More infiltration questions 1

[proud2banerd]

Okay,

I'm trying to design a couple of subsurface infiltration beds at a site in NJ that's just shy of 2.5 acres. (By the way, this is my first time working in NJ, not to mention the fact that I've still got a lot to learn in general about this stuff!)

According to the NJ standards, the bottom of the bed needs to be 3' above the water table, which ranges from 6' to 8' down in this area. What adds to the fun is that I've been asked to fully infitrate the 10yr post development volume increase. I was hoping to keep the beds simple, using perforated pipes and stone, but I'm not sure if that (or anything else) would be a good option for this.
I need to provide a 12" layer of topsoil above the bed.

I'm running into a bunch of headaches:

For starters, how far down do I need to put my perforated pipes? I'd like to at least get them so that the center o the pipe sits at or below the frost line (30"), but if I do this with a 12" pipe, my bottom of bed will be 2.5' above the wt (once I add in a 6" stone bed beneath the pipe.

Secondly, my beds are HUGE. I'm going to take a closer look at the volume reduction calcs, but at the moment I'm calculating that they need to be around 30' wide, and this was based on including 12" diameter pipes in the bed, spaced at 3' on center. It sounds so ridiculous to me to lay out that many pipes. Am I missing some big picture here?

BTW, I took a cursory look at Stormtech, but it looks like their system may be too deep as well.


The overflow for one of the beds needs to tie into a storm sewer. In my limited past experience, I've done this with an orifice, below the frost line. Since the bed is going to be so shallow, and since it needs to fully retain so much volume, I assume that that won't be feasible this time. I was thinking about directing the overflow over a weir and into a catch basin (tied into the storm system). Would that be crazy?

Thank you all for your help!!!

 

[TerryScan]

Be sure to check your regs and ask about requirements related to "the water table". Are they concerned about the MEAN water table...SEASONAL HIGH water table? This can be critical in the situation you are describing.

I work in coastal delaware where your scenario is not an unusual project. Here, one must be 3' above SEASONAL HIGH WATER TABLE. Which can be considerably more restrictive than the gw level encountered in a boring taken today. It can also be a difficult level to determine without long term monitoring...or obvious evidence in the soil investigation.

Here, we do not get too concerned with the frost line in stormwate applications. It is usually tough enough to fit the system between SHW el. and provide adequate cover. And yes, it takes a LOT more area for underground systems to provide the same storage as above ground systems. It is not unusual for the MAJORITY of space that is not building on a 2.5 acre lot to be needed for storage in an underground system with high gw.

 

[proud2banerd]

Thanks!

Can anyone recommend a good, online course that would provide a practical basics on stormwater management and BMP's?

I'd like to find a class specifically geared toward engineers that provides real-world, usable info. I know some people like to understand every fifth order differential equation used to derrive every constant or formula, but that stuff's not really for me.

I've also had mixed luck with DEP courses--the ones I've taken seem to come from a scientist's perspective, rather than an engineer's.

 

[RWF7437]

try pdhOnline.org

 

[pittsig]

Try these sites

http://www.bae.ncsu.edu/programs/extension/stormh2o/stormwater.html

http://www.bae.ncsu.edu/people/faculty/hunt/index.html

 

[lindbls]

What's the area going to be use for post-development above the infiltration beds? If just an open space with little or no traffic, check out the "Invisible Structures' Rainstore3" (

http://www.invisiblestructures.com/)or

similiar system. They are a stackable system of interlocking grids-I believe 4" high by 4'x4'. The claim >90% porosity and I believe H-20 loading. However, check with the manufacturer about placing beneath roadways. If the area will be only open space, consider a pond. A subsurface infiltration is really just a pond filled with a lot of very expensive material.

Keep in mind future maintenance. How will the 12" pipes be maintained? Subsurface infiltration systems have their applications, but often you need to look at the long term cost associated with them. Adequate pretreatment is critical, otherwise you'll end up with an expensive infiltration bed that fails in a few years.

 

[RWF7437]

See thread:

thread162-190501

Especially BLTSeattle's contribution

 

[jwkdesign]

It sounds like to me you are designing an underground detention / infiltration system if you have a positive outflow downstream controlled by a weir. Are you sure you need to be seperated from the SHW table by three feet? The NJ BMP only requires a 2' seperation from an infiltration basin, I believe both subsurface and surface application. This measurement is taken from the bottom of stone or sand replacement layer.

Remember, that in the wonderful state of New Jersey, you will be required to remove 80% TSS prior to the runoff entering the underground system!!