• Average retention:
37.1%
• Average peak runoff reduction: 61.8% • Average peak runoff delay:
47.2 minutes
Some of the most extreme rainfall events in recent memory occurred within this period of analysis, notably the June 2007 events that caused localised flooding in Sheffield: having retained 100% of the 12.8 mm that fell on 12th June, the subsequent 42.5 hour event (13 - 15th June) saw 99.6 mm of rainfall (peaking at 19.2 mm/hour). Yet runoff volume was still reduced by 12% and importantly peak runoff was reduced (by 26%) and delayed (by 30 minutes).
Importantly, the ZinCo green roof provided stormwater management benefits to every one of the 30 events; either reducing volume or reducing and/or delaying peak rates of runoff.
It is easy to see why green roofs are increasingly specified in urban developments: occupying otherwise- redundant urban roof space to improve the building’s performance (e.g. SUDS, thermal and acoustic insulation) and the environment (e.g. Urban Heat Island mitigation, biodiverse habitat). With green roofs tailored to deliver such diverse objectives, variable SUDS responses are inevitable.
Performance Differentials All drainage systems have a finite hydrologic capacity. However, a green roof’s capacity available at a storm’s outset (the Available Water Capacity or AWC) varies according to the antecedent storage volume. Evapotranspiration (ET) is critical in regenerating AWC, with the rate at which ET losses occur governed by (1) soil-water volume; (2) energy driving evaporation and transpiration; and (3) resistance to soil-water movements.
The climate affects two key determinants of ET losses: 1. Soil-water volume: influenced by rainfall volume and frequency; and
2. Energy driving evaporation and transpiration: dependent on solar radiation intensity.
Research findings will inevitably reflect the climate prevailing at that geographical location and/or at that time of the year. Such trends are evident in our 2008 research findings (as demonstrated in the graphs), with retention levels of 82% (of 80.3 mm rainfall) in the warmer summer months exceeding the 37% (of 88.5 mm) witnessed in the cooler, autumn months.
Yet, the incidence of precipitation can significantly affect the response as witnessed in June 2007, where significant monthly rainfall
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of 230 mm was largely concentrated across a small number of events, including one with 99.6 mm of rainfall. The retention percentage (22.7%) suffered as a result, however retention volumes (52 mm) remained high, relative to autumn and winter months.
The green roof specification - notably the substrate and the plant layer - largely governs the resistance to soil- water movements. Whilst maximum water-holding capacity defines the limits of the finite storage, the retention and release properties are most critical to the hydrologic response. The tenacity with which water is held in the substrate is a function of its structure and texture:
Texture: In relatively dry soils, thin films of water adsorb to the soil particles due to the very strong intermolecular forces between water and soil. Generally held with such tenacity that it is not available to plants, the scale of this force depends upon soil texture, with lower bonds formed with mineral aggregates than with organic matter.
Structure: Soil structure affects volumes held as pore- water. Pore-water is held by the greater intermolecular forces between water, compared to the water-air interface. These forces can be sufficient to resist gravity, but vary with pore-size – the greater forces being evident in smaller pores – and with soil-water volume – gravitational mass of higher volumes being most likely to induce downward flux.
Substrates should therefore be engineered with the appropriate balance of organic / mineral content and granular distribution to meet plants’ needs and benefit stormwater management. The choice of planting also affects the hydrologic response due to its impact on ET losses. Firstly, the plant’s architecture (i.e. the leaf surface area, exposure and shape) affects the amount of rainfall intercepted by the vegetation for evaporation back to atmosphere.
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