Flow Into An Infiltration Trench 2

[Ryb01]

I'm currently desiging an under ground storage facility, storing approximately 70,000ft3(2000m3). Will probably using Cultec Chambers, un-decided at this point. This SWM facility is being designed as one of many LID's onsite promoting infiltration.

We've been asked to allow major system runoff to infiltrate through the top of the facility. Currently we have a ditch/channel sized to convey major system flows to the facilty. I'm picturing a grass lined channel with a gravel bottom, gravel connected with the storage beneath.

I've been scratching my head to try and determine a reasonably accurate method of figuring out how to calculate a length and bottom area that I would need to "infiltrate" the flows into the facility as there would be flow in the channel (horizontally) and also into the gravel below(vertically).

Have a couple of ideas, but I'm not really comming up with any viable solutions. A couple of ideas for ex 1) Assume an area of the bottom of the channel has a void ratio of 40%, take the area and use an orifice equation, wouldn't be vaild as there's a 0.5% grade on the channel........2)Same approach as 1, use a weir equation, have doubts about the accuracy of this 3)Calculate flows through the gravel using darcy's law, not sure that equations can be applied in this fashion, as there's also flow through the channel 4) Modelling the bottom of the trench as a catchment in SWMM using Horton's infiltraiton with high infitration rates......

Has anyone completed a similar design? Any thoughts would be appreciated. Thank you in advance.

 

[beej67]

Neat! Sounds like a fun thing to design. Very interesting engineering challenge. Regardless of your design approach, you should make sure there's some contingency in your contract for periodic inspection in the first couple of years to make sure it's working properly, because no design approach is going to be "right."

I would NOT do the "40% void ratio assumed as an orifice" thing. Flow through porous media follows all sorts of different rules than orifice flow. Same for the weir option.

I would go with either the Darcy or Horton approach, and I'm interested to hear what some other engineers have done in this area.

The closest thing I've designed to what you're describing is a Florida style stormwater retention pond with a sandy bottom. Their design procedure calls for using Darcy flow, making a conservative assumption of the pressure gradient, and use the ratio of depth to bottom area to show an adequate recovery time. The recovery time is based on flow through the porous media via Darcy relationships.

The criteria for these things was in the "Florida Development Manual: A Guide to Sound Land and Water Management," although I can't for the life of me find a download link anywhere. I'd heard it was under revision, maybe that's why. I can email you a copy if you like, follow the link in my sig and I'll shoot it your way.

Potential hurdles you need to think about in advance: Will there be any suspended sediment or bed load advected into your ditch? If so, your gravel will clog over time, so you need to think long and hard about how to minimize that. Some kind of forebay might help.

Something else you might consider if you're VEing the thing, is those inverted open bottom chamber things StormTECH puts out. I've never done a design with them, but always wanted to. They seem like a very slick system.

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

http://www.campbellcivil.com

 

[SMIAH]

Here's a document on the design of sandy filters.
It could be similar that as the channel as a bottom slope of 0.05% (?).

 

[Ryb01]

Thank you beej67 and SMIAH for your replies.

We have a OGS u/s of the facility to treat the first flush, however, beej67 , you bring up a good point about addtional sediment clogging the gravel.

I have a copy of the florida development manual, I'll take a look through there and also the document that SMIAH has provided. We have a bit of time to work with to come up with a detailed design solution.

Thanks again for your help, I value your opinions.

 

[brandoncdg]

I am curious, when you contacted Cultec about using their product did they give any advice? Not sure what companies will still help basically design something for free so you spec their products out.

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading

http://www.civildevelopmentgroup.com

http://www.civildevelopmentgroup.com/blog

 

[beej67]

I just looked up the Cultec system, and it's very similar to the Stormtech system:

http://www.stormtech.com/

What they recommend, is something called an 'isolator row' for debris and trash management. Instead of catching their inflow through a gravel trench above like you intend to design, they ask you to direct flow into the first row of their chamber manifold via a normal manhole / storm drain pipe. Then you wrap that first row in filter fabric, and you install a maintenance manhole at the other end. All the trash/silt/debris ends up stuck in the first row, while filtered water percs through the manifold for storage, and you install a discharge control at the far end of your system. They have guides about routine maintenance for that 'isolator row.'

I did quite a bit of research on porous pavement systems in grad school, and one thing that always popped up with successful designs vs unsuccessful ones when it comes to infiltration, is failsafe overflows. If you're married to the idea of transferring the flow from surface flow into the chamber system via a gravel infiltration trench, consider putting a pedistal top inlet or drop inlet at the far end of your trench, maybe with its rim a little ways up from the trench invert, as an overflow to get water down into your chamber manifold if the gravel clogs.



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

http://www.campbellcivil.com

 

[amb2002]

I have used the Stormtech system beej67 mentions sucessfully on two sites. My local ADS/Hancor representative was very helpfully and provided a system layout drawing specifically for my site and construction detail drawings for my use. The rep was onsite to assist the contractor with any questions during the construction of the facility as well. I might add this assistance was all for free.

As brandoncdg mentioned, if you contact Cultec, they are likely to provide similar assistance.

I like beej67's idea of the pedistal top inlet at the downstream end of your channel.

You might consider consider breaking up your infiltration trench channel with short berms to create shallow ponding during small storm events.

The Kansas City chapter of APWA has a BMP design manual here:

http://kcmetro.apwa.net/chapters/kcmetro/specs/APWA_BMP_ManualAUG09.pdf

that details a infiltration trench design in a similar method to the one SMIAH provided.

 

[beej67]

Lucky you, amb2002. I never have gotten to use the dang things, although I've tried to spec them on half a dozen jobs. The price point never quite worked out. Either land is so expensive that there's a parking deck, in which case you're stuck with a concrete vault, or land is cheap enough to where you can afford to have a surface pond. Then nowadays land just isn't being sold.

I've done a few infiltration designs with non-proprietary systems just speccing out perforated pipe in stone matrixes, though, and they seem to work okay.

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

http://www.campbellcivil.com

 

[amb2002]

The times I've used the Stormtech system they were installed under parking lots to maximize the utilization of the site.

It's been my experience, regardless of the price of land, the owner never wants to leave adequate area to address stormwater.

 

[beej67]

Yup. My problem was we never ran into a site that had room for a parking lot and didn't have room for a pond. The highly urban stuff in Atlanta became so urban they wanted decked parking, or was so tight that underground utilities wrecked the potential drain field locations. And the suburban / rural stuff had enough unbuildable room in the corners that we could shoehorn the stormwater management there. Just unfortunate circumstances really, I'd always wanted to design and install one.

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

http://www.campbellcivil.com