CN Adjustments based on slope

[geosavvy]

Does the NRCS support any type of adjustments to CN values based on ground slope?

If not, are there any adjustment factors out there that are universally accepted by engineers?

I have a site that contains grades of 15-20% over a large portion of the site and I would be interested to know if higher CN values should be applied. Logically, it seems that they should.

 

[francesca]

I haven't ever seen a slope adjustment for an SCS curve number, though of course in the rational method the C is a function of slope. It could be that you might introduce "double counting" between your time of concentration (which is a function of slope) and an adjustment to the curve number.

Since a higher curve number would be a conservative measure, you should be safe as long as you document why and how you came up with your adjustment, depending on your use for the "answer".

 

[RWF7437]

The “484” in the name of the method is what the NRCS calls the “attenuation factor”. It may be thought of as a kind of average for a typical urban drainage basin with rolling topography. For other situations, some investigators suggest the following adjustments:


Basin Characteristics Attenuation Factor
Rural, flat 150
Rural, gently sloping 200
Rural, rolling hills 300
Mixed urban and rural 400
Mixed urban-rural, gently sloping 484
Urban, steeply sloping 575
Rational Formula 645

Because most developable urban basins are gently sloping and a mix of rural and urban land uses the 484 factor seems appropriate to use for the “developed” condition. But for the pre-developed conditions it may be advisable to adjust this factor to something closer to 300 to 150.

Good luck

 

[blueoak]

The curve number is to relate runoff volume from rainfall volume and (if I have this right) is more appropriately thought of as a loss potential. In this way it is much different than the rational C and no real comparison should be made. For more CN info see the NRCS paper:
ftp://ftp.wcc.nrcs.usda.gov/support/water/hydrology/Woodward.doc

For slopes of 15 to 20% it is unlikely that your watershed has low CN unless you have sandy forest soils. Typically good soils with low CN's are in the valleys with lower CN due to erosion.

As RWF7437 indicates it is the timing parameter that you are interested in. I really like his numbers for Kp, however these are difficult to come by. For watersheds with slopes greater than 6 to 10% I am used to 550 to 950 for Kp in my area. However, this is for detailed studies.

If you use an appropriate Tc I doubt your shed has the chance for great losses due to CN and your Kp will most likely not be controlling. I would recommend running your model with CN for each AMC value. AMC2 CN values are really just best fit where AMC1 and AMC3 are the bounding values and better for sensitivity analysis. These are generally accepted adjustments to curve number.

 

[LHA]

I have a nomograph of Rossmiller's conversion; given CN, avg Slope, Intensity, Recurrence Interval and %Impervious (whole number, not decimal ratio), you get C.

I can't find the nomo on line, but you could solve the equation back for CN, given C. It is a monster equation, here it is...good luck with the aspirin:

C = (7.2*(10)^-7)*(CN^3)*(RI^0.05)*{[(0.01*CN)^0.6]^-s^0.2}*[(0.001*CN^1.48)^(0.15-0.1*I)]*{[(IMP+1)/2]^0.7}

Engineering is the practice of the art of science - Steve

 

[geosavvy]

Thanks for the input.

I'm not sure higher CN is more conservative. It just depends on which aspect of the design you are referring to. For calculating the pre-developed runoff conditions, it seems a higher CN is less conservative since you would be "allowed" to match a much higher runoff peak then you would with a lower CN value. This would result in less required storage, a bigger orifice, and potential flooding downstream.

So my concern is that the peak flows are bumped up to account for steep slopes for the pre-development analysis... then the pond is designed such that it allows that same amount of flow post-construction. If the flows were incorrectly bumped up for pre-, then we would be alowing higher post- flows than the true pre- flows.

On the other hand, if the pre- runoff is acutally higher than calculated due to steep slopes, we will end up restricting post-development flows to an artificially low pre-developed peak flow, and will have a huge pond for no reason... not to mention it will have to be relocated to accomodate the additional storage required.

I am on the reveiwing agency end of this. I want to know the most "correct" way to approach this. The table in our manual simply states that the CN value for wooded areas is 60. If you just use 60, along with the standard 484 for the 'shape factor', then nothing accomodates the difference between flat sites and steep sites.

I guess I need to determine what types of sites the 484 should really be applied to. Gently sloping urban-rural is somewhat vuage to me. So is the 484 wrong to use on a wooded site? Wooded is not urban-rural to me... theres 0 impervious, even if its surrounded by some development outside of the basin. Being wooded would tend to decrease the number from 484... however, my site is 15-20% slopes... so that would tend to increase the number. Which one should be weighted more heavily?

I need to research this more I think. Once again, thanks for the input.

 

[blueoak]

geosavvy,
-Kp
The research I have seen seems to indicate that Kp (shape factor) is most highly influenced by watershed shape and slope. This would imply to some degree that a pre versus post wouldn't be too hurt by a bad Kp.

-Flat versus Steep
Your Tc is going to be the big factor.

-Interception
I think a huge impact may be the wooded part, especially for smaller storms. Your interception can be much more than anticipated. Coniferous can just hold water in both the trees and with a good build up of needles it can take a large storm to produce runoff. I lived in Pennsylvania and I remember a foot or two of leaves in the wooded areas.
I ran the hydrology for a 8 sq. mile watershed of conifers recently and unless I dropped the CN to some crazy value of 30 I couldn't come close to matching historical or regression values. I call my model the burned condition.

I really think a good initial intercept for your region may be the biggest factor.

I know some urban research on tree interception has been done at UC Davis:

http://www.fs.fed.us/psw/programs/cufr/publications.php

I like parks in basins. A lot of times a park takes up more space so it doesn't hurt the developer as much.
I would love to know what you figure out.

 

[Crystalct1]

As far as I have read in TR-55, which outlines to NRCS procedure, curve number has nothing at all to do with slope. Time of concentration will decrease with increasing slope. This will make the peak flows higher, accounting for the steepness in your watershed. Curve numbers are only adjusted for ground cover and hydrologic soil group, with poor coverage (heavy grazing or burning, so very little on ground) and more impermeable soils (group D - slate, clays) giving higher curve numbers.