Treatment options for roof runoff? 1

[ctbailey]

I have been hit upside the head by a State Regulator, saying that they now require all roof runoff to be treated. Just as they require parking lot storm water to be treated, (to remove oil,grit, etc.)

So, I ask the collective:

If a metal roofed building is constructed with gutters that are tied directly to an underground pipe network, have other designers been required to treat that roof runoff? and what method did you choose?

My original response was "that is ludicrous!" That did not go over well, however.

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[greenone]

Nope, haven't had that happen in CA yet. Just commercial roofs? Residential too? Is it retroactive? Let me know where so I can make sure I don;t get any projects there!

 

[tsgrue]

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ctbailey wrote: "If a metal roofed building is constructed with gutters that are tied directly to an underground pipe network, have other designers been required to treat that roof runoff? and what method did you choose?"

1) Is there a requirement for the roof to have gutters? I would remove those from the design if possible. You can still install deflectors at entrances.

2) Incorporate small bioretention areas for stormwater infiltration around the site and at building envelope, especially at the drip line. This shouldn't be a problem unless you have swelling clay problems (and, if so, you better handle them anyway). Straws, grasses, bushes, or trees can be used in these. (Saves on irrigation costs as well).

3) If you have to incorporate roof drains, then either direct them to the bioretention areas or (if that is not possible) to "soakaways" and "exfiltration trenches". Try a Google image search for "soakaway" and another for "exfiltration trench" and you'll see examples of these.

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[ctbailey]

This roof will be for a large building, which will house an indoor sports facility in New Hampshire.

I have tied the roof gutters into the overall drainage "system" because of the local requirement of post-development peak flows shall not be greater than the predvelopment flows.

I have proposed a uniformly graded-rock filled trench, on the order of 7feet wide, 6 feet high, and 340 feet long. The downspouts are to tie into an 18" perforated pipe, which will be embedded in the rock trench.

This trench was my attempt at designing an infiltration system.

Now the regulators tell me this rock infiltration system does not "treat" the runoff.

The system is drained by a 12" culvert, and the bottom of the trench is drained by a 6" perforated underdrain.

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[tsgrue]

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Exfiltration trenches will provide similar treatment as infiltration basins / bioretention areas, but without the advantage of the topsoil interactions. In simple terms, incorporate green mulch mixed with sand/loam into a layer around the exfiltration trench gravel exterior. This will provide additional treatment.

Also see the following for references...

On-Site Stormwater Management Guideline
New Zealand Water Environment Research Foundation

http://www.nzwerf.org.nz/publications/sw602/sw602.html

Development and Testing of a Partial Exfiltration Trench for In Situ Treatment of Highway Drainage
Transportation Research Board of the National Academies
Summary:

http://trb.metapress.com/content/r273816q2760242x/

Report:

http://trb.metapress.com/content/r273816q2760242x/fulltext.pdf

2007 January 30

Stormwater Technology Fact Sheet: On-site underground retention/detention
EPA 832-F-01-005 2001

http://www.epa.gov/npdes/pubs/sw_tech_fs_runoff.pdf

Transient Rainfall-Runoff Loadings to a Partial Exfiltration System: Implications for Water Quantity and Quality
ASCE J Environmental Engineering, 2005 July
J J Sansalone, Z Teng

Professor Simon Beecham
Professor in Sustainable Water Resources Engineering
University of South Australia

http://www.unisanet.unisa.edu.au/Staff/Homepage.asp?Name=Simon.Beecham

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[tsgrue]

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Below are additional resources...


Storm Sewer Design without Inlets, Using Perforated Pipes
thread162-194214

http://www.eng-tips.com/viewthread.cfm?qid=194214&page=1

http://www.duluthstreams.org/stormwater/toolkit/underground.html

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[ctbailey]

tsgrue:

Thank you very much for all the resources. I think it is time I seriously delve into these new fangled stormwater systems.

It appears the "good ole days" are long gone.

Thanks again,

Craig Bailey

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[bltseattle]

We've used "Stormfilter" to treat runoff from metal roofs as required in WA state, where the system has passed the state protocol for BMP testing and approval. There is an above-ground unit that can intercept downspout runoff for sites with HGL constraints.

 

[jartgo]

the short answer to your question is, "yes."

I am typically required to treat the water from any impervious surface, including, roofs, walkways, parking lots, gravel driveways, golf cart paths, etc.

I've found the best option is to break it up into really small drainage areas on the site and treat with individual bioretention or similar practices.

Meeting both quantity and quality regulations can be done by a single practice but it's usually my preference to separate the tasks and only size the treatment structures for the required treatment volumes. For a large site you might consider using a model so that you can get "credit" for the quantity reducing value you get out of all the small treatment storages throughout the site.

If you can drain the entire site to one wet pond (my ideal drainage area for this is about 10-20 acres) then you could meet quantity and quality requirements with the pond.

All treatment measure require specific design criteria (usually specified by the regulatory agency), as you found out with the infiltration trench, there are soil type requirements, distance from high water table, sideslope requirements, vegetation requirements etc.

Your regulators will likely have a design manual to help you out.

 

[sam74]

Here is a link to some details for infiltration applications

http://www.cityofknoxville.org/engineering/bmp_manual/ST-03.pdf