Infiltration bed and outlet structure

[joatmon22]

I'm designing an infiltration bed to take roof drains for a new house. I'm fairly green with this process.

The bed will consist of perforated pipes in a gravel bed wrapped in geotextile.

If some of you could provide tips on potential pitfalls, I would greatly appreciate it. (aside from the fact that I should provide an overflow and oversize the thing to begin with.)
Also, I can use some help in designing the outlet structure:
Larger storm events will empty into the public storm sewer. I plan to use a riser with orifices. Do any special measures need to be taken to ensure that there's no siphon effet (and if so, would providing a vent in the bed, just before the outlet structure do the trick--assuming that the outlet structure is also ventilated)?

Thanks for your time!

 

[LHA]

Why put orifices in the riser? Just size the diameter of the riser to handle the 100-yr inflow as a weir. Set the crest at the top of your infiltration volume. All design volume stays in, and no siphoning can occur.

Engineering is the practice of the art of science - Steve

 

[joatmon22]

Thanks for the response! Okay, here's another question, since I'm pretty green at this:
Is there a rule of thumb for distancing infiltration beds away from trees? There is a line of mature trees near the intended location of the bed. They appear to be a combination of maples (silver maples, I believe) and tall evergreens- I don't believe either have a reputation for problematic roots, but please let me know if I'm wrong. I'm planning to provide a 10' offset from the centerline of the tree-trunks to the nearest edge of the infiltration bed (this should also put the bed beyond the limits of the tree canopies).

Does anyone have any advice or comments?

Thanks!

 

[ACtrafficengr]

Leaves can clog your drains. Provide gutter caps and cleanouts!


"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[cvg]

the roots will go out to the drip line of the tree, 10 feet from the trunk will not be sufficient

 

[ACtrafficengr]

Also, considere the mature size of the trees. It sounde like they may well be immature and will grow larger. A 10' dripline is pretty small.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[blueoak]

I might try calling up to the Tahoe area, try the local NRCS offices or Soil and Water Conservation District(Natural Resource District). They got a bundle of money under Clinton for water quality and have been doing a lot of infiltration driveway and other projects.

http://www.tahoebmp.org/general.aspx

and

http://www.ntcd.org/bcp_moreinfo.html#

I would use large enough gravel (maybe up to small cobble) so the pipes are needed and not worry about roots. Call it bonus transpiration losses. Trees will suck a lot of water up if available. Other than some bad stuff hurting your tree (which is nice to know I guess) do you care if the roots go into your gravel? Bioretention plants trees in the infiltration areas. Just get an arborist to make sure you don't kill the trees.

 

[joatmon22]

Thanks, all good advice.

I will definitely run the design past the landscaper.

In the meantime, I've been thinking about this a lot--My bed is going to be roughly 70' long by 10' wide by about 4' deep. (We need to fully infiltrate a 25 yr storm). The bottom of the bed will be sloped at 1% toward one outlet structure at the end.

Originally, I was going to use 18" perforated pipes, but I was thinking that if I bump them up to 30", I might be able to reduce the chances of the roots choking the pipes.
(Bear in mind that the trees will be at least 10' away, and that they're mostly coniferous--I'm not a tree person by any means, but I believe their roots grow down, more than out).

Any comments?

 

[LHA]

I wouldn't slope the bottom. That will concentration your percolation and might lead to failure. Unless you are just sloping the pipe, that is OK.

With 30", your trench can be a little shorter, 5'-10', if space is an issue.

Engineering is the practice of the art of science - Steve

 

[joatmon22]

Thanks, Steve! Actually, the local code states that the bottom of beds need to be sloped at 1%. I'm glad you brought up the concentration issue (I am pretty green with this type of design). Is that a common problem with infiltration beds and trenches?

Double-ring tests indicate that we have a 1.75 in/hr infil rate at the proposed bottom of bed elevation.

Please let me know if you have any more comments.

Also, let me know if you have an thoughts on these trees.

Thanks again, everyone for all of your help!