CN Credit for Soil Ammendment
CN Credit for Soil Ammendment
(OP)
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.
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.





RE: CN Credit for Soil Ammendment
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
RE: CN Credit for Soil Ammendment
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
RE: CN Credit for Soil Ammendment
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
RE: CN Credit for Soil Ammendment
The Ia approach is interesting, I'm going to look into this a little more.
RE: CN Credit for Soil Ammendment
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: CN Credit for Soil Ammendment
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: CN Credit for Soil Ammendment
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.