derivation of CN's? 1

[civilsid]

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

 

[bpattengale]

Think about it this way you're adding impervious, but you're also adding a lush green lawn next to all of that impervious. Those lawns slow the runoff and allow more of it to be absorbed into the soil. With the pre-developed CNs you mentioned they lack substantial vegatative cover. Even the row crops which have a large ammount of vegetation still have large areas of exposed soil. Water will runoff of bare soil very quickly and not allow it to soak into the ground. Look at the flashy nature of streams and washes in the southwest. Even a small rain event can produce large flows in the creeks and dry washes.

Now if you're not adding that lawn then by all means use a higher curve number.

 

[psmart]

When using the average CN value for 1-acre residential lots, your lot must be 20% impervious, and the pervious portion must be "Open space - good condition". These are the assumptions of which the average CN values in the TR-55 table are based. (See TR-55 Table 2-2a footnote 2)

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

http://www.hydrocad.net

 

[waterwitch]

usually the existing conditions CN for a natural site is in the vicinity of 55 for wooded and thick brush

 

[cvg]

waterwitch - CN = 55 is applicable only for existing ground which is wooded with thick brush.

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.