Percent impervousness from CN?

[MapEngr]

Is it possible to calucalte an approximate value for percent impervousness if all one is given is a curve number?

 

[jgailla]

I wouldn't think so. Too much depends on the underlying soil.
You could back into a number if you had a good idea of the soil type, particularly which soil type was used by the person generating the curve number.
In this way, you would back into an estimate, but you wouldn't have any way of knowing if it was right.
Let's say you have a site with a curve number of 90.
You know that the underlying soils have a curve number of 82 and the impervious area has a curve number of 98.
Therefore your site would be 50% impervious.
You could really run into some problems doing this, however.

 

[psmart]

You would need to know the CN value for the pervious portion. Assuming that the impervious CN=98, you could then calculate the percentage of each CN.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Drew08]

Assuming:
CNcomposite x Atotal = CNp x Ap + CNimp x Aimp

Then:
%imp = (CNcomposite - CNp) / (CNimp - CNp)

Where:

CNcomposite = the site composite CN
Atotal = total area
CNimp = impervious CN (98)
Aimp =Impervious area
CNp = CN for pervious area
An = Area of pervious area