Planning considerations - Living Roofs in Integrated Urban Water Systems - page 85

3.5.5 USA National Stormwater Calculator
The USA's National Stormwater Calculator was publicly released in 2014 by the
uS EPA. The main engine behind the calculator is SWMM 5, but a number of
features simplify building and running a continuous simulation. The model
integrates with online databases to obtain local climate (rainfall and ET), soils and
topographic information to automatically populate many of SWMM's input data
requirements. Details regarding site layout are provided by the user, but extend
to relatively generic parameterization such as the fraction of total site area
occupied by each land use, and the fraction of a given land use managed by a
particular SCM, including GI SCMs. The user's manual suggests the model is
appropriate for screening level analysis of small footprint sites up to several dozen
acres in size with uniform soil conditions (Rossman 2014).
Default settings are provided for all elements of a generic living roof system
assembly. The depth of growing media and its saturated hydraulic conductivity
only may be modiied by the user. The model assumes a growing media porosity
of 45 percent, which is not editable, but the instruction manual claims the model
is not very sensitive to porosity value (Rossman 2014).
While the model calculates hydrology in 15-minute increments, the output is
agglomerated into daily statistics (rather than storm-by-storm) as peak low or
volume frequency spectrum curves. It also provides an analysis of behavior in
extreme events and subject to several climate change scenarios.
Again the beneit of a tool such as the National Stormwater Calculator is an
ability to perform a continuous simulation, which in theory provides a more
realistic characterization of performance according to a range of conditions. It
models living roofs not only at the building scale, but they can be integrated into
wider site or watershed design along with other SCMs. As it is a very new model,
there are not yet any publicly available studies demonstrating calibration or
validation of the National Stormwater Calculator speciically for living roofs.
3.5.6 Discussion: stormwater models
The CN method and the Rational Formula are simpliied estimators of hydrologic
response, but with proven histories in drainage design. The physical assumptions
underpinning the methods do not relect the hydrologically complex system such
as a living roof. In reality, living roofs are signiicantly engineered, pseudo-
pervious systems with constrained storage capacity and no physical connection
to the ground. It is worth iterating that the CN method and the Rational Formula
are intended for design to satisfy stormwater control objectives, as described in
Section 3.1.1 , rather than recreating historic lows.
Hydrologic models capable of continuous simulation theoretically provide a
more realistic picture of a stormwater system's response, by accounting for varia-
ble climate conditions and their effects on the living roof - namely by accounting
for the moisture content of the growing media/how much water storage is avail-
able at the onset of each rain event. They also provide a reasonable format for
assessing performance; for example frequency spectrum curves for runoff volume