Modeling Pavement
Standard pavement is usually modeled as a subcatchment with a curve number of 98. You can enter the CN value directly, or select it from the lookup table. The high CN value will cause virtually all of the rainfall to appear as runoff, as determined by the SCS runoff equation.
What about porous pavement?
Before attempting to model porous pavement, you need to evaluate the role of the pavement in your model, and how you expect the pavement to behave. There are three basic scenarios to consider:
1) No surface runoff
Under ideal conditions, porous pavement with a suitable base will accept (infiltrate) many inches of water, so there may be zero "runoff" in the traditional sense. In this case, you actually need to evaluate the "runoff" that is penetrating through the pavement. This requires the use of a high CN value (98) to capture most of the rainfall.
Once intercepted by the pervious surface, the water will take some time to travel through the base layers of the roadway, before ponding in the voids of the stone base. To simulate this behavior:
(A) Use an extended Tc value to simulate the travel time through the base (gravel, sand, etc).
(B) Route the "runoff" from (A) into a pond which represents storage in base material. Depending on the exact material, voids in the stone can be up to 40%. The "pond" discharge could be through exfiltration into the surrounding soil, and/or specific outlet devices, such as perforated pipe (modeled as a culvert and/or orifice array.)
Using these two steps together will simulate the significant reduction in the peak flow caused by the base material and underdrains. Note that the Tc path (A) must end at where the water enters the "pond" (B). Do not extend the Tc beyond this point, since the effects of the ponding are already being simulated with the pond node.
2) Complete surface runoff
In the worst case, the pavement could be fully blocked with ice or debris, in which case you would be modeling normal surface runoff with a standard CN of 98 and a relatively short Tc, much like standard pavement.
3) Partial runoff
If you expect partial runoff, you'll need to use an appropriate CN value to model the true surface runoff. A suitable CN value may be available from the pavement supplier. However, the value you achieve will depend largely on the base material, so it's important to follow manufacturers recommendations carefully. In some cases, you may be able to estimate a CN value based on the potential maximum retention of the roadway base, as described here.
An alternate for modeling partial runoff would be to use a CN value of 98, and then use a link to apportion the flow between surface runoff and infiltration. This would allow the use of a flow threshold and/or scale factor to determine the point at which some of the rainfall begins to run off the pavement rather than passing through.
Further comments
Porous pavement can be an effective component of stormwater management, as well as providing valuable groundwater recharge. A growing number of agencies are encouraging the use of porous pavement, and are developing standard designs and modeling procedures.
However, actual performance can vary widely depending on the exact construction, maintenance, and climate. When infiltration performance is uncertain, some agencies prefer to design for the "worst case" by treating all pavement as impervious, with a CN value of 98. Unfortunately, this tends to discourage the use of porous pavement in situations where it could still have real benefits. The approach you ultimately select will depend on the exact design and the requirements of the reviewing agency.
References
UNH Stormwater Center:
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Porous pavement fact sheet
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Iulia Barbu Dissertation
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Publications
National Ready Mixed Concrete Association:
Pervious Concrete - Overview
Pervious Concrete - Hydrological Design Considerations
Hydrologic Design of Pervious Concrete (5 parts)