After going through the Online Reference Guide for this credit a few times, and then watching the two videos that illustrate how to calculate the various parameters (and feeling like I was running in circles), I thought I'd share some revelations I had, and would love it if USGBC or others in this community could confirm my understanding.
Once you understand how to calculate the various storm events (in inches of rainfall) and the associated runoff volumes from different site surfaces, the next question is: how do you determine the volume of runoff that can be treated in your various LID strategies. For example, I have a project where the building is covering most of the site, and the project is planning to include a large rain garden along one length of the building to treat the runoff draining from the roof. But I don't really know how much rainwater volume that rain garden can treat.
While it doesn't say so explicitly, the online reference guide video suggests that the volume of rainwater treated is simply the volume of the LID strategy in question. That is, it's saying that if I have a 2' deep, 100 SF rain garden, then it can treat 200 CF of rainwater. That seemed like a gross oversimplification, so I emailed Michael DeVuono, one of the guest experts on this page, and he confirmed that that is indeed the case (thank you Michael) -- that the volume of rainwater treated is the volume of your LID feature. He adds that you can actually treat more that this volume with this LID feature since the soils underneath will accept some of the rainwater based on that soil's porosity.
So if I have a building with a 20k SF roof, and the storm event I'm managing is 1.5", then I need to treat 2500 CF of water from that roof (20,000 * 1.5/12). Which means that the volume of my LID feature should be 2500 CF. So if the LID I'm using is 2' deep, then the area of that LID should be: 2500 / 2 = 1250 SF.
It doesn't really say anywhere in the Reference Guide how to calculate the volume of water treated with a given LID, so I thought I'd share and see if there was agreement here.
Thanks!
Josh
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
July 23, 2015 - 8:11 am
Josh, I just want to add that the runoff is only treated if it infiltrates (and it should do this in a reasonable amount of time (96 hours)).
If you try to build a raingarden on clay soils, you will end up with 2 feet of standing water, which does not meet the intent of the credit. You are typically looking for soils with an infiltration rate greater tan 0.5" per hour. Of course there are other alternatives, such as an underdrained design you had shown me.
This isn't really spelled out in the Reference Guide because LEED is giving a lot of leeway to the civil engineer to demonstrate compliance.
Joshua Radoff
Renewable and Sustainable Energy Specialization Lead, MENVUniversity of Colorado Boulder
LEEDuser Expert
45 thumbs up
July 23, 2015 - 10:08 am
Thanks Michael, for the clarification. It seems that there's a big difference between having and underdrain, which conveys the treated rainwater to the city sewer (mimicking natural hydrology and run off rates as best as possible for an urban site), and one that infiltrates. I agree that infiltration is a straightforward compliance method. But the engineers we're working with are more comfortable with an underdrain which is more in line with typical urban design (around here anyway), so it's important to be able to give them assurance that such a drain would comply, and we seem to be agreeing that it does, and that this is still acceptable "LID", which is helpful.
Adam Stoker
Sustainable Infrastructure EngineerCity of Calgary
2 thumbs up
July 23, 2015 - 11:02 am
Be careful Joshua, I think you might be going down the wrong track. An 'underdrain' which connects to the storm sewer would very likely not meet the intent of the credit. This probably does not mimic the natural pre-development hydrology of the site and no special allowances are given for urban projects to only do 'as best as possible'. Special allowances are also not given for design teams that 'aren't comfortable' with certain technologies. The overall intent is to reduce volume and improve water quality and it seems like an underdrain might only accomplish the latter.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
July 23, 2015 - 11:00 am
Adam his underdrain is perforated.
Now remember, Josh. They need to look at the exfiltration hydrograph. This discarded volume needs to exceed to the required volume control.
But I am really only throwing darts without knowing the entire job.
Joshua Radoff
Renewable and Sustainable Energy Specialization Lead, MENVUniversity of Colorado Boulder
LEEDuser Expert
45 thumbs up
July 23, 2015 - 11:05 am
Adam,
I'm looking for some clarification on where LID ends and grey infrastructure begins. Also, while I had expected the calculations to start with evaluating what pre-development hydrology looks like, the credit doesn't appear to ask for documentation or calculation of that at all. You don't have to determine and replicate pre-development runoff or infiltration, for example.
Take a green roof, which would clearly be considered LID. If one uses a green roof to treat the rainwater incident upon it, can it then be released into the city storm sewer? I would think so, but there's not really any clarification on how to determine whether or not something has been "treated" by an LID in question, which is the nature of my comment. So since the credit isn't asking for infiltration explicitly, it seems that an underdrain would make a rain garden the same as a green roof -- the water is detained and treated as it moves through a planted medium, and then released off the site, much as it would in a natural condition -- except that there's no stream channel to drain to, just the street.
Also, while allowances obviously aren't made for "discomfort" with given technologies, it's also nice to be able to guide project teams towards solutions based on an understanding of what a credit is actually asking for.
Adam Stoker
Sustainable Infrastructure EngineerCity of Calgary
2 thumbs up
July 23, 2015 - 11:12 am
The green roof scenario as you describe likely wouldn't meet the credit requirements (despite involving LID/green infrastructure). The key bits of language from the requirements that lead me to believe this are "In a manner best replicating natural site hydrology processes" and "manage on site". Unless the natural site hydrology had water draining off-site (rare, but happens sometimes with sites on very shallow or no topsoil over bedrock, etc.), your project won't likely be able to use any technique which sends a significant portion of stormwater to the storm sewer. I do agree that the credit language should probably be more clear (or the reference guide could at least get more specific with some examples) in order to avoid confusion on this topic. Can't say that I'm 100% correct, but that's my interpretation.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
July 23, 2015 - 11:16 am
We are working to clean this up, and make this more understandable, and provide some better guidance.
Summary .... volume, don't exceed the pre with the post for the 95, 98, (or 85th percentile storm) .... however you do that is up to you.
WQ ... whatever that post-development runoff volume is for the 95, 98 (or 85th percentile is) it needs to be treated "somehow."
Theresa Backhus
Sites Technical Specialist, LEEDUSGBC
66 thumbs up
July 23, 2015 - 11:26 am
I think most of the question has been addressed already, but it's important to stress that v4 approaches stormwater differently than 2009. One key aspect of the v4 Rainwater requirements is the word "manage." All runoff volume (100%) for the calculated percentile (85th, 95th, 98th) must be managed on site, meaning that it must be infiltrated into the ground, captured and reused, etc. If any runoff from your chosen percentile storm is ultimately sent offsite (outside of your LEED project boundary)- whether to the storm drain, a water body, etc- it will not meet the credit requirements, regardless of the quality of that runoff when it leaves the BMP. The 85th percentile path was added because the SS TAG acknowledged that this will be more difficult for urban projects.
Joshua Radoff
Renewable and Sustainable Energy Specialization Lead, MENVUniversity of Colorado Boulder
LEEDuser Expert
45 thumbs up
July 23, 2015 - 11:36 am
Theresa,
Is this clarification written anywhere? It would be a bit of a shock for the project to learn that 100% of the storm event in questions must be actually _infiltrated_ since it doesn't say that anywhere in the credit language or in the reference guide. Nor does it appear that way in the video on the online RG that is the only real instruction on how this calculation should be performed. If it does and I've missed it, can you point out where that would be?
Thanks,
Josh
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
July 23, 2015 - 11:43 am
It doesn't need to be infiltrated, the post development volume just can not exceed the pre development, and there are a hundred different ways this can be accomplished. The obvious are raingardens and reuse. But you can lose post development volume other ways as well; such as soil amending lawn areas or planting trees where they did not previously exist.
Follow some of those techniques in the BMP manual I sent you , Josh. There is never one easy or correct solution. Most times it is finding the correct combination.
Manuel Martin Delgado
Buro Happold Polska Sp. z o.o5 thumbs up
September 2, 2015 - 8:38 am
In our project we have two rainwater tanks capturing all the onsite water run-off.
The first tank collects water from roofs and then this water is reused for flushing.
The second tank collects water run-off from ground surfaces. Water retention is the only aim of this tank. After the storm, water is discharged to vortex settlement tanks (on site) and afterwards to the city sewer system.
Would an underground retention tank + vortex settlement tanks before water is discharged to city sewage system meet the credit intent?
thanks.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
September 2, 2015 - 8:49 am
Are you showing a reduction in volume?
Manuel Martin Delgado
Buro Happold Polska Sp. z o.o5 thumbs up
September 2, 2015 - 9:00 am
Currently the whole plot has impervious surface (parking lot). We will catch all the post development water runoff during storm events. So the post-development run-off is less than for pre-development. later on, the water from the retention tank will be discharged after passing through the vortex chambers.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
September 2, 2015 - 9:03 am
You are definitely on the right track for Option 1 of the credit.