Thank you for all of the information provided on this forum. I am confident in all of our calculations up to the Storm Interval and Drawdown calculations. If I understand things correctly, the calculations are fairly simple:
Example
Storm volume: 119,000 liters
Storage Capacity: 148,000 liters
Dawdown Rate (reuse): 2,300 liters/day
Storage + (drawdown x 3 days) - Storm volume = capacity remaining (or runoff)
Storm 1
148,000 + 6,900 - 119,000 = 35,900 storage remaining after three days.
Storm 2
Storm volume - Capacity remaining - drawdown (1 day) = Runoff
119,000 - 35,900 - 2,300 = 80,800 liters of runoff (post-dev)
Assuming a fairly impervious Pre-dev site condition let´s assume a calculated runoff of 115,000 liters per 2 year 24 hour storm:
Pre-dev runoff: 115,000 liters
Post-dev runoff: 80,800 liters
Runoff reduction: 34,200 liters
Runoff reduction: 30%
Is it that easy? Am I missing something?
Thank you!
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
January 28, 2016 - 11:36 am
I am not following your post at all, but simply put, you need a minimum discharge rate of 0.02 cfs to drain that tank full in 72 hours.
Your theoretical 2 year storm, at that same drawdown rate will drain in 56 hours.
Leandro Silva
Novva Solutions17 thumbs up
January 29, 2016 - 9:09 am
Thanks, Michael, I will try to clarify. In case two, you must reduce runoff by 25% as compared to pre-development conditions.
From what I understand, the tank doesn't need to be empty. The storage capacity and drawdown rate must be sufficient to reduce post-development runoff by 25%. You have the two year 24 hour storm and, in our case, there is no runoff due to the tank capacity and drawdown rate, however three days later you have the same storm event and this is where you measure post-development runoff? Or due you continue with the deluge every three days? I guess it is the storm interval that has me confused. Thanks again.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
January 29, 2016 - 10:49 am
You need to reduce "volume" by 25% not rate.
You are not doing that if you are simply capturing runoff and slowly releasing it. Your site hydrology, the pre vs post analysis, should be completed by a civil engineer with experience in stormwater management.
You need to show that the tank has sufficient volume to handle a 2nd 2-year storm after a pre-determined time (it is usually 72 hours, but this isn't actually specified in LEED).
You need to come up with a plan for reuse of the captured water, and you also need to pass a straight face test. This thing needs to serve a purpose.
You need to show how you are reducing volume by 25% through reuse, infiltration, etc.
Leandro Silva
Novva Solutions17 thumbs up
January 29, 2016 - 2:56 pm
Thank you, please bear with me.
We are demonstrating a 25% reduction in Post-development runoff. This does not necessarily require capturing and using all of the storm water volume. Our drawdown is achieved through reuse of rainwater to flush toilets and urinals (based on WEp1 performance case flush fixture use).
Storm 1
119,000 liters, storm volume
148,000 liters, storage capacity
6,900 liters, drawdown per storm interval (2,300 per day x 3 days)
148,000 + 6,900 - 119,000 = 35,900 storage volume remaining after storm 1.
3 days later, Storm 2
119,000 liters, storm volume
35,900 liters, storage remaining
2,300 drawdown (1 day)
Storm Volume - Capacity Remaining - Drawdown = Runoff (in this case)
119,000 - 35,900 - 2,300 = 80,800 liters of runoff after Storm 2.
Pre-development runoff 108,000 liters
Post-development runoff 80,800 liters
Runoff reduction is 25.18%
Please let me know if this is correct and, again, thanks for your time.
Leandro Silva
Novva Solutions17 thumbs up
February 8, 2016 - 2:50 pm
Michael, can you comment on my last post, please.
Leandro Silva
Novva Solutions17 thumbs up
February 11, 2016 - 1:02 pm
I guess not. I wonder, what am I paying for?
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
February 18, 2016 - 3:31 pm
Hi Thiago,
I'm sorry I missed your post. Regardless, I feel I have provided you enough guidance to attempt the credit or have your civil engineer attempt the credit. I will offer guidance and my opinion, but I will not provide engineering direction, check your math, or do the work for you.
Best of luck with your project.
Leandro Silva
Novva Solutions17 thumbs up
February 19, 2016 - 10:54 am
Michael, I am probably not describing my doubt clearly and am certainly not asking you to check my math or do any of our work. My doubt concerns what the LEED NC reference guide says on page 97 in the example given. It says that "the captured rainwater must be drained within three days... for the tank to be emptied before the next storm. If the drainage rate is slower, full capacity cannot be assumed to be available during the 2-year 24-hour design storm." My doubt is whether or not the system capacity truly needs to be emptied before the next storm
The requirement for case 2 is, as you said, to reduce runoff volume by 25% compared to pre-development conditions and I probably confused things with the example calculations I gave.
My question remains if, at the time of the Second Storm Event, the following it true:
Storm volume - Storage capacity remaining - Drawdown = Runoff in the case that the system in not completely empty.
Thank you
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
February 19, 2016 - 11:55 am
In a nutshell, your storage volume needs to accommodate a 2nd 2-year storm that occurs in 72 hours.
Does the cistern need to be empty? If it was only sized to handle the volume of the 2-year storm, then yes it needs to be empty. If you oversized the thing, then it would not need to be empty because you have supplemental storage volume built in.
If you only sized for the 2-year volume, and the cistern does not drain within 72 hours (lets say you have 50% volume remaining on Day 3), then an equivalent volume would miss the cistern at the next storm event (because the cistern would be full), you would thereby have 50% of the storm volume leaving your site.
I personally would model this in HydroCAD as a back to back event, but your approach with a detailed description could work.
The first thing I would want you to prove is that fact that you can use all the water you are drawing down within 3 days, so be sure to check that.
Leandro Silva
Novva Solutions17 thumbs up
February 19, 2016 - 1:11 pm
That helps a lot, thanks very much.
Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11474 thumbs up
April 20, 2016 - 12:01 pm
Thiago, FYI, the LEEDuser forum is a free part of our site and our experts volunteer their time. Please be kind to them. See our FAQ for more info.
Ashley Hu
Jr. Sustainable Building AdvisorPerkins+Will
8 thumbs up
July 25, 2018 - 5:52 pm
Hello Thiago Bondini,
I am wondering if you were successful with achieving this credit with your calculation methodology?