Agriculture Reference
In-Depth Information
a percentage change) are useful for providing macro-scale expectations for
watershed management, and contribute invaluable evidence that living roofs
“work” to eficiently manage stormwater. In contrast, technical requirements and
objectives for stormwater design usually requires predictive capability to meet a
speciied level of discharge under a limited set of climate conditions.
Unfortunately, measures of percent-change or capture eficiency may be of
limited use to a stormwater design professional without greater investigation
linking operational conditions to performance.
Typically, the regulatory agency requires the hydrologic and hydraulic design
of an SCM to limit the peak discharge (outlow) rate or low volume to that
which occurred before the development took place, or some variation thereof.
The starting point for determining the footprint of an SCM is calculations to
quantify the lows before and after development. Mathematical models repre-
senting the hydrologic and hydraulic processes serve as tools to quantify the
effect of the SCMs and demonstrate compliance with regulatory requirements.
Section 3.8 discusses the most common calculation and modeling approaches
currently in practice for stormwater management and drainage design.
To date, a few models have been successfully calibrated and veriied against
ield data, with signiicantly varying levels of computational input requirements.
Selection of an appropriate model depends on the question requiring an answer.
Where only the runoff from a rooftop is of interest, a living roof speciic process
model is helpful, provided adequate data are available. Where the living roof is
part of a treatment train, or one component of several SCMs across a site,
compatibility of the process model with the site stormwater model may be
logistically problematic, and thus prohibitive for common usage. Likewise, issues
of time and scale are relevant. For eficiency and responsibility of practice, the
simplest model appropriate to the level of detail required or data available (either
site-speciic or from the literature) should always be chosen. For example, city-
wide conceptual planning strategies requires signiicantly less detail compared to
inal engineering design for actual sizing of individual SCMs and their installation.
In the former case, simple(r) assumptions of net retention or detention per event
may be adequate (assuming event-to-event variability is reasonably considered),
while in the latter, a process based model may be a better choice, depending on
locally relevant (or comparable) data availability and conidence.
As each living roof creates a unique combination of growing media character-
istics, plant palate and root structure, detailed modeling of one living roof's
behavior may not be transferable to another living roof. At the same time, the
differences from system to system may not result in signiicant performance dif-
ferences in the context of the more general stormwater design objectives. It is
only through development and testing of many living roofs that the inluence of
design variables will be understood, and a generic living roof model may be
accepted for universal application.
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