Green Roof Model Techniques 1

[SPW]

thread194-123522

The thread below was provided by "Bltseattle" however since it was closed I could not respond. I am interested in the methods you used to model green roofs. We are currently in the New England area and our local reviewers are not up to speed with these new stormwater LID practices. Any input you could provide would be greatly appreciated...



bltseattle (Civil/Environme) 18 May 05 16:39
It's interesting, as more low impact development practices (porous pavement, greenroofs, etc) are used it gets harder and harder to adapt CN methods that were developed for conventional, deep soils. For example, one CN may match the volume of runoff for the proposed BMP but yield too high of a peak runoff rate. So you have to look at the CN as well as the Tc, using the Tc to control the runoff peak to a reasonable value. Case in point is using the gravel CN for porous pavement, what is the appropriate Tc to go with a particular CN? Does it depend on the size of the storm (probably should!)? There's still work to be done in making these BMPs model-able. My firm has actually developed an alternative model for green roofs since CN methods are so poorly suited for the processes that occur in these systems, and many of our clients want to install greeen roofs.

 

[stanford94]

i just finished a building with a green roof. but i think it's against the rules to post names/emails, etc.....

 

[SPW]

Hey there stanford94, you mentioned you just finished a building with a green roof. Anything you can mention regarding the runoff design parameters?

 

[SPW]

psmart I see that you have marked this thread for notification. Any comments that you might have I would be interested in reading them?

 

[psmart]

SPW - Unfortunately, there are no standards for this type of modeling, so the engineer frequently has to make their own decision as to how to model each specific situation. I'm just as interested to learn about other's experiences as you are, which is why I marked this thread.

Unlike a conventional runoff simulation, modeling "green infrastructure" requires a consideration of the water that is captured, in addition to the surface runoff. This applies to green roofs, rain gardens, porous pavement, and other partially-permeable artificial surfaces.

We have some basic information on modeling rain gardens and porous pavement at the links below.

http://www.hydrocad.net/raingarden.htm

http://www.hydrocad.net/pavement.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[SPW]

Peter,
Thanks for the links. We are in the process of trying to understand how the green roofs and porous pavement type facilities can meet the latest LEED requirements as well as provide proper flood management within the site limits. And to top it off get the local and state reviewing agencies to buy off on it. To date they have not be up front with reviewing these items so the leg work will rest on the developer's consultants to determine what works or doesn't. Any knowledge on state's that have adopted a methodology?

Thanks again for your comments and I will be sure to update you (through this site) on our progression.

 

[brandoncdg]

What state/city are you in for the green roof projects? Besides LEED certification, was there any other reason to use the green roof?

There are some recent strange requirements in the City of Los Angeles relating to the cleaning of stormwater runoff from hardscape areas. I am wondering if a green roof is not considered hardscape.

Why not be on the safe side and treat the green non infiltration type of products as hardscape in your calcs?

CDG, Civil Engineering specializing in Hillside Grading in the Los Angeles area

http://www.CivilDevelopmentGroup.com