Agriculture Reference
In-Depth Information
Table 3.5
Summary of selected hydrologic models commonly applied for living roofs
Model
Basic approach for predicting
living roof hydrology
Beneits
Limitations
TR-55 Curve
Number Method
The living roof “replaces” a
runoff source area described as
an impervious surface (an
unvegetated, conventional
roof) which would normally
have
CN
= 98
• Easy to apply, with hand
calculation, spreadsheets, or
integrated into various
software packages
• Widely accepted by
permitting agencies
• Potential to apply for design
storms or in continuous
simulations within software
such as SWMM or HEC-HMS
• Limited data available to
develop robust
CN
s
• Not intended to recreate
measured discharge
hydrographs
• use of a single
CN
does not
represent the day-to-day
variability of living roof
performance
• unit hydrograph methods
commonly used to transform
runoff depth (volume) to
hydrographs do not apply
Rational Formula
The living roof “replaces” a
runoff source area described as
an impervious surface (an
unvegetated, conventional
roof) which would normally
have
C
v
≥
0.9
• Easy to apply, with hand
calculation, spreadsheets, or
integrated into various
software packages
• Widely accepted by
permitting agencies
• Limited data available to
develop robust runoff
coeficients (
C
v
)
• Not intended to recreate
measured discharge
hydrographs
• Should not be used to
determine runoff volume
• usually for design storms
only
uS EPA SWMM
5.1
Simulates water movement
through the living roof using
the Green-Ampt Iniltration
model
• Living roofs can be modeled
at the building scale, or
integrated into wider site or
watershed scale simulations
• Ability to run as design
storms or continuous
simulation
• Open source code, freely
available model
• Signiicant data input
required to set up and run
model
• Accuracy for living roof
applications has not been
veriied
(USA) National
Stormwater
Calculator
Simulates water movement
through the living roof using
the Green-Ampt Iniltration
model
• Living roofs can be modeled
at the building scale, or
integrated into wider site or
watershed scale simulations
• Continuous simulation
model
• Freely available
• Linked to online databases
to automatically populate
many data ields based on
project location
• Applicable only in the uSA
• Accuracy for living roof
applications has not been
veriied
• Fewer site design options
with less detail regarding site
coniguration compared to
SWMM 5.1
HYDRuS
Simulates water movement
through the growing media
using the Richard's equation
for variably saturated
conditions
• Accuracy has been
successfully calibrated and
veriied for several living
roofs
• Potential to apply for design
storms or in continuous
simulations
• Limited to building scale
applications
• Signiicant technical detail
required to set up and run
model
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