Secondly, the plant’s physiology affects the rate of transpiration of pore-water. For example, succulents such as sedum species, with Crassulacean Acid Metabolisms (CAMs), regulate their water demands in line with its supply – closing their stomata to conserve water during warm, dry conditions and consuming significant volumes when supply is plentiful.
Research highlights the importance of specification The wide variance in the response of 9 different green roof configurations to the same storm event is highlighted in the graph, where Beds 1 to 3 are vegetated with sedum, 4 to 6 with meadow flower and 7 to 9 non- vegetated. Beds 1, 4 and 7 have ZinCo H&L substrate; 2, 5 and 8 have ZinCo Sedum substrate; and 3, 6 and 9 have a LECA-based substrate mix.
different retention and release tendencies.
Equally apparent was the beneficial impact of a sedum carpet layer – attributable to controlled water consumption and 90% vegetated coverage – with retention increased by as much as 28% (compared to the same configuration without vegetation) and 19% (compared to a meadow flower planting strategy). This differential is expected to further increase in cooler, dormant seasons, when the sedum coverage will remain relatively unchanged, yet the meadow flower will shed its leaves and reduce its coverage before reproducing from its root stock the following spring.
Summary If green roofs are to become integral components of SUDS strategies, increasingly accurate performance predictions will be required. A physics-based approach would appear the only apparent means to account for all of the influential variables introduced above. As our research into green roof hydrologic performance continues to this end, it is important to stress that engineered green roofs, such as ZinCo systems, have been seen to retain and detain rainfall in all events – including even the largest events. The extent of the benefit will vary geographically and seasonally, but also critically according to the specification of the green roof.
The importance of an engineered approach is evident in the research data set – with retention levels seen to increase by as much as 39% when ZinCo’s engineered substrates were used. Indeed, ZinCo’s ‘Sedum substrate’, displaying a similar maximum water-holding capacity as the horticultural, LECA-based mix, increased retention by more than 34%. This can be attributed to the blends’
Alumasc, as ZinCo’s UK supply partner for nearly 20 years and a founder member of GRO, is at the forefront of the green roof industry, developing long-term engineered solutions for a sustainable built-environment. For further details on ZinCo green roof systems, please call 01744 648400 or visit
www.alumascgreenroof.co.uk.
Below: Rockliffe Park Clubhouse, Hurwurth, Near Darlington
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