We have a tall office building that has a cistern for rainwater harvesting. I have two related questions:
1. The amount of runoff reduced by stormwater harvesting system is based, I understand, on its storage volume, the rate at which the system is emptied, and the interval between storm events. How can I calculate the interval between the storm events? In the reference guide, example 2 "volume of captured runoff" the design storm interval is 3 days. Can the team use this as the default value for the project?
2. The volume of captured runoff is based on the area of collection surface. Considering that the building has terraces, does the team count the horizontal surfaces of the terraces in this calculation?
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
June 7, 2013 - 1:41 pm
A lot of discussion about time intervals today (see below). I do not necessarily agree with the 72 hour dewatering for a cistern, an above-ground basin, sure (its a mosquito thing), and as far as I know, the ref manual does not require all facilities to drain within 72 hours.
Recent research has shown that the chance of back to back 2-year storms within 3 days is less than 1%. If this system is for reuse, can you use all of the water that quickly? If you can, then sure use the 72 hours as your interval. But if you can not, I would make the argument that the interval between 1 and 2 year storms is 1 and 2 years. If the cistern is simply to control the runoff rate, I'd use the 72 hours.
Yes, you need to account for the terraces (or the paving below, whichever is greater). Just develop an impervious footprint, if the terraces are stacked above one another, i would not count all of them, just the upper most.
Jonathan Kirby
DesignerCroxton Collaborative Architects PC
3 thumbs up
October 25, 2013 - 12:49 pm
Michael, can you please direct me towards the recent research that you referenced above? I have searched the Villanova Urban Stormwater Partnership for information pertaining to the likelihood (less than 1%) of a 2-year storm occurring twice within a 3 day period. I need an excerpt of the research along with the source. Thank you for your assistance.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
October 28, 2013 - 1:19 pm
I'm not too sure exactly what presentation this came from, but this is simply a math problem.
You are seeking the probability that a 2 year storm will occur twice within 3 days.
Pn = 1-(1-P)^N
P = Chance of 2-year storm occuring any year - 0.50
N = 0.0082 years
Pn = 0.006