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# Should I use a Special CN or add a pond for modeling a stone area?

## Should I use a Special CN or add a pond for modeling a stone area?

(OP)
I have a subcatchment described in the attached photo.

The subcatchmet has a roof with five disconnected roof drains that spill onto stone (open graded ASTM #2 stone with no fines) areas. Each stone area has a catch basin. The catch basins are part of a longitudinal network which outfalls into an existing storm water pond. The stone depth is approximately 12". Assuming a porosity of the stone to be 0.54 I could assign a special CN value of 61 for the stone.

S=1000⁄(CN-10)
Where: S = depth in inches
If S is calculated as the available voids in the ASTM #2 stone, the CN can be estimated by rearranging the equation as:
CN=1000⁄(S+10)
CN=1000⁄(0.54*12"+10) = 61

I would like to take advantage of exfiltration (1.02 in/hour) beneath the stone. Should I use a CN value of 96 for the stone, use a pond depth of one foot, the surface area of the stone, the volume of the voids as my storage volume, and 1.02 in/hour for exifiltration. I'm afraid I'll be overestimating my storage if I use a value of 61 while incorporating the storage of the stone into a exfiltrating pond. If I use a pond for exfiltration before the flow enters the catch basins what CN value should I assign the stone, 96 or 61? The stone areas I would like to incorporate storage are about 0.1 acres in size each.

What is the best way to account for the storage and exfiltration the stone areas provide?

Thank you,

### RE: Should I use a Special CN or add a pond for modeling a stone area?

Since you're already modeling the ponding in the stone voids, I would use a high CN value (perhaps 98) to model the direct rainfall on the surface of the stone.

Peter Smart

### RE: Should I use a Special CN or add a pond for modeling a stone area?

Do what Peter says. Think about it this way - CN is basically a way to abstract the idea that some of the rainfall might not turn into runoff and end up in your storage node. If it falls from the sky directly into your storage node, then 100% of it gets into your storage node, so a CN reflecting imperviousness would be accurate.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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