Porous Paving 3

[keezii]

Does anyone know the CN used for porous paving.
Since the material is made of asphalt but behaves similarly to sandy soil, the determination of its CN is difficult to calculate.
Are there any formulas...
..or whats your best approach when designing porous paving systems under state regs for runoff and recharge?

 

[daddymem]

I asked this question myself many years ago. The best answer I got (and I got many from the guy that did Walden Ponds to our State DEP) was to use the CN for the gravel below. Perhaps things have changed by now.

Try these oldies:

http://www.epa.gov/ednnrmrl/repository/abstrac2/abstra2.htm

and
"Investigation of Porous Pavements For Urban Runoff Control" 11034 DUY 03/72 google it, I found a place that will sell you a copy for $10

If you hurry catch a seminar:

http://www.hotmix.org/view_article.php?ID=423

An article "Porous Asphalt The Right Choice For Porous Pavements" from the September/October 2003 Hot Mix Asphalt Technology magazine---see the link above for the NAPA

I ain't never seen a hearse with a luggage rack.-GS

 

[keezii]

hmm.. using the CN of the gravel within the storage area doesn't seem right, since thats about 70-80..
and im sure DEP will accept a CN that high!LOL
.. but whats the point of using porous asphalt if your CN is similar to that of regular asphalt!!
..from the actual performance of porous asphalt and its ability to infiltrate water, its look more in the range of sandy soils..30-40..but thats all speculation. TR-55 wont accept CNs below 40 which makes modelling even more difficult.

 

[daddymem]

One comment repeated over and over by many people on this subject was that maintenance was difficult but critical for these surfaces to operate as designed. We reluctantly gave up on our project, hopefully times have changed and you have better luck.

I ain't never seen a hearse with a luggage rack.-GS

 

[explortainment]

The paving projects I have done using porous asphalt will also work if the asphalt loses it's porosity or if the asphalt is sealed in the future...The intent is to monitor the porous asphalt performance and, if it stops performing, scrape and re-pave with standard asphalt. The difference is in the aggregate gradation mix, and the extra depth of stone below the asphalt.

 

[bltseattle]

It's interesting, as more low impact development practices (porous pavement, greenroofs, etc) are used it gets harder and harder to adapt CN methods that were developed for conventional, deep soils. For example, one CN may match the volume of runoff for the proposed BMP but yield too high of a peak runoff rate. So you have to look at the CN as well as the Tc, using the Tc to control the runoff peak to a reasonable value. Case in point is using the gravel CN for porous pavement, what is the appropriate Tc to go with a particular CN? Does it depend on the size of the storm (probably should!)? There's still work to be done in making these BMPs model-able. My firm has actually developed an alternative model for green roofs since CN methods are so poorly suited for the processes that occur in these systems, and many of our clients want to install greeen roofs.

All that said, for a porous pavement or paver system you can compute a CN by using the old SCS formula where S = 1000/CN - 10, with S as the potential moisture storage in inches.

So, for example, if you have 12 inches of gravel to store water that percolates through the asphalt, at 30% porosity (typ. for gravel) the equivalent CN would be 73.5. At 6-inches of gravel it works out to 84.7 This doesn't account for underlying soils, which should improve the performance/lower the CN, but it is easiest to demonstrate the CN attributable to the gravel layer alone.

For comparison, the City of Portland BES drainage manual (page C-5) treats it as gravel and allows CN= 76/85/89 for porous pavement over hydrologic soils group A/B/C respectively.

I would go with one of these CNs and also make the case of at least doubling the Tc compared to conventional pavement.