Introduction - Living Roofs in Integrated Urban Water Systems - page 2

The historic focus of an urban drainage system was to expediently remove or
dispose of runoff so as not to disrupt urban activities, damage structures or
threaten public safety. Expedient removal is no longer the only goal or cost. In
some cases, it is not the goal at all. Almost every aspect of the hydrologic cycle
(water's distribution and lux in a watershed) is modiied by urban development.
In a natural forested condition, 10-20 mm of precipitation may be intercepted by
the vegetated canopy and iniltrated (soaked) into the ground before stormwater
runoff is generated at the surface. In an urbanized condition, runoff may be gen-
erated from as little as 2 mm of rain. Thus, in urban settings, lows are generated
almost every time it rains, and pollutants are transported to receiving waters such
as streams, rivers, lakes, estuaries, bays and harbors. Increased low rates, runoff
volumes and occurrence of runoff along with how quickly runoff is initiated con-
tribute to channel erosion and instability, which degrades both physical and bio-
logical habitat structure by a process known as hydromodiication (US EPA 1993).
Studies show that marked alteration of channel low processes is associated with
declining ecological health, or degradation of the physical channel attributes
required for normal ecological functioning (Gippel 2001). Across the United
States, receiving water quality has largely been considered “degraded” for
decades; pollutants carried by urban runoff are largely discharged without treat-
ment. Altogether, hydromodiication and pollutant loadings compromise aquatic
habitat, infrastructure and property almost every time it rains.
Reducing or avoiding impacts from “everyday” rainfall events is increasingly
incorporated into policy, but has not historically been the focus. Since 2001, US
state and municipal agencies in Portland, Philadelphia, Seattle, Atlanta, Chicago,
New York, Pittsburgh, Washington State, California, Maryland, Vermont and Vir-
ginia have introduced policies and related design requirements. Signiicant legis-
lation enacted in 2007, Section 438 of the USA Energy Independence and
Security Act, requires extensive on-site runoff control from “everyday” events for
federal facilities undergoing new or redevelopment. Living roof technology is per-
fectly suited to mitigate these sorts of storm events.
In many older cities, “everyday” stormwater impacts to receiving environ-
ments are exacerbated or even superseded by combined sewer overlows (CSOs).
Combined sewers are intended to carry sanitary sewerage and stormwater runoff
through the same pipes to a municipal wastewater treatment plant. In many
major cities around the world, urban inill and densiication now generate lows
well exceeding the carrying capacity of the combined sewer network. By design,
overlow points discharge untreated runoff and sanitary sewerage into receiving
environments when the capacity of the sewer is exceeded during wet weather
(e.g., rain or snowmelt). While the intention is to prevent overloading the munici-
pal wastewater treatment facility, and causing even greater volumes of untreated
wastewater discharge, the impacts to local receiving environments can be devas-
tating. In Brooklyn, NY, modeling predicts CSO events to occur almost every time
it rains, without intervention (City of New York 2008). In New Jersey, the state
with the highest population density in the United States, as little as 5 mm of rain