk
Introduction The objective of this research is to investigate the hydrological processes occurring in extensive green roof systems through data collected at the University of Sheffield during a continuous monitoring program of nine different green roof configurations. While previous phases of the project focused on studying each green roof component under controlled conditions (Workpackages 1 and 2), this study analyses the behaviour of the whole system in field conditions.
Overview of Methodology The monitored test beds (TB) are characterized by three vegetation treatments (Sedum, meadow flower mixture, no vegetation) and three substrate options with different organic matter content and particle size distribution (Figure 1). Two commercial substrates, ZincoAlumasc Heather with Lavender (HwL) and Sedum Carpet (SC) substrate, and a Lightweight Expanded Clay Aggregate (LECA) based substrate have been monitored. Since April 2010, rainfall depth, flow rate and climatic parameters have been recorded. Since March 2011 water content vertical gradient measurements have been collected in four test beds, characterized by sedum vegetation and different substrates (test beds 1 to 3) and no vegetation and HwL substrate (TB7).
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Figure 1 –The experimental site at the University of Sheffield
Preliminary Findings Per event retention performance varied depending on the initial water content within the substrate and the characteristics of the rainfall event. A consistent behaviour was observed among the tested green roof configurations (Figure 2, left). A greater retention was associated with HwL and SC substrate compared with LECA, which is characterized by
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield coarser particle size distribution. Vegetated configurations showed consistently higher performance. Sedum vegetation resulted in higher retention performance than Meadow Flower, particularly evident on LECA substrate. The water content behaviour during dry weather periods due to evapotranspiration (ET) and evaporation from non-vegetated configurations showed similar results, in terms of spring and summer daily rates, to the ET study performed in a controlled climate chamber at the University of Sheffield (Poë and Stovin, 2012). The water content vertical profile (Figure 2, right) varied depending on the substrate characteristics and the presence of vegetation. A clear influence of the water content and temperature on the daily ET rates was observed. The presence of vegetation resulted in higher daily water loss. The water content behaviour in time for HwL and SC substrates is consistent with the moisture retention results obtained through the pressure plate extraction test (pF curves), thus suggesting the influence of the substrate characteristics. Further analyses of the data series are in progress. This data base will support the validation of the hydrological model implemented in this project. 14/5/12
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7 Sep 2011 - 8 mm 26 Aug 2011 - 11.2 mm 25 Aug 2011 - 14.8 mm 13 June 2010 - 20.2 mm 6 Sep 2010 - 31 mm 1 Oct 2010 - 19 mm
Figure 2 – Retention performance comparison by selected rainfall events (left). Rainfall, runoff and moisture content during May 2012 (right).
Further Reading Poë, S., and Stovin, V. (2012) Advocating a Physically-based Hydrological Model for Green Roofs: Evapotranspiration during the Drying Cycle. World Green Roof Congress 2012, Copenhagen. 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.