The influence of plant type on green roof rainfall retention

Abstract

Green roofs can mitigate the flood risk by reducing the volume of runoff through direct interception and subsequent evapotranspiration (ET), but the planting choices can influence the extent of this service. Glasshouse experiments were carried out in spring/ summer using simulated rainfall to compare the rainfall retention capacity of three physiologically active broadleaf species (Heuchera micrantha, Salvia officinalis and Stachys byzantina), which have previously shown to provide improved rooftop cooling, to an industry standard green roof species, Sedum spurium. Furthermore, the impact of varying ambient temperature and humidity conditions on the ability of these species to restore the substrate retention capacity through ET was also tested in a series of controlled-environment experiments simulating a range of potential UK summertime scenarios. Canopies alone retained up to 17% (Sedum) of the total rainfall in this study, with Salvia and Stachys also retaining in excess of 10%, and can make a substantial contribution to rainfall retention on a green roof. Rainfall retention was also strongly correlated with total ET in the preceding 72 h (R 2 = 0.94; P < 0.001). Species with high ET rates (Salvia and Stachys) were able to provide the greatest stormwater management service (up to 72% retention due to ET component). Furthermore, species 'rankings', in terms of ET and thus restoration of substrate retention capacity, were the same in all simulated potential UK summertime temperature and relative humidity scenarios, indicating that 'superior' species will be able to provide the greatest stormwater management provision in all climatic conditions.

Urban stormwater characterization, control, and treatment

“…Although sedums are the most common plant type used on extensive green roofs, there is much interest in vetting plant choices for improved retention. A study comparing three broadleaf species to the industry standard sedum found that two of the three species ( Salvia and Stachys ) demonstrated the greatest retention due to high ET rates between rainfall events (Kemp, Hadley, & Blanusa, 2019). Canopy interception, which was greatest for the sedum, was found to be a statistically significant factor in retention but was less important than ET and overall substrate water storage.…”

A simple method for sizing modular green–blue roof systems for design storm peak discharge reduction

“…For example, Jahanfer et al [14] reported peak flow reductions of 58% for "large rainfall events" but large was only defined as depths greater than 10 mm (0.4 in). Kemp et al [15] found vegetative canopies retained 2-17% of rainfall, but for a rainfall depth of only 9.3 mm (0.37 in). This pattern of analyzing smaller storms is very clearly shown by Stovin et al [11] who created detention design charts that show approximations of both peak discharge reduction and runoff delay across a range of rainfall depths.…”

On the impact of porous media microstructure on rainfall infiltration of thin homogeneous green roof growth substrates

“…Other experimental work on green roofs has focused on the plant ecology [8,9] with the aim of greater climatic resilience and possible additional benefits to storm water performance in terms of evapotranspirative efficiency. A final aspect of experimental research is the determination of the growth substrate material composition on detention and retention performance.…”