Would you consider 8" CABC with 3" of topsoil to be pervious for storm drainage purposes?

[zazapon]

We were discussing this in the office the other day and people are split on whether to consider compacted 8" CABC (crushed aggregate base course) with 3" of topsoil on top of it to be pervious or impervious. Some people say that since we have 3" of topsoil, that we should consider the entire thing to be pervious for storm drainage purposes. However, I tend to think that CABC underneath it would make it impervious since it is highly compacted and the water is not allowed to percolate down as fast as it normally would.

Our state definition (NC) considers gravel to be pervious but all the cities and townships I dealt with apply a more stringent definition and consider gravel to be impervious. I have emailed the state to see what their interpretation is since I could not find any good sources regarding this particular situation online. What do you all think?

 

[dik]

I would consider it to be pervious... and with a non-woven geotextile between... 3" of topsoil may desicate. You're allowing the soil to drain, by restricting the flow from the surface by some kind of vegetation.

Dik

 

[cvg]

for rational method around here C for compacted crushed road base would usually be considered mostly impervious. For instance:

landscaping with impervious undertreatment 0.85 - 0.95
gravel vehicular travel lanes and shoulders 0.70 - 0.88

with lightly compacted topsoil on top, there is a small amount of additional void space that could retain water, so you might use the lower range.

 

[civilman72]

In general, almost all construction materials have some value of imperviousness, even concrete. But for the purposes of infiltration and water quality attributes, compacted roadbase/gravel is generally not considered "impervious." The addition of topsoil may help improve the situation, but 3" is a fairly small media thickness to consider the overall section impervious.

 

[dik]

Water ponds on cmpt Gran A base... I was thinking that if it's topsoil, then there would be some vegetation on it... best for absorption.

Dik

 

[darthsoilsguy2]

in regions that have different permitting criteria to satisfy, i could see counting it as grass for a "water quality storm" which is around a 0.5 inch to 1 inch storm... but not in stronger storm event criteria concerning runoff flows. Gravel drives are absolutely considered impervious in my state