"URLX - Urban Land" Soil Type

[rlondeen]

Is anyone familiar with this HSG Soil Type, "URLX - Urban Land". I was recently pulling HSG maps from the NRCS website and there was a large residential development with this soil type that matched the boundaries of the development. Looks like pre-developed HSG soils were type B at one point.

Does this suggest that HSG soil types change with development? Does anyone here reflect post-developed conditions with type D soils? I have always chosen post developed CNs using existing HSG soil conditions with proposed developed / ground cover type and have never seen any other indication to do otherwise.

 

[Twinkie]

This is always one of those "grey areas" in SWM .... I typically model the urban soils based on their original underlying soil type. A lot of the Urban series soils around here are classified as such as well. Urban Land - Metapeake Complex , etc.

 

[beej67]

Technically the soils may change their HSG once they undergo mass grading, because the mass grading wrecks the soil profile and re-compacts everything. Presuming that the process switches the soils to a D type, though, isn't necessarily a good presumption. You wouldn't expect a central florida sandy silt to suddenly become clay simply due to mass grading activity.

In the end, this is something you need to feel out for your region, and will rely in part on how your regulators want to see it, if you're doing a pre/post stormwater analysis for development. In most of the areas I've worked, they want you to presume the same HSG for "Urban" land as the underlying soils used to be, or as the import soils were likely to be. I have had one mass grading project with no impervious buildout, though, where a regulator wanted me to presume the original B soils would be shifted into HSG C by the operation, which really wrecked my initial assumptions about my pond.

Feel it out with your regulator, and use your best judgment.


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

http://www.campbellcivil.com

 

[LincolnPE]

I understand the feeling to please the local regulators, but understand they have no real liability if something floods or is damaged by your design. I have learned this empirically through my expert witness work. What it comes down to, is that the attorneys do not go after them, ever...only the original engineer, developer and their insurance.

You need to fight for your ideas and judgments since your the PE signing... You will have to do that strongly in deposition if that occurs. The regulators can go home and sleep at night without worry about the project, but not you.

 

[beej67]

In a general sense I agree Lincoln.

For this particular question.. ..if you get sued over considering Urban to be the native soil type instead of a reduced number, I'd presume you could simply default to the standard of care, which in hydrology means doing it like the thousands of other engineers in the same region have done it in the past without it causing a huge problem. Any engineer in land development in areas of the country in which I'm familiar would presume Urban to be similar HSG to what it was before it was disturbed. The further truth is it's not likely that an erroneous presumption of Urban HSG is going to cause a design to fail any more than a dozen other assumptions, or than a rain event beyond the design capacity of the pond. Chances are pretty good if you're doing a stormwater pond for detention on existing Urban soils, that the existing site has already been developed and is undetained, so whatever you throw in the ground is going to be an improvement. And that'd be pretty easy to show in court.


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

http://www.campbellcivil.com

 

[LincolnPE]

Erroneous presumption of HSG doesn't hurt??

You said that just going from HSG B to C wrecked your pond...

OP: Don't just do what other engineers have done in the past. Many of them are now getting sued for it. I would use that latest NRCS information you have on your site soils, especially if it is more conservative.

 

[beej67]

You said that just going from HSG B to C wrecked your pond...

That was when the existing land use was HSG B forest, and the proposed land use was forced by the regulator to be considered HSG C grass. The temporary pond I designed for a simple mass grading project grew in size to one I would have done for a permanent pond for a fully developed site.

rlondeen:

If your existing Urban soil actually reacts as HSG C, but you presume it's HSG B, then your presumption is a conservative one, because your allowable discharge is set at a lower threshold. Therefore it's no big deal in almost any case to presume Urban land is a lower HSG for the purposes of redevelopment projects. You're actually doing your neighbors a favor with the presumption, and the assumption could easily be shown to be a conservative one in court. That presumes we're talking about a pre/post analysis for stormwater management of urban redevelopment projects. If you're talking about checking culvert capacity, or freeboard on an existing reservoir, or the like, then assuming a higher HSG for "urban" would be the more conservative assumption.

I would use that latest NRCS information you have on your site soils, especially if it is more conservative.

Unless things have changed recently, "Urban" soil classifications by their definition do not have a HSG category. That's what prompted the question. The engineer must make an assumption.

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

http://www.campbellcivil.com

 

[rlondeen]

Thanks guys. I didn't know this would devolve into argument. Its just dirt! just kidding.

This was helpful. I may run the hydrology calcs with type D soils and see how it impacts it. However, this project has been ongoing and previous designs used the NRCS "pre-developed" B soils. The maps were changed in this area just recently after we completed design based off of the B soils. Tying in current designs with just recently constructed design while using different assumptions could possibly look bad and negatively affect our overall design.

Unless things have changed recently, "Urban" soil classifications by their definition do not have a HSG category.

It actually did have a soil classification of D which is what really prompted the question.

 

[beej67]

Oh, well then take "D." Everywhere I've worked, Urban land is simply unclassified.

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

http://www.campbellcivil.com