Crushed stone CN

You may be able to use the SCS runoff equation to estimate the CN based on the potential maximum retention:


Be careful however. If the water that enters the peastone is going to drain along the sides of the driveway, then the peastone really doesn't effect the CN value, and you should just use the CN value for the underlying material.

If I were reviewing the plan, I would require you to use the CN for the higher of the driveway subbase or the in-situ peastone. For the subbase to hold the peastone up (not have it be pushed down into the subbase), it will have to fairly well compacted...almost to the point of being impervious. If the subbase will not be compacted, just use gravel, that is what will be there after a year of driving over it.

Due to the fact that most gravel drives and lots end up paved eventually, in my Township, gravel is (by Code) considered impevious, and CN = 98 no matter what. Check with the reviewer.

Engineering is the practice of the art of science - Steve

Look at the township regulations, for 3/4" gravel use a CN of 85. Go with the township regulations or whichever agency holds the final say in design. In NJ, stormwater rule NJAC 7:8 does not consider gravel as impervious but Land use regulations Program (LURP) considers gravel as impervious.