I've been wondering about what could be main reasons of the change made in LEED v4 regarding rainwater. If anyone could enlighten me, please share your expertise. As a non-expert, this is my wild guess.
LEED v3 is using 1,2-year 24hr design storm (worst case). It assumes that it's inevitable for runoff to occur. As long as, it maintains or reduces existing condition's runoff volume/rate, it's fine.
LEED v4 is using 95th/98th percentile. Excluding worst cases, project is required to make sure that all of the rainwater should be managed within the site. (what about rainwater falling on the edge of a project boundary?). This requires installation of natural practices such as bio-retention, and for urban projects that can't accommodate such features, it will be hard to satisfy the credit, since building owners don't like dirty ground water to be collected and sent to the water tank in the building.
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Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
187 thumbs up
May 21, 2013 - 8:38 am
Your 90% storms are generally referred to as your water quality storms. There has been significant research down over the past decade which supports the thought that this is the storm you want to match pre>post. It is becoming more and more common through local regs to infiltrate or retain this delta volume on site as well.
The implementation of the credit will be easy for some jobs, a little more difficult for others, but it is certainly not asking above what is becoming more and more standard practice in the SWM industry.
Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11478 thumbs up
May 21, 2013 - 9:22 am
Good comment, Mike.Jim, this is just the kind of thing that LEEDuser sought to elucidate in our report on Key Concepts in LEED v4. I'd recommend it.
Theresa Backhus
Sites Technical Specialist, LEEDUSGBC
66 thumbs up
May 21, 2013 - 11:55 am
Hi Jim,
Building on what Mike describes above, LEED is moving towards the “percentile of rainfall events” methodology because we feel that it paints a more realistic picture of a project’s rainfall conditions. This method is equally effective at managing runoff and controlling TSS, and it has been suggested that it may actually be more cost-effective. Also, the industry has been moving in this direction for some time now.
The v4 draft includes a special path for zero lot line, urban projects. We realize that these projects may have fewer opportunities for onsite LID/ green Infrastructure, thus the threshold is lower.