LEED Credit 6.1

[txageng98]

I'm working on a project in Dallas that is pursuing LEED Credit 6.1 - Stormwater Quantity. My questions are these......

- Does detention count as mitigation of stormwater volume for the appropriate storms? It obviously meets the intent for rate, but I'm not sure if it would qualify for volume.

- If detention does qualify, how do you calculate the pre-developed volume? Is this something you have to do in HEC-HMS or is there a down and dirty way to do it?

Thanks for any tips....

 

[bpattengale]

Detention does not count for the Quantity Credit, detention is only temporay. The intent of the credit is to mimic the predevelopment runoff volumes. Depending on the ammount of impervious area on the site there are different rules for the site. You'll need to use some form of retention such as infiltration, stormwater reuse(irrigation), pervious pavers, ect.

I'm assuming that if you're concerned with the quantity part of the credit the existing site is 50% or greater inpervious. If the existing site is less then 50% impervious you only have to meet the rate part for the credit.

The pre-developed volume would be the volume of runoff from the site as it sits today, before your project. You can use the SCS(NRCS now)Unit Hydrograph method to calc the runoff by hand or use a software such as HydroCAD(My personal choice) to calc the runoff. A lot of software is just an automated version of the SCS method.

If you're going to be doing a lot of this type of work or stormwater in general invest in HydroCAD. The software is relatively easy to use and works great for runoff volumes and pond sizing. Some of the more complicated system modeling is beyond the software's capacity but for most smaller sites it work excellent.

 

[txageng98]

bpattengale,

thanks for the response...i am in the situtaion where existing imperviousness is less than 50%. when reading the LEED reference guide, Case 1/Option 1 requires "implement a stormwater management plan that prevents the post development peak discharge rate and quantity from exceeding the predevelopment peak discharge rate and quantity for the 1yr/2yr 24hr storm".

i'm interpreting the "quantity" as volume and obviously "rate" is rate. how do you interpret "quantity"?

 

[bpattengale]

Sorry, I was looking at an older version of the Credit. You're correct in your iterpretation, Version 3 has changed to include the quantity control in Option 1 also. I haven't done a LEED project under the newest version.

I'd look at stormwater reuse as one of the more cost effective options to reduce the runoff. Most likely it would be reused as irrigation, but on the extreme end it can be used similarly as grey water for flushing of toilets.

Pervious pavement is also another option, assuming you have soils that will readily infiltrate water, or else it becomes more of a detention facility then retention.

Green roofs are another option, but I've yet to see one put in on a project where it was proposed. The liturature that I've read has them at about the same cost at traditional roofing systems, but usually the estimates from Contractors come in substatially over the traditional systems, plus they can increase the building's structural requirements depending on the type of system used. So green roofs are usually the first thing to get cut when the buget starts to grow. The cost factor could also be that there aren't a whole lot of contractors experienced with them in my area of the county (Minnesota).

In general getting LEED certified is not cheap, especially at the higher levels. Just a heads up when the cost start coming back.

 

[Trackfiend]

You could look up what's called "Credit Interpretation Rulings" or CIRS from the USGBC. I've worked on a few LEED projects in dealing with the stormwater credits, SSc6.1 and SSc6.2, although all of these were LEED version 2.2. The CIRS helped a great deal in seeing what other contractors or engineers were dealing with in trying to implement infiltration and flow rate control. The CIRS will give you a good feel for what is allowed per past credit rulings. A note of caution however, is that just because the ruling was made on one particular project does not automatically mean that it will or can be applied to your project.

On one of my past projects, there was an existing infiltration plan for the entire site (including the LEED project boundary). Since this plan was already in place and currently in use, I was able to use this as my quality control for credit 6.2 without having to design or implement a new strategy for the LEED project site.