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Instruction Manual
Stormceptor® CD Sizing Program Version 4.0.0
Appendix B
Modeling Parameters
SWMM models catchments and conveyance systems based on input rain, temperature,
wind speed and evaporation data. Only rain data were used in these analyses. The
default SWMM daily evaporation values (2.5 mm/day) were used. Evaporation data will
not be important in this analysis since the catchment area is small (< 10 ha) and has
minimal depression storage. The Horton equation was chosen for infiltration. The
method of infiltration chosen is unimportant due to the small amount of pervious area
(1%). Table 3 provides a list of the parameters used in the SWMM model.
Table 3. SWMM Area Parameters
Area - ha (ac)
variable
Imperviousness
99%
Width - m (ft)
variable
Slope
2%
Impervious Depression Storage - mm
4.7 (0.19)
(in.)
Pervious Depression Storage - mm
0.5 (0.02)
(in.)
Impervious Mannings n
0.015
Pervious Mannings n
0.25
Maximum Infiltration Rate - mm/h
62.5 (2.46)
(in/hr)
Minimum Infiltration Rate - mm/h (in/hr)
10 (0.39)
-1
Decay Rate of Infiltration (s )
0.00055
The width of catchment was assumed equal to twice the square root of the area.
RESULTS
EMC versus Build-up/Wash-off
The suspended solids removal results based on the build-up/wash-off model were
compared to those based on an EMC (124 mg/l; USEPA, 1983) to demonstrate the
sensitivity of the model to the different solids loading approaches. The use of an EMC
assumes an equal concentration of suspended solids in all of the stormwater that is
conveyed to the Stormceptor.
Figure 2 shows a comparison of results using an event mean concentration loading and
build-up/wash-off loading given the default particle size distribution and settling velocities
based on a specific gravity of 1.8.
S T O R M C E P T O R
B - 9/17
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