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Hyrologic Soils Groups and Ledge

Hyrologic Soils Groups and Ledge

Hyrologic Soils Groups and Ledge

(OP)
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!

RE: Hyrologic Soils Groups and Ledge

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
www.hydrocad.net
 

RE: Hyrologic Soils Groups and Ledge

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.  

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