derivation of CN's?
derivation of CN's?
(OP)
I am working on a residential subdivision approx. 100 acres in size and all lots are going to one acre or larger. I am using TR-55 to develop my Q's and notice that the CN for residential districts one acre in size or larger is CN = 68; all soils are Type "B" for this application.
Many "pre-developed" CN's are significantly higher, including Natural Desert (77), bare soil (86), row crops (range 70-81), etc. of course there are several where the CN is lower.
Can anyone give me an explanation? I can't imagine that going from "natural desert" to 1 acre lots can possible have a reduction in runoff since we will be adding impervious roads, rooftops, driveways, etc.
~Civilsid
Many "pre-developed" CN's are significantly higher, including Natural Desert (77), bare soil (86), row crops (range 70-81), etc. of course there are several where the CN is lower.
Can anyone give me an explanation? I can't imagine that going from "natural desert" to 1 acre lots can possible have a reduction in runoff since we will be adding impervious roads, rooftops, driveways, etc.
~Civilsid





RE: derivation of CN's?
Now if you're not adding that lawn then by all means use a higher curve number.
RE: derivation of CN's?
If your conditions are different, you should calculate a weighted CN value based on the actual impervious and CN values. For example, if the pervious portion is bare soil (HSG-B CN=86) then the weighted CN with 20% impervious would be 88, rather than 68 for 1 acres-lots with lawn.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: derivation of CN's?
RE: derivation of CN's?
Most of the desert around here has no trees and sparse vegetation. Residential developments generally have significantly more vegetation than open desert (even without grass). Desert landscaping is usually covered with gravel mulch which improves percolation and often incorporates small retention areas and curbs which retain water. However, if using a plastic weed barrier under the gravel, that increases runoff.