86 research outputs found
Moisture content behaviour in extensive green roofs during dry periods: the influence of vegetation and substrate characteristics
Evapotranspiration (ET) is a key parameter that influences the stormwater retention capacity, and thus the hydrological performance, of green roofs. This paper investigates how the moisture content in extensive green roofs varies during dry periods due to evapotranspiration. The study is supported by 29 months continuous field monitoring of the moisture content within four green roof test beds. The beds incorporated three different substrates, with three being vegetated with sedum and one left unvegetated. Water content reflectometers were located at three different soil depths to measure the soil moisture profile and to record temporal changes in moisture content at a five-minute resolution. The moisture content vertical profiles varied consistently, with slightly elevated moisture content levels being recorded at the deepest substrate layer in the vegetated systems. Daily moisture loss rates were influenced by both temperature and moisture content, with reduced moisture loss/evapotranspiration when the soil moisture was restricted. The presence of vegetation resulted in higher daily moisture loss. Finally, it is demonstrated that the observed moisture content data can be accurately simulated using a hydrologic model based on water balance and two conventional Potential ET models (Hargreaves and FAO56 Penman–Monteith) combined with a soil moisture extraction function. Configuration-specific correction factors have been proposed to account for differences between green roof systems and standard reference crops
The influence of substrate and vegetation configuration on green roof hydrological performance
A four-year record of rainfall and runoff data from nine different extensive (80 mm substrate) green roof
test beds has been analysed to establish the extent to which the substrate composition and vegetation
treatment affect hydrological performance. The test beds incorporated three different substrate components
with different porosity and moisture retention characteristics, and three different vegetation
treatments (Sedum, Meadow Flower and unvegetated).
Consistent differences were observed, with the vegetated beds showing higher levels of rainfall retention
and better detention compared with unvegetated beds. The seasonal Meadow Flower beds had
similar hydrological performance to Sedum-vegetated beds. There was a 27% performance reduction in
annual volumetric retention attributable to differences in substrate and vegetation. The beds with the
most porous/permeable substrates showed the lowest levels of both retention and detention.
As with previous studies, retention efficiency in all nine beds showed a strong dependency on rainfall
depth (P), with retention typically >80% for events where P < 10 mm, but significantly lower when
P > 10 mm. The effects of vegetation and substrate were most evident for rainfall events where P > 10 mm,
with the mean per-event retention varying between beds from 26.8% to 61.8%. On average, the test beds
were able to retain the first 5 mm of rainfall in 65% of events where P > 5 mm, although this ranged from
29.4% to 70.6% of events depending on configuration. In terms of detention, all but one of the test beds
could achieve runoff control to a green field runoff equivalent of 2 l/s/ha for more than 75% of events.
Detention was also characterised via the calibration of a reservoir-routing modelthatlinked net rainfall
to the measured runoff response. The parameter values identified here – when combined with a suitable
evapotranspiration/retention model – provide a generic mechanism for predicting the runoff response
to a time-series or design rainfall for any unmonitored system with comparable components, permitting
comparison against local regulatory requirements
Effects of recycled aggregate growth substrate on green roof vegetation development: a six year experiment
Green roofs have the potential to address several of the environmental problems associated with urbanisation, and can be used as mitigation for habitats lost at ground level. Brown roofs (a type of green roof) can be used to mitigate for the loss of brownfield habitat, but the best way of designing these habitats remains unclear. This paper reports an experiment to test the effects of different types of recycled aggregate on the development of vegetation assemblages on brown roof mesocosms. Five recycled aggregates were tested: (1) crushed brick, (2) crushed demolition aggregate, (3) solid municipal waste incinerator bottom ash aggregate, (4) a 1:1 mix of 1 and 2, and (5) a 1:1 mix of 3 and 2. Each was seeded with a wildflower mix that also included some Sedum acre and vegetation development was studied over a six-year period. Species richness, assemblage character, number of plants able to seed, and plant biomass were measured. Drought disturbance was the key factor controlling changes in plant assemblage, but effects varied with substrate treatment. All treatments supported a similar plant biomass, but treatments with a high proportion of crushed brick in the growth substrate supported richer assemblages, with more species able to seed, and a smaller amount of Sedum acre. Crushed brick, or recycled aggregates with a high proportion of crushed brick, are recommended as good growth substrate materials for encouraging brown roof plant diversity. This investigation demonstrates the importance of multi-year studies of green roof development for the generation of robust findings
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