LEED SS Credit 6.2 - Basic question about LEED Boundary 2

[mistermopar]

Hello,

I have what is probably a basic question about LEED criteria. I'll prefix my question by stating I am not a LEED guru in any way, just a couple of projects under my belt. I have a site with a defined LEED boundary, and it is my understanding that ALL of the LEED boundary is to be considered for the ss6.1 quantity calcs, whether all areas are developed or not (hopefully I'm not wrong already). However, I get confused when I get to the ss6.2 quality calcs. My LEED boundary includes a significant area of lawn space that is not developed at all (pre = post exactly), and those areas are also behind curb, so the lawns can't be loaded with TSS or pollutants in the pre or post condition. At the moment I'm looking at the LEED manual and it doesn't really spell out this condition. I can see two sides of the coin: 1-Use the LEED boundary for all LEED calculations, period, or 2-Eliminate the lawnspaces as we would in a regular development calc (which I'm assuming also eliminates the associated runoffs for those lawn areas from being considered in the "% of site treated" section of the ss6.2 credit form).

In other words, I guess I'm hung up with the "intent of LEED" vs. the poor logic of treating unloaded runoff. Are they wanting us to treat the entire LEED boundary no matter what, or can we remove undisturbed subbasins and just treat the real TSS loaded areas? If I don't remove those lawn areas then I would have to assign TSS removal values to the yard spaces that aren't loaded, which doesn't sound right either since the lawns aren't meant to be BMPs at all, but I don't see any talk in the LEED manual of exempting areas based on whether it makes sense or not!

I'm leaning toward ss6.1 using the full LEED boundary no matter what, and ss6.2 can remove unloaded areas, but definitely curious how others are doing those credits. Thanks!

 

[Twinkie]

In regards to 6.1, yes use the entire LEED boundary if you can, but undisturbed areas should be a wash pre/post (lawn>lawn CN, etc). I would have these areas be their own nodes in your model.

In regards to 6.2, the intent here is water quality. Not just mitigating what you paved, but improving what is already there.

You still have TSS from lawn areas, and if these areas are treated with fertilizers and such they are quite high. This credit really isn't all that hard to achieve, you only need to treat an inch across the site.

If this is an issue, why are all these undisturbed areas in your boundary?

 

[mistermopar]

Thanks Twinkie,

That I've been trying to get across to my architect (project LEED manager), however he seems to think that in this case the grass areas shouldn't be counted due to the barrier curb blocking parking lot runoff from entering the yardspace, and the fact it is unchanged pre & post. I didn't include this area in my original LEED boundary, but they wanted me to add it as they need more open space for other non-storm credits (this is an unconventional long/skinny project site/boundary). If a graphic helps, the current plan is attached, the yardspace being questioned is in bottom left of the plan in front of the existing building and east of the street (drainage area #3 on page 2).

The argument from their side is something like "it is consistent with the intent of LEED to "promote infiltration" by green areas, so how can grass count against us?" They are using the term that the lawn is "disconnected from the on-site drainage system" since the lawn drains directly out into the street gutter before and after the project (the lawn slopes downhill away from our structural BMPs deeper into the site, so we can't really bring it back to our bioswale). I'm afraid I don't have enough LEED projects under my belt to argue the point effectively either way, and I can say I do see the point they're trying to make, but the engineer in me wants to count all the LEED boundary area with no exceptions (I could be wrong on that). But they insist I'm wrong and the undeveloped yard area must not be counted. They have a great deal more experience in LEED than I do, and I see a point on both sides of the argument really, but I'm uneasy about what they're saying (they may be totally right, it just seems odd at this point).

Even if I "won" the argument, the form would want every area to be assigned a "% site treated by BMP" and then the %TSS removal rating for each area. So if I do end up counting the lawns in my LEED boundary, they are just plain yards with grass, and they don't have a structural BMP to treat that particular area, but the surface is all grass, which would provide a filtering effect. So would I count the grass lawn as a non-structural "grass filter strip" BMP or similar? BMP ratings that I can find online aren't really all that high for plain grass, like 61% effective, so the overall effect seems to me that if I would count the lawns, it hurts the site average since we're trying to get to the 80% weighted average on the form.

Thanks.

 

[beej67]

The USGBC's interpretations of their own rules don't often jive with common sense or science, when it comes to LEED boundaries and water quality. I've got probably two dozen LEED projects under my belt, and every one we have to either re-jigger the official LEED boundary to marry up with the treated area in the hydro study, or we have to jump down some other hoop to get the USGBC to buy it. I've had projects, for instance, where we had 2 acres in the LEED boundary, bypassed a quarter acre around water management, caught an extra acre of offsite runon and treated it, and they still wouldn't give us the credit because not all the LEED boundary was 'treated.'

One great angle that we use a lot - the LEED manual states that if a local criteria exists for water quality and downstream channel protection, that they'll defer to the local criteria. All of my LEED projects have been in Georgia, so we use the GSMM's TSS spreadsheet tool, and show a net 80% or more TSS reduction for the site through a combination of structural and nonstructural credits, and quote back at the USGBC their own language about local criteria overriding. Seems to work. Email me if you want to talk further offline.


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

http://www.campbellcivil.com

 

[Twinkie]

You need to remember, that LEED SWM credits are not a SWM ordinance. The intent is to improve your site.

You also need to remember, in most cases, these aren't SWM guys (or even engineers) who are reviewing your plans, they are just reading the rules from a book.

 

[mistermopar]

Thanks guys, much appreciated. I've noticed the disconnect between "engineering" versus "LEED" for sure, especially on the forms and the odd way (to me) that they do their voodoo magic. I like the idea of throwing back the design standards at them, I can certainly justify some things with that. I do struggle with the LEED mentality for sure on their forms, I wish we could just submit the calcs alone, the LEED forms and general/unspecific "guidelines" get me every time! LOL. Thanks again!

 

[Twinkie]

All this being said ... if you ever have real engineering issues, especially SWM that come up in your reviews, feel free to contact me. I have an ear at USGBC.