Delayed stormwater runoff
Even though green roof systems retain
stormwater, runoff will still occur after the
media becomes saturated. However, runoff is
delayed because it takes time for the media to
become saturated and for the water to drain
through the media. This delay can prevent
stormwater sewer systems from overflowing,
by allowing it to process runoff for a longer
time at a lower flow rate. Green roofs can
delay runoff between 95 min (Liu, 2003) and
4 h (Moran et al., 2004), compared with the
reference roofs for which runoff was nearly
instantaneous. After runoff begins on a green
roof system, the rate at which the rain leaves
the roof is slower than a nongreened roof
because of the nature of the green roofing
components. Liu (2003) found that when
initial rainfall was 2.8 mm/h, runoff from
the green roof was reduced to 0.5 mm/h.
North Carolina researchers found a 57% to
87% reduction in flow rates on a green roof
(Moran et al., 2005). By slowing down the
rate of runoff and turning it out over a longer
period of time, green roofs can help mitigate
the erosional power of runoff that does enter
streams, either through direct runoff or storm
sewers.