CN Credit for Soil Ammendment 4

[Twinkie]

I am working on a few intersection improvement projects (widening, turning lanes, etc), and I am trying to come up with an approach for SWM.

These improvements are all occurring within populated (developed) areas, and any ROW I need will be taken from the individual property owner. That being said, stormwater BMPs are not planned, but I still need to attempt to show some form of mitigation for the increased impervious.

All work is occurring within 'B' and 'C' soils. All disturbed areas that will revert back to lawn will be treated with an 8" soil amendment.

Do you think it is an acceptable approach to call the soil an 'A' soil after the amendment?

I also did a model that routed the lawn area to itself, as an 8-inch depth "basin" with a 17% void ratio. Does anyone have an opinion on that?

Thanks for your time.

 

[beej67]

Do you think it is an acceptable approach to call the soil an 'A' soil after the amendment?

Opinions will certainly vary on this forum, but I would not consider anything of the sort an "A" soil. A soils are sandy, usually coastal, and their ability to wick water doesn't stop 8 inches down. I wouldn't buy it.

Your second approach is the sort of thing I've done before, but not on a lawn. If it were a porous pavement with a defined engineered base course that was going to be maintained that way in perpetuity, instead of mowed once a month by the Orange Tractor Brigade, then the second approach might apply better. You may also be able to stab at that calculation by adjusting your initial abstraction instead of modeling it as storage.

Completely reforesting the area might put you in a legitimately lower CN, but that might be pricey.

The ugly truth is DOTs are sometimes held to lower standards than land developers are. LDs get put through the ringer on stormwater, and DOTs often just sorta mumble their way through the process and get approval. So while I would not consider either of your approaches adequate, they might get you by, depending on the other stakeholders in the design.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

sure, you could "amend" it, but it would be difficult, more like "replace" it with sand

Group A soils typically have less than 10 percent clay and more than 90 percent sand or gravel and have gravel or sand textures.

Group C soils typically have between 20 percent and 40 percent clay and less than 50 percent sand.

you would need to blend at a ratio of 20% native soil and 80% pure sand in order to get your percentage of sand back up to 90% where it needs to be

 

[beej67]

you would need to blend at a ratio of 20% native soil and 80% pure sand in order to get your percentage of sand back up to 90% where it needs to be

This. Further, HSGs are set based on the entire soil profile, not just the top few inches.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Twinkie]

The 'A' soil approach never really sat well with me, for reasons you both mentioned. Thanks for the replies.

The Ia approach is interesting, I'm going to look into this a little more.

 

[beej67]

Ia is a parameter that could be conceivably used for that purpose, but wasn't really built explicitly for it, so you'll want to make sure your use of it is very defensible.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[psmart]

Also be aware that the standard commonly used SCS/NRCS CN values are based on Ia=0.2S, so if you change the Ia/S ratio you need to use different CN values.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Twinkie]

Thanks again everyone. I'm actually going to back away from the Ia/S approach as well. I've found some research (from more than one source) that states than an Ia/S = 0.05 is actually a more accurate representation of site hydrology.

These improvements are all within one larger watershed. I think I am going to try to do a trade off somewhere else within the watershed and manage some uncontrolled runoff where I have room to construct BMPs on municipal land.