Planning considerations - Living Roofs in Integrated Urban Water Systems - page 74

outcomes in practice, comprehensive and complementary land-use planning
and SCM installation with a GSI approach is required.
• On a long- term basis, match up to 95 percent of the low rate frequency
spectrum or runoff volume frequency spectrum between pre- and post-
development. In other words, the frequency of occurrence of peak lows or
runoff volumes of the most commonly occurring events must be equivalent
pre- to post-development on a long-term basis, rather for every individual
isolated storm.
• Statistically assessing the range of hydrologic responses that may arise from
storms of similar depth and duration, but subject to contrasting antecedent
conditions.
The signiicant advancement in SCM design by continuous simulation compared to
the design storm approach is a theoretically more realistic understanding of system
performance over a range of climate and storm conditions, perhaps leading to
more resilient design. Arguably, two storms are rarely (if ever) identical in nature,
therefore it is of little value to match the hydrologic characteristics of every event.
On the other hand, over the long- term, between pre- and post- development, it
should be reasonable to mimic occurrences of similar ranges of coupled occurrence
and magnitudes of peak lows and volumes, i.e., with design for the frequency
spectrum. The historic challenge of applying continuous simulation for stormwater
design has been a lack of tools (software) and guidance to implement or interpret
continuous simulation, or readily-available location-speciic historic precipitation
and/or ET data with appropriate quality and resolution. This is changing rapidly. At
present, it is still a signiicant challenge to simulate the behavior or performance of
individual GI SCMs on a comparable basis.
Simulation models freely available in the public space and backed by
government agencies tend to enjoy widespread use. The US Army Corps of
Engineers provides HEC- HMS. On behalf of its community of practice, Seattle
Public Utilities has developed a continuous simulation using the Western
Washington Hydrologic Model (WWHM, a locally calibrated version of the
Hydrological Simulation Program- Fortran [HSPF]). The uS EPA Stormwater
Management Model (SWMM, discussed in Section 3.5.4 ) can be run for
continuous simulation. SWMM and HSPF have long histories of application for
drainage design - design of piped storm sewers and channels. In 2014, the US
EPA released a desktop continuous simulation tool called the National Stormwater
Calculator ( www.epa.gov/nrmrl/wswrd/wq/models/swc/) . This tool couples
SWMM with online databases to provide location- speciic weather, soils and
topography, among other information needed for basic site stormwater analysis
and planning using continuous simulation. In WWHM, SWMM and the National
Stormwater Calculator, recent development includes new routines or assumptions
regarding GI SCMs. Despite generally good theoretical development, veriication
of these models, or more speciically GSI-related functions, to reproduce “real”
ield observations is in its infancy, although it is an active topic for research.