Planning considerations - Living Roofs in Integrated Urban Water Systems - page 72

considered as a starting point to facilitate technology adoption within existing
stormwater engineering frameworks. As the body of knowledge linking design
to operation grows through research and practice, revision, redevelopment or the
introduction of altogether better estimators is anticipated and welcomed.
The discussion in the following sections presumes the reader is familiar with
overall frameworks. It should not be interpreted as stand-alone methodology or
a substitution for technical references, for example professional manuals of
practice, authoritative textbooks, or regulatory-endorsed design guides.
3.5.1 Design objectives for single event design storms and continuous
simulation
In planning stormwater systems, engineers use mathematical equations to model a
system design's inluence on performance. Design objectives usually require
calculations that show that runoff from a developed site incorporating land-use
planning and SCMs mimics runoff from the pre- development or natural condition
of the site, or satisies an arbitrarily determined limit. The overall number of SCMs,
and each individual footprint, shape and hydraulic coniguration are dictated by
satisfying the stormwater control objectives and physical site constraints. Some of
the most common quantitative hydrologic control objectives include:
• Match the post- development peak low to pre- development peak low rate
for a speciic design storm(s) (a combination of rainfall depth or intensity and
duration for a unique storm event). The 2-year, 10-year and 100-year, 24-hour
ARI are among the most common design storms in the United States, as per
the risk assessments anticipating damage to receiving waters, property and
loss of life described in Chapter 2 . Peak low mitigation is the primary
(perhaps sole) hydrologic objective in conventional stormwater management
planning and found in the majority of municipal or state regulations.
• Match the post- development runoff volume to pre- development runoff
volume for a speciic design storm(s). This objective is typically aimed at
small(er) depth design storms, such as the 75th-95th percentile events, and
achieved by retention and reuse. It is found where GI is promoted. For
example, Section 438 of the 2007 (uSA) Energies Independence and Securities
Act requires on-site retention of the 95th percentile design storm from all
federal facilities undergoing at least 464 m 2 of new or redevelopment.
• Retain x- mm of rainfall or runoff per event. Retroit of GI in CSO communities
is designed to alleviate pressure on over-capacity sewer networks, rather than
matching pre-development conditions. For example, New York City and
Philadelphia's GSI plans call for engineered systems that retain the runoff from
25 mm of rainfall (City of New York 2012; PWD 2009).
In terms of water quality treatment, the objective is usually to:
• Capture the water quality volume (WQV, the runoff produced by the
designated design storm for water quality treatment) in an SCM. The intention