The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 24

incentive for providing runoff retention at the source of runoff generation.
Typical municipal regulations have aimed to limit peak lows, requiring deten-
tion of runoff from larger, relatively infrequent rainfall events, such as the 2-10
year annual recurrence interval (ARI) 3 events. The intent is to prevent erosion
and mitigate lood risk. In terms of the rainfall spectral frequency analysis
( Figure 2.1 ), these storms are usually larger than the 95th percentile event. The
two-year ARI event was established as a design objective by simplifying research
attempting to identify the frequency of occurrence of critical storms that were
thought to determine the size and shape of the receiving streams (Brown and
Caraco 2001; Leopold et al. 1964; MacRae 1991; MacRae and Rowney 1992;
McCuen 2005; Roesner et al. 2001; USEPA 2004). Additional levels of deten-
tion are usually required to attenuate peak lows from the 25-100 year ARI.
Managing runoff from these storm events aims to protect property and prevent
loss of life. 4
Recent studies are demonstrating that the typical “one-size-its-all” manage-
ment approach of peak low control with retention or detention ponds has not
been entirely effective for mitigating the impacts of hydromodiication and urban
runoff (Bledsoe et al. 2012). For example, the frequency of runoff from an urban
catchment has been identiied as a key contributor to stream ecosystem degradation
(Walsh et al. 2009). Reducing runoff volumes by retaining runoff on-site, and
considering runoff timing and duration is also required. Where CSOs are a
problem, the most effective solution is to simply keep runoff out of the sewers.
These are all beneits afforded by living roofs.
Nonetheless, while living roofs are an active area of research regarding storm-
water performance, focus has largely been on characterizing retention, rather
than quantifying the processes contributing to detention. Even where GI is philo-
sophically embraced or incentivized, updating regulatory-endorsed stormwater
design protocols and calculation tools tends to lag. The designer may ind it com-
plicated to present a compelling case for living roof implementation where only
detention of larger storm events is required by the permitting authority. This topic
provides quantitative methods to address at least some of these challenges, but
acknowledges the early state of understanding the science behind, and develop-
ing predictive tools for, living roof hydrology.
2.4 How a living roof “works” To conTrol sTormwaTer runoff
The living roof's primary role in urban stormwater management is to prevent
runoff generation from rooftops. A living roof re-introduces some of the natural
“sponge” that is lost to impervious surfaces. In terms of the hydrologic cycle,
living roofs store precipitation for subsequent ET. In terms of typical stormwater
management goals, living roofs provide retention and detention, and effectively
increase the time of concentration, delaying runoff peaks and lowering runoff
discharge rates. These characteristics are related: rainfall retention almost always
coincidentally mitigates potential peak low rate, and delays its timing.