The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 20

permeable pavement are examples of hydrologic source controls, as surface
runoff simply does not occur for many storm events. Sometimes other SCMs,
such as bioretention and swales, are considered at- or near- source controls if
they are installed close to the point of runoff or contaminant generation, such
that runoff is well-managed before discharging off-site.
• Treatment usually refers speciically to managing contaminants. Contaminants
found in stormwater runoff vary in chemical, biological and physical
composition, as well as concentration. Where water quality is a concern, the
appropriate SCM for a given site should be selected at least in part based on
the mechanism(s) it provides for removing or reducing contaminant(s) speciic
to the site or receiving environment. For example, sediment is usually removed
well enough by gravitational settling, which can be provided by a pond or a
simple grassed swale, or by iltration through a ilter medium such as in
bioretention. Hydrocarbons, on the other hand, loat. Heavy metals such as
zinc and copper will often sorb to a ilter medium. Nitrogen removal usually
requires an anoxic (devoid of oxygen) environment, a narrow temperature
range, and a fair amount of time. Pathogens are destroyed by exposure to
ultraviolet rays from the sun. In the context of living roofs, the fundamental
question to consider with respect to water quality is: what, if any, is the source
of contaminants? This is explored in more detail in Section 2.6 .
2.1 THe urban waTer balance in municipal regulaTions
The integrity of aquatic ecosystems relies in part on a balanced distribution of
water amongst the components of the hydrologic cycle. In undisturbed environ-
ments, precipitation (rain, snow, sleet, etc.) is intercepted by a vegetated canopy
or soaked into the ground. Water captured by foliage is returned to the atmos-
phere directly by evaporation, or travels along plant elements as stemlow, ulti-
mately to seep into the ground near the base of the plant. The larger portion of
precipitation that passes through vegetation will iniltrate through the ground
surface to recharge deep groundwater supplies, or travel through shallow soil
layers towards streams to recharge base (dry weather) low. Evaporation from the
upper soil layers and from open water in surface depressions combined with
transpiration from plants (together, known as ET) return signiicant amounts of
temporarily stored water to the atmosphere. In an undeveloped setting with sig-
niicant vegetative cover, and gentle land slope, a relatively small fraction of the
total precipitation becomes surface runoff, which makes its way overland to
receiving waters such as streams, rivers, lakes, estuaries, harbors and coastal
environments.
Urban development has offset nature's typical distribution of water. Whereas
nature provides uneven topography that stores water in localized depressions for
evaporation and recharge, site grading with human development eases construc-
tion and long-term use. Often, little open space is left for iniltration. What open
space is preserved or created is usually disturbed; construction to create parks,