Hyrologic Soils Groups and Ledge 1

[daddymem]

I am a mid level civil engineer and I have been asked by one of my superiors to do what I think is a peculiar thing on one of my stormwater models.

The site in question has a large portion mapped as HSG D and a smaller portion mapped as HSG C. It is a wooded site with exposed and shallow ledge outcroppings.

For the existing conditions model, I am being asked to calculate the ledge area and use a CN 98 for these and use the CN for woods on the remaining portions for both HSG mapped areas. I thought the CN values, combined with HSG included such things as ledge assumptions. Am I wrong?

TIA!

 

[psmart]

The standard CN values are for uniform soil (without any ledge.) If the ledge is close enough to the surface to effect the runoff, you'll need to use a different CN value.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

rock outcrops need to actually be exposed otherwise they really shouldn't be considered impermeable. Even very shallow soil overburden can hold a lot of water - unless you are starting with AMC III.