Trash collection nets on the end of storm drain pipes

[beej67]

I have a client who manufactures a product that collects trash and debris in storm drain systems. There's some details and some high-tech-ness to the product, but the basics are that it's an expandable plastic net you bolt onto a discharge headwall, then unbolt it when it fills up, bolt a new one on, and carry it off to a landfill. They also offer a way to set them down in a concrete channel or ditch by building a weir wall, with several short sticks of pipe sleeved into the wall as orifices, to which you can bolt the nets.

In the client's attempt to market this product to private developers and municipalities, the municipalities often ask "how big a one do we need?" or "how do you design it?" Which is where I come in. Some municipalities even require that the design includes a "50% (or whatever) clogging factor," although this clogging factor isn't ever fully defined by the municipality.

So what I'd like to do in this thread, is describe my thoughts on how these things should be designed, and ask for feedback.

Clogging Factor -

As I understand HEC-22, they typically define clogging factor in terms of a direct reduction of the hydraulic capacity of an inlet or drainage appurtenance. So an inlet that conveys 5 cfs at a certain head with no clogging factor, conveys 2.5 cfs at the same head with a 50% clogging factor. (correct?) This approach seems like the most reasonable for me to use, especially since I don't have access to any field information comparing the capacity of one of these nets with its "percent fullness." Simply define "50% clogging factor" to mean "reduction in capacity by 50%."

Installation in an open channel via weir wall -

For the case where they wish to install something in a channel / ditch / etc, a simple and intuitive sizing method would be to treat the short stubs of pipe as orifices, apply the clogging factor to their capacity at a given head, treat the wall as a sharp crested weir, and calculate a head that gives a combined flow through the system equal to the design flow. Then the engineer can use that head as his tailwater condition for sizing upstream structures and checking HGLs through the upstream system. Would you folks agree that would be a conservative design approach?

Installation on a pipe end of a culvert or storm sewer network -

This gets trickier. It would seem to me that the thing to do is go with the orifice assumption again, apply the clogging factor, and determine a head that drives the design storm through that reduced capacity orifice. Then use that head as your starting tailwater for the HGL calculations in the system. Would you all agree that is a conservative design approach?

Now obviously in a perfect world we'd set one of these things up in a large fluids lab and measure the head-discharge relationship as we fill it full of trash, and use that field testing to describe the process in more detail, but I'm not sure my client has the money available to set something like that up without selling some of them first, so we need an outline for a design procedure.

Parallel question: What sort of liability am I taking on if I produce a document outlining this design procedure? If someone else follows the procedure, and it fails, can they come back on my client? What about me? If so, can I absolve myself from liability with some well phrased notes and caveats in the procedure documents?

Thanks in advance.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

If you stamp it, you assume some liability. By law, I believe you will be required to stamp your report so yes you would be at risk. However, it doesn't sound like a product I would recommend to any of my clients because of the increased flood risk. I especially don't like the idea of putting one across an open channel. In fact, any kind of trash rack on an open channel seems like a very bad idea.

 

[Ryb01]

I would have to agree with cvg....if you stamp it, you be exposing yourslf to some liability. I would think that this product, if viable, would need to go through a similar process to all the other SWM products on the market before being accepted and put into use. Being, testng, proper approvals and certification, etc. Investment in a product always seems to be a challenge to overcome. If this is a product that you feel will have promise, be upfront with your client let them know what it's going to take to get it infront of the right approval agencies. I would think that you could assist in this regard by setting up the contatcts and providing input where required. Personally, I would not be getting involved in any kind of testing procedures or certification process. At most, I may offer to provide some input (if asked) by an approval agency.

With any new product that enters the market, there's always concerns with functionality, practicality, feasibility, etc. CVG, hit on a couple of good points. This product sounds like it has some good characteristics but is it really applicable for SWM use? I would have concerns about placing this product in a channel and/or at a pipe outlet due to flooding, maintenance, feasibility, etc.

My two cents......

 

[beej67]

I could put them in touch with academic institutions, and facilitate getting the things lab tested in that way, but what else can I do? This sort of thing isn't exactly in ASTM's court, is it? If not, whose?

What do you mean exactly by "get it in front of the right agencies" ..? My experience, at least in the southeast, is that every municipality makes their own decisions about what sorts of stormwater BMPs they allow.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Ryb01]

beej67,

I would assume that your in the consulting business? In my mind the best thing you could do is consult and support your client in regards to the process involved to gain exposure to this product. Beyond that, I wouldn't suspect that there would be much more that you could do. Testing and certification of anything is a specialized field.

It may be in the ASTM jurisdiction. The only advice I could provide in this regard is to look into other SWM products and see if they have ASTM certification. Or call the ASTM just to inquire if "a SWM product" (not neccesarily your product) would need this type of certification. If so then, this is a road that your client will have to travel.

By "getting them infront of the right agencies" I thinking of a few, namely the NJ DEP and also EPA if applicable. Speaking to someone at either one of these agencies and or a local municipality, may be able to point you in the right direction. What kind of certification would they be looking for specifications, criteria, testing requirements, etc?

Again...just my two cents.

 

[beej67]

My experience with stormwater BMPs in the south, particularly Georgia, is that the Georgia EPD keeps their fingers out of certifying anything, and the municipalities do it individually. What's allowable on one site in one county won't be allowable on another site in another county. Typically the BMP manufacturers spend a lot of time and effort running around from one muni to another one convincing them that their product works. Sometimes they pal up with the municipality to do a pilot installation or whatever, and watch that for a year before the thing reaches final approval.

And my client is already doing that, and has a municipality who'd like to buy some to install them and see how they work, but wants to know how a "typical design" would work. That's all I'm really being asked to provide. I just want to be prepared for what happens next after I provide that.

So to your knowledge, there's no "independent agency" that would have their fingers in testing this thing other than perhaps ASTM?



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[francesca]

In Tennessee, TDEC does a SW BMP certification course and publishes a manual. The DOT requires certified people on all construction sites, prefers designers to be certified. The DOT sanctions products and municipalities, at least smaller ones, follow the DOT.

IMO, if you've got trash in your storm sewers, the problem is upstream. Catching it is just fixing the wrong problem.

 

[beej67]

Well I could travel across the country giving "Give a Hoot Don't Pollute" seminars, but it's really not my specialty.

We have a similar sort of certification scheme for erosion control in Georgia, which I have, and if the product was an erosion control product then I'm sure we'd go straight to the Georgia EPD. But to my knowledge, Georgia EPD has stayed out of the "certification of stormwater BMPs" issue. And even if they did, the big boogyman down here is TSS, not trash.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[dicksewerrat]

I concur with Francesca. The best way to deal with trash as it floats along is to get it out of the flow early. Self-cleaning trash guards on te upstream end of the pipes. If this item could be pushed off to the side of the flow without human interaction it might be received better by DOT's. Running out to every outfall after a sorm would tend to get expensive.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[beej67]

Cleaning trash out of 20 inlet nets might be more expensive than one outlet net. Regular street sweeping might be more expensive still. Walking a couple miles of creek with trash bags to get the crap out might be worse than even that. I don't really know these things, I do hydraulics, and if folks want to buy them I'm sure they'll run the numbers. Part of what my client is selling, I'm led to believe, is that it's cheaper to pick up one net with a crane truck than it is to spend all day pulling manhole covers cleaning out in-structure traps.

Again, my point with this thread is not to espouse the virtues of the product, it's to discuss the hydraulics of the product. I'd like to focus the conversation there, if that's okay with the group.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[hokie66]

Agree with Francesca. Stopping trash at the outfall does nothing to prevent trash clogging the drains at the infall, and can lead to increased local flooding. The outlet nets would be collected after the fact, which could be too late.

 

[Drew08]

I always considered trash as TSS, but that's another debate. I think for a proper design, some experimental basis is needed. A purely theoretical design won't be worth the trash it collects. If your client wants to sell this, he has to do his due diligence.

Flow capacity in my mind, whether in a channel or pipe, means head loss. So, I'd start by looking at the head loss through the net. Perhaps define head loss as [Loss = K x V^2/2g]. Determining the K part is the tricky part that will have to be determined experimentally for various degrees of "fullness". A scale model might be appropriate before a full sized test is done. As for what "clogging factor" means, perhaps it can be defined as a reduction in K based on the experimental data and expected trash content after some extended period of use without changing, with the assumption it will be neglected and not changed out when it ought to be.

The wrench in the idea is if one of these things is installed, the property it’s on changes ownership a few times, and the net is forgotten. What happens? Is there an overflow bypass? Or, will the culvert loose 100% capacity at some point? This is the first question I’d ask if I were the permitting agency.

 

[beej67]

I suppose I should have mentioned this earlier, but there's an overflow bypass built into the things. The unit is an aluminum pipe that sticks out from the headwall, and has a bunch of square orifices punched in it on all sides for flow bypass, then the net (which is expandable) bolts onto the open end of the pipe.

I have cut sheets on the things that specifies the orifice locations/sizes. It would be fairly easy to develop a head-discharge relationship for all the orifices neglecting the bag entirely, which I agree would definitely require experimentation. How accurate would an analysis like that (head-q relationship for the family of orifices) be? I do know the total area of orifice punch-outs exceeds that of the pipe cross section.

As far as ownership goes, most of their targets are municipalities so far, so they likely won't change much, and if they did, someone could just pop the bag off and not put a replacement bag on.


Also, trash is absolutely not TSS.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com