Planning considerations - Living Roofs in Integrated Urban Water Systems - page 83

regression results were compared against these individual sites, the runoff coefi-
cient simply does not systematically represent performance.
Performance variability is not represented by a design storm approach using a
simpliied hydrologic model intended for drainage design. The Rational Method
is not intended to characterize performance per se of a complex system such as a
living roof. Also similarly to the CN s in Table 3.3 , the C v values presented in Figure
3.7 are offered only as a starting point to enable implementation where regula-
tory agencies rely on the Rational Formula.
3.5.4 US EPA Storm Water Management Model (SWMM)
The uS EPA's Storm Water Management Model (SWMM) is one of the most
widely used stormwater models (Burszta- Adamiak and Mrowiec 2013; Gironás
et al. 2009). It is an open-source model freely downloadable from the US EPA
website. It can be applied for single-event or continuous simulation scenarios.
First introduced in the 1970s, SWMM enables site to watershed scale modeling
including overland low paths, surface runoff, storm and combined sewers,
various conventional SCMs, and more. While predominantly used as a hydro-
logic modeling tool, it also has water quality simulation capability. SWMM pro-
vides a range of options for hydrologic modeling of many of the components.
In other words, for developing at-grade runoff hydrographs, or pipe network
discharge hydrographs, the user is not limited to a runoff coeficient, but may
alternatively choose amongst more sophisticated theoretical or empirical
models for runoff generation and transport. GI techniques were explicitly intro-
duced in model algorithms in 2010, and then revised in 2014 to SWMM
version 5.1.
In SWMM 5.1, a living roof is considered a layered system including a surface
layer, “soil” (growing media) layer and drainage “mat” (which would be consid-
ered a drainage layer as per the terminology in this book) ( Figure 3.8 ). The user
must input technical information characterizing the overall roof (namely slope),
drainage mat (thickness, void fraction and roughness), and growing media. Char-
acterizing the growing media requires substantial technical information.
SWMM 5.1 assumes moisture moves vertically through the growing media
according to the Green-Ampt iniltration model. The relative merits of Green-Ampt
iniltration theory as applied to living roof systems were discussed in Chapter 2 .
Data needs for the growing media include: depth (allowable range 75-150 mm),
porosity, ield capacity, wilting point, hydraulic conductivity and suction head. These
data are speciic to the particular products used in a speciic growing media. Their
values will change amongst media of different compositions. Ideally, the growing
media supplier would provide this sort of technical information. If they are not
available, they should be measured in a laboratory. As physical characteristics of
engineered media differ signiicantly from natural soils, living roof parameterization
should not be assigned from literature values for natural soils.
In theory, SWMM assumes that the moisture content of the media must
exceed the ield capacity before runoff will be routed to the drainage mat, while