05b – Modeling Urban Retrofits
05b – Modeling Urban Retrofits
For this session, we will model two of the urban retrofit stormwater control measures the City of
Santa Monica has installed – 1) Porous Pavement and 2) Biofilters.
Porous Pavement:
The City of Santa Monica has installed porous pavement in some alleys, streets, intersections,
and gutters. Our example area has alleys. We will retrofit the alleys with porous pavement.
Start with the WinSLAMM data file created for the No Management Condition. Save the File
with a new name, such as “Porous Pavement”. All the information in the “Current File Data”
form will remain the same except the Site Description. Modify the Site Description so that it
reflects porous pavement in the alleys.
Porous Pavement is a Source Area Control in WinSLAMM. This means that only the runoff
from the single source area will be routed to, and treated by, the stormwater control measure.
Source Area Controls are accessed from the main land use window. To enter the Porous
Pavement form, double click on the intersection of the “Driveways 2” Source Area and the “P”.
The “P” stands for Porous Pavement.
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05b – Modeling Urban Retrofits
Enter the data below into the Porous Pavement form.
Please Note: The values used are average values. Your specific site may require different values.
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05b – Modeling Urban Retrofits
Run the model.
Porous Pavement Results:
Runoff Volume:
Runoff Volume Percent Reduction:
Particulate Solids Concentration:
Particulate Solids Yield:
Particulate Solids Percent Reduction:
Rv:
Approx. Urban Stream Classification:
Total Phosphorus:
Total Phosphorus Percent Reduction:
2.919E+06 cu ft
5.46%
78.19 mg/L
14,249 lbs
10.19%
0.67
Poor
2.00 lbs
9.45%
Because the Porous Pavement is applied at the Source Area, the reduction will be displayed in
the “Total Before Drainage System” row.
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05b – Modeling Urban Retrofits
To view the results for Pollutants other than Runoff Volume and Particulate Solids, click on the
“Pollutants” tab and then the “Yield (lbs)” tab.
A “Source Area total without Controls” is not available in the Pollutants results. Therefore, all
the Total Copper values will be the same if only Source Area controls are modeled. Also, a
percent reduction for the pollutants is not calculated automatically by the program. That will
need to be done by hand.
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05b – Modeling Urban Retrofits
Adding Biofilters:
Another stormwater control measure the City of Santa Monica usually installs is biofilters. For
this example, we will assume it is feasible to install one 75 square-foot biofilter per residence.
Save the File with Porous Pavement with a new name, such as “Porous Pavement and
Biofilters”. All the information in the “Current File Data” form will remain the same except the
Site Description. Modify the Site Description so that it reflects that biofilters have been added.
We are going to assume that only rooftops will drain to the biofilters (that all downspouts direct
runoff to the biofilter). Infiltration measures (except grass swales) are modeled as Biofiltration in
WinSLAMM. Because the biofilters will only serve the rooftops, we will model the biofilters as
Source Area Controls.
To enter the Biofilter form, double click on the intersection of the “Roofs 1” Source Area and the
“B”. The “B” stands for Biofilter.
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05b – Modeling Urban Retrofits
We will assume there are an average of 48 homes and therefore 48 biofilters.
The biofilters will be a total of four feet deep with one foot of rock fill, two feet of engineered soil (peat-sand mix), and one foot of
open-air storage. The outlet structure will include infiltration into the native soil and an overflow (modeled as a three-foot wide broad
crested weir.)
The biofiltration form with the data is shown below.
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05b – Modeling Urban Retrofits
The overflow is modeled as a broad crested weir. The overflow is set one half foot above the
top of the engineered soil.
Run the model.
Porous Pavement plus Biofiltration Results:
Runoff Volume:
Runoff Volume Percent Reduction:
Particulate Solids Concentration:
Particulate Solids Yield:
Particulate Solids Percent Reduction:
Rv:
Approx. Urban Stream Classification:
Total Phosphorus:
Total Phosphorus Percent Reduction:
2.082E+06 cu ft
32.57%
94.78 mg/L
12,317 lbs
22.37%
0.48
Poor
1.55 lbs
29.84%
As with the Porous Pavement, because the biofilters are applied at the Source Area, the
reduction will be displayed in the “Total Before Drainage System” row.
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