Curve Number (CN) for "Mulch" 2

[DASkurky]

I have to show the increase in runoff for timber harvesting a small patch of woods for a client in Atlanta, GA. We are felling the trees and grinding the stumps. We chose not to grub to preserve the integrity of the soil as much as possible, and to not trigger as many erosion control requirements.

The County is requesting to see the increase in runoff due to the change in land use. I'm using the SCS (CN) Method and a 55 for pre-developed conditions (woods, good condition, HSG "B") but I'm struggling to come up with a good post-developed land use.

We use the National Engineering Handbook for CN determination but nothing seems to apply very appropriately.

Any thoughts??

Thanks,
Dave Skurky, P.E.

 

[cvg]

the land will probably most closely resemble one of the following categories in table 9-1:

woods (poor condition) if you plant new trees or

pasture, meadow or brush if you don't replant

 

[TerryScan]

For the condition after the timber harvest, how about "Fallow - Crop residue cover (CR)" (good CN = 83 for HSG B)?

What is ultimately going to be done with the land? Is there any kind of construction to occur? If not, some kind of vegetation will establish. Will it be cut and maintained as an open area? or will it revert back to woods?

 

[cvg]

The curve numbers are for agricultural lands. Thus fallow is for a farm field which has been cultivated in the past and is currently not planted. It would assume there has been furrows created by the plow and cultivator. Unless there has been some terracing done in your forst plot, I don't believe this is a realistic curve number.

 

[RWF7437]

Have you considered measuring it on a sample plot of ground ?

 

[TerryScan]

cvg,

I am aware that fallow is for idle agricultural lands between crops. I still believe a freshly cut woods with stumps ground out and mulch residue is probably much closer to the fallow condition (good CN 83) than poor woods (CN 66). I believe the lower CN is the unrealistice curve number at this stage.

I would agree with the poor woods or pasture conditions for an ultimate land cover (vs immediately after harvest), which is why I asked the follow up questions.

 

[cvg]

well, if we are talking about "immediately" after harvest I agree with you. But I have to assume that there are some bmp's in place which likely include seeding with grass and possibly re-planting trees. Even without these, the grass will grow in within one season. This will approach "meadow or pasture" conditions very quickly.