It depends if you're trying to model surface runoff or infiltration, and under what conditions.
Under ideal conditions, porous pavement 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, and route it through the base layers, and perhaps an underdrain. This can be modeled by using a high CN value (98), together with a long Tc value, to simulate the travel time through the base and underdrain, or perhaps a "pond" to simulate the storage in the stone voids.
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.
A third option is to calculate an equivalent CN value, based on the potential maximum retention of the base.
Unfortunately, standards for modeling porous pavement are still developing. So your approach will depend on the exact design, and what you and the reviewing agency can agree on. Some states are starting to encourage porous pavement, and are developing standard designs and modeling procedures. As an example, see the work by the UNH Stormwater Center at
"Zero" runoff is highly unlikely for any type of surface during a design rainfall event even with perfect design and maintenance. It is not reasonable to assume that during your design storm the pavement is in "ideal" condition. In fact, unless your parking lot is dished out with no slope to promote runoff, I don't believe you could ever see "zero runoff". This certainly flies in the face of the research and even the manufacturers claims. You should evaluate "porous" pavement similar to normal pavement with a "small" reduction in runoff. The size of that reduction is still very much open for debate.
cvg: The linked document is intended to give a range of options on a controversial topic. The "zero runoff" scenario is presented as a limiting case, and not a specific recommendation. Professional judgment is required to determine the best approach in each situation.
That said, we are starting to see more instances of porous pavement being modeled with no surface runoff - as well as agencies that accept this approach. There is still an eventual discharge through exfiltration and/or underdrains, but it is considerably attenuated by the storage and travel-time through the base material.
I will not argue the zero runoff case as a limiting factor as that can only be applied to a designed pavement area which is intended to act as a retention basin, properly designed to hold the entire storm. Permeability of the pavement clearly degrades with time and some references say that it maintains only 20% of the original permeability after 30 years. It is also clear that runoff will generally occur on any surface unless the slope is very flat. I have seen 0.5% as a limiting slope for porous pavement in the references.
A few more references that may better illustrate the properties of porous pavement.
One problem with perviousness of HMA is that it can fill up rapidly - so that yes it will seem to take up water in a light rain, but when the gully washer comes it fills up and the permeability goes to zero - or rather to the permeability of the underlying material.
LCruiser: Good designs will provide a considerable amount of storage in the roadway base. New designs in this region (NH) are using at least 12" of stone, so they can hold several inches of rainfall, even without the immediate benefit of the underdrains.
with correct pavement slopes, good maintenance, suitable in-situ soils and a thick gravel layer, perhaps you could capture and store several inches of water during a storm. However, for the large design event - once your gravel storage is full the pavement CN may shoot back up to 90 like ordinary pavement. There is not a simple answer to the question of "what is the curve number for porous pavement"
psmart - what do you mean by "several inches"? Given that voids of a uniformly graded material are on the order of 30%, and "stone" as in base course I presume is well graded so those voids are filled with smaller pieces ad nauseam, how much rainfall can be stored in 12" of rock?
cvg - That is exactly how we designed it. Instead of several inches we are going to have 12" of #57 stone @ 40% voids. We will have enough volume to hold the 100 year storm. The stored water will then discharge at a lower rate than the pre-development runoff rate.
To be effective, the base must use a layer of clean stone with 30-40% voids. So 12" of stone will retain 4-5" of rainfall. This allows most storms to be fully retained, with gradual discharge through the underdrain. In effect, you have a 5" deep detention pond under every square-foot of pavement. This causes a dramatic reduction in peak discharge from the overall system. It easily exceeds the performance of a typical detention pond, and can actually reduce runoff below pre-development levels, especially if ground conditions permit infiltration from the base into the surrounding soil.
actually, it is not 5" deep under every square foot. Only under that portion at the lowest elevations which is flat. With sloped pavement, this results in only a small area which can store water. See attached sketch...
cvg: If you minimize the cross-slope you can avoid most of this effect. And the surface requires much less crown than traditional pavement (if any).
Different layers of base material will also slow the longitudinal flow so it doesn't form an immediate level pool, and underdrains can be placed at low points to prevent overflow.
Sure, there are plenty of engineering challenges, but there are also lots of benefits.
I agree with all the challenges and some of the benefits. However, my point is that 1) I would never use a curve number of zero for anything and 2) it takes significant effort to design and to estimate the runoff from porous pavement. There is no easy solution to the OP question of "what is the curve number"
